Fang Tao - Optimization of the annular heat pipes cooling system design and operating parameters

Optimization of the annular heat pipes cooling
system design and operating parameters
A Thesis
Presented to the Moscow Aviation Institute
By Fang Tao
Thesis Supervisor
Professor Igor Shirokov, Professor Nesterenko
JUN. 2019
Abstract
With the development of high-performance aero-engine technology, the
requirements for increasing the thrust weight ratio and life cycle of the engine
continue to increase. The working conditions of the combustor are getting worse.
Wall temperature gradient and maximum wall temperature are two important
factors limiting the service life of the flame tube, so advanced cooling method is
a key factor. In addition, the problem of pollutant discharge is becoming more and
more prominent. ICAO formulated severe standard for pollutant discharge. Low
pollution combustion technology has become an important research problem in
the field of aviation.
A lot of effective works have been done on the cooling method. The floating
wall combustor technology is considered to be the most ideal solution to improve
the combustor life, and the key design technology is about selecting the proper
cooling structure and connection structure form. About the low pollution
combustion technology, GE has developed from single annular combustor (SAC),
double annular combustor (DAC) to twin annular premixing swirler (TAPS). Lean
Direct Injection Combustor (LDI) is another promising technology.
Based on the double annular combustor of GE90, a new type of head
structure is proposed，which is a radical triangle staging floating wall combustor.
The initial design parameters of the combustor are determined by calculation. The
UG software is used for modeling, the ICEM software is used for meshing, and
the Fluent software is used for numerical simulation. The results were processed
using Origin, Tecplot, and CFD-post software. The flow characteristics,
combustion characteristics, fuel-gas distribution and NOx emissions are analyzed.
Keywords:
aeroengine, floating wall combustor, low pollution
combustion technology
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CONTENT
Abstract................................................................................................................... i
Chapter 1 Introduction........................................................................................... 1
1.1 Background ............................................................................................. 1
1.2 Study contents ......................................................................................... 7
Chapter II Structural Features of Floating Wall Combustor ................................. 9
2.1 Combustor development history and working principle ......................... 9
2.2 Problems with traditional flame tube structure ..................................... 13
2.3 Several measures to improve the life of the flame tube ........................ 16
2.3.1 Temperature increase of the material bearing ............................ 16
2.3.2 Using coating to enhance thermal protection ............................. 17
2.3.3 Advanced and efficient cooling technology ............................... 18
2.3.4 Adopting advanced and reliable structure .................................. 18
2.4 Floating wall flame tube development history ...................................... 19
2.5 Floating wall flame tube structure and working principle .................... 19
2.6 Typical structure and characteristics of floating wall combustor ......... 21
2.6.1 Overall layout of the floating wall flame tube ........................... 22
2.6.2 Floating wall cooling forms ....................................................... 24
2.6.3 Floating wall connection structure ............................................. 26
2.6.4 Floating wall combustor material selection ............................... 26
2.6.5 Floating wall combustor process................................................ 27
2.6.6 Floating wall combustor assembly ............................................. 28
2.7 Design of key structural features ........................................................... 28
Chapter III Design of Floating Wall Combustor ................................................. 32
3.1 Design scheme ....................................................................................... 32
3.2 Combustor structural parameter calculation ......................................... 34
3.2.1 Air flow distribution ................................................................... 34
3.2.2 Diffuser design ........................................................................... 35
3.2.3 Fuel distribution ......................................................................... 37
3.2.4 Combustor length and heat capacity .......................................... 38
3.2.5 Cyclone design ........................................................................... 39
3.2.6 Venturi design ............................................................................. 41
Chapter IV Numerical Simulation Equations and Models of Combustors ......... 43
4.1 Governing equations ............................................................................. 43
4.1.1 Continuity equation .................................................................... 43
4.1.2 Momentum conservation equation ............................................. 44
4.1.3 Energy conservation equation .................................................... 44
4.1.4 Component mass conservation equation .................................... 45
4.2 Turbulence model .................................................................................. 45
4.3 Combustion model ................................................................................ 47
4.4 Discrete phase model ............................................................................ 49
4.5 NOx model ............................................................................................ 50
4.6 Materials ................................................................................................ 51
4.7 Solution Methods .................................................................................. 52
Chapter V Flow Field Calculation and Analysis ................................................. 53
5.1 Combustor modeling and meshing........................................................ 53
5.1.1 Geometric modeling ................................................................... 53
5.1.2 Meshing ...................................................................................... 54
5.2 Combustion performance analysis ........................................................ 55
5.2.1 Flow field analysis ..................................................................... 55
5.2.2 Temperature field analysis.......................................................... 58
5.2.3 Fuel-air distribution characteristics ............................................ 60
5.2.4 Performance parameter .............................................................. 61
Chapter Ⅵ Summary........................................................................................... 63
References ........................................................................................................... 65
Acknowledgements ............................................................................................. 67
Chapter 1 Introduction
1.1 Background
Modern turbofan engines are mainly composed of compression components
(fans, high-pressure compressors), combustion components (main combustor,
afterburner), and expansion work components (high and low pressure turbines),
as shown in Figure 1-1 and Figure 1-2. The compressor, main combustor and
turbine are called the three core components of the areoengine. The working
conditions are harsh and the design is difficult. The matching performance of the
core machine directly affects the development prospect of the engine series.
Among them, the function of the main combustor component is to mix the
compressor outlet air and fuel in a certain ratio, and convert the chemical energy
of the fuel into heat energy through high-efficiency combustion, so that the cold
air of the combustor is turned into high-temperature gas to improve the total air
enthalpy. These gases expand and work at high speeds through the turbine and
exhaust nozzle, causing the engine to generate thrust or output mechanical energy.
Figure 1-1 military turbofan engine
1
Figure 1-2 civil turbofan engine
Since the advent of jet aircraft, the need to increase the engine's thrust weight
ratio has continued to increase as aircraft performance continues to evolve.
Studies have shown that increasing the engine pressure ratio (increasing the cycle
efficiency) and the turbine inlet temperature (increasing the unit thrust) is the most
effective and most direct way to increase the engine thrust weight ratio. Under
such demand, the design of the main combustor of modern aeroengines is
developing towards high temperature rise and high heat capacity combustion: inlet
pressure of combustor (P3), inlet and outlet temperature (T3, T4) and temperature
rise (△T=T4-T3) are getting higher and higher, as shown in Table 1-1, which
directly causes the working conditions of the combustor to become worse. At the
same time, with the requirements for the reliability, durability and economy of the
engine structure, the requirements of main combustor is greatly improved.
Table 1-1 Development of main combustor parameters
The main combustor is mainly composed of a combustor casing, a fuel
nozzle and a flame tube, as shown in Figure 1-3. Among them, the flame tube is
the key to whether the combustor can be reliable and durable, and it is one of the
2
core components of the main combustor. The flame tube works for a long time
under high heat load environment. The wall is subjected to high temperature and
uneven temperature gradient, which is prone to cracks, deformation and blockage,
which directly affects the reliability and life of the engine[1]. Therefore, the life
of the flame tube is listed as one of the important indexes of the main combustor
design, and foreign countries attach great importance to the reliability and
durability of the flame tube.
Figure 1-3 main combustor structure
It is believed that the highest wall temperature and wall temperature gradient
are two important factors limiting the service life of the flame tube, and these two
factors are directly related to the cooling mode and structural form of the flame
tube. Therefore, in order to obtain satisfactory flame tube life and reliability,
advanced cooling methods and reasonable and reliable structural solutions must
be adopted.
A lot of effective research work has been done on the cooling mode and
structural scheme of the combustor flame tube, from single tube to annular tube,
from single layer wall to double wall, from single cooling to composite cooling.
A large number of research results and experience have proved that the use of
floating wall combustor structure is an ideal solution to improve the life of the
combustor. It has been widely used in military and civil aviation engines, and with
3
the continuous improvement of knowledge and scientific and technological
progress, The research on the structure of the floating wall combustor has never
been interrupted, especially the key structure research that affects the working
characteristics of the floating wall.
The design of the floating wall combustor involves many disciplines such as
combustion, heat transfer, material, strength, process and optimization. These
disciplines are developed and improved independently in their respective basic
application research fields, but they are closely related and integrated in
engineeannular applications. The key structure of the floating wall combustor
refers to the important structural features that affect the performance, strength,
structural reliability and life of the floating wall combustor, including the cooling
structure characteristics and the connection structure characteristics. Reasonable
selection of key structural parameters to optimize wall temperature and stress
distribution is one of the key technologies for floating wall combustor design.
Based on the relevant foreign materials and application results, it is necessary to
study the key structure of the floating wall combustor and break through the key
technologies of the floating wall combustor design. The key structure research of
