An Experimental Research of the Performance of a

Trapped-vortex Combustor

He Xiaomin Zhang jinyu Xu jinsheng Su Junqing Wei minxiang College of Energy and Power Engineering, Nanjing University of Aeronautics and Astronautics,

Nanjing, 210016, P.R.China

Abstract This paper describes the performance of a

trapped-vortex combustor (TVC) which is a novel and compact gas turbine combustor with two cavities and a main zone. A 180mm wide rectangular TVC test rig, including diffuser, combustor case and liner, was designed and fabricated. Experiments were conducted to obtain the flow drag characteristics, ignition and lean blow out(LBO) performance and the combustion efficiency etc. at different inlet air flow rates(0.45kg/s~0.65kg/s), inlet temperatures (300K~500K) and overall fuel/air ratio with fixed inlet air pressure(about 0.108MPa). The total pressure loss increases from 3.05% to 5.19% with increasing the inlet air flow rates and inlet temperatures, however the flow drag coefficient varies little (about 24~25). Ignition limits and LBO limits are not so strongly dependent on the inlet air flow rates, but improve greatly when inlet temperature increases from 300K to 500K. Minimum overall fuel air ratio of ignition and LBO is 0.0055 and 0.0023 respectively. The combustion efficiency about 90%~95% was achieved at different overall fuel air ratio When only cavity was fueled. And it varies from 84% to 99.5% with overall fuel air ratio increasing from 0.0016 to 0.0037 when the primary fuel air ratio is fixed at 0.042 and the cavity and main zone were fueled together.

Nomenclature

σ＊ = total pressure loss coefficient ζ = flow drag coefficient FAR = Fuel air Ratio Pt = total pressure η = combustion efficiency

Ma = mach number ma = air flow rate ρ = density IOFAR = ignition overall fuel air ratio LBO = lean blow out cp = specific heat Hμ = calorific value

Introduction

Current gas turbine engine is becoming marginal to meet the increasingly severe, yet conflicting requirements for effective combustion, high thrust-to-weight ratio, enhanced parts durability and exhaust emissions. The engine combustor is a key part in developing future advanced designs. Advanced combustors are becoming shorter to meet the thrust-to-weight ratio goals. Shorter residence times in the combustion chamber may reduce the NOx emissions, but the CO and UHC emissions increase. Also, the partially-reacted fuel could escape the combustion chamber which imparts the effective combustion. To meet the conflicting requirements , a trapped-vortex combustor has been pioneered and developed for potential application in gas turbine engines.

For a conventional annular combustor, shown in Fig.1, the swirler is employed to establish recirculation zone which provides a continuous source of ignition by mixing hot products with the incoming fuel and air. The residence time in the combustor is a function of axial length of the system. The trapped-vortex combustor is shown in Fig.2. With difference from the conventional swirl stabilized combustor, the TVC is a staged combustor with two cavity zones(pilot zones) and a main zone.

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The pilot zone employ cavity to establish the stable trapped vortex, air and fuel are introduced into the cavities where it mixes and burns quickly. the hot gases spread out across the face of the dome serving as pilots for igniting the remaining fuel that is injected into the main air stream. The TVC design approach allows use of simple fuel injection techniques.

Fig.1 conventional gas turbine combustor

Fig.2 Trapped-vortex combustor The TVC concept is under development by the

Air Force Research Laboratory (AFRL) and GE Aircraft Engines (GEAE) under the auspices of Air force, Navy, and Strategic Environmental Research Development Program (SERDP) agencies. The flow structure in a cavity has been investigated many years ago which demonstrate that a vortex trapped in the cavity properly sized can reduce the pressure drag and the unsteady vortex motion1~3. The cavity has been originally employed as the flame holder for supersonic combustion4~6. For a low speed combustion, Hsu et al.(1995) have proposed a first generation Trapped-vortex combustor which flame stability is enhanced due to a stable vortex trapped inside the cavity7. Sturgess and Hsu (1997) found that the entrainment of the main annular flow into the

