Emission characteristics of a model combustor for aero gas turbine application

Experimental Thermal and Fluid Science 72 (2016) 235–248

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Emission characteristics of a model combustor for aero gas turbine application Lin Li, Yuzhen Lin, Zhenbo Fu, Chi Zhang ⇑ National Key Laboratory of Science and Technology on Aero-Engine Aero-Thermodynamics, School of Energy and Power Engineering, Beihang University, Beijing 100191, China

a r t i c l e

i n f o

Article history: Received 19 May 2015 Received in revised form 21 October 2015 Accepted 10 November 2015 Available online 2 December 2015 Keywords: TeLESS combustor Lean staged combustion Low emissions Empirical NOx model

a b s t r a c t A TeLESS concept combustor using lean staged NOx reduction technology has been proposed on the basis of previous studies. TeLESS means the Technology of Low Emissions of Stirred Swirl in this paper. As the power condition increases, the TeLESS combustor will work in low power mode, circumferentially staged mode, or high power mode. Meanwhile, each combustor dome is in either only pilot mode or two stages mode. Emission characteristics of TeLESS combustor have been investigated experimentally in a singlemodule rectangular model combustor. NOx emissions and combustion efficiency of the model combustor were measured under high pressures and temperatures up to 2.8 MPa and 845 K. Fuel stage ratio ranged from 1.5% to 34.9% in the tests. The fuel staging effects on NOx emissions, combustion efficiency and stability of the model combustor were studied under circumferentially-staged mode and high power mode conditions. Tests results showed that the optimal fuel stage ratio for the combustor varied with operating conditions. A new empirical NOx model, based on the idea of flame partition, was proposed for staged combustion. Combustor pressure, inlet temperature, and dome equivalence ratio, as well as the dome fuel stage ratio were used as input parameters in the NOx model. Finally, the empirical model was used to predict landing and take-off cycle NOx emissions for the TeLESS combustor. Ó 2015 Elsevier Inc. All rights reserved.

1. Introduction With the rapid development of civil aviation, the pollutant emissions from aero gas turbine engines are also increasing. These pollutant emissions have great influence on climate change and local air quality [1–5]. Thus, the international standards on civil aviation engine emissions set by International Civil Aviation Organization (ICAO) Committee on Aviation Environmental Protection (CAEP) have become more and more stringent. These standards cover the take-off, climb, approach, and taxi/ground idle phases of the engine operation, which is called landing and take-off (LTO) cycle. All the standards call for reducing LTO NOx emissions with no increase in other emission constituents [6]. In addition, the inlet temperature and pressure of aero gas turbine combustor are continuously increased with the progress of aero gas turbine technology [7]. As a result, the control of pollutant emissions generated by aero gas turbine combustor has been more challenging during the past few decades. Besides, gas turbine combustors adopting lean premixed prevaporized (LPP) [8,9], rich-quench-lean (RQL) [10–12], and lean direct injection (LDI) [13,14] combustion strategies are widely

⇑ Corresponding author. Tel./fax: +86 010 82316518. E-mail address: [email protected] (C. Zhang). http://dx.doi.org/10.1016/j.expthermflusci.2015.11.012 0894-1777/Ó 2015 Elsevier Inc. All rights reserved.

investigated to reach an ultra-low emission level. Combustors that use different low emissions technologies have been successfully applied to aviation engines. Emission data from production engines have proved that lean-burn technology can reduce more NOx emissions than rich-burn technology, especially at a high operation pressure ratio (OPR) [15–19]. Studies of fuel staging effects on combustion instabilities [20,21], flame structure [22], fuel/air mixing [23], as well as NOx reduction [13,24] have been carried out widely. However, there are only a few researches aimed at the fuel staging effects on emission characteristics of aero engine combustors. NOx emissions and combustion efficiency characteristics of single-sector and multisector aero gas turbine combustors were investigated by Yamamoto et al. [25–27]. However, fuel staging effects on NOx emissions were not discussed in details in their studies. Ignition performance and emissions of a single-sector combustor adopting lean staged combustion technology were investigated by Fu et al. [28,29]. Fuel staging effects on combustion efficiency and NOx emissions were studied experimentally, but the test conditions were rather limited in Fu’s paper. The effects of pilot fuel proportion on NOx emission in a gas turbine combustor were studied by Liu et al. [30]. But combustion efficiency was not measured and combustor inlet temperature and pressure range was very narrow in Liu’s study. On the other hand, lots of semi-empirical and empirical NOx models have

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Nomenclature EI F FAR H P T W _ m

emission index (g/kg) maximum rated thrust (kN) fuel-to-air ratio lower heating value (kJ/kg) pressure (MPa) temperature (K) air mass flow rate (kg/s) fuel mass flow rate (kg/s)

c f fl m p stoi

combustor fuel flame main stage pilot stage stoichiometric

Greek symbols fuel stage ratio U equivalence ratio p pressure ratio g combustion efficiency

a Subscripts oo ISA sea level static conditions 3 combustor inlet a air

been widely used to correlate experimental results of NOx emissions [24,31–38]. Empirical NOx models for aero gas turbine combustor were proposed by Mellor [36,37], Lefebvre [31], Rizk and Mongia [32], Tsalavoutas et al. [34]. However, few empirical NOx models has taken consideration of fuel stage ratio effects on NOx emission [24]. So it is valuable to find an empirical NOx model for staged combustion in aero gas turbine combustor. In this paper, a combustor with Technology of Low Emissions of Stirred Swirl (TeLESS) for aero gas turbine engine application was developed and tested. The model combustor was developed on the basis of previous studies carried out by authors [28,29]. Compared with former studies, the combustor is comprised of a new main stage and an optimized pilot stage and the combustor is designed according to new operating parameters. A large amount of experiments has been carried out to obtain the emission characteristics of the model combustor. Furthermore, the effects of fuel staging on pollutant emissions and combustion efficiency were investigated experimentally under different operating conditions. Correlation for NOx emissions were carried out after tests. One purpose of this paper is to study fuel staging effects on the combustor performance under a wide range of operating conditions, while taking NOx emission, combustion efficiency, and combustion stability into account. The other purpose is to propose an empirical NOx model including fuel staging effects for aero engine combustor using lean staged combustion technology. 2. Combustor configuration and experimental setup 2.1. Combustor configuration The TeLESS combustor tested in present work is a singlemodule rectangular combustor using lean staged combustion technology, as shown in Fig. 1. The key issue for low emissions is to maintain the combustion zone temperature within a fairly narrow band over the entire power range of the engine [38]. The dome airflow includes pilot swirler air, premixer air, and stirred air. According to the results of flow distribution experiment, dome airflow accounts for 65% of the entire airflow. There is no dilution hole on the combustor liner in TeLESS combustor, the rest airflow is all used for combustor liner cooling. Lean combustion has great advantage in reducing NOx emission and smoke under high power conditions. However, lean combustion technology could cause higher carbon monoxide (CO) and unburned hydrocarbon (UHC) emissions under low power conditions, such as idle and approach. This will lead to lower combustion efficiency and higher specific fuel consumption (SFC). Besides, the combustor should meet the ignition and stability requirements. In

