Simulated-fuel-jet/shock-wave interaction

Transcription

1 Simulated-fuel-jet/shock-wave interaction Frank Houwing 1, Alexis Bishop 2, Matthew Gaston 3, Jodie Fox 1, Paul Danehy 4, and Neil Mudford 5 1 Australian National University, ACT 0200, Australia 2 The University of Queensland, QLD 4072, Australia 3 University of Technology Sydney, NSW 2007, Australia 4 NASA Langley Research Center, Hampton, VA , USA 5 University of New South Wales, ADFA, ACT, 2600, Australia Abstract. This paper considers the breakdown of streamwise vorticity as a means of enhancing the rate of fuel-air mixing in supersonic combustion. In particular, the prospect is investigated of improving fuel-air mixing in a scramjet engine through baroclinic torque and the amplification of streamwise vorticity arising out of the interaction between an oblique shock and a fuel jet issuing from a hypermixing fuel injector. 1 Introduction This paper considers the breakdown of streamwise vorticity as a means of enhancing the rate of fuel-air mixing in a supersonic combustion ramjet (scramjet) engine[5]. In particular, the prospect is investigated of improving fuel-air mixing in a scramjet engine through the application of baroclinic torque and the amplification of streamwise vorticity associated with a fuel jet produced by a hypermixing fuel injector[7]. This is accomplished by studying the interaction of the flow from a hypermixing fuel injector with an oblique shock wave, using planar laser-induced fluorescence (PLIF) imaging of nitric oxide (NO) present in trace amounts in the simulated fuel. In order to study the flow free of the complicating effects of combustion, helium replaces hydrogen in the simulated fuel stream of the model scramjet combustor. Nitrogen replaces air in the simulated oxidant stream to eliminate NO there. LIF is then produced in mixtures of simulated fuel and oxidant but, ideally, not in the pure oxidant stream. 2 Theoretical Considerations on Shock-Vortex Interactions Figure 1 (a) is a schematic that represents a single fuel jet, with no accompanying streamwise vorticity, interacting with an oblique shock wave. Because the fuel jet contains fluid at a density lower than the surrounding coflow, the interaction with the oblique shock wave causes the jet to acquire streamwise vorticity through the phenomenon of baroclinic torque[8]. This is produced by the interaction of orthogonal components of the gradients of density and pressure. Specifically, the baroclinic source term in the vorticity transport equation is given by[4] ρ D ( ω ) = 1 ρ p. (1) Dt ρ ρ2

2 2 Frank Houwing et al. When a fuel jet is accompanied by streamwise vorticity, an important issue is how the shock wave and vorticity influence each other. The shock-vortex interaction generally leads to three-dimensional curvature of the shock front because of the strong gradients of total pressure and Mach number in a supersonic streamwise vortex[6]. Theoretical inviscid analysis[3] has shown that the magnitude of the vorticity component tangential to the shock front is increased during interaction with a shock wave, while the magnitude of the normal component is unchanged. This analysis shows that the vorticity jump δ ω across a general three-dimensional steady curved shock wave is given by δ ω t = n δ ω n =0, (2) [ t (ρ 1 V 1n ) δρ ] V 1t t V 1t δρ, (3) ρ 1 V 1n where: ω = ω t + ω n is the vorticity vector; V is the velocity vector; ρ is the fluid density; n is the unit vector normal to the shock surface; t is the surface gradient operator; subscripts t and n denote components tangential and normal to the shock surface respectively; δρ ρ 2 ρ 1 and δ ω ω 2 ω 1, where subscripts 1 and 2 denote conditions immediately upstream and immediately downstream of the shock front, respectively. A fuel jet produced by a hypermixing fuel injector is generally accompanied by multiple streamwise vortices created by the production of streamwise vorticity in the external flow over specially designed structural features such as compression and expansion ramps, vanes etc. Flow separation at the injector trailing edge places these vortices in close proximity to the fuel jet whereupon they proceed to Oblique shock wave Fuel jet Distorted jet (c) (a) fuel strut 131 fuel strut o wedge wedge (b) 8mm 22 16mm 7.5 o o side elevation fuel injector plan view (d) 8mm 16mm 24mm Fig. 1. (a) Fuel jet interacting with oblique shock wave. (b, c) Fuel injection strut and shock wave generator - side elevation and plan view. (d) Swept compression expansion ramp (SCER) hypermixing fuel injector. Dimensions in millimetres.

