Sarah A. Tedder, Yolanda R. Hicks, Kathleen M. Tacina, and Robert C. Anderson

Glenn Research Center, Cleveland, Ohio

Fundamental Study of a Single Point Lean Direct

Injector Part I: Effect of Air Swirler Angle and

Injector Tip Location on Spray Characteristics

NASA/TM—2015-218475

February 2015

AIAA–2014–3435
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Sarah A. Tedder, Yolanda R. Hicks, Kathleen M. Tacina, and Robert C. Anderson

Glenn Research Center, Cleveland, Ohio

Fundamental Study of a Single Point Lean Direct

Injector Part I: Effect of Air Swirler Angle and

Injector Tip Location on Spray Characteristics

NASA/TM—2015-218475

February 2015

AIAA–2014–3435

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Prepared for the

50th Joint Propulsion Conference

cosponsored by AIAA, ASME, SAE, and ASEE

Cleveland, Ohio, July 28–30, 2014
Acknowledgments

This work was supported by the Supersonics and Aeronautical Sciences Projects of the NASA Fundamental Aeronautics Program. 

We also thank Aimee Bogner, Derek Podboy, and Kurt Rusmisel for their technical support.

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NASA/TM—2015-218475 1 

Fundamental Study of a Single Point Lean Direct Injector 

Part I: Effect of Air Swirler Angle and Injector Tip 

Location on Spray Characteristics

Sarah A. Tedder, Yolanda R. Hicks, Kathleen M. Tacina, and Robert C. Anderson 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Abstract 

Lean direct injection (LDI) is a combustion concept to reduce oxides of nitrogen (NOx) for next 

generation aircraft gas turbine engines. These newer engines have cycles that increase fuel efficiency 

through increased operating pressures, which increase combustor inlet temperatures. NOx formation rates 

increase with higher temperatures; the LDI strategy avoids high temperature by staying fuel lean and 

away from stoichiometric burning. Thus, LDI relies on rapid and uniform fuel/air mixing. To understand 

this mixing process, a series of fundamental experiments are underway in the Combustion and Dynamics 

Facility at NASA Glenn Research Center. This first set of experiments examines cold flow 

(non-combusting) mixing using air and water. Using laser diagnostics, the effects of air swirler angle and 

injector tip location on the spray distribution, recirculation zone, and droplet size distribution are 

examined. Of the three swirler angles examined, 60q is determined to have the most even spray 

distribution. The injector tip location primarily shifts the flow without changing the structure, unless the 

flow includes a recirculation zone. When a recirculation zone is present, minimum axial velocity 

decreases as the injector tip moves downstream towards the venturi exit; also the droplets become more 

uniform in size and angular distribution. 

Nomenclature 

CFD computational fluid dynamics 

Dt venturi throat diameter 

FNUS injector flow number 

h distance upstream of venturi throat 

LDI lean direct injection, lean direct injector 

PIV particle image velocimetry 

PLS planar laser scatter 

r distance from LDI exit 

S swirl number 

SV swirler venturi 

Introduction 

For more than 40 years, NASA has sustained programs to reduce the environmental effects of 

aviation. A major focus of these programs has been reducing the emissions of nitrogen oxides (NOx). 

NOx emissions decrease the protective ozone layer in the stratosphere and increase smog and ozone in the 

lower troposphere (Ref. 1). To prevent damage to the protective ozone layer, NASA programs have 

focused on reducing NOx at cruise for supersonic flight. To reduce the emissions of NOx in the lower 
NASA/TM—2015-218475 2 

troposphere, NASA programs have also focused on reducing NOx emissions during the landing-takeoff 

cycle in subsonic flight. 

In addition to decreasing NOx emissions, NASA has also addressed the reduction of carbon dioxide 

emissions by increasing engine fuel efficiency. Increased fuel efficiency is typically achieved by 

increasing the engine operating pressure ratio, which increases combustor inlet temperature; however, 

NOx formation rates increase with higher temperatures. Without an improvement in combustor 

technology, higher efficiency engines will have higher, not lower, NOx emissions. Therefore, improved 

low-NOx combustor technologies need to be developed. 