cavity is minimal when a vortex is trapped inside the cavity8. The GE and AFRL team began work on the TVC in 19939. A prototypical high pressure capable TVC test rig was designed in 1996 and the test was finished in 1998.Test conditions covered conventional gas turbine engine cycles. TVC test rig performance exceeded all initial expectations: 50% improved ground ignition, altitude relight, and lean blow out margins compared to levels of a conventional swirl stabilized combustor. NOx emissions reductions of 40% to 60% compared to the 1996 ICAO standards . Effective combustion efficiency above 99% was maintained over a 40% wider range than conventional combustors, Maximum temperature rise of 1489 K on cavities only achieved at an operating pressure of 3.4 atmospheres and operating temperature of 533 K. Maximum temperature rise of 1669 K on cavity + main achieved at an operating pressure of 14.6 atm. and operating temperature of 811 K. Also, many experiments and numerical simulations were performed to know about the combustion mechanism of the TVC10~13. Apart from these, Douglas (2000,2005) and Sturgess(2003) have discussed the application of the TVC as a RQL combustor14~16. Dale (2004) has investigated SiC-SiC Ceramic Liner for The Trapped Vortex Combustor Concept and Joseph Zelina(2002,2004) has designed the Ultra compact combustor(UCC) based on the TVC17~19.

It should be pointed out that all of works discussed above were performed based on the liner of the TVC which the cavity and main air were supplied and controlled separately.

Experimental Descriptions A schematic diagram of the TVC is shown in

Fig.3. The air enters into the liner through seven flow passages that one is for the main zone, two is for the each cavity and two is for cooling. The ratio of air flow rates for main zone, cavities and cooling is 50%, 30% and 20% respectively. The cooling air is distributed on the cavity front wall, back wall and combustor exit wall. Fig.4a is a TVC UG image with sidewall removed and fig.4b is a photo of the

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Rectangular TVC test rig in this study. The TVC test rig consists of combustor case、liner、fuel system and ignition system , and it was designed dependent on the former studies20~22. The parts of the TVC were made from 304 stainless steel. The TVC is 180mm wide and 248mm long without considering the length of the diffuser. Splitter plates were employed in the diffuser to separate proportionately the incoming air into cavities and the main zone. A bluff-body was placed in the front of the main zone that serves as the flame holder for the main zone. Also it is helpful to aid in mixing the hot products from the cavities with premixed fuel and air injecting into the main zone. In the main inlet, three flow passages are divided by the vertical struts which are used to transport the hot gas of the cavity into the main zone. Total nine pressure atomizers are employed in the TVC, and for each cavity three injectors are set on the front wall just down the primary air inlet. Another three injectors are placed in the front of the bluff-body which is help to mix the fuel and air entering the main zone. The air entering the cavity split two parts, one is introduced into cavities from the front wall and the other from the back wall. The main fuel is injected on the face of the bluff-body to form fuel film, and then is mixed with inlet main air. The main fuel and cavity fuel are controlled separately. An igniter (~6J at 2 spark per second) is set in the middle of the outer cavity. The photo of TVC test system is shown in fig.5. The air was provided by four air compressors which maximum air mass flow rate is 1.6kg/s and maximum pressure is 0.8MPa. Air is routed through the electrical heater that can deliver 1.0kg/s air flow rates to the test section at temperature up to 600K. Two identical kerosene fuel system was used ,one is for the pilot zone at maximum fuel rate of 100g/s (~3Mpa), and the other is for main zone at maximum fuel rate of 150g/s(~7Mpa). Air and fuel flows are metered with turbine flow-meters. Total pressure rakes with 3 probes are placed at the combustor inlet and exit, and water manometers are used to measure the pressure. The K-type thermocouple rakes and the water-cooled B-type thermocouple rakes with 5

probes are employed to measure inlet and exit temperature distribution along the radial direction. Data acquisition system with 24 channels record the signal of temperature delivered from the thermocouples.