3 Simulated-fuel-jet/shock-wave interaction 3 influence the development of the mixing layer. The jet-vortex-shock interaction for these jets is therefore highly complex, which is why experiment is crucial to the study of the behaviour of the interaction. One such injector, known as the swept compression expansion ramp (SCER) injector[2], produces a pair of counter-rotating streamwise vortices for each fuel jet. The presence of these streamwise vortices close to the jet/coflow interface promotes mixing in the near wake by spreading out the jet thereby increasing the interfacial area between the two fluids. In addition, the counter-rotation of the two vortices tends to split each jet in two[1] even without the presence of an intersecting shock. This previous work provided motivation for the current study, which seeks to determine whether interaction of the simulated-fuel jets with an oblique shock wave can increase both the streamwise vorticity and jet bifurcation and thereby promote mixing beyond what can be achieved with the hypermixing injector alone. 3 Experiment 3.1 Flow Conditions and Model The experiments were performed using the Australian National University s freepiston shock tunnel T3, which generates a supersonic nitrogen flow to simulate the coflowing oxidant stream around the model fuel strut. A conical nozzle with a 125 mm diameter exit and 35 mm diameter throat produces a free stream flow with a static pressure of 40 kpa, a static temperature of 700 K and Mach number of 4.8. Figures 1 (b and c) illustrate the experimental apparatus which produces the desired interactions in the tunnel test section. As this figure shows, a fuel injector is mounted in the aft section of a horizontal sharp-nosed flat plate which simulates a fuel strut in a scramjet engine. The strut is at zero incidence and zero yaw with respect to the oncoming flow. A second sharp-nosed flat plate is set at incidence below the fuel strut. The leading edge of this second plate is slightly forward of the fuel strut trailing edge and parallel to it. The attached oblique shock off this lower plate then intersects the helium jets issuing from the fuel strut. The supersonic free stream flow passes over the fuel strut. The simulated fuel consists of a mixture of 1% CO 2, 1.5% NO and 97.5% He injected from the fuel injector at a Mach number of 1.64, a pressure of 40 kpa and a temperature of 160 K. CO 2 is an efficient quencher that reduces the fluorescence lifetime of NO. It is added to the mix to provide sufficient quenching to avoid motional blurring, which is the loss of spatial resolution caused by flow motion during fluorescence emission. The emphasis of the current work is on the imaging of these simulated-fuel jets rather than of the whole flowfield. Therefore the simulated fuel is doped with the PLIF target species (NO) while the simulated coflowing oxidant stream is nominally free of NO.