floating wall is carried out. The purpose is to analyze the key structural features
of the floating wall, find out the corresponding laws of the dimensional parameters
of these key structural features and the wall temperature, and optimize the design
of the key structural dimensions to obtain the optimal efficiency. The floating wall
combustor structure scheme provides technical support for the application of the
floating wall combustor.
The floating wall combustor is one of the most advanced advanced structural
technology solutions adopted by high-performance advanced military and civil
aviation engines at home and abroad. Its design technology has always been the
frontier of technical research in related fields at home and abroad. From the late
1980s, the floating wall combustor structure began to be successfully applied to
aero engines. With the problems exposed duannular the use and the solutions
4
taken, the research work is deepening and the applicability of the project is getting
higher and higher, and domestic and foreign experts and scholars are studying the
key structure of the floating wall combustor and improving the cooling
performance. The work has never stopped, the purpose is to make the advanced
cooling structure of the floating wall more perfect.
For the floating wall cooling structure, the typical structural schemes
currently used mainly include two types of cooling: impact + spfueler convection
and impact + dense hole divergence. Foreign countries have carried out a lot of
detailed research work, but out of technical blockade. There are few published
materials, and some of the research results are included in the literature [2-8]. The
United States is in the "Engine Hot End Component Technology (HOST)
Program" [9], "Efficient Engine (E3) Research Program" [10] and "High
Performance Turbine Engine Technology Integration (IHPTET) Program" [11]
and other research projects. The structural reliability, service life and design
analysis techniques of floating wall flame tubes (or block type, tile type flame
tubes) were studied. In the HOST plan, the impact + divergence gas film
composite cooling structure is adopted to minimize the fatigue crack and thermal
stress of the flame tube, and the low cycle fatigue life damage mode of the flame
tube is basically eliminated, and the satisfactory flame tube life is obtained, and
the research result is obtained. Verification was carried out on the flame tube of
the TF30-P-100 engine [12]. E3 plans to study two kinds of floating wall flame
tube structures of impact + gas film (GE company) and reverse parallel flow fin
wall (P&W company). In the flame tube life estimation analysis process, the finite
element method is used to maximize the elastic stress. The conversion to the
equivalent strain, and then the equivalent strain is used to predict the probability
of crack generation, and the elastic analysis method is used to predict the cycle
life, and the prediction effect is satisfactory. These research results have been
applied to the F119 and V2500, PW6000 and other military and civil aviation
engine flame tubes. The design life is 9000h and the repair life is 18000h. The
5
IHPTET program has mainly verified the feasibility of using a floating wall flame
tube with impact + film cooling method to cool with very low cooling air in the
case of high temperature rise and wide fuel-gas ratio range, and has made
satisfactory progress.
GE's low-pollution combustion technology[15]-[17] develops from single
annular combustor, double annular combustor to TAPS, with the nitrogen oxide
falling.
Figure 1-4 GE Combustor Technology Evolution
About TAPS, It has been developed for three generations from TAPS1 to
TAPS3 with structure optimizing to get a less cooling flow and a lower pollutants
emission.
6
Figure 1-5 TAPS combustor
1.2 Study contents
In view of the above problems, this paper proposes a new type of radial
staging combustion chamber, combined with floating wall technology, using UG,
ICEM, Fluent and other software to analyze the flow field characteristics and
combustion performance. The main research contents of this paper are as the
figure 1-6:
7
Floating wall
combustion chamber
design
Aerodynamic and
structural parameter
calculation
Flow
characteristic
Combustion
characteristic
Flow field
analysis
Fuel
distribution
Modeling and meshing
NOx
emission
Numerical simulation
Figure 1-6 study contents
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Chapter II Structural Features of Floating Wall
Combustor
This chapter is based on the working principle of the combustor. The history
of the flame tube is traced back to the origin of the floating wall flame tube. Based
on the analysis of the structural composition and working principle of the floating
wall combustor, the structural characteristics of the floating wall combustor are
analyzed and refined. Design features and parameter selection rules of key
structural features of floating wall combustor.
2.1 Combustor development history and working principle
Since the birth of the 20th century aviation gas turbine engine, the
aerodynamic layout and structural form of the combustor have evolved with the
improvement of engine performance and overall layout. In the evolution of this
long-term development, there are several forms of DC, reflow and baffle in the
combustor [1], see Figure 2-1 - Figure 2-3; where the DC combustor is the most
widely used, its flame volume The cavity structure evolves from a single tube and
a annular tube to a single annular shape and a double annular structure. See Figure
2-4 - Figure 2-7. The cooling structure is developed by a single layer film cooling
to develop a double wall and a multi-slant hole divergence. Composite, highefficiency cooling forms such as laminates and floating walls are shown in Figure
2-8.
9
1. External cases; 2. Flame tube; 3. Fuel nozzle; 4. Compressor final stage guide vane; 5.
Internal cases 6. Turbine guide vanes; 7. Compressor and turbine connecting shaft
Figure 2-1 DC combustor structure
1. Air inlet; 2. External casing; 3. Flame cylinder outer annular;
4. Nozzle; 5. Internal cases; 6. Flame tube inner annular;7. Turbine guide
Figure 2-2 Returning combustor structure
1. the rear wall of the flame tube; 2. the front wall of the flame tube;
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3. the shaft of the compressor;4. Centrifugal fuel pan; 5. cases; 6. Intake duct;
7.Turbine guide vanes; 8. turbine; 9. turbine shaft
Figure 2-3 Baffle combustor structure
Figure 2-4 Single tube combustor Figure 2-5 annular tube combustor
Figure 2-6 annular combustor Figure 2-7 Double annular combustor
(a) wave film-cooling （b）round hole film-cooling （c）Sheet metal hole film-cooling
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（d）interlayer film-cooling （e）impingement film-cooling （f）effusion film-cooling
（g）Laminated cooling
Figure 2-8 Flame tube cooling forms
The working principle of the combustor is shown in Figure 2-9 - Figure 210. The working principle is: the air flow from the combustion inlet is decelerated
by the diffuser after the pressure is decelerated, and divided into two parts under
the shunt of the flame tube cap. (single tube or annular tube combustor) or three
air streams (annular combustor), one outer annular enteannular the two channels
(the annular channel formed by the casing and the flame tube), one enteannular
the inner annular of the two channels, one passing through the flame tube The
head vortex enters the flame tube to participate in combustion, wherein the airflow
enteannular the inner and outer annulars of the two passages is arranged through
the regular openings along the axial direction of the flame tube, respectively
enteannular the flame tube for participation in combustion and blending gas
regulation Temperature field and cooling flame tube.
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Figure 2-9 Working principle of single tube or annular tube combustor
Figure 2-10 Working principle of annular combustor
It can be known from the working principle of the combustor that the flame
tube is the key to the successful combustion of the combustor. The working
conditions are harsh, and the proportion of air must be properly distributed in a
limited space, which not only ensures efficient combustion, but also ensures that
the flame tube works safely and reliably. The excellent design of the flame tube
structure is directly related to the performance of the combustor.
2.2 Problems with traditional flame tube structure
In the development process of the flame tube, the slotted pure gas film
cooling flame tube is undoubtedly the most typical, the most widely used and most
developed structure, see Figure 2-11. The flame tube of this structure is generally
formed by stacking a plurality of annular structures which are segmented in the
13
axial direction, and forms a convergent slot structure similar to the louver at the
superimposed position, and the cooling air enters from the front end of the slot
and exits from the slot ( The tongue piece flows out at a high speed, forming a gas
film at the rear end of the tongue to protect the flame tube wall. The flame tube of
the structure has good overall rigidity and high precision of cooling flow control,
and has the disadvantages of single cooling mode, low cooling efficiency, large
demand for cooling flow rate, and uneven cooling of the flame tube wall surface
in the axial direction. A large temperature gradient is formed, resulting in a large
thermal stress, resulting in low cycle fatigue failure and reduced flame life.
Figure 2-11 Typical film cooling flame tube
As the engine performance improves, the combustor inlet temperature (T3)
and temperature rise (ΔT=T4-T3) are continuously increased (see Figure 2-12).
The increase in inlet temperature means that the cooling potential of the flame
tube cooling gas is reduced, and the temperature rises. Increasing means that the
combustion air is increased, and the amount of cooling air in the flame tube is
greatly reduced (see Figure 2-13). At the same time, due to the limitation of weight
index, the temperature rise is increased and the size of the flame tube is required
to be smaller and smaller, which results in a significant increase in the heat
capacity of the flame tube (Fig. 2-14). The radiation and convection heat transfer
between the gas and the flame tube wall are obviously intensified. . These factors
14
directly cause the thermal protection of the flame tube wall surface to become
more and more serious and important. According to the analysis, even if the pure
gas film cooling potential is maximized, the wall temperature of the flame tube
will exceed 1000 ° C, and the existing flame tube material is difficult to bear [13].
Figure 2-12 Combustor outlet temperature change [13]
Figure 2-13 The change of combustor temperature rise [13]
15
Figure 2-14 Change in heat capacity of the combustor [13]
The above analysis shows that the traditional pure film cooling single-wall
flame tube structure cannot be applied to high temperature rise and high heat
capacity combustor design, and this structure itself has many shortcomings. In
order to improve the working reliability of the flame tube, extend its For life, more
advanced cooling methods, structural designs, materials and processes must be
used.
2.3 Several measures to improve the life of the flame tube
In order to improve the life of the flame tube and meet the needs of advanced
combustor design, experts in various fields such as cooling, structure, materials
and technology have proposed various improvement measures from their own
fields.
2.3.1 Temperature increase of the material bearing
Significantly increasing the temperature of the material is mainly achieved
by developing new materials. Due to the constraints of new materials development
16
cycle, high cost, inadequate engineeannular application verification, etc., the
development speed often fails to meet the needs of combustor technology
development. Figure 2-15 shows the relationship between the specific gravity of
various materials on the engine and the aging. It can be seen that with the
advancement of materials technology, the application of new materials such as
toughened ceramics or C/C composite materials on aeroengines The bigger it is.
The use of such materials instead of superalloy materials can not only significantly
increase the temperature of the flame tube (about 1600K ~ 1900K), but also
significantly reduce the weight of the flame tube (about 30% to 40%) due to its
density advantage. It is a future high-performance engine. An important technical