Fig.3 A schematic diagram of TVC

(4a) TVC UG image

(4b) a photo of a TVC Fig.4 Trapped-vortex combustor test rig

Fig.5 Experimental System of TVC

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RESULTS AND DISSCUSSION For seven flow passages linking to the liner, the

air velocity of each passage is different. The Mach number of the main inlet (just at the exit of the bluff-body) and the primary air inlet is shown in Fig.6. It illustrates the velocity of the main air is much greater than that of the primary air, and also greater than that of the dome inlet in the conventional combustor at given air flow rate and inlet temperature. In this study, discussion is carried out on the basis of inlet air flow rates.

0.44 0.46 0.48 0.50 0.52 0.54 0.56 0.58 0.60 0.62 0.640.21

0.22

0.23

0.24

0.25

0.26

0.27

0.28

0.29

0.30

0.31

0.32

0.33

0.34

Ma

ma ( kg/s)

T=370K T=420K T=460K

(6a) main air inlet

0.44 0.46 0.48 0.50 0.52 0.54 0.56 0.58 0.60 0.62 0.64

0.095

0.100

0.105

0.110

0.115

0.120

0.125

0.130

0.135

0.140

0.145 T=370K T=420K T=460K

Ma

ma (kg/s) (6b) primary air inlet

Fig.6 inlet Ma versus air flow rates Flow Drag Characteristic

The cold flow drag characteristic is an important performance for aircraft gas turbine engines. Experiments were conducted to obtain the total pressure loss coefficient (σ＊ )and the flow drag coefficient( ζ ) which are used to evaluate the characteristic of the flow drag. σ＊ and ζ are defined as below:

234

3

34*

5.0

ref

t

t

t

Up

pp

ρςσ Δ

=Δ

= 　

Pt3 and Pt4 represents the inlet and exit total pressure(Pa),ρis the inlet density(kg/m3), and Uref

is reference velocity. Fig.7 shows the influence of inlet air flow rates

and inlet temperatures on the total pressure loss, and Fig.8 shows the influence on the flow drag coefficient. The total pressure loss increases linearly from 3.05% to 5.19% when the inlet air flow rates increase from 0.45kg/s to 0.65kg/s at ambient inlet temperature, and it varies from 4.45%to5.97% with inlet temperature increasing from ambient temperature to 460K at a fixed air flow rate. The flow drag coefficient(24~25) varies little with the inlet air flow rates and inlet temperature. The inlet velocity becomes faster when inlet air flow rates and inlet temperatures increase, which increasing the total pressure loss. the air flow has been already at the full developing state in the test which makes the flow drag coefficient change little. The total pressure loss and flow drag coefficient is almost the same as the conventional swirl stable combustor. however, as shown in fig.6, main inlet velocity of TVC is twice even third higher than that of the conventional combustor, the flow drag performance will be improved if decreasing the inlet main velocity.

0.44 0.46 0.48 0.50 0.52 0.54 0.56 0.58 0.60 0.620

1

2

3

4

5

6

7

σ* (%)

ma (kg/s)

(7a) Ti＝281K

4

260 280 300 320 340 360 380 400 420 440 460 4800

1

2

3

4

5

6

7

σ* (%)

Ti (K) (7b) ma=0.55kg/s

Fig.7 Total Pressure Loss versus the inlet airflow rates and inlet temperatures

0.44 0.46 0.48 0.50 0.52 0.54 0.56 0.58 0.60 0.62

10

12

14

16

18

20

22

24

26

28

30

ζ

ma (kg/s)

(8a) Ti＝281K

280 300 320 340 360 380 400 420 440 46010

12

14

16

18

20

22

24

26

28

30

ζ

Ti /K

(8b) ma=0.55kg/s Fig.8 Flow drag coefficient versus the inlet airflow

rate and temperature Ignition performance

The ignition performance is affected by many factors including inlet velocity, temperature, pressure, fuel/air ratio(FAR),spark energy and spark location etc.. Experiments were conducted to study how the

ignition FAR varied with inlet air flow rates and inlet temperatures.