In order to reduce NOx emissions, NASA is exploring fuel-lean front-end concepts. Historically, 

turbine engines for aircraft use rich-front-end combustors in order to ensure stable combustion in flight. In 

these combustors the primary combustion zone is fuel-rich—only part of the combustion air enters 

through the fuel/air mixers; the rest enters through downstream dilution jets, allowing combustion to be 

completed (Refs. 2 to 4). In contrast, lean-front-end combustors operate fuel-lean throughout: All of the 

combustor air except that used for liner cooling enters through the combustor dome. Lean front end 

combustion concepts include lean, premixed, prevaporized (LPP), lean partially premixed, and lean direct 

injection (LDI) combustion (Refs. 2, 5 to 9). This paper focuses on an LDI combustion concept. 

LDI and other lean front end combustion concepts minimize local flame temperature. This keeps NOx

emissions low because NOx is an exponential function of local temperature. To minimize NOx emissions, 

fuel-lean combustion needs to avoid local near-stoichiometric zones where the flame temperature is high. 

Avoiding these zones requires rapid and uniform fuel/air mixing. LDI accomplishes this mixing in part by 

replacing one traditionally-sized fuel/air mixer with multiple smaller fuel/air mixers. However, previous 

experiments have shown that decreasing the size and increasing the number of fuel/air mixers alone does 

not sufficiently improve mixing: fuel/air mixer design is also important. 

Multiple fuel/air mixer designs have been studied. There are several ways an individual fuel/air mixer 

can be constructed. On the air side, a radial, axial, or discrete jet swirler may be used. With any of these 

types of swirlers, the swirl number can be varied. A venturi can be placed downstream of the air swirler, 

or the venturi can be omitted. On the fuel side, a simplex or air assist-atomizer can be used. For a simplex 

atomizer, flow number (i.e., effective flow area) can be varied. In addition, once a fuel/air mixer design 

has been chosen, the size and number of the mixer elements can be varied. 

In order to choose the best fuel/air mixer design for an LDI combustor, it is critical to understand the 

fuel/air mixing process. Understanding fuel/air mixing requires knowledge both of large overall features 

such as recirculation zones and precessing vortices, and of smaller eddies where much of the mixing 

occurs. To better understand the mixing process, the NASA Fundamental Aeronautics/Aeronautical 

Sciences project is conducting a series of tests in the 5-atm Combustion and Dynamics Facility (CDF) 

flametube rig at NASA Glenn Research Center. 

The baseline swirl-venturi (SV) LDI fuel/air mixer (Refs. 2 and 6) was used for these studies. The 

baseline SV-LDI fuel/air mixer consists of an axial swirler followed by a venturi. Fuel is injected through 

a simplex fuel injector; the fuel injector tip is at or just upstream of the venturi throat. 

This first set of experiments examines cold flow mixing using water. It studies the effect of air swirler 

angle and fuel injector tip location on the cold flow droplet size and velocity fields. Testing was done at a 

temperature of 700 K and a pressure of 517-kPa. Water was used in place of jet fuel to prevent autoignition during long run times. Diagnostic measurements included particle image velocimetry, 30 kHz 

high-speed video images of planar laser scatter, and shadowgraph images for droplet size measurement. 

The results from these measurements are used to examine the effects of air swirler angle and injector tip 

location on the spray distribution, recirculation zone, and droplet size distribution. 
NASA/TM—2015-218475 3 

Experimental Facilities and Hardware 

Combustion and Dynamic Facility (CDF) 

All testing was done at NASA Glenn Research Center in the Combustion and Dynamics Facility 

(CDF). A schematic of the CDF test rig is shown in Figure 1. The CDF is orientated vertically, with the 

flow going down. The CDF can supply non-vitiated air preheated up to 810 K at air flow rates up to 

0.35 kg/sec and pressures up to 517-kPa. The combustor test section has a circular cross-section 

nominally 7.62-cm in diameter. Three sets of double-paned windows, spaced 90° apart around its 

circumference, are used to gain optical access to the water-cooled combustor. The two opposing windows 

are aligned to provide access in the x direction, while the third window is on the –y side of the combustor. 