order to meet the requirements of emissions, combustion stability, and efficiency, the combustor dome is comprised of two stages – pilot stage and main stage. And Fig. 2(a) is the photo of two stages. The pilot stage is designed to adopt a diffusion combustion mode. It has primary influence on easy and reliable ignition during ground starting, rapid relighting of the combustor after a flameout in flight, lean blowout limits, combustion efficiency, and pollutant emissions at low power conditions [28,29]. To ensure combustion stability, the pilot stage is fueled over the entire power range of engine. The configuration of main stage is shown schematically in Fig. 2(b). The main stage is a novel annular premixer, using a prefilmer to realize plain-jet film concept in swirl crossflow [39,40]. In the annular premixer, the prefilmer has two advantages. One is to form a fuel film to enhance atomization and improve spray uniformity in the circumferential direction. The other is to control the penetration of the liquid fuel jet so that no fuel will splash on the wall at high liquid–air momentum flux ratios under high power conditions. The main stage adopting a premixed flame is used to reduce NOx emission at high power conditions. The uniformity of air/fuel mixture has great influence on the pollutant emissions in the combustion zone [41–43]. The air/fuel mixing must be achieved rapidly without auto-ignition and flashback in the annular premixer. This is because auto-ignition time of aero kerosene is in the order of milliseconds under take-off conditions. As a result, the residence time in the annular premixer is limited to several milliseconds or less [44,45]. In current study, the residence time of airflow is less than 0.7 ms in the annular premixer under take-off condition. On the other hand, the annular premixer is designed to be convergent at the exit to avoid flashback. 2.2. Experimental setup The experimental facilities for the combustion and emission tests are shown in Fig. 3. The experimental setup includes an air supply system, a fuel control system, a cooling water delivery system and data acquisition equipment. The experiment system has been improved based on the previous one, described in Ref. [28]. Before the air for combustion entering in the model combustor installed in the test section, it is heated to a high temperature by the air heater. The air mass flow (Wa) is measured by an orifice plate flowmeter in front of the air heater. The measurement precision of the orifice plate is ±0.5% in the test condition range. The temperature (T3) and pressure (P3) parameters are obtained by K-type thermocouple and pressure transducer, respectively. Besides, the kerosene is delivered to the model combustor by two independent fuel pipelines. The pilot and main stage fuel mass flow rates

L. Li et al. / Experimental Thermal and Fluid Science 72 (2016) 235–248

237

Fig. 1. Schematic diagram of the single-module combustor.

(a) Pilot and main stage coomponent

(b) Schem matic diagram m of annular premixer

Fig. 2. Schematic diagram of pilot and main stages.

Fig. 3. Schematic diagram of experimental facilities.




 _ p;dome and m _ m;dome are measured by a coriolis mass flowmeter in m each pipeline. The measurement precision of the fuel coriolis mass flowmeter and air orifice plate flowmeter are ±0.5% and ±2%, respectively. The uncertainty of dome equivalence ratio is about ±1.9% in current study. The gaseous emission measurement system strictly conforms to the emission measurement standards by ICAO. Gas sampling was achieved by a water-cooling sampling probe located in the downstream of the test section. A pipe was used to connect the sampling probe and the gas analysis system. The temperature of

the sampling pipe was held constant at 160 °C by an electrical heating tape to ensure that all the sample components in the pipe were in gas phase. Subsequently, the sample gas entered five parallel gas sampling analyzers, namely an O2 analyzer (SIEMENS ULTRAMAT 6), a non-dispersive infrared (NDIR) CO2 analyzer (SIEMENS ULTRAMAT 6), a NDIR CO analyzer (SIEMENS ULTRAMAT 6), a chemiluminescent NO–NOx analyzer (CAI Model 600), and a flame ionization detector unburned hydrocarbons (UHC) analyzer (Baseline MOCON Series 9000). The measurement ranges of five analyzers are 0–25%, 0–20%, 0–50,000 ppm, 0–1000 ppm, and

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0–2000 ppm, respectively. Note that O2, CO, and CO2 were measured through the dry method. The accuracies of the three SIEMENS ULTRAMAT 6 analyzers are all ±2% of the measured values, while that of both the NO–NOx and UHC analyzers is ±1% of the measured values. While the manufacturer repeatability of each analyzer is ±2%, the test repeatability is about ±3%. Before the test, each analyzer was calibrated by the standard gas. The errors of all apparatus were within ±1%. A home-made smoke measurement test rig was used to collect the smoke particles in the exhaust gas. The emission index (EI) can be calculated according to formulas proposed by ICAO [6]. On the other hand, gas analysis method was used to calculate combustion efficiency, as shown in Eq. (1).

gc ¼ 1 

EICO HCO þ EIUHC HHC 1000Hf

ð1Þ

where H is the lower heating value; EICO and EIUHC are emission indexs for CO and UHC. In current study, the lower heating values of CO, UHC and kerosene are as follows: HCO = 10,114 kJ/kg, HHC = 50,179 kJ/kg, Hf = 43,530 kJ/kg. 2.3. Test conditions As mentioned above, each dome in an annular TeLESS combustor is comprised of two stages. A combustor dome is in only pilot mode (OPM) while only the pilot stage is fueled. On the other hand, the combustor dome is in two stages mode (TSM) while both of the two stages are fueled. Therefore, there are two operation modes for each dome. As a result, staged combustion can be achieved by dome operation mode in an annular combustor. As shown in Fig. 4, the annular TeLESS combustor has three different combustion modes with the power condition increasing. The three combustion modes are low power mode (LPM), circumferentially-staged mode (CSM), and high power mode (HPM). All the domes are in only pilot mode while the annular combustor is working in low power mode. On the other hand, each dome is in two stages mode while the annular combustor is working in high power mode. However, the adjacent domes of an annular TeLESS combustor are in only pilot mode and two stage mode, respectively, while the annular TeLESS combustor is working in circumferentially-staged mode. That is to say there are half domes in only pilot mode and half domes in two stages mode in this case. The fuel can be divided into two parts in circumferentially-staged mode and high power mode. One part is provided to pilot stages and burned by means of diffusion combustion mode. The other

part is provided to main stages and burned in premix combustion mode. In staged combustion, fuel stage ratio a has an important influence on the emissions and combustion efficiency [22,24– 26,30]. As shown in Eq. (2), fuel stage ratio a is defined to describe fuel allocation in an annular TeLESS combustor.

_ m

a¼ _ p_ mp þ mm

with 0 < a 6 1

ð2Þ

_ p and m _ m represent the fuel mass flow rate of pilot In Eq. (2), m stages and main stages in annular combustor, respectively. However, a single-module rectangle combustor was experimentally studied in this paper. Dome fuel stage ratio adome was used to represent fuel allocation between pilot stage and main stage in the single-module TeLESS combustor. The definition of adome is shown in Eq. (3).

_ m

p;dome adome ¼ _ _ m;dome mp;dome þ m

with 0 < adome 6 1

ð3Þ

_ p;dome and m _ m;dome represent the fuel mass flow rate of pilot where m stage and main stage in the single-module TeLESS combustor, respectively. Provided that the number of domes in an annular TeLESS combustor is 2n. Thus, the conversion between a and adome is as shown in Eq. (4).

adome

8 _ p;dome _ m m > ¼ p;dome ¼ 1 OPM dome in LPM; CSM > > m_ p;dome þm_ m;dome m_ p;dome þ0 > < 1m _ p;dome _p m 2a ¼ m_ p;dome þm_ m;dome ¼ 1 m_2np þ1m_ m ¼ 2 a TSM dome in CSM n 2n > > > 1 > _ p;dome _ m m : ¼ 1 m_ 2nþ 1p m_ ¼ a TSM dome in LPM _ _ m þm p;dome

m;dome

2n

p

2n

m

ð4Þ The dome fuel stage ratio adome is equal to fuel stage ratio a when the annular TeLESS combustor is in low power mode and high power mode. However, the values of adome and a are not equal when the combustor is working in circumferentially-staged mode. Emission tests were carried out on the test rig as mentioned above. In current study, emission tests were performed as the combustor inlet pressures (P3) up to 2.8 MPa and temperatures (T3) up to 845 K. The dome equivalence ratio Udome ranged from 0.07 to 0.69. All the test conditions are listed in Table 1. The definition of dome equivalence ratio Udome is shown in Eq. (5).

Udome ¼

_ p;dome þ m _ m;dome Þ=W dome ðm FARstoi

Fig. 4. Schematic of staged combustion.

