Zhuohui J. He, Kathleen M. Tacina, and Chi-Ming Lee

Glenn Research Center, Cleveland, Ohio

Robert R. Tacina

Glenn Research Center, Cleveland, Ohio

Phil Lee

Woodward FST, Inc., Zeeland, Michigan

Effects of Spent Cooling and Swirler Angle on a

9-Point Swirl-Venturi Injector

NASA/TM—2014-218117

April 2014
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Zhuohui J. He, Kathleen M. Tacina, and Chi-Ming Lee

Glenn Research Center, Cleveland, Ohio

Robert R. Tacina

Glenn Research Center, Cleveland, Ohio

Phil Lee

Woodward FST, Inc., Zeeland, Michigan

Effects of Spent Cooling and Swirler Angle on a

9-Point Swirl-Venturi Injector

NASA/TM—2014-218117

April 2014

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Prepared for the

8th U.S. National Combustion Meeting

sponsored by the Western State Section, Combustion Institute

Park City, Utah, May 19–22, 2013
Acknowledgments

This research was funded by NASA’s High Speed Research (HSR) Fundamental Aeronautics/Aeronautical Sciences programs.

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Level of Review: This material has been technically reviewed by technical management. 
NASA/TM—2014-218117 1 

Effects of Spent Cooling and Swirler Angle on a 

9-Point Swirl-Venturi Injector 

Zhuohui J. He, Kathleen M. Tacina, and Chi-Ming Lee 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Robert R. Tacina*

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Phil Lee 

Woodward FST, Inc. 

Zeeland, Michigan 49464 

Abstract 

This paper presents multipoint Lean-Direct-Injection (LDI) emissions results for flame tube 

combustion tests at an inlet pressure of 1034 kPa and inlet temperatures between 835 and 865 K; these are 

the combustor inlet conditions that the High Speed Research (HSR) program used for supersonic cruise. It 

focuses on one class of LDI geometry, 9-point swirl-venturi LDI (SV-LDI). Two parameters are 

compared in this paper: the use of dome cooling air and the swirler blade angle. Dome cooling air is 

called “spent cooling” and is at combustor inlet conditions. Three cooling variations are studied: cooling 

at the venturi throat, cooling at the dome face, and no cooling at all. Two swirler blade angles are studied: 

45° and 60°. The HSR 9-point SV-LDI emissions are also compared to a similar 9-point SV-LDI design 

which was used in the later ultra-efficient engine technology (UEET) program. The HSR and UEET 

designs cannot be compared directly due to different UEET combustor conditions. Therefore, this paper 

uses previously published UEET correlation equations to make comparisons. Results show that using a 

45° swirler produces lower NOx emissions than using a 60° swirler. This is consistent with the later 

UEET results. The effects of spent cooling depend on swirler angle, spent cooling location, and the test 

conditions. For the configuration with 45° swirlers, spent cooling delivers lower NOx emissions when it 

is injected at the throat. For the 60° swirler, spent cooling does not have much effect on NOx emissions. 

These results might be caused by the location and the intensity of the flame recirculation zone. 

Introduction 

Burning hydrocarbon fuel produces many effects on the environment. One of the major pollutants is 

nitrogen oxides (NOx). Through several complex chemical reactions, nitrogen oxides emissions have 

caused problems like photochemical smog, greenhouse gas generation, as well as depletion of the 

protective ozone layer in the stratosphere (Ref. 1). In aircraft engines, nitrogen oxides reduction relies on 

advanced combustor design. Low-NOx combustor designs have been investigated by NASA in 

collaboration with engine and fuel injector companies since the 1970s; since the 1990s, NASA has 

investigated Lean Direct Injection (LDI) combustion concepts. The LDI concept is designed to burn fuel 

as lean as possible to avoid high flame temperatures because NOx emissions are an exponential function 

of combustion flame temperature. To avoid local hot spots, fuel is mixed with air as quickly as possible. 

 

*

 Retired. 
NASA/TM—2014-218117 2 

In order to avoid the problems of flashback and premature autoignition that plague premixed lean 

combustion concepts at high temperatures and pressures, in LDI fuel is injected directly into the 

combustion zone. 

