Hypersonic Inlet for a Laser Powered Propulsion System

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1 Hypersonic Inlet for a Laser Powered Propulsion System Alan Harrland, Con Doolan, Vincent Wheatley, and Dave Froning Citation: AIP Conference Proceedings 1402, 145 (2011); doi: / View online: View Table of Contents: Published by the American Institute of Physics

2 Hypersonic Inlet for a Laser Powered Propulsion System Alan Harrland, Con Doolan, Vincent Wheatley and Dave Froning * School of Mechanical Engineering, The University of Adelaide, SA, Australia, 5005 Centre for Hypersonics, The University of Queensland, QLD, Australia, 4072 Abstract. Propulsion within the lightcraft concept is produced via laser induced detonation of an incoming hypersonic air stream. This process requires suitable engine configurations that offer good performance over all flight speeds and angles of attack to ensure the required thrust is maintained. Stream traced hypersonic inlets have demonstrated the required performance in conventional hydrocarbon fuelled scramjet engines, and has been applied to the laser powered lightcraft vehicle. This paper will outline the current methodology employed in the inlet design, with a particular focus on the performance of the lightcraft inlet at angles of attack. Fully threedimensional turbulent computational fluid dynamics simulations have been performed on a variety of inlet configurations. The performance of the lightcraft inlets have been evaluated at differing angles of attack. An idealized laser detonation simulation has also been performed to validate that the lightcraft inlet does not unstart during the laser powered propulsion cycle. Keywords: Hypersonic inlet, Stream traced inlet design methodology, Laser induced detonation wave. PACS: Kk INTRODUCTION The idea of Laser Propelled Lightcraft Vehicles was first conceptualised in the early 1970 s [1] as a means of achieving low cost earth to orbit payload launches. Laser Propelled Lightcraft Vehicles have continued to have been investigated by the Air Force Research Laboratory (AFRL) [2]. Power is transmitted to the Lightcraft from a ground based laser, and atmospheric air is utilised as the propellant during flight in the sensible atmosphere.when the Lightcraft exits this region of usable atmosphere, the laser is then used to heat an ablative fuel source providing anaerobic propulsion. Current conceptual and experimental designs of Lightcraft are explained in [2]. This design consists of three main sections; the forebody, the engine cowl and the afterbody, as indicated in Fig. 1. The conical forebody acts as both an aerodynamic shape for providing lift to the craft, and a supersonic ramp to compress the incoming air before it enters the engine cowl for laser induced detonation. The engine cowl acts as both an inlet and an impulsive thrust surface for the detonation wave. The parabolic afterbody acts as both a primary receptive optic for the incident laser beam, and an expansion nozzle for the heated exhaust flow. Current research into lightcraft inlet Beamed Energy Propulsion AIP Conf. Proc. 1402, (2011); doi: / American Institute of Physics /$

3 designs is somewhat limited, with the typical configurations being similar to that of the Lightcraft Technology Demonstrator concept [3]. This research aims to investigate and extend the work on hypersonic lightcraft inlets to provide a stable and efficient platform for the pulsed laser detonation engine. A stream traced inlet design methodology has been produced for the lightcraft concept, with numerical simulations performed to compare the performance of the new inlet design to traditional lightcraft inlets. Performance at on and off design Mach numbers and angles of attack have been investigated through three dimensional viscous numerical simulations. Future gun tunnel testing is also planned to verify the numerical results. FIGURE 1. Current lightcraft configuration INLET UNSTART Inlet unstart at supersonic speeds is a significant issue associated with fixed geometry inlet configurations. Unstart can typically be defined as a supersonic inlet operating in an abnormal state [4], and is characterised by an abrupt decrease in captured mass flow rate and total pressure recovery. Severe increases in aerodynamic and thermodynamic loads due to violent shock system oscillations and prominent pressure fluctuations also occur. These processes have highly detrimental effects on the thrust generated by the engine, and may cause catastrophic damage to the craft [5]. Hypersonic inlets are designed to operate always in a started state, however this state is typically highly sensitive, and a number of factors can lead to unstarted conditions in inlet operation. Unavoidable physical processes within the hypersonic inlet flowfields, such as strong-shock/boundary layer interactions, over compression from low operating Mach numbers, changes in angles of attack or even small perturbations in atmospheric conditions lead to a situation where inlet unstart is often difficult to avoid. To maintain the lightcraft performance at a range of flight Mach numbers, altitudes and vehicle angles of attack, a fixed geometry inlet that does not unstart is required. It must also be ensured that if unstart does occur, the inlet can be successfully self-starting. It is also essential that the inlet can withstand external influences from the pulsed laser detonation propulsion system. 146