ð5Þ

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L. Li et al. / Experimental Thermal and Fluid Science 72 (2016) 235–248 Table 1 Tests conditions for model combustor. Test conditions

P3 (MPa)

T3 (K)

Udome

Dome mode

a (%)

adome (%)

Case1 Case2 Case3 Case4 Case5 Case6 Case7 Case8 Case9

0.57 1.30 1.30 1.90 1.75 2.80 1.15 2.64 2.25

533 654 654 650 650 811 746 845 842

0.30–0.42 0.07–0.13 0.62–0.69 0.59 0.62 0.59 0.56 0.65 0.65

OPM in LPM OPM in CSM TSM in CSM TSM in HPM TSM in HPM TSM in HPM TSM in HPM TSM in HPM TSM in HPM

100 20.0–35.0 14.8–35.0 1.6–19.3 1.5–18.3 2.1–10.0 10.0–30.0 10.0 10.0

100 100 8.0–21.2 1.6–19.3 1.5–18.3 2.1–10.0 10.0–30.0 10.0 10.0

_ p;dome and m _ m;dome are fuel mass flow rates of pilot stage and where m main stage in the single-module TeLESS combustor; Wdome is the dome airflow; FARstoi is the stoichiometric fuel-to-air ratio. In Table 1, ‘‘case1” represents low power mode conditions and ‘‘cases4–9” represent high power conditions. In addition, ‘‘case2” and ‘‘case3” represent the conditions of only pilot stage fueled domes and two stages fueled domes in circumferentially-staged mode, respectively. The effects of fuel stage ratio on emissions were investigated experimentally under ‘‘cases2–7” conditions. The ranges of fuel stage ratio are also listed in Table 1.

3. Results and discussions

3.2. Fuel stage ratio effects on NOx emissions and combustion efficiency

3.1. NOx emissions and combustion efficiency in only pilot mode The model combustor is in only pilot mode under ‘‘case1” and ‘‘case2” conditions. Diffusion combustion is dominant when the model combustor dome is in only pilot mode. Emissions and combustion efficiency results of ‘‘case1” condition are shown in Fig. 5. Fig. 5 shows the effect of Udome on the Emission Index of NOx, UHC, and CO in low power mode. The EINOx increases from 4.3 to 6.6 g/kg with the Udome varying approximately from 0.30 to 0.42. EINOx has increased more than 50% with Udome increasing, which means that EINOx is sensitive to dome equivalence ratio variations in low power mode. This is because the average temperature of combustion zone becomes higher as dome equivalence ratio increasing. On the other hand, the CO and UHC emissions decrease as the Udome increasing. There is a 80% reduction in EICO and a 93% reduction in EIUHC with Udome increasing from 0.30 to 0.42.

100.0

65 60

99.5

55

45

EINOx

40

EICO

99.0

EIUHC 98.5 Combustion efficiency

35 30

98.0

25 20

97.5

15 10

Combustion efficiency (%)

50

Emission Index (g/kg)

Combustion efficiency was calculated based on EICO and EIUHC data by Eq. (1). As shown in Fig. 5, the combustion efficiency increases from 97.4% to 99.6% while dome equivalence ratio varying approximately from 0.30 to 0.42. The results shows that the emissions of the combustor are sensitive to dome equivalence ratio variations. The combustion efficiency of modern aircraft should be higher than 99% to meet efficient combustion and emissions requirements [38]. In order to meet both low NOx emission and high combustion efficiency requirements, the dome equivalence ratio of the combustor should be around 0.36 under low power mode.

The effects of fuel stage ratio on NOx emissions and combustion efficiency of the model TeLESS combustor were studied under ‘‘case2”, ‘‘case3” and ‘‘case4” conditions. The value of fuel stage ratio changed from high to low during the tests. This is because lean blowout might occur in pilot stage while the fuel stage ratio is too low. A decrease in the fuel stage ratio means more fuel would be burned in premixed main flame. The fuel stage ratios are listed in Table 2. 3.2.1. Emission results under circumferentially-staged mode The results of ‘‘case2” and ‘‘case3” will be discussed in the first because they represent only pilot stage fueled dome and two stage fueled dome in circumferentially-staged mode, respectively. After that, NOx emission and combustion efficiency under high power mode will be discussed, including ‘‘cases4–7” conditions. Fig. 6 shows the emissions and combustion efficiency variations with different fuel stage ratios under ‘‘case2” and ‘‘case3” conditions. Test results show that emissions variations of ‘‘case2” and ‘‘case3” are totally different as the fuel stage ratio decreasing. The fuel stage ratio decreased from 34.9% to 14.8% during the tests. There is no emission data when the fuel stage ratio is equal to 14.8% under ‘‘case2” condition. That is because the pilot flame had been blown out under that condition. Emissions of the combustor are very sensitive to the fuel stage ratio variations under ‘‘case2” condition. As shown in Fig. 6(a), EINOx under ‘‘case2” condition almost linearly increases with the Table 2 Fuel stage ratios under cases2–7 conditions. Test condition

97.0

5 0

96.5 0.30

0.32

0.34

0.36

0.38

0.40

Dome equivalence ratio, Φ

0.42

dome

Fig. 5. Dome equivalence ratio effects on emissions and combustion efficiency.

Case2 Case3 Case4 Case5 Case6 Case7

Fuel stage ratio a 1 (%)

2 (%)

3 (%)

4 (%)

5 (%)

34.9 34.9 19.3 18.3 9.9 29.9

29.6 29.6 14.3 15.4 7.9 23.6

25.2 25.2 9.6 10.1 4.9 19.7

20.0 20.0 4.9 5.2 2.1 14.4

– 14.8 1.6 1.5 – 9.8

L. Li et al. / Experimental Thermal and Fluid Science 72 (2016) 235–248

9.5

110

Emission Index, NOx (g/kg)

9.0

NOX case3

8.5

Eimission index, CO & UHC (g/kg)

NOX case2 8.7

8.0 7.5

7.4

7.0 7.1

6.5

7.1

7.0

7.2

7.0

6.3

6.0 5.5 5.0

4.9

100

100 99

90

98

80 CO _ case2 UHC_case2 CO _ case3 UHC_case3

70 60

η case2 η case3

96

50 40

95

30

94

20 93

10

92

0

4.5 15

20

25

30

35

97

Combustion efficiency (%)

240

15

20

Fuel stage ratio (%)

25

30

35

Fuel stage ratio (%)

(b) EICO, EIUHC and combustion efficiency

(a) NOx emission

Fig. 6. Fuel stage ratio effects on emissions and combustion efficiency under ‘‘cases2–3” conditions.

Φmain -TSM

0.85

0.22 0.20

0.7

0.72

0.18

0.6

0.16

0.61

0.14

0.5 0.49

0.129

0.12

0.4 0.113

0.3 0.2

0.10

0.36 0.095

0.08

0.074

15

20

0.06 25

30

35

Fuel stage ratio (%) Fig. 7. Fuel stage ratio effects on equivalence ratio under ‘‘cases2–3” conditions.

fuel stage ratio increasing from 20% to 34.9%. Fig. 6(b) shows that both EIUHC and EICO at ‘‘case2” condition decreases sharply as the fuel stage ratio decreasing from 34.9% to 20%. Then the combustion efficiency under ‘‘case2” condition decreases from 98.9% to 92.6% correspondingly. The results under ‘‘case2” condition show that a decrease in fuel stage ratio is beneficial to NOx emission reduction for only pilot fueled dome in circumferentially-staged mode. However, it is disadvantageous to stable and efficient combustion. As shown in Fig. 7, the local equivalence ratio varies from 0.85 to 0.36 under ‘‘case2” condition. This will reduce the combustion stability of pilot flame. In fact, when the fuel stage ratio was 14.8%, the pilot flame was blown out during the test. On the other hand, emissions of the combustor under ‘‘case3” condition are insensitive to fuel stage ratio variations. Fig. 6(a) shows that EINOx at ‘‘case3” condition varies between 7.0 and 7.2 while the fuel stage ratio increases from 14.8% to 34.9%. Meanwhile, both CO and UHC emissions under ‘‘case3” condition keep at a very low level with different the fuel stage ratios. Thus the combustion efficiency exceeds 99.8% in the whole range under ‘‘case3” condition. Both NOx emission and combustion efficiency are insensitive to fuel stage ratio variations under ‘‘case3”

conditions. This could be explained by the equivalence ratio variations with fuel stage ratio decreasing. As shown in Fig. 7, the local equivalence ratio of pilot stage decreases from 0.85 to 0.36 when the fuel stage ratio varying from 34.9% to 14.8%. This will cause the temperature of pilot diffusion flame decreasing, which would result in NOx emission reduction. However, both main stage and dome equivalence ratios increase with the fuel stage ratio decreasing. This would increase NOx emission. As a result, NOx emission did not change much under ‘‘case3” condition as the fuel stage ratio varies. The adjacent domes in an annular TeLESS combustor are in only pilot mode and two stages mode, respectively, while the combustor is working in circumferentially-staged mode. In an annular combustor, there are interactions between the two flames of adjacent domes. Cai et al. [46] found that confinement had a great influence on the flow characteristics of the swirl cup. The flame interaction could effect the NOx emission of burner, which depends on the distance between neighbouring burners [47]. In addition, Yamamoto et al. found that the LTO NOx emission of multi-sector combustor was almost same with the single-sector combustor [27]. In this paper, the interactions between the two flames of adjacent domes are ignored. The average results of ‘‘case2” and ‘‘case3” conditions are weighted by fuel mass flow, which are used

100.0 8 99.9 7

7.4 6.8

7.0

7.1

99.8

6 99.7 5

99.69

99.6

99.62

4

99.5 3 99.45

99.4

2 99.3 Emission index, NOx Combustion efficiency

1 99.24

99.2

0 20

25

30

35

Fuel stage ratio (%) Fig. 8. Average results of ‘‘case2” and ‘‘case3” condition.