The windows are flat and have a small offset away from the combustor circumference. The windows 

measure 6.1-cm tall (axial direction) by 5.8-cm wide (azimuthal). Figure 2 shows a cross sectional detail 

drawing of the combustor, which defines the coordinate system and shows the window positions. 

Figure 1.—Schematic of the test hardware. 

This schematic is not to scale.

Figure 2.—Cross sectional detail of combustor. 

Shown by red arrows are the window 

orientation and definition of the coordinate 

system used. Z is into the page.
NASA/TM—2015-218475 4 

LDI Hardware 

As illustrated in Figures 3 and 4, the baseline SV-LDI fuel/air mixer consists of a simplex pressureswirl fuel injector and an air passage with a six-bladed, helical axial air swirler followed by a convergingdiverging venturi section. The diameter of the venturi throat (Dt) is 13.0 mm. The simplex fuel injector is 

inserted through the center of the air swirler. The fuel injector tip is placed at one of three distances 

upstream of the venturi throat, h/Dt: 0, 0.16, or 0.31 throat diameters. The injector flow number, FNUS,

(as defined by Lefebvre (Ref. 10)) is 0.7. The helical axial air swirlers have six blades. Each blade has an 

inside diameter of 9.4 mm and an outside diameter of 22 mm. The blades angles for three configurations 

studied here are 45°, 52.5°, and 60°. The calculated swirl numbers, as defined by Beer and Chigier 

(Ref. 11), are 0.59, 0.77, and 1.02, respectively. 

This baseline SV-LDI fuel/air mixer is nominally 2.54-cm in diameter. However, the flametube is 

7.62-cm in diameter. In this study, the fuel/air mixer was mounted in the center of the flametube and 

surrounded by co-flow air; the co-flow enters the flametube through a screen that simulates the pressure 

drop across the fuel/air mixer. The test hardware is shown in Figure 4. 

Figure 3.—Schematic drawing of a single SV-LDI fuel/air mixer. 

a) b) 

Figure 4.—Cross section of the single SV-LDI module. Detailed drawings showing a cross section of the single 

SV-LDI module in its spool piece; (a) and a bottom view that shows the screen inserted around the module (b). 
NASA/TM—2015-218475 5 

TABLE 1.—TEST CONDITIONS 

[Common conditions for both test points, 500 and 700: Air: Pressure = 517.1 kPa, Temperature = 700 K. 

The water was at room temperature, approximately 298 K.] 

Test point Reference velocity, 

m/s 

Air flow rate, 

kg/s 

Water flow rate,

kg/h 

Water/Air ratio 'Pinjector,

kPa 

500 9.1 0.107 3.89 0.100 672 

700 12.2 0.143 4.09 0.008 738 

Test Conditions 

Results from two test conditions are presented in the results and discussion section. These test 

conditions will be referred to as test points 500 and 700. These conditions are detailed in Table 1. The 

combustor air inlet temperature and pressure were the same for both test points 700 K and 517 kPa. The 

test point 500 has a lower reference velocity and a higher water to air ratio than the 700 test point. The 

water was injected at the same ambient temperature (~298 K) for both test conditions. 

Optical Instrumentation 

For the work presented here, we measured water droplet velocities and sizes in preheated air. 

Two-dimensional velocity measurements were obtained using particle image velocimetry (PIV). The 

shadowgraph technique was used to obtain drop sizes and instantaneous velocity measurements of water 

spray. Planar laser scatter (PLS) was used to visualize liquid drops at a high frame rate and to develop 

limited pseudo time-series PIV. High frame rate PLS images were also used to develop spray pattern 

histograms. All techniques were laser-based. The lasers were located in a separate room and the light was 

transmitted to the test rig using mirrors, and then conditioned to the appropriate measurement volume via 

combinations of lenses. The details of the experimental setups are outlined below. 

Standard PIV and PLS, used similar optical arrangements. The laser light for PIV and PLS entered 

and exited the combustor in the x-direction via the two opposing windows at 0° and 180°, and the camera, 

which received the 90°-scattered light was positioned to collect the light via the window at 90°. For 

shadowgraphy, a line-of-sight technique, the light source and camera were positioned on opposite sides, 

using the two opposing windows. 