The LDI geometry that is studied in this paper is called 9-point Swirl-Venturi. With this geometry, 

fuel is injected into the combustor through nine small fuel injectors. Each injector is accompanied by an 

air swirler that generates highly turbulent air to break up the fuel and promote quick mixing. Throughout 

this paper the fuel injector and air swirler will be referred to as an “element” or a “fuel/air mixer.” 

These experiments were conducted in a flame-tube combustor at high-speed-research (HSR) 

supersonic cruise conditions. Two parameters were investigated in this study: the use of cooling air and 

the swirler blade angle. The cooling air is called “spent cooling” and is injected at combustor inlet 

conditions. Flame produced by this LDI geometry has a very short flame length (~25-mm) (Ref. 2). To 

address the concern that the flame would be too close to the dome and damage the hardware, spent 

cooling air is applied. There are two spent cooling variations and a configuration with no spent cooling. 

The effects of these spent cooling methods on NOx emissions were studied with injectors that configured 

of either 45 or 60 blade angle swirlers. 

Four configurations were compared in this report: 45 blade angle swirlers with both types of spent 

cooling, 60 blade angle swirlers with one type of spent cooling, and 60 blade angle swirlers with no 

spent cooling. These HSR 9-point SV-LDI emission results were also compared to those from a similar 

9-point SV-LDI design which was used in the later ultra-efficient engine technology (UEET) program. 

The HSR and UEET designs could not be compared directly due to different UEET combustor conditions. 

Instead, two previously published UEET correlation equations were used to predict the NOx values at 

HSR conditions, and then both results were compared with each other. 

Experimental Setup and Procedure 

All the tests were conducted in CE-5, a flame-tube combustion test facility at the NASA Glenn 

Research Center (Ref. 3). A drawing of the CE-5 flame-tube is shown on Figure 1. Nonvitiated air was 

pre-heated to a maximum temperature of 865 K and a maximum pressure of 1380 kPa. Commercial Jet-A 

aviation fuel was used. The fuel flow rate was measured by a turbine meter, and air flow rate was 

measured by a venturi meter. The fuel-air mixture was injected into a flame tube that contains ceramic 

Figure 1.—Drawing of CE-5 flame tube. 
NASA/TM—2014-218117 3 

liners with no liner cooling. A gas probe was placed downstream of the injector to collect combustion 

products for analysis. The rest of the combustion products were cooled down to 450 K by mixing with 

sprayed water before exiting to a low pressure exhaust system. The combusted gas samples were analyzed 

according to the standard gas-analysis procedure, SAE-ARP1256 (Ref. 4). As for NO and NO2, the 

simultaneous chemiluminescence method was used. 

The SV-LDI module described here contains nine identical fuel/air mixers in a 3 by 3 array 

occupying a 76.2 by 76.2-mm square area, as shown in Figure 2. The nine fuel/air mixers replace a single 

conventional fuel injector. Each fuel/air mixer consists of an air passage with a helical axial air swirler 

followed by a converging-diverging venturi section (Fig. 2(a)). For the HSR SV-LDI configurations, the 

swirlers have five blades; for the UEET configurations, the swirlers have six blades. A simplex fuel 

injector is inserted through the center of the air swirler; the fuel injector tip is at the venturi throat. 

Two types of spent cooling are used in the HSR SV-LDI configurations: cooling at the dome face 

(“straight through”) and cooling at the venturi throat (“at throat”). For both types of spent cooling, the 

cooling air was at the combustor inlet conditions. Figure 2(b) shows straight through spent cooling (ST). 