Combustion efficiency (%)

Dome equivalence ratio

0.8

Φpilot -TSM & OPM

Dome equivalence ratio

Φdome -OPM Φ dome -TSM

Emission index, NOx (g/kg)

0.24

0.9

L. Li et al. / Experimental Thermal and Fluid Science 72 (2016) 235–248

to represent the results for the annular combustor in circumferentially-staged mode. Average NOx emissions and combustion efficiencies are shown in Fig. 8. As shown in Fig. 8, the average results show that there is only 8.8% increase in EINOx as the fuel stage ratio varies from 20% to 34.9%. It is not very effective to reduce NOx emissions through the change of fuel stage ratio. Furthermore, a decrease in fuel stage ratio is harmful to combustion efficiency and stability. The combustion efficiency decreases from about 99.7% to less than 99.2%, which means a waste of fuel and more CO and UHC emissions. In addition, local equivalence ratio of pilot swirl cup is 0.85 when fuel stage ratio is 35%, which could ensure stable combustion for only pilot fueled domes in circumferentially-staged mode. In general, combustion in the TeLESS combustor is the most efficient and stable while the fuel stage ratio is 35%. As a consequence, the optimal fuel stage ratio should be 35% while the combustor is in circumferentially-staged mode.

15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

100.0 99.9 case4 case5 case6 case7

99.8 99.7 99.6

case4_CO case5_CO case6_CO case7_CO case4_UHC case5_UHC case6_UHC case7_UHC

99.5 99.4 99.3 99.2

Combustion efficiency (%)

CO (g/kg)

3.2.2. Emission results under high power mode The effect of fuel stage ratio on emission characteristics of the model combustor in high power mode is studied experimentally under ‘‘cases4–7” conditions. The dome equivalence ratio was kept constant under each condition during the test. The range of fuel stage ratio under each condition is listed in Table 2. EIUHC, EICO and combustion efficiency of the model combustor are shown in Fig. 9. The combustion efficiency of the model combustor is insensitive to fuel stage ratio variations when the combustor is working in high power mode. Both EIUHC and EICO are less than 0.8 over the entire range of fuel stage ratio under ‘‘cases4–7” conditions. Thus, the combustion efficiency of the model combustor is higher than 99.9% with different fuel stage ratios in high power mode. So combustion efficiency will not be chosen as a factor to determine the optimal fuel stage ratio for TeLESS combustor under high power mode. The fuel stage ratio effects on NOx emission, when the combustor is working in high power mode, are shown in Fig. 10(a). Although the fuel stage ratio ranges of ‘‘cases4–6” are different, the NOx emission variations with fuel stage ratio decreasing are similar. The NOx emission first decreases to a lowest value and then increases as the fuel stage ratio decreasing to around zero. In addition, the optimal fuel stage ratio, which makes the model combustor generating the lowest NOx emission, is different for each condition. As shown in Fig. 10(a), the optimal fuel stage ratio for

241

case4, case5 and case6 conditions are 4.9%, 10.1% and 4.9%, respectively. Liu et al. [30] also found that the NOx emission first decreases and then increases with the pilot fuel stage ratio variations in their study. And they indicated that pilot flame contributes less thermal NOx than the main flame while the pilot stage equivalence ratio is less than 1. This means that the NOx emission will increase with fuel stage ratio decreasing when the pilot stage equivalence ratio is less than 1. So the pilot stage equivalence ratio should be around 1 when the fuel stage ratio is optimal for NOx reduction. However, the test results in current study do not agree with the conclusion proposed by Liu. As shown in Fig. 10(b), the local pilot stage equivalence ratios are less than 0.8 while the fuel stage ratio range are between 1.5% and 19.3% under ‘‘cases4–6” conditions. And the pilot stage equivalence ratios corresponding to the optimal fuel stage ratios for NOx reduction under ‘‘cases4–6” conditions are 0.20, 0.43 and 0.20, respectively. These results shown in Fig. 10 demonstrate that the diffusion pilot flame can still contributes more thermal NOx even if the pilot stage equivalence ratio is in a very lean range. In order to find the reason why the optimal fuel stage ratios for ‘‘cases4–6” conditions are different, NOx emission variations with local equivalence ratio of main stage will been analyzed next. The total fuel mass flow was kept constant under each condition during the test. As shown in Fig. 11(a), the main stage equivalence ratio increases as fuel stage ratio decreases. And the NOx emission variations with main stage equivalence ratio are shown in Fig. 11 (b). Under ‘‘cases4–6” conditions, the local equivalence ratio of main stage is 0.69 ± 0.01 for each case while the model combustor has the lowest NOx emission, as shown in Fig. 11(b). The results of ‘‘cases4–6” conditions suggest that the diffusion pilot flame contributes more thermal NOx than the premixed main flame while the main stage equivalence ratio is less than 0.69. This means that a decrease in fuel stage ratio is beneficial to NOx reduction for the model combustor only when the main stage equivalence ratio is less than a certain value. In current study, this value is approximately equal to 0.69. And this can give a reasonable explanation why NOx emission index monotonically decreases as the fuel stage ratio varies from 29.9% to 9.8% under ‘‘case7” condition, as shown in Fig. 11(a) and (b). On the other hand, the local equivalence ratio of pilot stage is about 0.4 when fuel stage ratio is equal to 10% under ‘‘cases4–6” conditions, as shown in Fig. 10(b). This value is close to lean blowout limit for a single swirl cup [48,49]. However, combustion in the model combustor is still stable as the fuel stage ratio decreases to 1.5% under ‘‘cases4–6” conditions. This means that decreasing fuel stage ratio to nearly zero would not cause extinction while the model combustor is working in high power mode. But the pilot nozzle must be cooled with a certain mass flow rate of fuel to avoid carbon deposition and coking. In current study, the mass flow rate of fuel for the pilot stage should not be less than 5 kg/h while the combustor is working in high power mode. Thus, two factors must be taken into consideration to determine the optimal fuel stage ratio for the combustor in high power mode. One is that the local equivalence ratio of the main stage should be as close to 0.69 as possible. The other is that the mass flow rate of pilot stage fuel should not be less than 5 kg/h. 3.3. NOx correlation

99.1 99.0 0

5

10

15

20

25

30

Fuel stage ratio (%) Fig. 9. Fuel stage ratio effects on EICO, EIUHC and combustion efficiency under ‘‘cases4–7” conditions.

Empirical NOx models could play an important role in the design and development of new combustors. In this paper, a new empirical NOx model including fuel staging effects was proposed to correlate experimental data on NOx emissions for the model combustor. The new empirical NOx model is based on the idea of flame partition. Experimental results under only pilot mode and

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0.9

15 case4_NOx

14 13.7

13.5

12

case6_NOx

11

10.3

10 9

9.4

8

7.5

7

6.4 5.7

6

6.4

5.1

5

5.5

4

case4 case5 case6

0.8

case5_NOx

0.7

Local equivalence ratio

Emission index, NOx (g/kg)

13

13.9 14.0

0.6 0.5 0.4 0.3 0.2

4.9

4.4

0.1

3 2

0.0 0

5

10

15

20

0

5

Fuel stage ratio (%)

10

15

20

Fuel stage ratio (%)

(a) NOx emission

(b) Pilot stage equivalence ratio

Fig. 10. Fuel stage ratio effects on NOx emission and local equivalence ratio under ‘‘cases4–6” conditions.