PIV data were obtained using a dual head, frequency-doubled Nd:YAG laser operating at 15 Hz. The 

laser pulse width was 3-5 ns. The spatially-overlapped laser beams were expanded into vertically-oriented 

sheets using a cylindrical lens. The sheets were approximately 45-mm-high by 0.3-mm thick, entered the 

combustor through the 180° window, and exited through the 0° window. The sheet axis was aligned with 

the flow direction. Images were acquired through the 90° window, collecting light scattered from water 

droplets that passed through the laser sheet, to capture liquid phase velocity. To collect the light, we used 

an f = 105-mm, f/4 lens, outfitted with a 532-nm narrowband interference filter. An interline transfer 

CCD camera (1600u1200 pixels) captured the scattered light from adjacent laser pulses onto two 

individual frames. This collection scheme imaged a region that spanned roughly 50-mm in the x-direction 

(along the laser sheet axis) by 38-mm axially (z-direction). The time between frames was typically 5 μs. 

All synchronization was handled using a LaVision, Inc. programmable timing unit, PTU-9. Crosscorrelation methods were then applied to create a displacement vector field for each image pair. 

LaVision’s DaVis software version 7.2 was used to collect the images and version 8.2 was used to 

produce 2-D fields of the axial-horizontal (z-x) velocity components in the y = 0 plane. 

Shadowgraph images were produced using the same laser and camera. However, for the shadowgraph 

measurements the laser was expanded into a cone using a spherical lens, which in turn struck a 

fluorescent plate, producing a bright, yellow-orange background field. Light was collected on the opposite 

side with a long-range catadioptric microscope and stored on the camera. The system magnification was 

such that each image provided a field of view of approximately 2.2-mm in the y-direction, by 1.7-mm 

high (z-direction), with a depth of field of approximately 1-mm. Each image was considered to emanate 
NASA/TM—2015-218475 6 

from a “point” in space, and the whole optical system was traversed in 1- or 2-mm increments in the 

x-direction, along y = 0, with occasional data collected along x = 0. 

PLS images were acquired using a continuous wave Nd:YAG laser with a total power output of 

~1.1 W. As with PIV, the laser beam was formed into a vertical sheet (approximately 40-mm high, 

positioned along y = 0). The laser sheet entered via the 0° window and exited through the 180° window. 

We used a 12-bit, grayscale, high-speed camera that has a CMOS array with 1024u1024 pixel resolution. 

The high-speed camera was set up square to the rig and focused on the vertical plane at y = 0. The highspeed camera frame rate and resolution are variable. The camera can frame as fast as 5400-frames/s 

(5.4-kps) at full resolution (which provides the maximum field-of view image size), and faster at lower 

resolutions. A frame rate of 30-kps and a resolution of 512u352 pixels was used. The light was collected 

using an f = 60-mm, f/2.8 lens. The default exposure is 1/(frame rate), or 33.3 μs, which was used for all 

PLS images. 

Results and Discussion 

The effects of air swirler angle and injector tip location on the velocity field and droplet size 

distribution are presented in this section. First, we discuss effects of the air swirler angle on the spray 

distribution, droplet size, and recirculation zone. We then discuss the injector tip location effect on spray 

distribution, droplet size, and recirculation zone. 

Air Swirler Angle 

This subsection examines the effect of air swirler angle when the injector tip is located at the venturi 

throat. 

Spray Distribution 

An even distribution of droplets in the injector spray improves the mixing of fuel and oxidizer, 

reducing production of NOx by minimizing the occurrence of pockets of increased temperature caused by 

local peaks in fuel air ratio. One important parameter used to characterize the droplet distribution is the 

spray angle. 

The spray angles can be observed qualitatively by examining the measured PIV velocity fields. 

Shown in Figure 5(a) to (c) are PIV velocity fields at y = 0, at test condition 500, with the injector tip at 

the venturi throat. These results indicate qualitatively that the spray angle of the droplets increases with 

increasing swirler angle. 