approach to weight loss design. At present, the feasibility of applying such
materials to the combustor flame tube has been preliminarily verified abroad, but
it is still in the material exploration stage in China, and it will take some time to
apply it.
Figure 2-15 Change in specific gravity of various materials [13]
2.3.2 Using coating to enhance thermal protection
The current thermal insulation coating technology can only increase the
temperature of the material by 30 ° C ~ 100 ° C, the heat insulation effect is very
17
limited, it has little significance for greatly improving the service life of the flame
tube, and the coating is prone to defects such as falling off and slag, serious It can
cause engine failure.
2.3.3 Advanced and efficient cooling technology
The development of advanced cooling technology, the main idea is to
improve the cooling efficiency, so that the limited cooling air volume can fully
exert the cooling potential, effectively reduce the wall temperature and
temperature gradient of the flame tube, thereby reducing the thermal stress and
improving the life of the flame tube. At present, the main advanced cooling
methods studied at home and abroad include sweating, laminate cooling or
composite cooling [3]-[4] methods such as impact + reverse convection + gas film
composite cooling [2] and impact + diffusion (divergence).
2.3.4 Adopting advanced and reliable structure
The development of advanced structural forms, the idea is to fully release
thermal stress through a reasonable structural design, reduce mechanical failure,
and extend the life of the flame tube. The structural design of the flame tube is
related to the cooling form used. When the composite cooling technology is
adopted, the flame tube generally adopts a double wall structure, such as a
laminate structure and a floating wall structure. With these several solutions as the
development direction, a large number of meticulous and fruitful research work
has been carried out at home and abroad. Multi-inclined hole sweating film
cooling flame tube, layer flame tube, floating wall flame tube and ceramic
composite flame tube have been successively carried out. It has provided an
important technical approach for the advancement of combustor technology.
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2.4 Floating wall flame tube development history
Analysis of the causes of the short life of the traditional flame tube can be
found, mainly due to the high internal stress caused by the thermal load and
mechanical load (pressure) of the overall annular structure duannular operation.
If the split double-layer structure is adopted, the inner and outer layers
respectively bear the thermal load and the load of the casing, and the inner layer
is divided into a plurality of blocks by the integral annular structure, so that the
thermal stress is fully released, and the service life is necessarily multiplied. At
the same time, after the inner and outer layers are separated, the inner and outer
layers can be made of different materials and processes respectively. The inner
layer is made of high melting point material and casting process suitable for mass
production, while the outer layer is made of low-melting material with low price
and suitable for precision. Controlled machining process. In addition, the twolayer structure creates conditions for advanced composite cooling technology,
which allows for better cooling and significantly reduces cooling air usage. Under
the guidance of this complete breakthrough of the traditional flame tube design,
in the mid-to-late 1980s, the Americans first proposed the floating wall structure
and succeeded in the gas transfer section of the TF30-P-100 engine flame tube. A
transformation test was carried out. Taking this as a starting point, the structure of
the floating wall combustor has been widely used in foreign military and civilian
engines (F100, F119, PW6000, V2500, etc.) with its excellent comprehensive
performance. It is regarded as an ideal solution for long-life combustors.
2.5 Floating wall flame tube structure and working principle
The floating wall flame tube is a double-layer structure, which is mainly
composed of a casing (beaannular wall), an inner wall (floating tile), a sealing
gasket, a connecting structure (studs, a card slot, a hook, etc.), as shown in Fig. 216. The outer casing is a unitary annular structure similar to a conventional single19
walled flame tube structure; the inner wall is composed of a number of segments
shaped along the axial and circumferential segments (the shape of which is similar
to that used in buildings, so it is often referred to as a tile Between the splicing,
there are gaps between the adjacent tiles; the tiles are generally connected to the
outer casing by means of studs, card slots or hooks; there is a main positioning in
the connection layout of each tile, and the rest is Auxiliary positioning, which
ensures that the floating tiles can expand freely when heated; seals are used at the
joints to ensure accurate control of the proportion of cold air enteannular the flame
tube.
Figure 2-16 the structure of the floating wall flame tube
When working, the outer casing is mainly subjected to mechanical load
(called load-beaannular wall); because it is located in the two passages of the
combustor, the working environment temperature is low, which significantly
alleviates the high-temperature thermal fatigue generation rate. The inner wall
mainly receives the thermal load generated by the high-temperature gas in the
flame tube. Because it is divided into independent tiles, each tile can freely expand
to the periphery with its positioning center when heated, and fully release the
thermal stress, thereby prolonging the service life of the tile. At the same time,
after the inner wall is divided into a plurality of tiles, the crack can be restricted
to expand only in a single tile, further delaying the rate of fatigue crack
development.
The floating wall flame tube structure not only reduces the overall thermal
stress level of the flame tube, but also, because the tiles can be replaced separately,
it is beneficial to reduce the maintenance cost and significantly prolong the cycle
life of the flame tube. Both of these advantages give the floating wall flame tube
satisfactory reliability and durability. At the same time, the inherent double wall
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structure of the floating wall also creates favorable conditions for the efficient
composite cooling form, which can further reduce the wall temperature of the
flame tube and reduce the resistance of the flame tube to high performance
temperature resistant materials.
2.6 Typical structure and characteristics of floating wall
combustor
Since the construction of the floating wall combustor structure has been
applied, the problems exposed duannular the use and the solutions taken have
been accompanied by the deepening of the understanding and the advancement of
science and technology. Various forms have been developed in terms of cooling
methods and connection structures. Figure 2-17 shows several typical floating
wall flame tubes currently in service or under study in foreign countries.
(a) V2500 flame tube
(b) PW6000 flame tube
(c) PW4098（TALON I）flame tube
（d）PW6000（TALON II）flame tube
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(e) TALON X flame tube
(f) RR lean burning flame tube
(g) GE high-efficiency energy-saving flame tube (h) P&W high-efficiency energy-saving flame tube
Figure 2-17 Typical floating wall flame tube structure
2.6.1 Overall layout of the floating wall flame tube
Each company has its own characteristics in the overall layout design of the
floating wall flame tube, but in the axial and circumferential segmentation of the
flame tube, it is mainly determined by the geometrical factors such as the length
and diameter of the flame tube. The main features of the overall layout of the
floating wall flame tube are as follows:
(1) Consideannular the flame tube section (main combustion zone,
afterburning zone and blending zone) and the rigidity of the tile structure, process
forming and other factors, the axial section of the floating wall is generally in the
range of 2~6, each section of the tile axis. The length of the direction is within
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30~100mm.
(2) The number of circumferential segments of floating tiles is generally
related to the number of flame cylinder heads, main combustion holes and
blending holes to facilitate the circumferential layout. Generally, the
circumferential expansion length of floating tiles is within 100~300mm.
(3) The gap between the circumferential directions of the tile is mainly
determined according to the factors such as the tile size, the highest wall
temperature and the processing error. The purpose is to ensure that the tile can
freely expand when working, no interference occurs, and between the tiles The
protection of the beaannular wall is not enough due to the excessive clearance,
which causes the beaannular wall to be ablated by the gas.
(4) The adjacent tiles are overlapped in the axial direction in a misaligned
manner to ensure that a slot is formed between the rear section of the section of
the tile and the front section of the lower section of the tile, thereby creating a film
at this location to protect the edge of the tile from being Ablation.
(5) The adjacent tiles are sealed in the circumferential direction by sealing
ribs to reduce the amount of air leakage. The ribs are designed at the edge of the
two circumferential sections of the tile, and the thickness is generally close to the
wall thickness of the tile.
(6) In order to avoid the circumferential gap of the adjacent tiles in the
transition state being too large, the beaannular wall is surrounded by the gas in the
axial direction of the corresponding position, and the tiles of the axially adjacent
segments are shifted by a certain angle in the circumferential layout.
(7) In order to enhance the rigidity of the annular beaannular wall on the
basis of being as thin as possible to reduce the weight, a step annular is generally
designed between the circumferential sections of the beaannular wall.
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2.6.2 Floating wall cooling forms
The double wall structure inherent in the floating wall can fully utilize the
cooling airflow, which is a necessary condition for the composite high-efficiency
cooling method. Therefore, since the appearance of the floating wall structure, it
has basically adopted the form of composite cooling structure. The typical ones
include impact + spfueler array convection cooling, impact + light plate
convection cooling and impact + dense hole divergence film cooling. See Figure
2-18. In the E3 research program, GE and P&W have verified the two cooling
schemes of the impact + dense pore gas film and the reverse parallel flow fin wall
(which can be regarded as impact + light plate convection), and the impact +
spfueler array cooling form. It has been applied in V2500 and F119.
Figure 2-18 Several typical floating wall cooling structures
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Figure 2-19(a) is a working principle diagram of the convective cooling
floating wall cooling structure of the impact + spfueler array. The principle is that
the cooling air from the outlet of the diffuser of the combustor flows in the two
channels formed by the outer wall and the casing. And along the rows of impact
holes on the beaannular wall, sequentially enter the forced convection cooling
channel formed between the beaannular wall and the tile. The airflow first impacts
the position of the facing tile light board (the position of the gas storage tank in
the figure), then flows to both sides of the tile through the spfueler column, cools
the tile and the spfueler column, and finally forms a gas film from the adjacent
tiles. The slot is outflowed, and a protective air film is formed on the front and
rear edges of the hot side of the tile (being the high temperature gas side), and the
high temperature gas is isolated from the hot wall of the floating tile, so that the
wall temperature of the floating tile is kept at a low level. The working principle
of "impact + light plate convection cooling" is the same, except that the convective
air flow does not pass through the spfueler column, and the heat exchange effect
will be worse.
(a) Impingement/Column/Convection
(b) Impingement/Effusion Cooling
Figure 2-19 Working principle of the floating wall cooling structure
Figure 2-19(b) is a schematic diagram of the working principle of the impact