15

1.0 case4 case5 case6 case7

0.8

14 13

Emission index, NOx (g/kg)

Main stage equivalence ratio

0.9

0.7 0.6 0.5 0.4 0.3 0.2 0.1

12 11 10 9 8 7 6 5 4

0.0

case4 case5 case6 case7

3 0

5

10

15

20

25

30

0.45

0.50

0.55

0.60

0.65

0.70

Fuel stage ratio (%)

Main stage equivalence ratio

(a) Main stage equivalence ratio variations

(b) NOx emission variations

0.75

Fig. 11. NOx emission variations with main stage equivalence ratio under high power mode.

two stages mode were correlated, respectively. The parameters used in the empirical NOx model include combustor pressure (P), combustor inlet temperature (T3), dome equivalence ratio (Udome) and dome fuel stage ratio (adome). In this paper, combustor pressure is equal to combustor inlet pressure minus combustor liner pressure drop. Finally, the obtained empirical correlations were applied to predict LTO NOx levels for the TeLESS combustor. 3.3.1. NOx correlation for Only Pilot Mode data There is only pilot flame while the model combustor is in only pilot mode. And diffusion combustion is dominant under this condition. The model combustor is in only pilot mode under ‘‘case1” and ‘‘case2” conditions. The empirical NOx model for test data of only pilot mode is selected from one of NOx models proposed in Ref. [34], as shown in Eq. (6). Compared to NOx models in Ref. [34], dome equivalence ratio Udome is used instead of fuel-to-air ratio (FAR) in current study.

EINOx ðg=kgÞ ¼ a  expðb  T 3 Þ  Pn  Ucdome

ð6Þ

where P is combustor pressure, MPa; T3 is combustor inlet temperature, K; Udome is dome equivalence ratio; In addition, a, b, c and n are undetermined coefficients for Eq. (6). The results of NOx correlation for OPM data are shown in Fig. 12, as follows. In Fig. 12, EINOx values calculated by Eq. (6) are compared to experimental EINOx values. The prediction results show that the deviation between calculated and measured EINOx values is less than ±13%. The R2 (coefficient of determination) of nonlinear fitting by correlation shown in Eq. (6) is 0.9483. The results shows that the empirical NOx model shown in Eq. (6) can give a good prediction for NOx emission of only pilot mode. Moreover, the values of undetermined coefficients in Eq. (6) are listed in Table 3. It is generally found that NOx / Pn in conventional combustors, where the exponent n has values ranging from around 0.5 to 1.42 [38]. However, the exponent value of pressure varies widely in different empirical correlations. In this paper, the exponent value of combustor pressure is 1.28 while the model combustor is working in only pilot mode.

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L. Li et al. / Experimental Thermal and Fluid Science 72 (2016) 235–248

respectively. So the NOx emissions of pilot flame and main flame can be calculated by Eqs. (10) and (11).

10 EINOx-OPM

9

±13% deviation

7 6

ð10Þ

2 EINOx;main ¼ a2  expðb2  T 3 Þ  Pn2  Ucdome;main

ð11Þ

where Udome,pilot is the dome equivalence ratio when only pilot flame exist; Udome,main is the dome equivalence ratio when only main flame exist. They can be calculated by Eqs. (12) and (13).

R2=0.9483

5 4

Udome;pilot ¼

_ p;dome =W dome m FARstoi

ð12Þ

Udome;main ¼

_ m;dome =W dome m FARstoi

ð13Þ

3 2

P3 =0.57, 1.3 MPa

1

Φdome=0.07-0.42

0

T3=533, 654 K

0

1

2

3

4

5

6

7

8

9

Besides, EINOx,interaction is simply assumed to be proportion to the product of EINOx,pilot and EINOx,main, as shown in Eq. (14).

10

EINOx (measured), (g/kg)

EINOx;interaction ¼ m 

Fig. 12. Calculated EINOx by correlation versus experimental EINOx for Only Pilot Mode.

ð14Þ

1þc1 EINOx ¼ a1  expðb1  T 3 Þ  Pn1  Uc1 dome  adome

Table 3 Values of undetermined coefficients for Eq. (6).

2 þ a2  expðb2  T 3 Þ  Pn2  Ucdome  ð1  adome Þ1þc2

þ m  expðb1  T 3 þ b2  T 3 Þ

2

a

b

c

n

R

5.1371

0.0031

0.93

1.28

0.9483

3.3.2. NOx correlation for Two Stages Mode data Both diffusion pilot flame and premixed main flame exist while the model combustor dome is in two stages mode. The model combustor is working in two stages mode under ‘‘cases3–9” conditions. Fuel staging has a significant influence on NOx emissions under these conditions. However, there are few empirical models that include the fuel staging effects on NOx emission. A NOx model that includes the effect of fuel stage ratio was proposed by Han [24], as shown in Eq. (7).

EINOx ðg=kgÞ ¼ a  expðb  T fl þ d  af Þ  Pn  far

EINOx;pilot EINOx;main  a1 a2

Combining with definitions of adome and Udome by Eqs. (3) and (5), the new empirical NOx model can be expressed by Eq. (15).

c

ð7Þ

where P is combustor pressure, MPa; Tfl is flame temperature, K; far is fuel-to-air ratio of the combustor; af is the fuel stage ratio. However, the prediction results obtained by this empirical NOx model are not good enough for staged combustion [24]. In this paper, a new empirical NOx model is proposed to correlate emission data for staged combustion. As shown in Fig. 1, there are two flames in the model combustor in two stages mode, which are diffusion pilot flame and premixed main flame. Thus the NOx emission of combustor is assumed to be comprised of two parts in the new NOx model. One part is generated by diffusion pilot flame and the other part is generated by premixed main flame. And the total NOx emission is equal to fuel-massflow-weighted average of this two parts. The NOx model is shown in Eq. (8).

EINOx ¼ adome  EINOx;pilot þ ð1  adome Þ  EINOx;main

ð8Þ

However, the interaction between diffusion pilot flame and premixed main flame is ignored by the empirical model shown in Eq. (8). In order to account for this deficiency, EINOx,interaction is added to Eq. (8). Thus, a new empirical NOx model, which contains flame interaction effect, has been obtained, as shown in Eq. (9).

1þc2 1 þc 2 1  Pn1 þn2  Ucdome  a1þc dome ð1  adome Þ

where a, b, c, m and n are the undetermined coefficients. When the Eq. (6). That is to say, both only pilot mode and two stages mode data can be correlated by the new empirical NOx model shown in Eq. (15). But the undetermined coefficients are different for only pilot mode and two stages mode data. Fig. 13 shows the correlation results by the new empirical model for NOx emission data under ‘‘cases3–7” conditions. The deviations between calculated and measured EINOx values are approximately within ±15% for most of the data. In addition, values of undetermined coefficients in Eq. (15) are listed in Table 4. The R2 (coefficient of determination) of nonlinear fitting is 0.9804. The correlation results for the data of two stages

20

ð9Þ where adome is the dome fuel stage ratio; EINOx,pilot and EINOx,main are NOx emissions generated by pilot flame and main flame,

EINOx-TSM

18

±15% deviation

16 14 12

R2=0.9804

10 8 6 P3=1.15-2.80 MPa

4

T3=654-845 K Φdome=0.56-0.69

2 0

EINOx ¼ adome  EINOx;pilot þ ð1  adome Þ  EINOx;main þ EINOx;interaction

ð15Þ

adome = 1, the NOx model has the same form as the one shown in

EINOx (calculated), (g/kg)

EINOx (calculated), (g/kg)

8

1 EINOx;pilot ¼ a1  expðb1  T 3 Þ  P n1  Ucdome;pilot

α=1.5%-35%

0

2

4

6

8

10

12

14

16

18

20

EINOx (measured), (g/kg) Fig. 13. Calculated EINOx by correlation versus experimental EINOx for Two Stages Mode.