4 4 Frank Houwing et al. Figure 1 (d) shows the SCER hypermixing fuel injector used here. It is based on the design used in previous investigations by Gaston et al[2] and Fox et al[1]. It contains six fuel-jet ports, with each port located in the base of a swept compression ramp and flanked by expansion ramps. The ramp height is 8.0 mm, the port diameter is 4.0 mm and the port centreline is midway up the ramp. 3.2 Fluorescence Imaging System The two optical arrangements (A and B) of the laser and camera are illustrated in Fig. 2 (a and b). The laser sheet illuminates one vertical streamwise section and four horizontal streamwise sections of the flow, one section at a time. In each case, the line of sight of the CCD camera, which captures and focuses the fluorescence from the illuminated planes, is perpendicular to the laser sheet. The five images are denoted as follows. Vert is the image of the vertical section which intersects the SCER central injector ports as shown in Fig. 2 (c). Horiz1 is the image of the horizontal section coincident with the strut horizontal plane of symmetry, as shown in Fig. 2 (d). Horiz2 is the image of the horizontal section which passes through the centres of the three SCER upper injector ports. The horizontal section imaged in Horiz3 lies just above the tops of the SCER upper ports and that imaged in Horiz4 lies just above the tops of the SCER upper compression ramps. Imaging Configration A Injector strut Shock tunnel nozzle Jet FLOW imaged region: 576 x 384 pixels 78.1 x 52.1 mm Flow Laser sheet h Intensified CCD Camera (a) Imaging Configration B Intensified CCD Camera illuminated region (100 mm wide) (c) Shock tunnel nozzle Flow Injector strut Laser sheet Jet (d) FLOW imaged region: 576 x 384 pixels 78.1 x 52.1 mm h (b) illuminated region (100 mm wide) Fig. 2. Optical configurations: (a) For capture of the vertical section images, (b) For capture of horizontal section images. (c) Illuminated and imaged regions of vertical section, (d) Illuminated and imaged regions of horizontal sections; case shown is symmetry plane section. The sharp nosed flat plate at incidence and its accompanying oblique shock are omitted for clarity.

5 4 Results and Discussion Simulated-fuel-jet/shock-wave interaction 5 Figure 3 shows a set of the images produced by PLIF, as described above. These images all employ the same logarithmic greyscale in which light and dark indicate high and low fluorescence signal intensity respectively. It is apparent from the figure that some features, such as shock waves, are visible far from the jet even though the flow there should be free of NO. A spectral filter in front of the ICCD camera severely attenuates the scattered laser light so that Rayleigh scattered light, which is of low intensity in the tunnel, should be eliminated. A Mie scattering signal should be coarser in appearance. It therefore seems likely that it is LIF, implying that minute quantities of oxygen may have (a) (b) (c) (d) (e) (f) (g) (h) Fig. 3. PLIF images of simulated fuel. Oncoming flow is left to right. (a) Vert planebase injector, oblique shock. (b) Vert SCER injector, no oblique shock. (c) Horiz2 SCER injector, no oblique shock. (d) Vert SCER injector, oblique shock. (e) Horiz1 SCER injector, oblique shock. (f) Horiz2 SCER injector, oblique shock. (g) Horiz3 SCER injector, oblique shock. (h) Horiz4 SCER injector, oblique shock.

6 6 Frank Houwing et al. contaminated the test gas leading to NO production in the shock tube. Whatever the signals origin, it is not of concern for our present purpose as the image of the seeded jet is clear and laser attenuation due to NO in the simulated oxidant stream will be negligible. In any case, it is helpful for interpretation to see these otherwise invisible features. Figure 3 (a) shows Vert of a single helium jet interacting with the oblique shock. This jet issues from a plane base injector installed in place of the SCER injector. Apart from this change, the optical and flow arrangements are as shown in Fig. 2 (c). Figure 3 (a) shows the jet to be largely unaffected by the shock, apart from turning with the flow as it enters the shock. In particular, little or no jet bifurcation is observed. The absence of such bifurcation is supported by horizontal plane images not presented here. This jet and mixing layer are accompanied by very little streamwise vorticity as a plane base injector has no compression or expansion ramps to generate it. It is therefore an example of the interaction illustrated in Figure 1 (a). Figures 3 (b and c) show images Vert and Horiz2, respectively, of jets issuing from the SCER with no oblique shock present. Fig. 3 (d) shows the Vert image of the SCER central jets interacting with the oblique shock wave while the remaining images are horizontal views of the same flow realised in other shots. A number of observations can be made from comparison of these images. Firstly, the interacting jets have spread out in the vertical plane and turned ahead of the plane of the incident oblique shock. This suggests that the otherwise straight oblique shock has been distorted into a curved shock. This curvature is not visible in the images but this is not surprising as the PLIF signal is relatively insensitive to pressure and temperature for the chosen laser transitions and conditions. The curved shock is expected to be similar to that of a bow wave on a blunt body. Downstream of the shock, the jets bifurcate and consequently disappear rapidly from the illuminated vertical plane. Figure 3 (e) shows the Horiz1 image of the SCER horizontal symmetry plane. Upstream of the interaction region, only small amounts of jet fluid are transported into this plane by turbulent mixing in the mixing layer. Hence low PLIF signal levels are observed here. Downstream of the interaction region, the helium jets originating from the lower ports are bifurcated by the oblique shock wave and deflected upwards. They consequently intersect the illuminated plane and produce the stronger signals seen on the right of the figure. The Horiz2 image in Figure 3 (f) clearly shows the initial stages of the jet bifurcation before the jet material moves upwards and out of the illuminated plane. Before bifurcating, the jets spread rapidly taking up shapes reminiscent of that predicted for flow in the vicinity of a bursting vortex[5]. The Horiz3 and Horiz4 images in Figure 3 (g and h) show the bifurcated jets further downstream of the interaction region as their deflection by the shock carries them upwards. Clearly, the SCER jets show much stronger spreading and bifurcation than the plane base injector jet when acted upon by the oblique shock. This implies that, in the flows examined here, streamwise vorticity amplification and inter-