4 INLET DESIGN METHODOLOGY The stream traced inlet design methodology is a technique used to design hypersonic inlets using fictitious flow turning bodies to produce a suitable compressible flow field. This technique is traditionally applied to conventionally fuelled scramjet engines [7], but has been modified to suit the requirements of lightcraft. With the stream traced inlet design methodology, specific desired inlet design conditions in the engine can be stipulated, and an inlet geometry generated to suit. The stream traced methodology can be applied, in its simplest form, from two dimensional planar inlets [8] to complicated three dimensional geometries [9]. This design methodology has been chosen for the lightcraft design due to the performance of the resulting inlets at off-design conditions [10]. FIGURE 2. Tracing of streamlines through a cross section of the axi-symmetric generating flow field, overlaid on contours of Mach number In the stream traced inlet design methodology, the designer begins by specifying the desired flow conditions they wish to be present in the inlet isolator of the vehicle. For a scramjet engine, the designer will choose a desired pressure within the combustor at a certain inlet entrance Mach number. With the lightcraft design it is required that air (the working gas) be delivered to the laser induced detonation process at specific densities, as this determines the initial pressure of the laser induced detonation wave, and hence the generated thrust [6]. The inlet entrance conditions are dependent on the flight conditions of the lightcraft, and also the forebody geometry. As this study focuses on the stream traced portion of the inlet, we assume a simple conical forebody. If lower drag is required, this could be replaced by a more complicated forebody geometry. The free stream flow conditions for the lightcraft have been determined from a trajectory study (based on [11]), defining the flow properties entering the generating flow field for the inlet design conditions. The initial compression due to the forebody alters the inlet entrance conditions from the free stream conditions, and must be accounted for in the inlet design. Once these design parameters have been established, a fictious generating flow field is created to perform the required compression of the inlet flow. For three dimensional inlet designs (including axi-symmetric inlets) inward turning, internal axi- 147

5 symmetric flow fields with conical compression following a conical shock created by the inlet lip of the generating flow field are used due to the inherent isentropic compression [12]. A reflected conical shock from the centreline then produces a near uniform flow that is parallel to the free stream, as shown in Fig. 2. The inlet geometry is then created by choosing any stream surface of the generating flow field as the solid inlet wall. The desired generating shape is typically defined by selecting a capture area either upstream of the compression shock or at the end of the compression field. The streamlines that pass through this defined capture area are then followed downstream to the end of the compression field, defining the inlet shape. The streamlines used to generate the lightcraft inlet are shown in Fig. 2 with conical forebody and parabolic afterbody attached to demonstrate how the stream stracing methodology is applied to the lightcraft problem. The streamlines that produced the required inlet cowl shock cancellation at the expansion into the isolator were used to form the inviscid inlet geometry. A viscous correction was applied to allow for the growing boundary layer within the inlet, using a flat plate analytical approximation [13]. If unaccounted for, a growing boundary layer will reduce the cross sectional area available to the flow, thereby increasing the pressure rise beyond the intended design value. The final lightcraft configuration with stream traced inlet is shown in Fig. 3. FIGURE 3. Final lightcraft geometry with stream traced inlet The stream traced inlet design methodology is manually intensive, and thus requires automation. A script (streamtracer) has been written that, via the input of the generating flow field velocity field and the capture area geometry, produces three dimensional viscous-corrected stream traced inlet geometries. It is then a matter of meshing and simulating the resulting geometry in any CFD code. The inter-interoperability of the streamtracer code, meshing software and CFD software allows rapid generation of inlet designs. Minor modifications can be performed in a matter of minutes, ensuring a highly efficient design process. INLET NUMERICAL SIMULATIONS To establish the design conditions for the lightcraft inlet, a trajectory study was performed [14]. The conditions used for the inlet design were chosen to be at the point where aerobic propulsion system transitions to the ablative anaerobic system, i.e. at the 148