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L. Li et al. / Experimental Thermal and Fluid Science 72 (2016) 235–248

Table 4 Values of undetermined coefficients for Eq. (15). a1

b1

n1

c1

a2

b2

n2

c2

m

R2

3.4985

0.0042

0.88

0.36

0.3614

0.0074

0.85

4.77

0.0066

0.9804

30

10 R2=0.9442 R2=0.89

25

20

15

10

P3=0.1, 0.3, 0.5, 0.7 MPa

5

R2=0.8943 R2=0.8377

Current study Ref.[24]

EINOx (calculated), (g/kg)

NOx (calculated), (ppm)

Current study Ref.[24]

8

6

4

2

P3=0.575 MPa

T3=650 K

T3=523 K

α=3%-20%

α=5%-73%

0

0 0

5

10

15

20

25

30

0

2

4

6

8

10

EINOx (measured), (g/kg)

NOx (measured), (ppm)

(a) Correlation for NOx emission in Ref.[24]

(b) Correlation for NOx emission in Ref.[30]

Fig. 14. Comparisons between two empirical NOx models.

mode show that the new empirical model proposed in this paper could give a good prediction for the NOx emission of the model combustor. In order to further validate the empirical NOx model shown in Eq. (15), the model was used to correlate the NOx emission data in Refs. [24,30], respectively. Besides, correlation results are compared with the results obtained by the empirical model proposed by Han [24]. As shown in Fig. 14(a) and (b), the empirical NOx model proposed in current study can correlate the NOx emission data better than the model proposed by Han [24]. The results show that the new empirical NOx models proposed in this paper could give a good prediction for NOx emission in staged combustion. 3.3.3. LTO NOx prediction for the TeLESS combustor Empirical models would be used to predict LTO NOx emissions for the TeLESS combustor. However, the test results and empirical NOx models in this paper were obtained through a single-module combustor. The interactions between the two flames of adjacent domes are ignored in this paper. Although the prediction results of the single-module combustor cannot represent the LTO NOx level for the annular combustor, the results could show the potential of NOx reduction through TeLESS combustion concept. Operating conditions of LTO cycle are calculated by in-house programs for the combustor. The pressure ratio and maximum rated thrust of the turbofan engine, which will use the TeLESS combustor, are listed in Table 5. In addition, LTO NOx level has been calculated according to CAEP/6 standard [6]. In order to determine dome equivalence ratio and dome fuel stage ratio for the combustor, following assumptions are made in staged combustion strategy: (1) Combustor dome airflow ratio is Table 5 Parameters for the hypothetical aero gas turbine engine.

poo

Foo (kN)

CAEP/6, LTO NOx (g/kN)

33.0

137

65.0

The bold indicate the engine pressure ratio at take off condition.

Table 6 Operating conditions of the single-module TeLESS combustor under LTO cycle. Phase

Dome mode

T3 (K)

P (MPa)

adome (%)

Udome

Take-off Climb Cruise Approach Approach Idle

TSM in HPM TSM in HPM TSM in HPM TSM in CSM OPM in CSM OPM in LPM

844 811 747 654 654 533

3.19 2.80 1.22 1.31 1.31 0.59

12 6 7 21.2 100 100

0.645 0.591 0.556 0.613 0.130 0.353

equal to 65%; (2) The fuel stage ratio value is 35% while the TeLESS combustor is working in circumferentially staged mode; (3) When the TeLESS combustor is working in high power mode, the optimal fuel stage ratio is determined by the two factors discussed in Section 3.2.1. The operating conditions of the combustors under LTO cycle, such as combustor inlet temperature (T3), combustor pressure (P), dome equivalence ratio (Udome) and dome fuel stage ratio (adome), are listed in Table 6. And the parameters for only pilot mode dome and two stages mode dome under approach condition are listed separately in Table 6. Values of Udome with different dome airflow ratios are also listed. As listed in Table 6, the ranges of combustor inlet temperature, combustor pressure, dome equivalence ratio are 533–844 K, 0.59–3.19 MPa and 0.121–0.699, respectively. Furthermore, NOx emission of the entire LTO cycle can be calculated by the empirical NOx model proposed in this paper. Fig. 15 shows the prediction results of LTO NOx level for the combustor. In Fig. 15(a), the vertical axis represents the LTO NOx emissions of the combustor, which is expressed as a percentage of the CAEP/6 NOx regulatory limit. The prediction results show that LTO NOx emission of TeLESS combustor is 40.7% of CAEP/6 standard when the dome airflow ratio is 65%. Fig. 15(b) shows LTO NOx emissions of several production aircraft engines. Different types of low emissions combustors have been used in the engine, such as TAPS, DAC, Talon, and Phase 5 combustors. Emissions data are obtained from

245

130

60

Idle Approach Climb Take-off

50

120 110

Dp/Foo NO x (g/kN)

NOx emissions against CAEP/6 standards (%)

L. Li et al. / Experimental Thermal and Fluid Science 72 (2016) 235–248

40

30

20

100 90 80

CAEP/6 50% of CAEP/6

CA

GEnx-1B(TAPS) GE90(DAC) CFM56(TI) PW(Talon II) Trent XWB(Phase5 Tiled)

EP

/6

TeLESS

70 60 5 0%

50

of C

AEP

/6

40 30

10

20 0

10 15

TeLESS combustor

20

25

30

35

40

45

50

55

60

πoo

(a) LTO NOx of different conditions

(b) LTO NOx level of several turbofan engines [6, 17, 18]

Fig. 15. Prediction results of LTO NOx for the TeLESS combustor.

Table 7 Experimental uncertainties of gas emissions, combustion efficiency and dome equivalence ratio. Test conditions

EICO (g/kg)

dEICO (g/kg)

EIHC (g/kg)

dEIHC (g/kg)

EINO (g/kg)

dEINOx (g/kg)

g (%)

dg (–)

Udome (–)

dUdome (–)