7 Simulated-fuel-jet/shock-wave interaction 7 action with the shock is a more powerful mixing enhancement mechanism than baroclinic torque. 5 Summary and Conclusions The interaction of a shock wave with helium jets issuing from a hypermixing fuel injector has been visualised using fluorescence imaging. The observed bifurcation of the jets is attributed to the effects of baroclinic torque and the shock-induced amplification of streamwise vorticity, particularly the latter. The jet displays behaviour consistent with the presence of a bubble shock standing out from the incident plane oblique shock. It has yet to be determined whether this feature is produced by vortex bursting or by variations in total pressure in the mixing flowfield. Further work is required to ascertain whether or not the streamwise vortices have burst. 6 Acknowledgment This work was supported by a Small ARC Grant No. F00134 funded through the Australian National University. The technical expertise provided by Mr Paul Walsh and Mr Paul Tant is gratefully acknowledged. References 1. J. S. Fox, A. F. P. Houwing, P. M. Danehy, M. J. Gaston, N. R. Mudford, S. L. Gai: Mole-Fraction-Sensitive Imaging of Hypermixing Shear Layers, J. Prop. Power 17(2) (2001) 2. M. J. Gaston, N. R. Mudford, A. F. P. Houwing: A Comparison of Two Hypermixing Fuel Injectors in a Supersonic Combustor, AIAA Aerospace Sciences Meeting, Reno, NV, AIAA Paper No (1998) 3. W. D. Hayes:, The vorticity jump across a gasdynamic discontinuity, J. Fluid Mech., 2, (1957) 4. F. E. Marble, E. E. Zukoski, J. W. Jacobs, G. J. Hendricks, I. A. Waitz:, Shock Enhancement and Control of Hypersonic Mixing and Combustion, AIAA/ASME/SAE/ASEE 26th Joint Propulsion Conference, Orlando, FL, AIAA Paper, (1990) 5. A. Nedungadi, M. J. Lewis:, Computational study of the flowfields associated with oblique shock/vortex interactions, AIAA Journal, 34 (12), (1996) 6. M. K. Smart and I. M. Kalkhoran, Effect of shock strength on oblique shockwave/vortex interaction, AIAA Journal, 33 (11), (1995) 7. I. A. Waitz, F. E. Marble, E. E. Zukoski:, Vorticity generation by contoured wall injectors, AIAA/SAE/ASME/ASEE 28th Joint Propulsion Conference and Exhibit, Nashville, TN, USA, AIAA Paper, (1992) 8. J. Yang, T. Kubota, E. E. Zukoski:, An Analytical and Computational Investigation of Shock-induced Vortical Flows, 30th Aerospace Sciences Meeting and Exhibit, Reno, NV, AIAA Paper, (1992)