+ dense hole divergence film cooling floating wall cooling structure. The principle
is that the cooling air from the outlet of the diffuser of the combustor flows in the
two channels formed by the outer wall and the casing. And sequentially enter the
cooling holes uniformly formed along the beaannular wall into the cooling
passage formed between the beaannular wall and the tile. The airflow first impacts
the position of the tile light plate facing the front, and then flows directly from the
25
film hole near the corresponding position of the tile to form a uniform continuous
protective film on the hot side of the tile. The cooling wall structure using this
cooling form has better cooling efficiency than the former two, and foreign
countries are applying it to the newly developed floating wall combustor.
2.6.3 Floating wall connection structure
From the working principle of the floating wall combustor, the reason for its
long life is that in addition to the advanced cooling method, the most important
thing is that the inner heat-receiving tile can freely expand when heated, releasing
thermal stress. In order to ensure the free expansion of the tile when it is working,
and to ensure its reliable installation, it does not fall due to mechanical vibration
when the engine is working. It is very important to design a reasonable connection
structure. Figure 2-20 shows a schematic diagram of several typical floating wall
connection structures used abroad.
(a) Card slot type connection structure (b) Hanging type connection structure (c) Stud type connection structure
Figure 2-20 Typical floating wall connection structure
2.6.4 Floating wall combustor material selection
The selection principle of the floating wall combustor is to ensure the use
and ensure the price is within the acceptable range. The characteristics are:
(1) The beaannular wall is generally selected from ordinary high temperature
alloys with temperature resistance below 800 °C. The main requirement for
26
material properties is High specific strength, high hardness and good
machinability.
(2) Tiles generally use high-quality superalloys with temperature resistance
above 850 °C. The requirements for material properties are heat resistance,
corrosion resistance and oxidation resistance, moderate rigidity and good casting
performance.
(3) The connecting parts (nuts and washers) are generally made of materials
with a temperature resistance comparable to that of the beaannular wall but with
higher hardness and specific strength.
2.6.5 Floating wall combustor process
The selection principle of the floating wall combustion process is to ensure
economy and high efficiency. Its main features are:
(1) The beaannular wall contour processing technology generally selects the
finishing or milling process to ensure the contour accuracy. It is usually heat
treated after processing to eliminate processing stress and adjust material
properties to an excellent state.
(2) The profile of the tile is generally directly molded by a precision casting
process, and the joint structure is machined.
(3) In order to increase the heat resistance of the tile, prevent the stud
connection from bonding at high temperatures, and the friction and wear of the
beaannular wall when the floating tile expands, it is usually selected to use a
surface strengthening process for the structural member, such as: resistance to tile
spraying Thermal coating, spraying anti-adhesive coating on the joint structure,
spraying wear-resistant coating on the beaannular wall, etc.
(4) The film hole is usually processed by electric spark or laser to improve
the processing efficiency.
27
2.6.6 Floating wall combustor assembly
In order to achieve the ideal design concept of the floating wall, in addition
to ensuannular that the machining accuracy of each component meets the
requirements, there are also some precautions when assembling the floating wall
combustor:
(1) Select a reasonable tightening force for the joint position of the tile to
ensure free sliding of the hot working tile and ensure that the assembly stress is
not excessive; the tightening force is usually selected according to experience and
calculation and test results.
(2) Check the tile size profile duannular assembly to ensure uniform gap
between adjacent tiles after installation.
(3) Minimize the number of disassembly and assembly to ensure the
reliability of the tile connection structure.
2.7 Design of key structural features
It can be known from the working principle of the floating wall that it is very
important to optimize the design of the key structural features to achieve floating
and reliable floating tiles. The key structural features of the floating wall
combustor include the cooling structure features and the connection structure
characteristics. The cooling structure characteristics determine whether it has an
ideal temperature distribution, and the connection structure characteristics
determine whether the stress distribution is optimal.
As mentioned above, the current cooling forms of foreign floating wall
combustors mainly include three types: impact convection column array
convection film, impact + light plate convection film and impact + dense hole
divergence film. In summary, floating wall cooling The structural features mainly
include: beaannular wall thickness, impact hole diameter, impact hole and wall
angle, impact height, tile wall thickness, film hole diameter, spfueler diameter,
28
film hole wall angle, impact hole and Air film hole arrangement (unit number ratio
and geometric arrangement), film slit height, spfueler height, tile circumferential
and axial clearance. The design features of the cooling wall combustor cooling
structure are as follows:
(1) The wall thickness of the beaannular wall of the floating wall combustor
and the wall thickness of the tile, in addition to the cooling design requirements,
also depend on the pressure of the flame tube, mechanical load and weight limit,
etc., often based on traditional film cooling The design of the flame tube is
designed for initial design, and the optimal wall thickness value is optimized by
factors such as cooling, structural strength and weight.
(2) The cooling mode is selected as the mainstream impact + spfueler array
convective gas film is the mainstream, has been widely used; impact + dense hole
divergence film as an improved type will be applied to the newly developed
floating wall combustor, and impact + convection As the original structure, film
cooling has gradually been eliminated.
(3) When selecting the convective film cooling form of the impact + spfueler
array, the arrangement and quantity selection of the impingement holes are mainly
determined according to the pressure drop control of the flame tube and the
distribution of the cooling gas flow. Generally, each section of the tile corresponds
to 1~2 rows. Impact hole, the position of the impact hole is located in the middle
of the axial direction of the tile; there is no absolute correspondence between the
arrangement of the spfueler column and the arrangement of the impact hole. The
goal is to arrange as many rows of spfueler as possible to increase the heat transfer
of the tile. area.
(4) When the impact + dense hole divergence film cooling form is selected,
the arrangement of the impact hole and the air film hole mainly has several forms
of positive row, fork row and oblique row, as shown in Fig. 2-25. The positive row
distribution form is simple, but the cooling effect is slightly poor, and the fork row
and the oblique row cooling effect are better, but the oblique row is not conducive
29
to the actual processing operation, so the fork row form is widely used. After the
relationship between the impingement hole and the film hole arrangement is
determined, the impact hole and the film hole have a fixed unit ratio relationship.
In principle, under the premise of obtaining the maximum impact hole velocity,
the film velocity is kept as large as possible. Taken together, the ratio of the impact
hole to the film hole unit is 1:3, which is the most typical and the best.
(a)Positive row
(b) fork row
(c) oblique row
Figure 2-25 Typical arrangement of the impact hole and the film hole
(5) After the cooling mode is selected, the value of the impact hole diameter,
the impact height, the pore diameter of the gas film hole and the diameter of the
spfueler column can be optimized by heat transfer analysis. Generally speaking,
the impact aperture and the slot height are mutually constrained. When the ratio
of the slot height to the impact aperture is approximately a certain value, the
impact effect is the best; the large impact aperture causes the slot height to be too
large, which bannulars difficulties to the structural design. It also causes a large
secondary eddy current, which deteriorates the cooling capacity of the slot; too
small an impact hole will cause the spfueler column to be overcrowded. The pore
size of the gas film depends on the pressure drop of the beaannular wall and the
pressure drop of the tile to obtain a satisfactory air film blowing ratio. The
diameter of the spfueler column controls the minimum clear spacing of the
spfueler column. Under the premise that the minimum net spfueler column
spacing can meet the process requirements, the number of spfueler columns is
30
increased as much as possible. The height and diameter ratio of the spfueler
column also have an optimal value.
31
Chapter III Design of Floating Wall Combustor
3.1 Design scheme
A double annular floating wall combustor is designed. The combustor I
study is based on GE90. As figure 3-1 shows, it is a Double annular combustor. It
has a pilot stage in the outer annulus and a main stage in the inner annulus.
Figure 3-1 GE90’ Double annular combustor (DAC)
As Table 3-1 shows, When working in low power conditions, only pilot stage
works, When working in big power conditions, pilot stage and main stage both
works. [19]
Table 3-1 operating mode
Based on the GE90 combustor, as figure 3-2 shows. I do some change. Firstly,
the middle sector is cancelled.
32
Figure 3-2 the middle sector is cancelled in new structure
In addition, as shown in the figure 3-3, staging method changes from radical
staging to radical triangle staging. Doing so could reduce the amount of cooling
air.
Figure 3-3 radical triangle staging structure
The combustor design includes the main components and flow distribution
design. In order to facilitate the research of the subject, the design of the
combustor is assumed as follows: 1) The air is looked at the ideal gas, and the
ideal gas state equation is used in the calculation for the density pressure
calculation; 2) In the design, the diffusion process is observed Is adiabatic,
therefore, the total temperature is unchanged; 3) do not consider the specific
installation structure of the bolt.
The known initial design parameters are:
a) Cruise altitude H=11km;
b) Mach number M = 0.8;
33
c) Bypass ratio m=8.04;
d) Pressure ratio π = 43;
e) Compressor outlet velocity V2 = 120m/s;
f) At H=0 km, the whole engine air intake ma=527kg/s;
g) The total temperature of the combustor outlet Tt3=1707K;
3.2 Combustor structural parameter calculation
3.2.1 Air flow distribution
The air distribution is as figure 3-4.
Figure 3-4 Air flow distribution
It is known that when the height H=0, the standard atmospheric table can be
obtained: temperature T0=288.2K, atmospheric pressure P0=1.0133e5Pa.
When the cruising altitude is H=11km, the standard atmospheric table can be
obtained: temperature T11=216.7K, atmospheric pressure P11=0.227e5Pa, and
sound velocity c11=295.1m/s.
It is known that when H=0, the total engine intake air amount G0=527kg/s,
According to the formula
G11 T11 G0 T0
=
P11
P0
G11 = 136.15kg / s
From the ducted ratio m=8.04, the connotation air volume is obtained.
34
Gin =
136.15
= 15.06kg / s
9.04
The amount of intrinsic air is divided into two parts: the inflow into the
combustor and the inflow into the turbine. In this paper, the amount of air in the
combustor accounts for 90.4%, so
ma = 0.904Gin = 0.904*15.06 = 13.6kg / s
The airflow in the combustor is divided into three parts: the cyclone airflow,
the outer annular cavity cooling hole airflow, and the inner annular cavity cooling
hole airflow. The floating wall cooling method used herein can greatly reduce the
cooling gas flow rate. In this design, it is stipulated that the cooling gas flow rate
does not exceed 30%, and the cyclone air flow ratio is not less than 70%.
3.2.2 Diffuser design
Set the inlet port of the inlet to 0 and the outlet to 1 according to
P0 * = P0 (1 +
 −1
2