Case1

56.0 51.7 27.7 37.7 28.7 30.0 34.3 31.5 30.8 13.7 11.1

±1.5 ±1.4 ±0.8 ±1.0 ±0.8 ±0.8 ±1.0 ±0.9 ±0.9 ±0.4 ±0.3

10.48 9.45 4.78 5.63 3.46 3.55 4.01 4.46 2.98 1.18 0.76

±0.22 ±0.20 ±0.10 ±0.12 ±0.08 ±0.08 ±0.09 ±0.10 ±0.06 ±0.03 ±0.02

4.35 4.21 4.43 4.57 4.76 4.88 4.98 4.74 5.11 5.63 6.57

±0.09 ±0.09 ±0.10 ±0.10 ±0.10 ±0.11 ±0.11 ±0.10 ±0.11 ±0.12 ±0.14

97.49 97.71 98.81 98.48 98.93 98.90 98.74 98.76 98.94 99.55 99.66

±0.06 ±0.05 ±0.03 ±0.03 ±0.03 ±0.03 ±0.03 ±0.03 ±0.03 ±0.01 ±0.01

0.308 0.320 0.350 0.351 0.355 0.356 0.358 0.360 0.361 0.389 0.419

±0.006 ±0.006 ±0.007 ±0.007 ±0.007 ±0.007 ±0.007 ±0.007 ±0.007 ±0.007 ±0.008

Case2

26.9 44.5 66.5 94.8

±0.8 ±1.2 ±1.8 ±2.6

3.99 8.38 19.2 42.3

±0.09 ±0.19 ±0.4 ±0.9

8.72 7.38 6.32 4.90

±0.20 ±0.16 ±0.14 ±0.11

98.92 98.00 96.24 92.92

±0.03 ±0.05 ±0.08 ±0.15

0.141 0.124 0.104 0.081

±0.003 ±0.002 ±0.002 ±0.002

Case3

0.904 0.287 0.227 0.131 0.070

±0.026 ±0.008 ±0.006 ±0.004 ±0.002

1.131 0.792 0.749 0.525 0.374

±0.024 ±0.017 ±0.016 ±0.011 ±0.008

7.15 7.01 7.09 7.01 7.10

±0.15 ±0.15 ±0.15 ±0.15 ±0.15

99.849 99.902 99.908 99.936 99.955

±0.003 ±0.002 ±0.002 ±0.001 ±0.001

0.675 0.698 0.710 0.737 0.750

±0.013 ±0.013 ±0.013 ±0.014 ±0.014

Case4

0.217 0.259 0.352 0.203 0.098

±0.006 ±0.007 ±0.010 ±0.006 ±0.003

0.0278 0.1162 0.0685 0.0561 0.0394

±0.0006 ±0.0025 ±0.0015 ±0.0012 ±0.0009

9.42 6.42 4.86 4.42 5.50

±0.20 ±0.14 ±0.11 ±0.10 ±0.12

99.992 99.981 99.984 99.989 99.993

±0.001 ±0.001 ±0.001 ±0.001 ±0.001

0.630 0.645 0.634 0.650 0.660

±0.012 ±0.012 ±0.012 ±0.012 ±0.012

Case5

0.208 0.165 0.182 0.100 0.478

±0.006 ±0.005 ±0.005 ±0.003 ±0.014

0.182 0.1254 0.0743 0.0550 0.0485

±0.004 ±0.0027 ±0.0016 ±0.0012 ±0.0011

10.35 7.47 5.06 5.71 6.44

±0.22 ±0.16 ±0.11 ±0.12 ±0.14

99.974 99.982 99.987 99.991 99.983

±0.001 ±0.001 ±0.001 ±0.001 ±0.001

0.701 0.675 0.676 0.688 0.670

±0.013 ±0.013 ±0.013 ±0.013 ±0.013

Case6

0.161 0.458 0.277 0.226

±0.005 ±0.013 ±0.008 ±0.006

0.684 0.534 0.561 0.524

±0.015 ±0.012 ±0.012 ±0.011

14.0 13.9 13.5 13.7

±0.3 ±0.3 ±0.3 ±0.3

99.917 99.928 99.929 99.934

±0.002 ±0.002 ±0.002 ±0.001

0.591 0.595 0.591 0.570

±0.011 ±0.011 ±0.011 ±0.011

Case7

0.611 0.687 0.599 0.713 0.583

±0.017 ±0.020 ±0.017 ±0.020 ±0.017

0.690 0.690 0.653 0.651 0.591

±0.015 ±0.015 ±0.014 ±0.014 ±0.013

13.9 11.82 10.10 7.22 5.98

±0.3 ±0.26 ±0.22 ±0.16 ±0.13

99.906 99.904 99.911 99.908 99.918

±0.002 ±0.002 ±0.002 ±0.002 ±0.002

0.610 0.598 0.609 0.611 0.617

±0.011 ±0.011 ±0.011 ±0.011 ±0.012

Case8

0.289

±0.008

1.52

±0.03

16.8

±0.4

99.818

±0.004

0.646

±0.012

Case9

0.108

±0.003

0.897

±0.019

16.6

±0.4

99.894

±0.002

0.648

±0.012

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L. Li et al. / Experimental Thermal and Fluid Science 72 (2016) 235–248

the ICAO Aircraft Engine Emissions Databank on the website [18]. These data have been marked with hollow triangle in Fig. 15(b). Besides, the prediction result of the LTO NOx for the TeLESS combustor is also plotted in Fig. 15(b). The data is marked with a black solid triangle. The prediction results indicate that TeLESS combustor has great potential to reduce LTO NOx emissions below 50% of CAEP/6 standard for aero gas turbine engine.

Appendix A. Experimental uncertainties analysis In order to find the error for the data, the uncertainty analysis method proposed by Kline [50] is used. The result R of the experiment is assumed to be calculated from n independent variables, x1, x2,   , xn as shown in Eq. (16).

R ¼ f ðx1 ; x2 ;    ; xn Þ

ð16Þ

4. Conclusions

According to the root-sum-square method, the basic equation of uncertainty analysis can be represented by Eq. (17).

Emission tests and NOx correlation work have been carried out to study the emission characteristics of the TeLESS combustor in this paper. The obtained results are listed as follows:

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi  2  2  2 @f @f @f dR ¼ dx1 þ dx2 þ    þ dxn @x1 @x2 @xn

1. NOx emissions and combustion efficiency are very sensitive to dome equivalence ratio variations when the TeLESS combustor is working in low power mode. Both NOx emissions and combustion efficiency increase as the dome equivalence ratio increases in the current study. In order to meet both low NOx emissions and high combustion efficiency requirements, the dome equivalence ratio of the TeLESS combustor should be about 0.36. 2. The average results in circumferentially-staged mode show that both NOx emissions and combustion efficiency are reduced as the fuel stage ratio decreases. However, there is only 8.3% reduction in NOx emissions as the fuel stage ratio decreasing to 14.8%. Besides, reducing the fuel stage ratio would cause extinction while only pilot stage is fueled. Considering efficient and stable combustion for combustor dome in only pilot mode, the optimal fuel stage ratio should be 35% in circumferentiallystaged mode. 3. Combustion efficiency of the model combustor is insensitive to fuel stage ratio variations and keeps the value higher than 99.9% in high power mode. And flame extinction will not happen even if the fuel stage ratio is reduced to 1.5%. So combustion efficiency and stability need not be taken into consideration while determining the optimal fuel stage ratio in high power mode. Besides, the tests results indicate that diffusion pilot flame is dominating the NOx emissions over the premixed main flame even if the pilot stage equivalence ratio is in a very lean range. A decrease in fuel stage ratio is beneficial to NOx reduction for the model combustor only when the main stage equivalence ratio is less than a certain value. The optimal fuel stage ratio in high power mode is a tradeoff between NOx emissions and pilot nozzle cooling. 4. A new empirical NOx model based on the idea of flame partition was proposed in this paper. This model can give a good prediction of NOx emissions for the TeLESS combustor. The new empirical model was used to correlate NOx emission data from other authors. The correlation results show that the new empirical model could give a better prediction than a former empirical model. Finally, the prediction results show that TeLESS combustor has great potential to reduce LTO NOx emissions below 50% of CAEP/6 standard.

K¼

ð17Þ

where dxi is the uncertainty of one variable; dR is the uncertainty of the experimental result R. The uncertainties of experimental results, including emission index of gas emissions, combustion efficiency and dome equivalence ratio, will be discussed and quantified next. A.1. Uncertainty analysis for emission index of gas emissions Emission index of gas emissions are calculated by the formulas proposed by ICAO [6], as shown in Eq. (18)–(25).

 EICO ¼

½CO ½CO2  þ ½CO þ ½HC



! 103 M CO ð1 þ TðP0 =mÞÞ M C þ ðn=mÞM H



!

ð18Þ  EIHC ¼

½HC ½CO2  þ ½CO þ ½HC

103 M HC ð1 þ TðP0 =mÞÞ M C þ ðn=mÞM H ð19Þ



EINOx

½NOx  ¼ ½CO2  þ ½CO þ ½HC



! 103 M NO2 ð1 þ TðP0 =mÞÞ M C þ ðn=mÞMH ð20Þ

where

P0 =m ¼

2Z  ðn=mÞ 4ð1 þ hvol  ½TZ=2Þ

ð21Þ

and

Z¼

2  ½CO  ð½2=x  ½y=2xÞ½HC þ ½NO2  ½CO2  þ ½CO þ ½HC

ð22Þ

In this paper, CO and CO2 concentration are measured on a ‘‘dry” basis. Therefore it is necessary to make interference corrections. That is,

½CO ¼ K½COd

ð23Þ

½CO2  ¼ K½CO2 d

ð24Þ

where K can be expressed as follows:

f4 þ ðn=mÞT þ ð½n=mT  2hvol Þð½NO2   ð2½HC=xÞÞ þ ð2 þ hvol Þð½y=x  ½n=mÞ½HCgð1 þ hd Þ 