For straight-through spent cooling, four holes were drilled straight-through the dome plate in between the 

nine elements. Figure 2(c) and (d) show spent cooling at the throat (AT). For cooling at the venturi throat, 

the cooling air was split among the nine elements. Four small holes were used to cool the center element, 

and one larger hole was applied in the venturi throat for other eight elements. 

a) b) 

c) d) 

Figure 2.—Experimental hardware: (a) drawing of a fuel/air mixer, (b) drawing of a 9-point injector with straight 

through spent cooling, (c) and (d) picture and drawing of a 9-point injector with spent cooling at the throat. 
NASA/TM—2014-218117 4 

No spent cooling was used in any of the UEET 9-point configurations. The correlations that were 

used to calculate the UEET NOx emissions are given in Equations (1) and (2). For the UEET 

configuration with 45 swirlers (Ref. 5): 

0.56

194 5.07

3

0.59 EINOx 0.11 3 %







 



      

P

P P e

T

 (1) 

For the configuration with 60 swirlers: 

0.56

194 1.69

3

0.59 EINOx 0.013 3 %







 



      

P

P P e

T

 (2) 

Equations (1) and (2) were used to predict the UEET 9-point NOx emissions at the HSR inlet 

temperatures, inlet pressures, equivalence ratios, and mass flow rates. 

Results and Discussion 

Testing of the HSR configurations showed that the spent cooling was not needed at HSR conditions: a 

post-testing visual inspection of the 60 configuration with no spent cooling showed no damage. In the 

testing with the UEET configurations, post-testing inspections also showed no damage, and thermocouple 

measurements showed acceptable dome temperatures. Nevertheless, spent cooling might be needed at the 

higher inlet pressures of next generation aircraft engines, especially when burning alternative fuels with a 

shorter ignition delay time. Therefore, understanding the effects of spent cooling may be important for 

future SV-LDI designs. 

Air and fuel pressure drops are important parameters in designing the aircraft combustor. For the 

simplex fuel nozzle, fuel pressure drop across the injector affects the fuel drop size (Ref. 6). Higher the 

fuel pressure drops, the smaller is the fuel-drop size, which results in a better fuel-air mixture. The same 

type of simplex fuel injector was used in all four HSR configurations. As shown in Table 1, the flow 

numbers for these four configurations were about the same, which were around eighteen. The flow 

number is a function of fuel pressure drop across the injector and fuel mass flow rate and is defined as 

FN = fuel mass rate/sqrt(ΔP). Therefore, fuel drop sizes at a given fuel flow rate should be similar among 

the four HSR configurations. However, the flow numbers for the UEET configurations were about forty 

percent higher than these four HSR configurations. According to the correlations found in Lefebvre 

(Ref. 6), this 40 percent higher flow number will result in the UEET fuel drop sizes being about 

30 percent higher than the HSR fuel drop sizes at a given mass flow rate. 

TABLE 1.—AVERAGE ACD AND FLOW NUMBER FOR EACH CONFIGURATION OF THE 9-POINT INJECTORS 

45/ST 45/AT 60/ST 60/no cooling UEET 45 UEET 60

ACd 1.745 1.644 1.477 1.37 1.48 1.35 

FN(total) 17.9 17.16 18.49 18.24 26.1 26.1 

Figure 3 compares the effects of mass flow rate and two types of spent cooling methods on the NOx 

emission for mixers configured with 45 swirlers at various equivalence ratios (Φ) (Fig. 3(a)) or adiabatic 

flame temperature (Fig. 3(b)). For a typical fuel/air mixer configuration, NOx emissions mainly depend 

on fuel-air ratio, combustion inlet air temperature and pressure, as well as the air pressure drop across the 

dome. When NOx data was plotted against the adiabatic flame temperature, the effect of the combustion 

inlet temperature was minimized, allowing for easier comparison. Comparing the NOx emissions of the 

two spent cooling methods, the straight-through spent cooling produced higher NOx than spent cooling at 

the throat (at least 25 percent higher). This is consistent with the result expected from the UEET NOx 

correlations. At a given air mass flow rate, the straight through configuration’s larger effective area (ACd) 

decreased the air pressure drop across the dome. Based on the UEET correlations, NOx emissions decrease 
NASA/TM—2014-218117 5 

(a) (b) 

Figure 3.—Straight-through spent cooling vs. spent cooling at the throat for a 9 point injector with a 45 swirl angle 

(a) EINOx vs. , (b) EINOx vs. Tad (K). 

with increasing pressure drop: NOx is proportional to pressure drop to the –0.56 power (Ref. 5). 