In the test, inlet pressure was fixed(about 0.108Mpa) and only pilot zone was fueled. When combustion was originated within the outer cavity by the spark plug, the flame was transported into the inner cavity along the vertical struts. The ignition process is regarded as sucess only the outer and inner cavity combusting together. At a given inlet temperature, the ignition experiments were conducted by keeping inlet air flow rates and increasing slowly fuel flow rate until the ignition succeed. Then a different flow rate was selected and the procedure repeated at the same inlet temperature. Finally, this process was repeated again with a different inlet temperature. 4 inlet air flow rates and 3 inlet air temperatures were varied in the experiment.

Fig.9 and Fig.10 illustrates the impact of inlet air flow rates and inlet air temperatures on the ignition overall fuel air ratio(IOFAR). In the experiment, only the outer cavity fuel is used to calculate the IOFAR because only one igniter was placed in the outer cavity, although the fuel introduced into the outer and the inner cavity is supplied simultaneously with the same fuel system. At a fixed inlet temperature, The IOFAR isn’t sensitive to the inlet air flow rate, and the ignition performance improves slightly with the inlet temperature rising. The ignition performance is strongly dependent on the flow field in the cavity. At given inlet temperature, the primary velocity changes little. For example, with inlet air flow rates increasing from 0.45 to 0.65kg/s, the Mach number of primary air varies from 0.101 to 0.135 at inlet temperature 420K. At a given inlet air flow rate, increasing inlet temperature is useful to improve the fuel vaporization, fasten reaction velocity and flame propagation. minimum IOFAR is 0.0055 at the following condition: ma=0.609kg/s, Ti=460K.

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0.44 0.46 0.48 0.50 0.52 0.54 0.56 0.58 0.60 0.62 0.640.000

0.002

0.004

0.006

0.008

0.010IO

FAR

ma (kg/s)

Ti=370K

Ti=420K

Ti=460K

Fig.9 Ignition FAR versus inlet airflow rates

360 380 400 420 440 460 4800.000

0.002

0.004

0.006

0.008

0.010

IOFA

R

Ti (K)

ma=0.456Kg/s

ma=0.501Kg/s

ma=0.558Kg/s

ma=0.609Kg/s

Fig.10 Ignition FAR versus inlet temperature

Lean Blow Out performance

The procedure of the LBO experiment is similar with the ignition limit experiment. After the TVC was ignited and burned stable, the fuel flow rate decreased slowly until the LBO occur. Four inlet air flow rates and three inlet temperatures were varied in the experiment.

Fig.11 and Fig.12 shows the effect of inlet air flow rates and inlet temperatures on the LBO limits. Resemble with the ignition characteristics, the LBO limits are not so strongly dependent on the inlet air flow rates, and improves with the inlet temperatures rising. In the test, minimum LBO overall FAR(LBO OFAR) is 0.0023 at the following test condition: ma=0.558kg/s, Ti=460K.

0.44 0.46 0.48 0.50 0.52 0.54 0.56 0.58 0.60 0.62 0.640.000

0.002

0.004

0.006

0.008

0.010

LBO

OFA

R

ma(kg/s)

Ti=370K

Ti=420K

Ti=460K

Fig.11 LBO limits versus inlet airflow rates

360 380 400 420 440 460 4800.000

0.002

0.004

0.006

0.008

0.010

LBO

OFA

R

T (K)i

ma=0.456kg/s

ma=0.501kg/s

ma=0.558kg/s

ma=0.609kg/s

Fig.12 LBO limits versus inlet temperature

Combustion Efficiency The exit temperature of two position was

measured which one was placed directly downstream of a fuel injection plane and the other was positioned midway between two adjacent fuel injection planes. Combustion efficiency is defined as below:

f

fpffinpairairoutpt mH

TcmTcmTmc

μ

η−−

=

Tin and Tout represent the inlet and exit air temperature(K), Tf is a inlet fuel temperature, Cp is the specific heat.