 

a) b) c) 

Figure 5.—Average Velocity Fields. Average velocity fields at y = 0 for water spray measured with PIV measured 

at test condition 500 with injector tip at the venturi throat. Increasing air swirler angle from left to right: (a) 45q, 

(b) 52.5q, (c) 60q. The flow is from top to bottom. 
NASA/TM—2015-218475 7 

We also measured the spray pattern using the high-speed images of PLS. For each condition we 

analyzed 1000 high-speed image frames using IDL (Ref. 12) and software (Ref. 13) which finds and 

measures roughly circular features within an image. For each of the features, a distance, r, and angle 

relative to the vertical measured from (x, y, z) = (0, 0, 0) were determined. Spray pattern histograms were 

developed by creating software “bins” which were 3° wide with an r that ranged from 10 to 30 mm. The 

software counted the features found in each bin throughout the 1000 frames analyzed. Results of this 

analysis are shown in Figure 6, where all three plots are shown with the same scale. These histograms 

clearly indicate that the spray was a hollow cone, as expected from a simplex fuel injector. They also 

indicate that the number of droplets found in the center of the hollow cone increases with increasing 

swirler angle. 

Using the droplet distribution histograms, we define the spray angle as the difference between the bin 

angle where the total count reached 5 percent and the angle where the count reached 95 percent. These 

calculated spray angles are plotted versus swirler angle in Figure 7. The trend in Figure 7 closely matches 

the trends observed in the PIV velocity field in Figure 5 (increasing spray angle with increasing swirler 

angle). In addition, in Figure 7, one can note that the spray angle is higher at point 500 than at point 700 

for swirler angles of 45q and 52.5q, but not for 60q.

Figure 6.—Histograms of Droplet Distributions. Occurrences of droplets at angles relative to center of air mixer 

exit at test condition 500 with injector tip at the venturi throat. Increasing air swirler angle from left to right: 

(a) 45q, (b) 52.5q, (c) 60q. 

Figure 7.—Effect of air swirler angle on spray angle. Spray angle 

distribution versus air swirler angle at test conditions 500 and 700. 

The injector tip is at the venturi throat. 

 

60

80

100

120

140

40 45 50 55 60 65

Spray Angle (Degrees)

Air Swirler Angle (Degrees)

500

700

a) b) c)
NASA/TM—2015-218475 8 

Figure 8.—Sauter mean diameter versus radial position. Plot of 

droplet diameter versus radial location, x, for test condition 500 

with the injector tip at the throat of the venturi.

Droplet Size 

The Sauter mean diameter (SMD) is plotted versus radial position for each swirler angle in Figure 8. 

These measurements were collected at test point 500 with the injector tip at the throat. The SMD is 

measured at points along a line perpendicular to the flow at an axial location, z = 11 mm, downstream of 

venturi exit. The droplet size distribution is similar for swirler angles 45q and 52.5q with smaller droplets 

occurring towards the center of the spray and larger droplets near the outer edges. For these swirler angles 

the SMD ranges from 80 μm to below 20 μm. For swirler angle 60q, the size distribution is narrower, 

spanning from 30 to 60 μm across the scan. This may be a result of the presence of the recirculation zone 

produced by the 60q swirler angle as discussed in the next section. 

Recirculation Zone 

In gas turbine combustors, swirl is used to improve fuel/air mixing and to mix the combustion 

products with the unburned fuel and air, thereby decreasing flame length and increasing flame stability. 

Usually, the swirl is strong enough to cause reverse flow and a central recirculation zone forms 

downstream of the fuel/air mixer (Ref. 10). Previous work on swirling jets has shown that a central 

recirculation zone forms when the swirl number is greater than about 0.6 (Refs. 10, 11, and 14). 

Furthermore, a diverging flow passage has been shown to increase the diameter and mass of fluid in the 

recirculation zone (Ref. 11). 