Ma 2 )  −1
We can get
P1* = P0 * = 0.346atm
The compressor boost ratio is 43, then
P2 * =  P1* = 14.88atm
In addition,
T1* = T0 +
V0 2
236.12
=216.7+
=244.5K
2Cp
2*1004
Set the outlet position of the compressor to 2, and calculate the formula
according to the efficiency of the compressor:
k =

k −1
k
k

2

1
−1
T
−1
T
35
We can get
T2*=799.3K。
According to the formula
T2 * = T2 +
V2 2
2Cp
，
then T2=792.2K，
then c2 = kRgT2 = 1.4  287  792.2 = 564.2 m s ，
V2
= 0.213 ，
c2
then
Ma2 =
then
k +1
Ma2 2
2
=
=0.23
k −1
2
1+
Ma2
2
Check the aerodynamic function table, q( ) = 0.3549
According to the formula
m=K
P2 *
Aq ( ) ，K=0.0404，
T2 *
The diffuser inlet area can be obtained as A = 17941.8mm2 。
As figure 3-5 shows，The Dump Diffuser[19] is designed to reduce speed
and increase static pressure. which includes a pre-diffuser and a Sudden expansion
zone. The structural parameters of the diffuser are listed in the table 3-2.
36
Figure 3-5 Dump Diffuser
Table 3-2 design parameters
3.2.3 Fuel distribution
.
In the overall design of the engine，it has been known ma , T2* , T3* ,c ，to get
the fuel mass flow。Can be derived from the efficiency formula:
=
c H u − CpT 3T3 * −Cp f 2T f 2 *
L0 (CpT 3T3 * −CpT 2T2 *)
H u =43000kJ / kg ，CpT 2 =1096.648 J / (kg  K ) ，CpT 3 =1268.33J / (kg  K ) ，c =0.98 ，
Cp f 2 =2200 J / (kg  K ) ， T f 2 = 375K
then
 = 2.14
Since the head air volume accounts for 70%, the head residual gas coefficient
is 1.5. then, Fuel flow in the combustor:
mf =
ma
13.6
=
= 0.432kg / s
 * Lo 2.14*14.7
This paper calculates the cruise state, and sets the pilot stage and the main
37
stage fuel flow ratio to 19:31. The calculation of fuel – gas ratio is shown in the
table 3-3.
Table 3-3 fuel-air ratio
3.2.4 Combustor length and heat capacity
All combustors must have sufficient volume and length to accommodate a
low velocity flame holding zone. The volume and length depend on the volumetric
heat release rate, combustion residence time and other load parameters, and most
importantly the flow pattern is fully established, which determines the shape of
the combustor. Usually, measured by the length to height ratio of the combustor.
With the development of combustion technology, the length-to-height ratio of the
combustor has grown from the early 6 to the current 2 (short annular combustor).
In this paper, The combustor dimensions are shown in figure 3-6. The length-toheight ratio of the combustor is 176.6/128=1.38.
Figure 3-6 combustor dimensions
38
Reducing the size and quality of the combustor is a must for the aerospace
vehicle power unit. This requires burning as much fuel as possible in the smallest
possible space, ie, the combustion intensity is high.
The parameter for measuannular the degree of utilization (or combustion
intensity) of the combustor volume generally adopts the concept of so-called heat
capacity. In unit pressure and unit combustor volume, the amount of heat actually
released by the fuel enteannular the combustor is called the heat capacity within
one hour. The definition is:
Qvc =
3600c m f Hu
P2* Vc
The heat capacity of the main combustor of a modern aeroengine is generally
0.7 ~ 2 MJ / (m3  Pa  h) the heat capacity of the flame tube is 1.2 ~ 6.5MJ / (m3  Pa  h) .
Substituting each parameter into the formula, Vc = 0.0257 m3 where the heat
capacity of the flame tube designed in this paper can be obtained.
Qvc = 1.75MJ / (m3  Pa  h)
3.2.5 Cyclone design
Primary cyclone
The cyclone design is the key to the design of the combustor head and will
directly affect the flow field in the flame tube. The main function of the cyclone
[24] is: 1) the high-speed rotary jet is formed after the air passes, and the fuel is
atomized; 2) the combustor recirculation zone is formed to ensure the ignition
success and the flame is stabilized; 3) as the combustor head intake The device
ensures that the head residual coefficient is reasonable.
The relationship between the inner diameter of the cyclone ds1 and the outer
diameter ds2 is as follows:
d s1
= 0.45 ~ 0.78
ds2
39
The primary cyclone designed in this paper has an outer diameter ds2 of 8.4
mm and an inner diameter ds1 of 4 mm.
then
d s1
= 0.48
ds 2
The length L is as follows:
L
= 1 ~ 1.5
d s 2 − d s1
In this paper, the first-order cyclone L=4.5mm.
The swirl number SN of the cyclone is mainly affected by the blade
mounting angle β, and has the following relationship:
SN =
2
atg
3
（2- 8）
3
r
1−  
R
among them， a =  2 (r is the inner radius of the cyclone and R is the
r
1−  
R
outer radius of the cyclone)，for aero-engine, 1.1  a  1.35 ，In this paper, the firstorder cyclone a=1.15，  = 32 ，then Swirl number
S N = 0.5
The thickness of the blade is selected according to the required effective flow
area and processing method of the cyclone. In this paper, the thickness of the first
order swirler blade is taken as t=0.5mm.
one of the basic principles of cyclone design is that there is no light
transmission, the criterion is:
 ds 2
n
then，the number of blades n = 8 。
40