In the test, the inlet pressure and inlet temperature were fixed at 0.108Mpa and 460K respectively. Inlet air flow rates varied from 0.45 to 0.7kg/s, overall fuel/air ratio varied from 0.01 to 0.033, and there are two fuel supplying manners including cavity fueled only and cavity + main zone fueled together.

The results are plotted as a function of OFAR shown in fig.13. When the cavity fueled only, the

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maximum average temperature rise is about 943K, and the efficiency isn’t very high(about 91~95% ) which is not dependent on the inlet air flow rates and overall fuel/air ratio as shown in fig.13(a). The fuel/air ratio in the cavity zone changes from 0.0335 to 0.056 which is favorable for the stable combustion. Perhaps it is reason that causes the efficiency varying little. Inlet primary velocity varies from 40m/s to 65m/s which is fast slightly to get the high combustion efficiency , and whereas the fast main inlet velocity (103.2m/s~142.02m/s) quenches the burning gas transporting from the cavity. When the cavity and main zone was fueled together and the cavity fuel air ratio is fixed at 0.042, the maximum average temperature rise is about 1283K. The combustion efficiency is about 84%~99.5%, and it decreases with decreasing the overall fuel/air ratio, especially overall FAR less than 0.025, as shown in fig.13(b). in the experiments, the cavity fuel/air ratio is fixed at 0.042, the main FAR vary from 0.011 to 0.03 when overall FAR changes from 0.0203 to 0.0335. The main FAR is too lean to release enough heat to keep effective combustion , especially the main velocity is fast.

summary and conclusions Trapped-vortex combustor(TVC) is an novel

and compact gas turbine combustor. To evaluate the TVC performance, A 180mm wide rectangular TVC was designed and fabricated, which includes combustor case、liner、diffuser and nine pressure atomizers. Experiments were conducted to get characteristics of the flow drag, ignition and LBO performance and combustion efficiency.

In experiments, inlet air pressure was fixed at about 0.108MPa, inlet air temperature and inlet air flow rates varied from 300 to 500K and from 0.45 to 0.65kg/s respectively, the overall fuel air ratio varied with the different fuel flow rates at a given inlet air flow rate.

In the test, the total pressure loss is from 3% to 6% and the flow drag coefficient is 24~25 at different inlet air flow rates and inlet temperature which is nearly the same as the conventional annular

combustor. Ignition OFAR and LBO limits are not strongly dependent on the inlet air flow rates, but improved with increasing inlet temperature. The flame can not keep stable only the main zone fueled, even inlet Mach number is under 0.1. The minimum FAR of ignition and LBO is 0.0055 and 0.0023 respectively. When only cavity fueled, the combustion efficiency is 90~95% and is not strongly dependent on the inlet air flow rates and overall fuel/air ratio. When cavity and main zone was fueled together, the maximum average temperature rise is about 1283K. and the combustion efficiency is about 84%~99.5% which decreases with decreasing the overall fuel/air ratio.

0.010 0.012 0.014 0.016 0.018 0.02070

75

80

85

90

95

100

η(%)

overall FAR

ma=0.456 kg/s

ma=0.501 kg/s

ma=0.558 kg/s

ma=0.609 kg/s

(13a) only cavity fueled

0.010 0.015 0.020 0.025 0.030 0.035 0.04070

75

80

85

90

95

100

η/%

overall FAR

ma=0.456 kg/s

ma=0.501 kg/s

ma=0.558 kg/s

ma=0.609 kg/s

(13b) cavity and main zone fueled together

Fig.13 Combustion efficiency versus the overall fuel air ratio（Ti=460K）

acknowledgement This work was supported by the Aviation

Industry corporation of P.R.China. The authors would like to thank Prof.Wan jiahua for his helpful

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discussion, and Dr.Zhang penggan, Mr.sujuanqin and Mr.Xu jingsheng for assistance in experimental work.

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