Since the SV-LDI geometry includes a converging-diverging venturi, a central recirculation zone 

would be expected as long as the swirl number was near 0.6. Therefore, the minimum swirler vane angle 

was chosen to be 45°, corresponding to a swirl number of 0.59 (Ref. 6). However, subsequent optical 

diagnostic measurements (Ref. 15) and CFD calculations (Ref. 16) showed that with a swirler angle of 

45° no central recirculation zone formed. This was true both for a single 2.54-cm SV-LDI fuel/air mixer 

in a 2.54-cm by 2.54-cm square duct (Refs. 15 and 16) and an array of 3 by 3 fuel/air mixers in a 7.62-cm 

by 7.62-cm square duct (Ref. 16). In fact, the experiments conducted by Fu (Ref. 15) on a single 2.54-cm 

SV-LDI in the 2.54-cm by 2.54-cm square duct showed that a strong central recirculation zone did not 

form until the swirler angle was well above 45°. In this experiment, Fu (Ref. 15) varied the swirler angle 
NASA/TM—2015-218475 9 

from 40° to 65° in 5° increments. No central recirculation zone formed until the swirler angle was 55°; 

and even then, the central recirculation zone did extend to the venturi exit—it started about 22-mm 

downstream of the venturi exit. The central recirculation zone did not extend to the venturi exit until the 

swirler angle was 60°, S = 1.02 (Ref. 15). For a 3 by 3 array of fuel/air 60° swirler angle mixers, previous 

CFD calculations (Ref. 16) and experimental measurements (Ref. 17) also showed a recirculation zone 

extending to the venturi exit. 

The experiment presented in this paper differs from these previous experiments (Refs. 15 to 17) as it 

was done on a single 2.54-cm SV-LDI fuel/air mixer in a 7.62-cm circular duct. A single fuel/air mixer 

was studied to allow for detailed measurements to be taken without interaction from surrounding fuel/air 

mixers. However, this means that the boundary conditions, i.e., confinement, were significantly different 

than it would be in a more realistic case of seven 2.54-cm fuel/air mixers in the same 7.62-cm circular 

duct. Previous research (Refs. 11, 14, 15, and 18) has shown that confinement can significantly affect the 

velocity field and recirculation zone. Therefore, to partially simulate the effects of multiple fuel/air 

mixers, a coflow was used. Although this experiment had a coflow whereas the previous research did not 

(Refs. 16 and 17), the current results are consistent: a recirculation zone was observed for the 60° swirler 

but not for the 45° swirler. There was also no recirculation zone for the 52.5° swirler. 

In this work the value and location of the minimum axial velocity for various swirler angles was used 

as an indicator of the effect swirler angle (shown in Fig. 9). This was done since the neither the 45° 

swirler nor the 52.5° swirler case produced a recirculation zone. Figure 9(a) shows that the minimum 

axial velocity decreases with increasing swirler angle. In Figure 9(b) for both the 45° swirler and the 

52.5° swirler, the minimum axial velocity is at the downstream edge of the field of view, z § 36 mm; this 

means that the actual location of the minimum axial velocity may be farther downstream and that the 

minimum velocity may be lower. For the 60° swirler, the location of the minimum axial velocity (i.e., 

greatest reverse flow) is much farther upstream. 

a) b)

Figure 9.—Effect of Swirler Angle on Minimum Axial Velocity. The injector tip is at the venturi throat and test condition 

is 500. Shown are a) minimum axial velocity and b) axial location of this minimum axial velocity versus swirler angle. 

Ͳ15

Ͳ10

Ͳ5

0

5

10

15

40 45 50 55 60 65

Minumum Axial Velocity

(m\s)

Swirler Angle (Degrees)

0

10

20

30

40

40 45 50 55 60 65

Axial location of min.

velocity (mm)

Swirler Angle (Degrees)
NASA/TM—2015-218475 10 

Injector Tip Location 

This subsection examines the effect of the injector tip location on spray distribution and droplet size 

distribution for all three swirler angles, and the recirculation zone for the 60q swirler angle. 

