f

39'' A I A N ASME/SAE/ASEE/Joint Propulsion Conference and Exhibit AIAA-2003-4785 Huntsville, Alabama July 20-23, 2003

A Novel Swirlinq lniector for lmprovinq Mixinq in Hiqh Speed Flows

SMurugappan and E.Gutmark Department of Aerospace Engineering and Engineering Mechanics

University of Cincinnati. 45221 -0070

Abstract

A novel swirling injector capable of controlling the spreading rate and mixing in high speed flows has been designed and tested. The Controlled Supersonic Swirling Injector (CSSI) could be used to affect mixing both in the core and the shear layer of the jet. The CSSI employs the superior mixing characteristics of swirl and an active control mechanism to manipulate the growth rate and spreading characteristics of the jet. Supersonic swirling flows could be characterized by diamond shock patterns with flow recirculation observed in the compression- expansion region. The strength of the flow reversal is governed by a competition between the tangential momentum due to swirling motion and the axial momentum of the high speed jet. The baseline underexpanded swirling jet was compared with three other control cases. Control case b which used 0.9% of the base flow was found to be most effective in manipulating the shear layer characteristics. It reduced the spreading rate by 34% over the first 1.6 jet diameters as compared to the baseline. For 1.6cz/Djet<4, there was a 46% increase in the slope of the spreading rate than the baseline underexpanded swirling jet. For z/Djet>4, the spreading rates of both the controlled and baseline cases follow a similar trend. The local turbulent kinetic energy was comparable in magnitude with the baseline in the outer shear layer with the addition of control. There was a decrease in K.E in the first diameter of the jet which indicates that the control was effective in suppressing the initial spreading rate. In case c where the control mass was increased to 5.8% of the primary flow the spreading rate was suppressed by 66% when compared to the baseline in the first 1.6 jet diameters. For z/Djet>1.6 the slopes of the spreading rates was comparable in magnitude to the baseline. Though the effect of control was evident with case a, it was not effective as case b.

Introduction

Most energy for propulsive power comes from burning fuels. Factors such as combustion efficiency, reduction of emissions, improved flammability limits, noise reduction are all governed by the completeness of the mixing process between fuel and air. The technical challenge of improving mixing in high speed flows

stems from the inherently low growth rates of supersonic shear layers.

It was observed by Papamoschou and Roshko [ 11 that the ratio of the compressible to incompressible spreading rates of the shear layer asymptote at 0.2 for convective Mach number (Mc) greater than 0.8. In addition, the strong coupling between the density and pressure gradient produces vorticity due to the baroclinic torque, which makes some of the mixing techniques used in subsonic flows ineffective at high speed flows. Scramjet propulsion concept requires rapid mixing between fuel and air, due to the limited time and space available for mixing and reaction in a scramjet combustor. A major difficulty is achieving simultaneous penetration of the fuel jet into the high- speed air stream and intense mixing between them to ensure efficient combustion.

There are two widely used fuel injection strategies in scramjet combustors. 1) Transverse injection, 2) Parallel injection. Transverse injection may produce good near field mixing, but is inevitably accompanied by shocks, which reduce the total pressure. Parallel injection on the other hand could provide good fuel air mixing, but has poor penetration characteristics [2]. Due to inherent drawbacks in the two injection techniques, many researchers have suggested passive and active injectors to enhance mixing at high speeds.

Swirl is used in combustion systems to enhance mixing and provide flame stabilization in both subsonic and supersonic flows. In high-speed flow, compressibility effects reduce amplification rates of the spanwise vortices and increase three-dimensional instabilities, thereby producing preferential streamwise vortices. These vortices are less effective in entraining ambient flow into the mixing layer; therefore energy extraction from the mean flow is reduced and less energy is being transferred to small-scale vortical structures through vortex stretching. As a result, turbulent energy is reduced at all scales. In contrast to the spanwise structures, the streamwise vortical structures are amplified which increases the three- dimensional character of compressible shear layers. It has been shown that the stabilization of spanwise vortices by compressibility effects causes the turbulent mixing layer growth to asymptote to a value that is roughly 20 % of the corresponding incompressible value [ 1, 3, 41. By capitalizing on the increased natural

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39th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit20-23 July 2003, Huntsville, Alabama

AIAA 2003-4785

Copyright © 2003 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

amplification of three dimensional structures in the flow, addition of swirling motion was shown to improve mixing [5,6-81. However, the efforts to use swirl to enhance mixing, did not address the issue of penetration. The CSSI has the potential of increasing the initial penetration of the swirling fuel jet and enhancing mixing there after.