6 TABLE 1. Flight conditions simulated in numerical study Flight Mach No. 5 8 Altitude (km) Pressure (Pa) Temperature (K) Air density (kg/m 3 ) greatest air breathing velocity. This occurred at a flight Mach number of 8, and an altitude of 35 km. Further to these conditions, it was desirable to test the inlet s performance at off design values of Mach number and angle of attack. Therefore simulations were performed at angles of attack of 0, 3 and 6 for flight Mach numbers of 5 and 8. The lower Mach number simulations were to validate the inlet would be able to operate over an accelerating portion of the trajectory. The conditions simulated can be seen in Tab. 1. FIGURE 4. Inlet flow field contours of density at 0 angle of attack for a flight Mach number of 8 The final inlet design flow field is shown in Fig. 4. This image highlights the curved shock off the cowl lip coming to rest on the expansion of the conical forebody into the isolator. Fig. 4 also shows the conical forebody shockwave misses the cowl lip. This was intentionally done to reduce the cowl lip heating. A relatively weak expansion wave can be seen at the beginning of the isolator, which proceeds to propagate down the length of the inlet. It is difficult to obtain a completely cancelled shock structure within the inlet [10], and the weakness of the expansion wave is considered a very good result. The images shown in Fig. 5 show contours of Mach number for the axi-symmetric stream traced inlet at angles of attack of 0, 3 and 6. These highlight the weakening of the shock of the leeward side, with the converse strengthening of the windward shock. A very promising result from these numerical simulations is the absence of unstart at non-trivial angles of attack. There is negligible difference in the strength of the shock train in the isolator as the angle of attack increases, indicating boundary layer separation due to shock interaction is unlikely. The oblique shock strengthens within the isolator as angle of attack increases, as shown in Figure 6. This shock interacts with the boundary layer but does not appear to affect the operation of the inlet at the conditions simulated. 149

7 FIGURE 5. Contours of Mach for stream traced axi-symmetric inlet at 0 (top), 3 and 6 (bottom) angle of attack 150

8 FIGURE 6. Density contours on the outer wall of the isolator at 0 (top) and 6 (bottom) angle of attack for a flight Mach number of 8 FIGURE 7. Inlet flow field contours of density at 6 angle of attack for a flight Mach number of 8 151

9 FIGURE 8. Contours of density at exit of isolator for 0 (top), 3 and 6 (bottom) angle of attack for a flight Mach number of 8 152