L
tan 
Then the flow area of the cyclone is  ( R 2 − r 2 ) cos  − nt ( R − r ) = 17.1mm2 。
Secondary cyclone
The secondary cyclone is calculated in the same way as the primary cyclone,
as follows:
The secondary cyclone has an outer diameter ds2 of 28.8 mm and an inner
diameter ds1 of 18.2 mm.
d s1
= 0.63
ds 2
The secondary cyclone has a length L = 11 mm.
The swirl number SN of the cyclone is mainly affected by the blade
mounting angle β, and has the following relationship:
SN =
2
atg
3
（2- 8）
In this paper, the second-order cyclone a=1.24，  = 45 ，then Swirl number
S N = 0.83
The thickness of the secondary cyclone blade is taken as t = 1 mm.
The number of blades n, one of the basic principles of cyclone design is that
there is no light transmission, the criterion is:
 ds 2
n

L
tan 
then， n = 9 。
Then
the
flow
area
of
the
secondary
cyclone
is
 ( R 2 − r 2 ) cos  − nt ( R − r ) = 228.84mm 2 。
3.2.6 Venturi design
The venturi is a circular tube that is firstly contracted and expanded. The
41
section with the smallest area after contraction is called the throat, which helps to
prevent the nozzle from depositing carbon, and the swirling air passes through it.
Whether there is a venturi tube has a great influence on the combustion
performance of the flow field, the spray field and the combustor. At home and
abroad, some scholars have analyzed the influence of the venturi on the flow field,
and concluded that the structure of the venturi can limit the fluid flow, and can
delay the mixing of the inner and outer swirls, thus reducing the droplets. The
distribution range significantly improves the uniformity of the droplet size
distribution and the overall atomization quality of the fuel, and the venturi
structure has a pre-film atomization effect on the fuel.
The schematic diagram of the venturi tube parameters is shown in the figure.
The main parameters include the venturi contraction angle 1 , the divergence angle
 2 and the turning angle  0 . Two main parameters can be determined.
If the turning angle is too small, the shrinkage angle and the expansion angle
will be larger, and the flow separation will further increase the flow resistance loss;
if the turning angle is too large, the throat diameter will be increased, the cyclone
function will be weakened, and it is difficult to form in the flame tube. Stable
recirculation zone. Generally, the turning angle is as follows:
90   0  120
In this paper, the first  0 = 120 ，the second  0 = 115 。
42
Chapter IV Numerical Simulation Equations
and Models of Combustors
The combustion process of fuel-air mixing in an aero-engine combustor is
extremely complex. Fluent and other CFD software provides many control
equations and mathematical models for simulating transport phenomena under
complex geometries. In this paper, turbulence models, combustion models,
discrete phase models and pollution models are used in the numerical simulation
process. The control equations and physical models used in this design will be
analyzed in detail below.
4.1 Governing equations
The flow field in the aero-engine combustor follows the continuity equation,
the momentum conservation equation, the energy conservation equation and the
component mass conservation equation [25].
4.1.1 Continuity equation
The flow condition in the combustor satisfies the law of conservation of
mass, which can be described as: the mass of the flow micro-element increases in
unit time, equal to the net inflow quality of the micro-element in unit time. The
expression is:
 
+
( u i ) = S m
t xi
（4- 1）
This equation is a general form of conservation of mass and is suitable for
both compressible and incompressible flows. The source term in the formula is
the quality of the sparse phase added to the continuous phase or the user-defined
mass source term.
43
4.1.2 Momentum conservation equation
The momentum conservation equation is the equation of motion, also known
as the N-S equation. The fluid flow problem satisfies the law of conservation of
mass momentum, which is essentially Newton's second law. The content is: the
rate of change of fluid momentum in the micro-body is equal to the resultant force.
In the inertial coordinate system, the momentum conservation equation along a
certain direction of the coordinate system:


p  ij
( u i ) +
( u i u j ) = −
+
+ g i + Fi
t
x j
xi c j
（4- 2）
In the formula ， p is static pressure ；  ij is Stress tensor ，
  u i u j
+
  x j xi
 ij =   
 2 u l
 − 
 ， g i 、 Fi is the gravity volume force and other
 3 x ij
l

volume forces (if it is due to the interaction between the two phases)。
4.1.3 Energy conservation equation
The flow problem in the combustor contains heat exchange, which satisfies
the law of conservation of energy. The law is essentially the first law of
thermodynamics. The content is: the rate of change of the energy of the microelement is equal to the net heat flux and volume force of the micro-body, and the
surface force to the micro-body. The sum of the work done.
The energy equation solved by Fluent is as follows:



T
( E) +
(ui (  E + p)) =
(keff
−  h j J j + u j ( ij )eff ) + Sh （4- 3）
t
xi
xi
xi j
In the formula， keff is Effective thermal conductivity， k eff = k t + k ； J j is
Diffusion flux of j  ；The first three items on the right side of the equation are heat
conduction, component diffusion, and viscous dissipation; S h is a source term
44
containing chemical reaction heat and other volumetric heat sources;
E = h−
p