A swirling fuel jet has high penetration rate as long as the vortex core remains coherent. However, mixing rates between the swirling jet and the ambient flow are reduced in this mode. Above a critical swirl number, the vortex core becomes unstable and the vortex breaks down. This leads to vortex core disintegration and the production of high turbulence levels, which increase mixing. The swirl number is defined by the following integral:

SwirlNumber = 1 ]uwrzdr (Rz-R, ) u’rdr

where u and w are the axial and tangential velocity components, respectively, r is the tangential coordinate measured from the jet axis, and R1 and R2 are the boundaries of the zone in which the vortex is contained. Vortex breakdown occurs when the axial momentum is reduced relative to the azimuthal momentum. A patented technique to control the location of vortex breakdown is based on energizing the axial momentum by a secondary jet injection into the vortex core [9]. It was used successfully to delay vortex breakdown over delta wings.

Shock structures that are inherent in high- speed flow are yet another way of inducing vortex breakdown [ 101. The interaction of the streamwise structures with the planar, oblique or normal shock front causes vortex bursting due to the adverse pressure gradient imposed by the shock waves generating high turbulence which enhances mixing.

Cutler and Doerner [l 11 studied the combined effect of swirl and injection angle of a supersonic light gas injected into a supersonic air stream. The mass fraction of the injectant in the cross-stream was used to characterize mixing and penetration. They observed a minimal increase in mixing in the near field with a slight reduction in penetration. Lobed mixers [ 121 introduced an array of streamwise vortices into a planar or axisymmetric shear layer. These vortices were produced by a convoluted splitter plate (lobed surface) that generated a transverse mixing layer by displacing the flow, without separation in opposite directions inside neighboring channels to enhance mixing. Clemens and Mungal [13] employed shock waves which interacted with the flow separating at the splitter plate tip to produce a streamwise vortex that enhanced mixing rate of the shear layer. Dolling et a1 [14] mounted cylinder and wedge shaped vortex generators

upstream of a splitter plate trailing edge in a turbulent shear layer, The observations indicated that the vortex generators had an insignificant and even a certain adverse effect on the mixing rate. Elongated jets, such as elliptic and rectangular, is yet another strategy that has been used to enhance mixing, due to their asymmetric vortex dynamics that lead to axis switching [ 151. Multi step jets with several rearward-facing steps in a circular nozzle introduce multiple inflection points in the mean velocity profile. These have been shown to augment turbulence level by six fold, hence provide a stable combustion zone with higher energy release compared to a circular nozzle 1161. These passive control techniques could be used efficiently to improve mixing or penetration, but their effectiveness is limited in the operating envelope of the scramjet. Hence there is a need for active control techniques.

The CSSI consists of a high speed swirling flow with an active control mechanism used to manipulate the growth rate and spreading characteristics of the jet. A fraction of the swirling flow energy is required to modify the flow field behavior of the CSSI. The device is compact, rugged and simple and provides the dual benefits of transverse and parallel injection. Swirl, inherently has a very high spreading rates but poor penetration characteristics in the cross flow. Tests of CSSI in quiescent flow indicates that it could be capable of providing high penetration or a stable jet in the first few jet diameters with subsequent higher rates of mixing and entrainment downstream.

The current study evaluates the flow field characteristics of the CSSI in quiescent flow. The effect on spreading rate and mixing was evaluated at three control conditions. These were compared with an underexpanded swirling flow. Tangential and axial velocity measurements spanning 10 jet diameters have been used to compare the behavior of the CSSI with its baseline counterpart.

Experimental Setup

The baseline case which corresponded to a swirling underexpanded jet was operated at a pressure ratio, (R=Pstatlc/PamblenJ R of 2.72. This corresponded to a fully expanded Mach number of 1.29. The primary mass flow (=0.325 Ibhec) in the CSSI was set to be the same as baseline case. Both the CSSI and the underexpanded swirling jet exit into quiescent flow. The three control cases used varying amounts of the primary CSSI flow. The details of its construction and design have been provided in a patent that is pending. A 2D-LDV system was used to measure the axial and . tangential velocity components. The LDV used a 300 mW Argon-ion laser. The signal processor had 180 MHz Doppler frequency for high speed flow measurements and a 120MHz bandwidth for high .

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turbulence flow measurements. Laskin nozzle with olive oil was used to seed the flow. The mean diameter of the seeding particles was 1 micron, which gives a stokes number close to 1 so that seeding follows the main flow. The transmitter is a 60 mm probe with beam splitting and frequency shifting optics. The jet was attached through a fixed support to a biaxial-slide 3D-traverse. A LABVIEW program was used to synchronize the traverse with the signal processor. A TTL sent from the signal processor after recording the velocity at a certain location initiated the movement of the traverse to the next position. After a predetermined time the LDA recorded the data. This process continued until entire flow field spanning 10 jet diameters downstream was measured.