10 The inlet flow field can be seen in Fig. 7 for the 6 angle of attack case. Flow uniformity at the exit of the isolator is also good, with no strong discontinuities in the density profile. The density profiles at the exit of the isolator are shown in Fig. 8. At 6 angle of attack, there is an approximate difference in density of 50œ across the exit of the isolator which relates to a reduction in the initial pressure of the laser induced detonation wave of 20œ between the most windward and leeward positions of the inlet. This may present control issues from an increased turning moment acting due to the imbalance of incoming air compression. This may be exacerbated by an asymmetrical laser induced detonation wave profile, of which the effects would require further attention. The Mach 5 inlet simulations have also produced some interesting results. It appears from the simulations that the stream traced axi-symmetric inlet can operate sufficiently well at lower Mach numbers, avoiding inlet unstart. Figure 9 shows contours of Mach number for angles of attack of 0, 3 and 6 at the Mach 5 flight conditions. LASER INDUCED DETONATION WAVE SIMULATIONS Although the primary focus of this research is concerned with the design of the hypersonic inlet, the lightcraft propulsion system is highly complex and an integrated approach to all aspects of operation is required. The purpose of the hypersonic inlet is to deliver air to the laser detonation process at optimal conditions; the performance of this process is determined by the quality of the inlet design. However the performance of hypersonic inlet is highly dependent on the laser detonation process; the two facets of the lightcraft vehicle are intrinsically linked and must be considered as a whole. Our concern with the laser detonation process is the effect that the resulting detonation wave structure, and propagation, will have on the incoming compressed air flow. There is a risk of the detonation wave propagating back through the isolator and disrupting the hypersonic inlet flow field. This will in turn restrict the refreshment of the air being provided to the laser detonation process. It is essential to ensure that the design of the inlet is robust enough to adequately refresh the isolator after each laser pulse cycle. Hypersonic shock tunnel experiments with laser induced detonation waves [15] have shown that the detonation waves interact with the inlet flow field of traditional inlet designs. The inlets unstart due to the expanding detonation wave. In these experiments the laser deposition energy was significantly lower than those required for practical hypersonic flight, and it can be reasonably expected at flight laser powers the effects of the detonation wave on the inlet flow field will be drastically increased. To investigate the inlet sensitivity to the laser detonation process, an idealised numerical model of the laser induced detonation wave present in the lightcraft has been produced. This model does not take into account real gas effects, such as the dissociation, ionisation and recombination of the resulting plasma generation - rather a simple two dimensional model of the relaxing of the high pressure laser induced detonation wave is applied to the specific lightcraft geometry. Experimental evidence has shown that the time scale of plasma formation is significantly less than that of the detonation wave formation [16], it is therefore deemed acceptable to assume the plasma is formed instantaneously, having little effect on the surrounding fluid. Based on the work outlined by [6], the initial properties of the blast wave created by each laser pulse cycle can be 153

11 FIGURE 9. number of 5 Contours of Mach number for 0 (top), 3 and 6 (bottom) angle of attack for a flight Mach determined. These values are then patched into a region of cells in a CFD simulation representing the initial state of the laser induced detonation wave. A two-dimensional axi-symmetric turbulent transient simulation was performed to record the time depen- 154

12 TABLE 2. Laser induced detonation wave initial conditions Initial radius 5 mm Pressure 1,364,000 Pa Velocity 10,320 m/s Temperature 18,000 K dent flow history of the detonation wave. The initial inlet flow field was employed to represent the conditions present during cruise flight conditions at zero degrees angle of attack, Mach number of 8 and no laser induced detonation. The values listed in Tab. 2 were then patched into a cell zone of radius 5 mm representing the completely cylindrically evolved laser induced detonation wave. Figure 10 shows the transient progression of the modelled detonation wave as it expands and relaxes over the lightcraft afterbody. The high pressure region due to the laser induced detonation wave can also be seen to move up into the inlet isolator, where it remains until it is exhausted by the inlet flow. The detonation wave must remain in the isolator for a period less than the laser pulse cycle, as it presents a blockage to the incoming air flow. For the current inlet configuration, it takes approximately 1.5x10 4 seconds for the initial inlet flowfield to re-establish. The final design of lightcraft would require an optimisation study to minimise this time while maintaining sufficient compression and robust inlet operation. Another promising aspect emerging from these simulations is that the hypersonic inlets successfully restart between laser pulses. It is imperative this occurs, otherwise the inlet flow field will not be re-established between pulses. This would be highly detrimental to the laser induced detonation process. CONCLUSION The results of a three dimensional stream traced axi-symmetric inlet design study show that a stable inlet configuration for the lightcraft platform is possible. The inlet remains in a started state at both off design Mach numbers and non-trivial angles of attack, while achieving significantly higher levels of compression than certain traditional lightcraft inlet designs. The inlet also behaves particularly well when subject to a modelled laser induced detonation wave employed in the propulsion system. The inlet remains in started operation throughout the modelled detonation wave cycle, and is able to successfully refresh the working gas after each laser pulse. ACKNOWLEDGMENTS The authors would like to thank the Asian Office for Aerospace Research and Development for its continued support of this research. 155

13 FIGURE 10. Contours of pressure for modelled detonation wave evolution at 1x10 5 second increments for a flight Mach number of 8 and 0 angle of attack 156

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