+
u i2
。
2
4.1.4 Component mass conservation equation
The aero-engine combustor is a multi-component system in which each
component satisfies the component mass conservation law. The content is: the
amount of change in the mass of a component in the system per unit time, and the
system interface. The net diffusion flow is equal to the sum of the components
produced or consumed by the chemical reaction. The equation is:
 (  cs )
+ div (  ucs ) = div ( Ds grad (  cs ) ) + S s
t
（4- 4）
Where cs is the volume concentration of the component, which Ds is the
diffusion coefficient of the component, which S s is the mass produced by the
chemical reaction of the component per unit volume per unit time.
4.2 Turbulence model
The turbulent flow is highly complex, and its motion has the characteristics
of non-steady state, irregularity, and rotation. The turbulent flow is a flow
phenomenon that must be considered when designing the combustor. The
turbulent flow has the characteristics of randomness, turbulent diffusion and
energy loss.
Here are a few typical turbulence models included in Fluent:
(1) Spalart-Allmaras model
(2) k-ε model
a.Standard k-ε model
b.RNG k-ε model
45
c.Realizable k-ε model
(3) k-ω model
a. Standard k-ω model
b. Pressure correction k-ω model
c. Reynolds stress model
d. Large eddy simulation model
Each turbulence model has its corresponding applicable working
conditions. Choosing an accurate model is conducive to rapid convergence and
improved calculation accuracy. We need to select the appropriate model to
calculate according to the actual problem to be studied. The following points are
mainly considered when selecting a model: whether the fluid is compressible,
the calculation accuracy, the computer hardware condition, and the calculation
time.
In the numerical simulation of aero-engine combustion, the most widely
used is the k-ε model [26]. As mentioned above, the k-ε model is divided into
three categories, and the previous experiments are compared [27]. The standard
and achievable k-ε model predictions are in good agreement with the data, while
the RNG turbulence model performs poorly.
The.Realizable k-ε model can be used to calculate the viscosity using the
content related to rotation and curvature. It is suitable for the flow of free flow
and swirling uniform shear flow containing jet and mixed flow. The jet engine
and the mixed flow are contained in the aero-engine combustor. There are many
reflow zones of different sizes in the flow field. Therefore, for the numerical
simulation of this paper, the Realizable k-ε modelcan be selected.
Turbulent flow near the wall is more susceptible, there are two ways to solve
this problem: 1) wall function method. The wall function is used to relate the
physical quantities of the turbulent core zone to the near-wall element; 2) the low
Reynolds number model. The method of modifying the turbulence model is
adopted, and the grid is used to solve the viscous-effect region. The flow in the
46
near-wall region of the combustor designed in this paper is not highly unbalanced,
the force and dissipation are relatively small, and the application conditions of the
standard wall function are basically In addition, the aeronautical combustor has a
high structural complexity, a large number of meshes, and a large amount of
calculation. Therefore, this paper selects the standard wall function contained in
the Fluent software to solve.
4.3 Combustion model
The combustion process in aero engines is particularly complex, including
fuel atomization, evaporation, fuel and gas mixing and reaction [28]. For diffusion
combustion, general finite rate models and non-premixed combustion models can
be used in numerical simulations. Among them, the former is applicable to
turbulent or laminar, premixed or non-premixed conditions, but it is not applicable
to the rapid chemical reaction in the aviation combustor. Therefore, this paper
selects the non-premixed combustion model for simulation and uses the
probability density function. To reflect the role of chemical reactions and
turbulence. Before Fluent is solved, the relevant parameters of the PDF model
need to be set. The paper is set to: working pressure is 14.42atm, air inlet
temperature is 792.2K, fuel inlet temperature is 375K, and air component is 23%
O2 and 77%. N2, the fuel is aviation kerosene (C12H23).
(1) Balanced mixed fraction / PDF model
The non-premixed combustion model does not solve the single component
equation, but solves the transport equation of 1~2 conserved quantities (mixed
fraction). The concentration of each component is obtained by premixing the
fractional field. Thermochemical calculations in prePDF can be tabulated to
facilitate querying in Fluent. The role between turbulence and chemistry is
considered by the probability density function (PDF).
The basis of the non-premixed combustion model is that under a certain
47
degree of simplification, the instantaneous thermochemical state of the fluid is
related to a conserved quantity, ie the mixing fraction f. The mixed fraction can
be written as the atomic mass fraction as:
f =
Zi − Zi , ox
Z i , fuel − Z i , ox
(4-15)
Where: Zi - the elemental mass fraction of element i. The subscript ox
represents the value at the oxidant stream inlet and the fuel represents the value at
the fuel stream inlet. If the diffusion coefficients of all components are equal, the
mixing fraction in the formula is the same for all elements. Therefore, the mixing
fraction is derived from the elemental mass fraction of the fuel stream. Note: This
mass fraction includes all elements from the fuel stream, including inert
components such as nitrogen, as well as oxidizing components such as oxygen
mixed with fuel.
(2) Mixed fraction transport equation
In the aeroengine, under the assumption of the same diffusivity, the
composition equation can be simplified to a single equation for the mixed
component f. The source term in the composition equation is removed, so f is a
conserved quantity. The average (time average) mixed fraction equation is:



( f ) +   ( v f ) =    t f  + S m + S user
t
t

（4-16）
In the formula, the source term Sm only refers to the mass being introduced
into the gas phase by liquid fuel droplets, and the Suser is any user-defined
source term.
In addition to solving the average mixture fraction, FLUENT also solves a
conservation equation for the average mixed fraction mean square:
(
) (
)
(
)




 f  2 +   v f  2 =    t  f  2  + C g  t  2 f − C d  f  2 + S user
t
k
t

（4-17）
Where: f  = f − f ，  t = 0.85 ，Cg=2.86, Cd=2.0, Suser defines the source
item for the user.
48
(3) Relationship between mixing fraction and component mass fraction,
density, and temperature
When using the PDF method to express the combustion model, there is a
simple quantitative relationship between conserved quantities under certain
assumptions. Therefore, after the spatial distribution of a certain conserved
quantity is obtained, other conserved quantities can be solved according to the
boundary value. For adiabatic single fuel/oxidant reaction systems, the
instantaneous values of mass fraction, density and temperature are only related to
the instantaneous value of the mixing fraction f:
i = i ( f )
（4-18）
For non-adiabatic situations, the relationship is:
i = i ( f , H * )
（4-19）
Among them, H* is instantaneous enthalpy
T
H * =  m j H j =  m j   c p , j dT + h 0j (Tref , j )
 Tref , j