Spreading rate and mixing characteristics of underexpanded swirling jet was compared with the three other control cases. The three control cases involved adding 0.3, 0.9 and 5.8% of the primary CSSI mass flow.

Results and Discussions

Characterization of Baseline Swirling Flow

The swirl number for the baseline case was computed from the tangential and axial velocities. It was found to be 0.6. Compressible effects and the presence of shocks in non-ideally expanded flows modify the flow field in underexpanded swirling flows. The flow structure is governed by a competition between the swirling motion and shock strength. Strongly. swirling subsonic flows exhibit flow recirculation near the nozzle exit. The presence of the recirculation region causes stronger velocity gradients, higher levels of turbulence and higher rates of mixing and entrainments [17]. Figure 1 shows the centerline axial velocity profiles as the pressure ratio, R, of a swirling nozzle is increased. It could be observed that for R=1.02 the flow is reversible until 2 jet diameters. As the pressure ratio, R , increases the flow accelerates close to the nozzle exit. It remains negative in magnitude for R=1.63 until 2 jet diameters. For R> 1.63 the profiles exhibits an oscillatory pattern. This indicates the presence of alternating flow acceleration and deceleration. Underexpanded jet flows exhibit diamond shock pattern where as underexpanded swirling flows indicates alternating compression and expansion regions with flow reversal. For R=2.45, the flow reverse velocities in the first two shock cells, where as for R=3.0 the flow has a negative velocity only in the first shock cell. The magnitude of the reverse velocity is lower than that for R=2.45. For 3.2<z/Djet <4.8 the effect of swirl and shock strength decayed and the flow field behaves as a fully developed subsonic circular jet for all cases.

Figure 2a and b shows the baseline tangential and axial velocity profiles for the underexpanded swirling nozzle. The mean axial velocity profile indicates a velocity deficit region extending until 4 jet diameters. Flow reversal could be observed close to the nozzle exit (z/Djet=0.4). The maximum mean tangential velocities are comparable in order of magnitude with the maximum mean axial velocities. This could provide higher tangential mixing, a, characteristic of swirling flows. Figure 3 a and b shows the axial and tangential turbulence fluctuations. The presence of wake region in the core causes higher shear with stronger velocity gradients in the core of jet as compared to the outer shear layer. There is a 61 and 75% decrease in the axial and tangential rms in the outer shear layer as compared to the jet core. The local turbulence kinetic energy (shown in figure 4) was found to be concentrated along the jet core and outer shear layer. Local K.E was 4 times in the core as compared to the outer shear layer. Strongly swirling subsonic flows (Swirl number > 0.6) are characterized by high levels of turbulence in the recirculation region and outer shear layer [17]. It could be noted that high speed swirling flows exhibit similar turbulence characteristics.

Spreading Rate and Mixing Characteristics

Figures 5-7 (a and b) shows the axial and tangential mean velocities for the three control cases. In case a (figure 5 a and b), effect of control caused an addition of axial momentum in the core of the jet. At z/Djet=O.4, the reverse velocities observed in the core of the jet, becomes positive with control. The effect of control decayed after 1.2 jet diameters. The axial velocity profile resembles that of a swirling jet with a constant velocity bias added in the core of the jet. The tangential velocity were similar in magnitudes for the baseline and case a for z/Djet>0.8. In control case b (figure 6 a and b), the axial velocity had a top hat profile within 0.8 jet diameters indicating a low spreading rate. This initial low spreading characteristics is beneficial for the fuel injectors in scramjet since it could provide superior penetration characteristics without being washed away by the cross flow air stream. For dDjeb0.8, the flow has a velocity defect region similar to the baseline. This would correspond to stronger velocity gradient, larger shear, hence better mixing. The combination of initial low spreading rate coupled with subsequent larger growth rate is preferred for better penetration and mixing characteristics in a scramjet. Since the magnitude of the tangential mean velocity is comparable to the baseline swirling case and the effect of control on the mean tangential velocity was minimal, it could provide enhanced mixing in the tangential direction. In case c (figure 7 a and b) the axial velocity profiles resembles that of a turbulent jet.