Spray Distribution 

To examine the spray distribution, PIV results along the y = 0 combustor position are shown at both 

conditions 500 and 700 in Figures 10 and 11 for swirler angle 45q, and in Figure 12 at test condition 500 for 

swirler angle 60q. The flow is from top to bottom. From left to right in these figures, h/Dt is 0, –0.16, and 

–0.31. In Figures 10 and 11, the results show that the spray angle increases as the injector tip is moved 

farther upstream of the throat. For the 60° swirler (Fig. 12), it is harder to discern by eye, but the trend 

appears to be opposite. A more quantitative consideration of the effect is shown in Figure 13, which shows 

plots of spray angle versus injector tip location calculated from the histograms as described in the spray 

distribution section above. These graphs show agreement with the visual observations made for the PIV 

images, and emphasize that there is only a minor change of spray angle for the 60° air swirler with injector 

tip position (Fig. 13(c)). 

a) b) c) 

Figure 10.—Average Velocity Fields, 45q swirler, Condition 500. Average velocity fields for water spray measured with 

PIV at test condition 500 with 45q swirler angle. Injector tip distances upstream of the throat, h/Dt are a) 0, b) 0.16, 

and c) 0.31. The flow is from top to bottom. 

a) b) c) 

Figure 11.—Average Velocity Fields, 45q swirler, Condition 700. Average velocity fields for water spray measured with 

PIV at test condition 700 with 45q swirler angle. Injector tip distances upstream of the throat, h/Dt are a) 0, b) 0.16, 

and c) 0.31. The flow is from top to bottom.

a) b) c) 

Figure 12.—Average Velocity Fields, 60q swirler. Average velocity fields for water spray measured with PIV at test condition 

500 with 60q swirler angle. Injector tip distances upstream of the throat, h/Dt are a) 0, b) 0.16, and c) 0.31. The flow is 

from top to bottom. 
NASA/TM—2015-218475 11 

To understand the effect of the injector tip location further, the histograms from the 60q swirler are 

examined. Figure 14(a) to (c) show histograms of the droplet distributions for the range of spray angles 

measured for test condition 700 with h/Dt of 0, –0.16, and –0.31. The histograms show that the 

distribution becomes more uniform as the injector tip is moved closer to the throat. 

Droplet Size 

Figure 15 shows the SMD for the various injector tip locations, measured across the flow at the axial 

location z = 11 mm (11 mm downstream of venturi exit). The plots show that as the injector tip moves 

downstream the droplet sizes are larger in the center region of the of the flow from ~ x= –10 mm to 

10 mm. This is most evident for Figure 15(a) for swirler angle 60q, test condition 700. This trend is also 

seen for Figure 15(b) for swirler angle 52.5q, test condition 700. There are many possible reasons for 

increase in uniformity of the droplet sizes such as: a simple shift of the flow downstream or droplet 

coagulation. To determine the cause of these trends, droplets sizes need to be measured at several more 

axial locations. 

Recirculation Zone 

The recirculation zone generated using the 60° swirler was characterized based on its size and 

minimum velocity. The velocity field for the 60° swirler flow, as seen in Figure 12, shows that the region 

measured with the PIV in this experiment does not include the stagnation point of the recirculation zone. 

The field of view only includes the top portion of the primary zone as shown in the drawing 

representation of the flow in Figure 16. Because of this limited field of view, the outer bounds of the 

recirculation zone cannot be determined. Hence in order to measure the width of the recirculation zone, 

the zero velocity line is used instead. The maximum width of zero velocity area of the recirculation zone 

is plotted versus the injector tip location in Figure 17(a). This plot shows that the recirculation zone has a 

width that is almost constant, but is slightly wider with the injector tip at the throat. The location of this 

maximum width plotted versus the location of the injector tip is shown in Figure 17(b). The location 

shows a shifting of the maximum width downstream as the injector tip proceeds downstream, which 

reflects a simple shifting of the flow downstream. 

Figure 13.—Effect of Injector Tip Location on Spray Angle. Plots of spray angle versus location of injector tip diameter 

measured in h/Dt. From left to right the plots are for swirler angles 45q, 52.5q, and, 60q. 