Assuming the dependence of NOx on air pressure drop was similar between UEET and HSR 

configurations, straight-through spent cooling should produce 8 percent higher NOx emissions than 

cooling at the venturi throat. 

Figure 3 also includes the predicted NOx values for the UEET 9-point configuration with 45

swirlers. The predicted UEET NOx emissions were lower than the HSR configuration with straightthrough spent cooling, but about the same with spent cooling at the throat for an air flow rate of 

0.603 kg/s. At a higher air flow rate (0.726 kg/s), spent cooling at the throat had lower NOx than 

predicted UEET NOx emissions. When cooling air is applied at the venturi throat, the air jets might 

promote faster fuel air mixing, and thus lower NOx emission. 

In Figure 4, the straight-through spent cooling was compared with no cooling on the HSR 9-point 

configuration that was configured with 60 swirlers. These data indicated straight-through spent cooling 

produced slightly higher NOx emissions than no cooling, but the difference was small. In addition, with a 

higher air mass flow rate, the NOx emissions were lower. For this 9-Point-Swirl Venturi design, fuel was 

injected into the high turbulent region that was created by the air swirler. As the air velocity increases, the 

fuel-air mixing is enhanced by stronger air turbulence. The combustion residence time is also reduced to 

shrink the NOx emission. Figure 4 also compares these experimental data to the predicted NOx values of 

a UEET 9-point configuration with 60 swirlers. Overall, the predicted UEET NOx values were lower 

than experimental HSR data. 

Figure 5(a) compares two straight through spent cooling configurations; one had 45 swirlers and the 

other had 60 swirlers. Using the 45 swirlers, the NOx emissions were lower. This was consistent with 

the later UEET results as shown on Figure 5(b). According to the past LDV measurements and recent 

CFD studies (Refs. 7 and 5), the differences between using the 60 swirler and the 45 swirler were the 

intensities and locations of the flame recirculation zone in the combustor. The 60 swirler produced a 

more stable recirculation zone, and its location was on the axis of the swirler in the gas stream. When the 

hot recirculation zone is located on the axis of the swirler, fuel and air might be burned before uniform 

mixing could occur. Thus, more NOx is produced. The 45 swirler produced a relatively weak 

recirculation zone, and its locations were attached to the dome face in between the swirlers, which gives 

more time for fuel and air to mix before burning. This could be the reason for the lower NOx emissions 

when using the 45 swirler. 
NASA/TM—2014-218117 6 

(a) (b) 

Figure 4.—Straight-through spent cooling vs. no spent cooling for a 9 point injector with a 60 swirl angle. 

(a) EINOx vs. , (b) EINOx vs. Tad (K). 

(a) (b) 

Figure 5.—(a) Comparison of the HSR straight through spent cooling configurations with 45 and 60 swirlers, and 

(b) Comparison of UEET configurations with 45, 52, and 60 swirler angles (Ref. 6). 

The effects of spent cooling depend on spent cooling location, swirler angle, and test conditions. 

By applying spent cooling at the venturi throat, the cooling air is coming out at the same location as the 

swirled air (or combustion air). It will change the swirl number, and thus affect the air flow dynamic 

inside the combustor. The straight-through spent cooling method applied cooling air through the dome 

plate at the locations between the fuel injectors. These locations were at the same location of the 

circulation zones that the 45 swirlers produced. Since the recirculation zones were relatively weak, 

applying straight-through spent cooling at the dome plate might change the location of the recirculation 

zone and lead to the change in NOx emission. For configuration with 60 swirlers, the recirculation zone 

was strong and it was located on the axis of the swirlers in the gas stream. As a result, straight-through 

spent cooling did not show much effect on this configuration. 
NASA/TM—2014-218117 7 

Conclusions 

This paper explores the effects of dome cooling and swirler angle on SV-LDI low NOx combustion 

configurations. This dome cooling is called “spent cooling” and its original intent was to protect the 

fuel/air mixer hardware from the flame burning too close to the dome plate. However, testing and 

thermocouple measurements at the dome plate showed that the spent cooling was not needed at HSR or 

UEET conditions studied here. But spent cooling might be needed at the higher inlet pressures of next 

generation aircraft, especially when burning alternative fuels with a lower ignition delay time. Therefore, 

understanding the effects of spent cooling may be important for future SV-LDI designs. 