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It suppressed the spreading rate by 38% for z/Djet< 1.6. The wake region is completely eliminated. Hence the vorticity is suppressed which degrades mixing. The effect on the tangential components was minimal after 1.2 jet diameters; it was similar in trend to the baseline tangential velocity field. Figures 8 a and b shows the axial and tangential rms for control case b. It could be observed that the maximum axial and tangential turbulence levels were 68% and 60% lower than the baseline in the first jet diameter. This provides supportive evidence that the case b has lower growth rate as compared to the baseline near the nozzle exit. The local turbulence K.E for case b is shown in figure 9. The local K.E in the outer shear layer was comparable in magnitude with the baseline near the nozzle exit. The local K.E dropped with control near the core, another, “indication of suppressed mixing” near the CSSI exit.

Figure 10 shows the jet half width as a function of the axial location for all the cases. Both these quantities are normalized by the jet diameter. The slopes of the spreading rates from plot 10 are shown in figure1 1. The flow field was divided into three zones, O<z/Djet<l.6, 1.6<z/Djet<4 and z/Djet>4. The baseline underexpanded swirling flow had comparable slopes in different regions of the flow field. Case a was able to provide 19% reduction in slope near the nozzle exit. This was accompanied by a 14% increase in slope between l.G<z/Djet <4. Control case b was identified to be optimal, since it had better effects of lower spreading rate of 34% until 1.6 jet diameters and 46% enhancement in growth rate for l.G<z/Djet <4 as compared to baseline. Though control case c was effective in minimizing the spreading rate near the nozzle, there was minimal improvement in spreading rate in the region 1.6<z/Djet <4. For z/Djet >4, the slopes of the spreading rate was similar in magnitude for all the control cases when compared to the baseline.

Conclusions

A novel supersonic swirling injector has been designed and tested in quiescent flow. Unlike in subsonic swirling flows, underexpanded swirling flows are associated with the interaction of shocks and flow reversal. Flow recirculation regions which is a fundamental characteristic of subsonic swirl flows is also observed in supersonic cases. A baseline supersonic swirling case was compared with three other controlled cases. It was observed that fraction (0.3-6%) of the primary flow mass added to the CSSI is capable of modifying the jet characteristics. Case b was found to provide superior performance with a low spreading rate initially and subsequent larger spread. Case a and case c had either low or high amounts of control mass, hence it was unable to attain the desired effect. The

effect of control on the tangential flow field was minimal for all the cases studied.

Acknowledgements

The financial support of DAGS1 AFRL/PR is gratefully appreciated. The authors would like to acknowledge Drs. J. Donbar, M. Gruber and T.A. Jackson from AFRLPR for their help and support. We appreciate the guidance given by Russell Dimicco, for his help in data analysis and setting up the experiments.

References

l.Papamoschou, D., Roshko, A,, ‘‘ The Compressible Turbulent Shear Layer: An Experimental Study”, Journal of Fluid Mechanics, Vol. 197, pp. 453-477, 1988. 2. Northam, G.B., Greenberg, I., and Byington, C.S., “Evaluation of Parallel Injector Configurations for Supersonic Combustion,” AIAA paper 89-2525, 1989. 3. E. Gutmark, K. Schadow, and K. Wilson. “Effect of Convective Mach Number on Mixing of Coaxial circular and Rectangular jets,” Physics of Fluids A, Vol. 3, No. 1, pp. 29-36, January 1991. 4. K. Schadow, E. Gutmark, and K. Wilson. “Compressible Spreading Rates of Supersonic Coaxial Jets,” Experiments in Fluids, Vol. 10, pp. 161-169, 1990. 5,“Mixing enhancement in supersonic free shear flows”, E.J.Gutmark, K.C.Schadow, K.H.Yu, Annual review of fluid mechanics, Vol. 27, pp 375-417, 1995. 6.“An experimental study of compressible turbulent mixing enhancement in swirling jets”, J.W.Naughton, L.N.Cattafesta, G.S.Settles, Journal of Fluid Mechanics,

7. “Mixing of Swirling Jets in Supersonic duct flow”, D.K.Kraus, A.D.Cutler, Journal of propulsion and power, V01.12, No.1, 1996. 8. “Near field flow of supersonic swirling jets”, A.D.Cutler, B.S.Levy, D.K.Krauss, AIAA Journal, Vol. 33, No.5, 1995. 9. E. Gutmark and S. Guillot, “Control of Vortex breakdown via near core blowing,” US patent No. 6,138,955, 2000. 10 “Aspects of shock wave induced vortex breakdown”, I. J.Kalkhoran, M.K.Smart, Progress in aerospace sciences, Vol. 36, pp. 63-95, 2000. 11. Cutler, A.D., and Doerner, S.E., “Effects of Swirl and Skew upon Supersonic Wall Jet in Cross Flow“, Journal of Propulsion and Power, vo1.17, No.6, pp.