j
j
（4-20）
4.4 Discrete phase model
The combustion process in the combustor involves the process of fuel
fragmentation, evaporation, etc., including gas-liquid two-phase flow. For the
numerical calculation of multiphase flow, there are two methods of EulerLagrange and Euler-Euler, because the fuel volume fraction in the combustion
zone is small. In accordance with the applicable conditions of the Euler-Lagrange
method, this paper selects the discrete phase model (DPM) based on the EulerLagrange method.
The pre-combustion stage and the main combustion stage of the combustor
are provided with an air atomizing nozzle. The basic principle is that the impact
of the auxiliary air around the nozzle causes the droplet to be broken smaller and
the atomization effect is better. Select the air-blast-atomizer nozzle type in Fluent.
49
The particles are droplet type, and set parameters such as spray point coordinates,
injection direction, temperature, mass flow, nozzle inner and outer diameter, spray
cone angle and relative speed.
4.5 NOx model
This paper only analyzes the NOx in the pollutants generated by the
combustor. Fluent contains three models of NOx formation: Thermal NOx
(thermal mechanism), Prompt NOx (transient mechanism), Fuel NOx (fuel
mechanism). One difference between the three mechanisms is that at temperatures
below 1573 K, Thermal NOx is produced spaannularly, but at higher temperatures,
Thermal NOx emissions are much higher than the other two generation
mechanisms, as shown in Figure 4-1. In this design, the average temperature of
the combustor is above 1900K, and NOx is mainly generated by Thermal NOx.
The following will mainly analyze Thermal NOx.
Figure 4-1 Effect of temperature on pollutant emissions
Zeldovic proposed the mechanism of Thermal NOx formation, which
Fenimore extended to form the following three reactions:
N 2 + O  NO + N
（a）
N + O2  NO + O
（b）
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N + OH  NO + H
（c）
There are three chemical bonds in N 2 that will cleave at high temperatures,
so reaction a determines the rate of NO formation. The concentration of O is
obtained from the reaction d, and the O2 cracking also requires high temperature
conditions.
O2 +M  NO + H
（d）
The rate of NOx formation can be expressed by the following formula:
d [ NO]
= k1[O][ N 2 ] + k2 [ N ][O2 ] + k3[ N ][OH ] − k−1[ NO][ N ] − k−2 [ NO][O] − k −3[ NO][ H ]
dt
Where k1, k2, and k3 are the rate constants of the reactions a, b, and c, and k-1,
k-2, and k-3 are the reaction constants of the reactions a, b, and c.
4.6 Materials
The Floating wall uses GH3536[14], as table 4-1 shows.
Table 4-1 GH3536
The Beaannular force wall uses GH4169[14], as table 4-2 shows.
Table 4-2 GH4169
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4.7 Solution Methods
The inlet of the combustor adopts the mass flow inlet, the outlet is set
as the pressure outlet, and the wall surface is set as the solid wall boundary.
In the calculation of the hot flow field, aviation kerosene is used as a fuel
and is ejected from a pneumatic atomizing nozzle. The discrete format uses
first-order precision, and the velocity and pressure coupling uses SIMPLE
algorithm. SIMPLE (Semi-Implicit Method for Pressure-Linked Equations)
is a semi-implicit method for solving pressure-coupling equations. It is an
unstructured algorithm. The Simple algorithm is mainly used to solve the
incompressible flow field, and can also be used to solve the compressible
flow field.
52
Chapter V Flow Field Calculation and Analysis
In this chapter, based on the data calculated in the previous section, the UG
software is used to model the floating wall combustor model, the model is
imported into the ICEM to draw the grid, and finally iteratively solved in Fluent.
The numerical simulation results were compared to investigate the effects of
aerodynamic structural parameters on the combustion characteristics of the
floating wall combustor, temperature distribution, combustion product
concentration, combustion efficiency and pollutant emissions.
5.1 Combustor modeling and meshing
5.1.1 Geometric modeling
The type of combustor designed in this paper belongs to the annular
combustor. Because it has periodic circumferential variation, only one single head
is selected for modeling, which can effectively reduce the workload. The structure
of the cyclone, diffuser, flame tube and casing is drawn by the UG software [20]
according to the initial parameters, and finally assembled into a complete
combustor, as shown in Figure 5-1, wherein the structural model of the cyclone is
shown in Figure 5-2.
53
Figure 5-1 Combustor geometry model
Figure 5-2 Two-stage axial cyclone
5.1.2 Meshing
The .x_t file generated by UG modeling is imported into the ICEM software
[21] meshing grid. Because the flow field in the combustor is complex and
irregular, it is difficult to generate a structured grid. This paper uses a more
adaptive unstructured grid to divide. The body mesh type is Tetra/Mixed.
Tetra/Mixed generates a tetrahedral mesh by default, and performs proper mesh
encryption on the swirler blades and cooling holes, and uses a larger mesh for the
machine and other parts. This is beneficial to control the number of grids, to
ensure that the grid of critical flow fields is observed, while reducing the amount
of calculation. As shown in Figure 4-3, the total number of generated grids is
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approximately 4 million.
Figure 5-3 Combustor meshing
5.2 Combustion performance analysis
5.2.1 Flow field analysis
Reasonable flow characteristics in the combustor contribute to good
combustion performance. For the combustor, it is necessary to form a reasonable
flow field structure of the main combustion zone to meet the requirements of
ignition and flame stability, and also to control the flow loss within a certain range.
Through the calculation of the Fluent iteration, the velocity cloud map and the
streamline graph are obtained.
Figure 5-4 is the velocity cloud and streamline diagram of the central section
of the combustor in the cold state. It can be seen from the figure that a part of the
airflow enters the flame cylinder through the head swirler, and the other part enters
the flame cylinder through the cooling hole. Under the action of the air
enteannular the flame tube by the cyclone, a recirculation zone is formed, two
central vortices in the recirculation zone, and angular vortices are formed on both
sides of the recirculation zone. The velocity in the recirculation zone is relatively
55
small, which is beneficial for ignition and flame stabilization. The maximum axial
length of the recirculation zone is approximately below the second floating wall.
图 5-4 flow field
Fluent read intake air volume of each part of the cyclone is shown in Table
5-1, which is slightly deviated from the preset intake air quantity, but basically
meets the design requirements, and the air flow distribution of the combustor is
realized.
Table 5-1 Actual distribution ratio of air flow and design distribution ratio
As shown in the figure5-5, the pre-diffuser flows close to the wall surface,
and no flow separation occurs. Two stationary vortex are formed in the Sudden
expansion zone to facilitate air flow distribution.
56
Figure 5-5 Diffuser flow
As shown in the figure5-6, the cooling air flows against the floating wall,
which helps to form a film.
Figure 5-6 Near wall flow
As shown in the figure5-7, a section is selected downstream of the cyclone,
and the right side is the velocity vector diagram on the section. It can be seen that
the upper and lower cyclones each form their own flow field, and the two flow
fields do not interact with each other.
57
Figure 5-7 downstream flow field
5.2.2 Temperature field analysis
Temperature distribution is one of the main indicators for evaluating the
performance of aero-engine combustors. The temperature and variation in the
combustor have an impact on the performance parameters of the entire engine.
Therefore, the analysis of the combustor temperature field is of great significance
in the combustor design process. In this paper, two sections of the upper and lower
cyclones are selected. The temperature distribution on the section is shown in
Figure 5-8. The maximum temperature is 2293.7K.
Figure 5-8 Combustor cross section temperature distribution
In this paper, the wall cooling adopts the floating wall structure, and the cooling
method is Impingement/Convection cooling. It can be seen from the figure 5-9 that
the maximum temperature of floating wall is 1138.5K. The highest temperature
point appears closer to the cyclone because this is where the combustion takes place.
58
Figure 5-9 Floating wall temperature distribution
Figure 5-10 shows the beaannular force wall temperature distribution. The
maximum temperature is 907.3K, which is less than 970K and achieves design
requirements. Also the highest temperature point appears closer to the cyclone
because this is where the combustion takes place.
Figure 5-10 beaannular force wall temperature distribution
As figure 5-11 shows, The outlet maximum temperature is 1817.5K .The
59
outlet average temperature is 1712.3K.
Figure 5-11 outlet temperature distribution
5.2.3 Fuel-air distribution characteristics
Figure 5-12 is the fuel and gas distribution cloud map in the combustor.
The fuel mass fraction can reflect the ratio of fuel to oxidant. It can be seen from
the figure that the vicinity of the injection point is locally rich and the mass
fraction is rapidly reduced.
Figure 5-12 fuel distribution
60
5.2.4 Performance parameter
There are many important parameters for evaluating combustion
performance, and the combustion efficiency and the outlet temperature field are
mainly analyzed.
(1) Combustion efficiency
There are many calculation methods for evaluating combustion efficiency,
and the values calculated by various methods are different. By the gas analysis
method, the volume fraction (molar fraction) of each component in the
combustion products at the exit of the combustor is measured, and the combustion
efficiency can be obtained. The combustion efficiency calculation formula for gas
component analysis is as follows
B =
xCO2 + 0.531xCO − 0.319 xCH 4 − 0.397 xH 2
xCO2 + xCO + xUHC
In the formular： xCO2 ， xCO ， xCH 4 and xH 2 separately represent the volume
fraction of CO2 ，CO ，CH 4 and H 2 ；xUHC represent the volume fraction of UHC
except CH 4 ，we use xC12 H 23 to calculate。The Combustion efficiency is
 B = 0.99
（2）Hot spot indicator
The hot spot index refers to the maximum value of the exit temperature of
the combustor T4max . The ratio of the excess of the average value to the room
temperature rise of the combustion is called the outlet temperature distribution
coefficient (OTDF).
OTDF =
T4max − T4 ave 1817.5 − 1712.3
=
= 0.12
T4 ave − T3ave 1712.3 − 792.2
This value is usually in the range of 0.25~0.35. The lower the better, the
paper is 0.12, which indicates that the outlet temperature distribution is relatively
61
uniform.
62
Chapter Ⅵ Summary
Based on the double annular combustor, a new type of head structure is
proposed ， which is a radical triangle staging combustor. The initial design
parameters of the floating wall combustor are determined by calculation. The UG
software is used for modeling, the ICEM software is used for meshing, and the
Fluent software is used for numerical simulation. The results were processed using
Origin, Tecplot, and CFD-post software. The main conclusions of this paper are
as follows:
1. The combustor forms a recirculation zone in both the outer annular and
the inner annular, which is good for stable combustion of the flame, but the shape
of the recirculation zone is asymmetrical. The axial maximum length of the
recirculation zone is at the connection position of the second floating wall and the
third floating wall.
2. In addition to forming the vortex in the recirculation zone, the structure
also forms a pair of stable corner vortices.
3. The pre-diffuser flows close to the wall surface, and no flow separation
occurs. Two stationary vortices are formed in the sudden expansion zone to
facilitate air flow distribution. The function of the cap is to help the air to be
smoothly diverted, so that 30% of the air flows into the annular passage, 70% of
the air flows into the cyclones.
4. The cooling air below the floating wall forms an adherent flow, and the
floating wall achieves the desired cooling effect.
5. Near the injection point is rich combustion. The area with relatively large
oil-gas ratio is mainly close to the cyclone position, and the lean oil combustion
is realized downstream of the cyclone.
6. The highest temperature in the combustor is generated in a place with high
oil-gas ratio. The maximum temperature is 2293.7K.
7. The maximum temperature of floating wall is 1138.5K, which is less than
63
1170K and achieves design requirement. The bear annular force wall‘s maximum
temperature is 907.3K, which is less than 970K and achieves design requirement.
The highest temperature point appears closer to the cyclone because this is where
the combustion takes place.
8. The outlet maximum temperature is 1817.5K. The outlet average
temperature is 1712.3K. The outlet temperature distribution coefficient (OTDF)
is 0.12.
9. Nitrogen oxides are mainly distributed in the high temperature zone of the
combustor. The temperature in these zones is above 1900K, which is consistent
with the thermal NOx formation mechanism.
10. In summary, the new combustor designed in this paper has reached the
design goal, which has a promising head design structure.
64
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Acknowledgements
Time flying, it has been almost a year since I came to Russia. In a year's time,
I would like to sincerely thank everyone around me for their supports, which
encourage me to go forward in my study.
Thanks to the guidance and help of the Professor Igor Shirokov and Professor
Nesterenko. They are very patient and kind person. From the topic selection to the
present, they have devoted a lot of time to me. Every time I met questions, they
helped me solve problems carefully, which let me have a deep understanding of
combustor design.
Thanks to Tang ZhongFu, we have the same mentors, but different topics.
There is a mutual encouragement between us. We make progress together.
In the past year in Russia, I have encountered many difficulties due to
language barriers. Thanks to the help of my classmates, I quickly got used to life
here.
Thanks to my family, the road to learning is long, and the support of my
family is the biggest driving force for me to move forward.
The development of the aviation industry requires us to take the
responsibility. I will work hard to achieve my dreams!
67