The data presented in this paper showed the effect of applying spent cooling at two locations. The 

first location was at the dome face in between fuel/air mixers and the second location was at the venturi 

throat. The effects of spent cooling depend on spent cooling location, swirler angle, and test conditions. 

For fuel/air mixers configured with 45 swirlers, NOx emissions were significantly higher when the 

cooling was applied at the dome face than when it was applied at the venturi throat. For the fuel/air 

mixers configured with 60 swirlers, NOx emissions when spent cooling was applied at the dome face 

were not significantly different from NOx emissions when no spent cooling was used. 

HSR results also indicate that decreasing the swirler angle from 60 to 45 decreases NOx emissions: 

the straight through spent cooling configuration with 45 swirlers had lower NOx emissions. This is 

consistent with results from the UEET configurations, which had no spent cooling. 

References 

1. Wey, Chowen, C., and Maurice, Lourdes Q., Exploring Aviation’s Environmental Impact, AIAA2003-5993, 1st International Energy Conversion Engineering Conference (IECEC). 

2. Hicks, Yolanda R., Heath, Christopher M., Anderson, Robert C., and Tacina, Kathleen M. 

Investigations of a Combustor Using A 9-Point Swirl-Venturi Fuel Injector: Recent Experimental 

Result, ISABE-2011-1106, 2011. 

3. Bianco, J., NASA Lewis Research Center’s combustor test facilities and capabilities, AIAA-1995-

2681, 1995. 

4. SAE ARP1256-2006, Procedure for the Continuous Sampling and Measurement of Gaseous 

Emissions from Aircraft Turbine Engines. 

5. Tacina, Robert, Lee, Phil, and Wey, Changlie, A lean-direct-injection combustor using a 9 point 

swirl-venturi fuel injector, ISABE-2005-1106, 2005. 

6. Lefebvre, Arthur H, Gas Turbine Combustion, Taylor and Francis, Philadelphia, 2nd edition, 1998. 

7. Ajmani, Kumud, Mongia, Hukam, Lee, Phil, CEF Best Practices To Predict NOx, CO and Lean 

Blowout for Combustor Design, ASME Turbo Expo 2013, San Antonio, Texas, USA. 
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Effects of Spent Cooling and Swirler Angle on a 9-Point Swirl-Venturi Injector

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13. SUPPLEMENTARY NOTES

Robert R. Tacina, retired.

14. ABSTRACT

This paper presents multipoint Lean-Direct-Injection (LDI) emissions results for flame tube combustion tests at an inlet pressure of 1034

kPa and inlet temperatures between 835 and 865 K; these are the combustor inlet conditions that the High Speed Research (HSR) program

used for supersonic cruise. It focuses on one class of LDI geometry, 9-point swirl-venturi LDI (SV-LDI). Two parameters are compared in

this paper: the use of dome cooling air and the swirler blade angle. Dome cooling air is called “spent cooling” and is at combustor inlet

conditions. Three cooling variations are studied: cooling at the venturi throat, cooling at the dome face, and no cooling at all. Two swirler

blade angles are studied: 45° and 60°. The HSR 9-point SV-LDI emissions are also compared to a similar 9-point SV-LDI design which was

used in the later ultra-efficient engine technology (UEET) program. The HSR and UEET designs cannot be compared directly due to

different UEET combustor conditions. Therefore, this paper uses previously published UEET correlation equations to make comparisons.

Results show that using a 45° swirler produces lower NOx emissions than using a 60° swirler. This is consistent with the later UEET results.

The effects of spent cooling depend on swirler angle, spent cooling location, and the test conditions. For the configuration with 45° swirlers,

spent cooling delivers lower NOx emissions when it is injected at the throat. For the 60° swirler, spent cooling does not have much effect on

NOx emissions. These results might be caused by the location and the intensity of the flame recirculation zone.

15. SUBJECT TERMS

Nitrogen Oxide emission; Lean-Direct-Injection (LDI); Aircraft engine combustions

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