12. Presz, W.M., Gousy, R., Morin, B.L., “Forced Mixer Lobes in Ejector Designs”, AIAA Paper 86- 1614, 1986.

V O ~ . 30, pp.271-305, 1997

1327-1332,2001

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13. Clemens, N.T., and Mungal, M.G., “Effects of Side Wall Disturbances on the supersonic mixing layer”, Journal of Propulsive Power, Vol 8, No.1, pp. 249-251, 1992. 14. Dolling, D.S., Fournier, E., and Shau, Y.R., “Effects of Vortex Generators on the Growth of Compressible Shear Layer”, Journal of Propulsion and Power, Vol.8,

15. Gutmark, E.J., Schadow, K.C., Wilson, K.J., “Non- Circular Jet Dynamics in Supersonic Combustion”, Journal of Propulsion and Power, vo1.5, pp. 529-533, 1989. 16. Schadow, K.C., Gutmark, E., Parr, D.M., Parr, T.P., Wilson, K.J., Ferrell, G.B., “Enhancement of Fine Scale Turbulence for Improving Fuel Rich Plume Combustion”, Journal of Propulsion and Power, Vol. 6,

17. Syred, N., and Beer, J.M., “Combustion in Swirling Flows: A review”, Combustion and Flame, Vol. 23, pp.

No.5, pp. 1049-1056, 1992.

NO 4, pp. 357-363, 1990.

143-201, 1974.

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+ R=2.45 -x- R=2.72 -t R=2.99

+R=2.18-R=1.63

ZlDjet

-200

Figure 1: Centerline Axial Velocity as a Function of Normalized Axial Location for Varying Pressure Ratios.

-1

500 T I

300 -

400 i !I

z/Djet=O 4 * z/Djet=O 8 300 r r7

200 +z/Djet=l 6 + z/Djet=Z 8

- r/Djel=4 - z/Djet=6 4

- r/D et=l 0

> 3

-100

-200 1 d .&

2a

2b

Figure 2: a, Baseline Mean Axial Velocity as a function of Normalized Radial Location. b, Baseline Mean Tangential Velocity as a Function of Normalized Radial Location

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8

7

6 -

5 T

4:

3 -

2 -

1 -

O Y 2 - 1 0 1 2 3

RID 4

1 0

9

8

7

6

5

4

3

2

1

O 2 1 0 1 2 3 4 RID

~~

3a 3b

Figure 3: a, Baseline Axial RMS Contours. b, Baseline Tangential RMS Contours

8

7

6 + W

5 5

N 4 \

3

2

1

2 1 0 1 2 3 4 5 6 RID

Figure 4: Baseline Local Turbulent Kinetic Energy Contours

50

5a

-3 -2 -1 0 1 2 3 WDjet

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-+ z/Djet=O 4 --h z/Djet=O 8

++ z/Djet=l 6 + z/Djet=2 8

-z/Djet=4 - dDjet=6 4

WDjet

450

400

350 * z/Djel=O 4 A z/Djet=O 8

+z/Qet=l 6 300 -t- z/Djel=2 8

u - z/DjeI=4 $250 - z/Djet=6 4

-+- z/Djel= 10

3 -z 200

150

100

50

0

25

-3 -2 1 0 1 2 3 WDjet

- z/Djet=4 - z/Djet=6 4

1 '

-200

-250 ___

RlDjet

5b

6a

6b

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350 400 i 300 {

3

150

O L -3 -2 -1 0 1 2 3

RlDjet

300 1 200 -3 100 1

-loo 1

+ z/Djet=l.6 z/D~et=2.8

-z/D~el=4 - z/Dpt=6.4

/ I

It A

-300 RlDjet

7a

7b

Figure 5-7: a, Axial Mean Velocity Profiles b, Tangential Mean Velocity Profiles Figure 5 a & b: Case a, Figure 6 a & b: Case b, Figure 7 a & b: Case c

' - 2 . 1 0 1 2 3 4 5 6 7 8 RID

Figure 8:a, Axial RMS Contours b, Tangential RMS Contours for Case b 9

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U 01

B 1 N

0

7

6

5

4

3

2

1

O - 2 -1 0 1 2 3 4 5 RID

Figure 9: Local Turbulent Kinetic Energy Contours for Case b

1.65

1.45

1.25

*) Baseline

-7- Case a I 0.85

0.65

0.45

0.25 L i 0 1 2 3 4

z/Djet

Figure 10: Jet Half width as a Function of Normalized Axial Location for all Cases

16

14

3 12

; 10 TI

P

In - 0 8 6

4

2

Baseline Case a Case b Case c

Figure 11: Slopes of Spreading Rates for all Cases 10

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