60

80

100

120

140

0 0.1 0.2 0.3 0.4

Spray Angle (Degrees)

Diameters upstream of throat (h/Dt)

500

700

a)

60

80

100

120

140

0 0.1 0.2 0.3 0.4

Spray Angle (Degrees)

Diameters upstream of throat (h/Dt)

500

700

b)

60

80

100

120

140

0 0.1 0.2 0.3 0.4

Spray Angle (Degrees)

Diameters upstream of throat (h/Dt)

500

700

c)

a) b) c) 

Figure 14.—Histograms of Droplet Distributions of Swirler Angle 60q. Occurrences of droplets at angles relative to 

center of air mixer exit at test condition 700 with injector tip at the venturi throat. Injector tip distance upstream of 

the throat in throat diameters: (a) at throat, (b) 0.16, (c) 0.31. 
NASA/TM—2015-218475 12 

a) b)

Figure 15.—Droplet Size Distribution Effect of Injector Tip. a) Plot of droplet size verses versus x (radial) location 

for a range of injector tip locations at test condition 700 and swirler angle 60q. b) Plot of droplet size versus radial 

location for a range of injector tip locations at test condition 700 and swirler angle 52.5q. 

Figure 16.—Drawing of the recirculation zone. Drawing 

of the flow with a recirculation zone. The field of view 

for this experiment only include the area above the 

dashed line. 

a) b)

Figure 17.—Measures of Recirculation Zone Size. a) Plot of maximum width versus injector tip location diameter 

upstream of throat. b) Plot of location of maximum width versus injector tip location. Swirler angle 60q and test 

condition 500. 

0

10

20

30

40

50

60

70

80

Ͳ25 Ͳ20 Ͳ15 Ͳ10 Ͳ5 0 5 10 15 20 25

Sauter Mean Diameter (μm)

x location (mm)

0.31 h\Dt

0.16 h\Dt

0 h\Dt

0

10

20

30

40

50

60

70

80

Sauter

Ͳ20 Ͳ15 Ͳ10 Ͳ5 0 5 10 15 20

Mean Diameter (μm)

x location (mm)

0.31 h\Dt

0.16 h\Dt

0 h\Dt

Flow

Stagnation point

Min. axial

velocity

MaxWidth

Outer bound of recirculation zone

Zero velocity line

14

15

16

17

18

19

20

0 Ͳ0.1 Ͳ0.2 Ͳ0.3 Ͳ0.4

Maximum width (mm)

Injector Tip Location (h/Dt)

15

17

19

21

23

25

0 Ͳ0.1 Ͳ0.2 Ͳ0.3 Ͳ0.4

Location of Maximum

width (mm)

Injector Tip Location (h/Dt)
NASA/TM—2015-218475 13 

Figure 18.—Measures of Strength of Recirculation for 60q Swirler. a) Plot of minimum axial velocity and b) axial 

location of minimum velocity versus injector tip location. Test condition 500. 

The minimum axial velocity of the recirculation zone was also measured and is shown in Figure 18. 

Figure 18(a) shows that the minimum axial velocity increases as the injector tip is moved further 

upstream the throat. This trend shows that the location of the injector tip may have an effect on the flow 

structure when a recirculation zone is present. The location of this maximum reverse flow is shown in 

Figure 18(b) plotted against the location of the injector tip. This plot shows a shifting of the maximum 

velocity downstream as the injector tip is moved downstream. 

Summary 

The swirler angle has a large effect on the spray angle and the formation of a recirculation zone as 

shown in References 15, 19, and 20. The larger the swirler angle the wider the spray angle and the more 

likely the formation of the recirculation zone. Recirculation only occurred for swirl number S = 1.02, 

corresponding to the 60° swirler. The injector tip location appears to only shift the flow without changing 

the structure for most flows. For flows with a recirculation zone, the droplets became more uniform in 

size and angular distribution the closer the injector tip was to the venturi throat. This relationship between 

injector tip location and recirculation zone was also shown by considering the minimum axial velocity. 

The minimum axial velocity increases as the injector tip was moved upstream towards the venturi throat. 

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a) b) 

Ͳ5

Ͳ4

Ͳ3

Ͳ2

Ͳ1

0

0 Ͳ0.1 Ͳ0.2 Ͳ0.3 Ͳ0.4

Min. Axial Vel. (m/s)

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