 ð2 þ hvol Þ 2 þ ðn=mÞð1 þ hd Þ ½COd þ ½CO2 d  ð½n=mT  2hvol Þ 1  ½1 þ hd ½COd

ð25Þ

L. Li et al. / Experimental Thermal and Fluid Science 72 (2016) 235–248

The variables and constants are listed as follows: MAIR MHC MCO M NO2 MC MH R S T [HC] [CO] [CO2] [NOx] [NO] [NO2] hvol hd m n x y

Molecular mass of dry air 28.966 g, MAIR = (32R + 28.1564S + 44.011T) Molecular mass of exhaust hydrocarbons, taken as CH4 = 16.043 g Molecular mass of CO = 28.011 g Molecular mass of NO2 = 46.008 g Atomic mass of carbon = 12.011 g Atomic mass of hydrogen = 1.008 g Concentration of O2 in dry air, by volume = 0.2095 Concentration of O2 and rare gases in dry air, by volume = 0.7902 Concentration of CO2 in dry air, by volume = 0.0003 Mean concentration of exhaust hydrocarbon vol/vol Mean concentration of CO vol/vol, wet Mean concentration of CO2 vol/vol, wet Mean concentration of NOx vol/vol, wet = [NO + NO2] Mean concentration of NO vol/vol, wet Mean concentration of NO2 vol/vol Humidity of ambient air, vol water/vol dry air Humidity of exhaust sample leaving ‘‘drier”, vol water/ vol dry air Number of C atoms in characteristic fuel molecule Number of H atoms in characteristic fuel molecule Number of C atoms in characteristic exhaust hydrocarbon molecule, x = 1 Number of H atoms in characteristic exhaust hydrocarbon molecule, y = 4

There are five measurands used to calculate the emission index of gas emissions, which are [CO2], [CO], [HC], [NO2] and hd. Let x1 = [CO2], x2 = [CO], x3 = [HC], x4 = [NO2], x5 = hd. Therefore, the uncertainty of emission index, dEI, can be expressed as follows: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi  2  2  2  2  2 @f @f @f @f @f dEI ¼ dx1 þ dx2 þ dx3 þ dx4 þ dx5 @x1 @x2 @x3 @x4 @x5 ð26Þ

According to the accuracies of the gas analyzers, the uncertainties of measurands can be obtained: dx1 = ±2%x1, dx2 = ±2%x2, dx3 = ±1%x3, dx4 = ±1%x4, dx5 = ±2%x5. The experimental uncertainties of emission index are listed in Table 7. A.2. Uncertainty analysis for combustion efficiency and dome equivalence ratio Combining Eqs. (1) and (17), the uncertainty of combustion efficiency, dg, can be represented by Eq. (27).

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi s 2  2ffi HCO HHC dg ¼ dEICO þ dEIHC 1000Hf 1000Hf

ð27Þ

The values of dEICO and dEIHC can be obtained in Table 7. Similarly, the uncertainty of dome equivalence ratio, dUdome, can be expressed as follows:

dUdome

247

The measurement precisions of the fuel coriolis mass flowmeter and air orifice plate flowmeter are ±0.5% and ±2%, respectively. Thus, uncertainties for the measurands can be obtained: dmp,dome = ±0.5%mp,dome, dmm,dome = ±0.5%mm,dome, dWdome = ±2%Wdome. The uncertainties of combustion efficiency and dome equivalence ratio are also listed in Table 7. According to the results listed in Table 7, the uncertainty of emission index for gas emissions is within ±2.9%. Besides, uncertainties of combustion efficiency and dome equivalence ratio are within ±0.1% and ±1.9%, respectively. References [1] D.S. Lee, D.W. Fahey, P.M. Forster, P.J. Newton, R.C.N. Wit, L.L. Lim, B. Owen, R. Sausen, Aviation and global climate change in the 21st century, Atmos. Environ. 43 (2009) 3520–3537. [2] D.S. Lee, G. Pitari, V. Grewe, K. Gierens, J.E. Penner, A. Petzold, M.J. Prather, U. Schumann, A. Bais, T. Berntsen, D. Iachetti, L.L. Lim, R. Sausen, Transport impacts on atmosphere and climate: aviation, Atmos. Environ. 44 (2010) 4678–4734. [3] O. Dessens, M.O. Köhler, H.L. Rogers, R.L. Jones, J.A. Pyle, Aviation and climate change, Transp. Policy 34 (2014) 14–20. [4] A. Khodayari, S.C. Olsen, D.J. Wuebbles, Evaluation of aviation NOx-induced radiative forcings for 2005 and 2050, Atmos. Environ. 91 (2014) 95–103. [5] Report on Voluntary Emissions Trading for Aviation, International Civil Aviation Organization, 2007. [6] International Standards and Recommended Practices, Annex 16 to the Convention on International Civil Aviation, Volume II, Aviation Engine Emissions. International Civil Aviation Organization, Montreall, 2008. [7] H.C. Mongia, Engineering aspects of complex gas turbine combustion mixers part V: 40 OPR, in: 9th Annual International Energy Conversion Engineering Conference, AIAA 2011-5527, American Institute of Aeronautics and Astronautics, San Diego, California, 2011. [8] I. Akira, Y. Masanori, K. Manabu, A. Norihito, N. Yasushi, K. Masaru, Research and development of a LPP combustor with swirling flow for low NO(x), in: 37th Joint Propulsion Conference and Exhibit, AIAA 2001-3311, American Institute of Aeronautics and Astronautics, Salt Lake City, Utah, 2001. [9] P. Gokulakrishnan, M.J. Ramotowski, G. Gaines, C. Fuller, R. Joklik, L.D. Eskin, M. S. Klassen, R.J. Roby, Experimental study of NOx formation in lean, premixed, prevaporized combustion of fuel oils at elevated pressures, GT 2007-27552, American Society of Mechanical Engineers, Montreal, Que, Canada, 2007, pp. 441–449. [10] S. Sam, V. Jeffrey, M. Kian, Low emission technology for small aviation gas turbine engines, AIAA International Air and Space Symposium and Exposition: The Next 100 Years, AIAA 2003-2564, American Institute of Aeronautics and Astronautics, Dayton, Ohio, 2003. [11] M. Randal, C. Albert, S. William, S. Domingo, The Pratt & Whitney TALON X low emissions combustor: revolutionary results with evolutionary technology, in: 45th AIAA Aerospace Sciences Meeting and Exhibit, AIAA 2007-386, American Institute of Aeronautics and Astronautics, Reno, Nevada, 2007. [12] A. Stefano, C. Luigi, B. Francesco, T. Raffaele, Analysis of a low-emission combustion strategy for a high performance trans-atmospheric aircraft engine, in: 46th AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit, AIAA 2010-6549, American Institute of Aeronautics and Astronautics, Nashville, TN, 2010. [13] M.T. Kathleen, C. Clarence, J.H. Zhuohui, L. Phil, C.M. Hukam, K.D. Bidhan, A second generation swirl-venturi lean direct injection combustion concept, in: 50th AIAA/ASME/SAE/ASEE Joint Propulsion Conference, AIAA 2014-3434, American Institute of Aeronautics and Astronautics, Cleveland, OH, 2014. [14] K.M. Tacina, C. Wey, NASA Glenn High Pressure Low NOx Emissions Research, NASA Glenn Research Center, Cleveland, OH, United States, 2008. [15] H.C. Mongia, On continuous NOx reduction of aero-propulsion engines, in: 48th AIAA Aerospace Sciences Meeting Including the New Horizons Forum and Aerospace Exposition, AIAA 2010-1329, American Institute of Aeronautics and Astronautics, Orlando, Florida, 2010. [16] R.P. Frank Haselbach, Hot end technology for advanced, low emission large civil aircraft engines, in: 28th International Congress of the Aeronautical Sciences, 2012. [17] F. Michael, T. Doug, S. Richard, C. Clayton, D. Will, Development of the GE aviation low emissions TAPS combustor for next generation aircraft engines, in: 50th AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition, AIAA 2012-0936, American Institute of Aeronautics and Astronautics, Nashville, Tennessee, 2012.

vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi !2ffi u 2  2 m þ m 1 u 1 1 p;dome m;dome t ¼ dmp;dome þ dmm;dome þ  dW dome FARstoi W dome W dome W 2dome

ð28Þ

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