Characterization of a Dual-Mode Scramjet Combustor via Stereoscopic Particle Image Velocimetry

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1 Characterization of a Dual-Mode Scramjet Combustor via Stereoscopic Particle Image Velocimetry Author: Brian Advisor: Chris Goyne and Jim McDaniel University of Virginia Abstract The following research paper presents an up-to-date status of the testing of a Dual-Mode Scramjet (DMSJ) model which is experimentally operated at UVa s Aerospace Research Laboratory. The optical technique Stereoscopic Particle Image Velocimetry (SPIV) is applied to the combustor of the DMSJ with a ramp fuel injector configuration. SPIV is utilized to measure the three-dimensional, combusting, turbulent flow field. In particular quantities such as instantaneous velocity, average velocity, vorticity, turbulence intensity, turbulent kinetic energy and Reynolds Stresses can be calculated. Measurements were conducted at two planes in the combustor downstream of fuel injection and at the exit plane of the DMSJ. Two different fuel conditions were chosen such that measurements are taken while the DMSJ is operating in scramjet and ramjet mode with the goal of comparing both modes of operation. These measurements will provide insight into the complex flowfield which will allow improvements to DMSJ computational tools. Introduction Dual-Mode Scramjets (DMSJ) are of interest for hypersonic flight vehicles for high speed, long-distance strike or two-stage access to orbit. As part of NASA s Aeronautics Research Mission Directorate, the Reusable Airbreathing Launch Vehicle (RALV) project, seeks to enable sustained hypersonic flight through the earth s atmosphere. 1 The proposed research supports the mission directorate goal through investigation of the fundamental physics of supersonic combustion in a DMSJ. Advances in this supersonic combustion research will lead to propulsion technology for Two- Stage-To-Orbit Turbine-Based Combined Cycle (TSTO/TBCC) vehicles. A DMSJ combines the capabilities of both ramjet and scramjet engines and enables the propulsion system to operate at peak efficiency across a broad range of Mach numbers. The principal advantage of a DMSJ is that there is no need to carry large oxidizer tanks like those of traditional rockets and this offers a decrease in payload fraction. In addition, the DMSJ has the capability of operating over a wide range of Mach numbers from 3-20 with a fixed geometry flow path. At speeds in the Mach number range 3-5 the DMSJ operates in ramjet mode which is characterized by subsonic flow through the combustor. At speeds above Mach 5, the DMSJ operates in scramjet mode and flow through the combustor remains supersonic. Transitioning to scramjet mode above Mach 5 is advantageous to avoid excessive pressure rise and performance losses due to a normal shock wave system. The DMSJ does not produce static thrust so it is necessary to utilize a combined cycle such as the turbine-based combined cycle (TBCC). In the TBCC, the turbine jet engine powers the flight Figure 1. TBCC Concept 2 vehicle to Mach 3-4 at which point the vehicle transitions to the high-speed flow path DMSJ. The features of the TBCC concept and flow paths are illustrated in Figure 1. 1

2 and extender sections as the DMSJ in Figure 1. The combustor being tested features a 10 degree unwept ramp with hydrogen fuel injected at the base. This unique facility is an electrically-heated, continuous flow, direct-connect tunnel which is capable of simulating flight Mach numbers to 5. Since the facility is electrically heated, the flow is free of any contaminates such as water or carbon dioxide. The continuous flow capability allows unlimited duration testing but typical run times are on the order of hours with steady state heating and fuel conditions. Long duration testing enables a comprehensive set of measurements to be taken. The facility flow conditions are presented in Table 1. Table 1. Facility Test Conditions Figure 2. UVA Supersonic Combustion Tunnel As presented in Figures 1 and 2, a DMSJ is comprised of a constant area isolator section which serves to prevent combustor-inlet interaction and provide an area for gradual pressure rise. If the pressure rise due to combustion is high enough, a normal or oblique shock train develops in the isolator. The location and control of the shock train is of main concern because if it reaches the inlet, unstart conditions develop. Unstart conditions prevent airflow necessary to the engine for thrust and shocks in the inlet reduces the total pressure required for performance. Downstream of the isolator is the combustor which typically injects hydrogen or a hydrocarbon via a ramp fuel injector or cavity flameholder which both act to mix the fuel with air. Finally the combusted flow expands through a nozzle providing thrust to the aircraft. Experimental Facility and Flow Conditions The DMSJ flow path is tested experimentally in a Mach 2 Supersonic Combustion Tunnel at the University of Virginia s National Center for Combined Cycle Hypersonic Propulsion. As shown in Figure 2, the supersonic combustion tunnel is constructed of the same isolator, combustor, Parameter Air Fuel Error Equivalence ratio % Total pressure (kpa) % Total temperature (K) % Mach number* Static pressure* (kpa) Static temperature* (K) *Property at nozzle exit determined using nozzle areas and isentropic flow assumption Research Objectives The primary objectives of my PhD research are to conduct DMSJ experiments to advance the understanding of dual-mode transition and supersonic combustion flow regimes. This will be accomplished in the present research study by: 1) Designing new test sections that a. Enhance the optical access in the isolator and combustor section of the DMSJ in order to allow more advanced laser diagnostics. b. Employ a modular fuel injection wall for both ramp fuel injector and cavity flame holder flow-paths. 2) Utilizing advanced laser diagnostics, such as Stereoscopic Particle Image Velocimetry (SPIV), to further understand the flow physics in the combustor of a DMSJ which will: a. Provide reacting flow turbulence statistics and the advancement of 2

3 fuel-air mixing and flame holding techniques b. Enable performance improvements and control of mode-transition 3) Providing a comprehensive benchmark dataset for the development and validation of DMSJ computational models and comparison to the Hy-V flight experiment. Hardware Design A key goal for the NCHCCP is to apply a comprehensive suite of laser diagnostic techniques to the DMSJ in order to measure necessary flow properties. Therefore, the main objectives in the design of the test-section hardware were to achieve a modular combustor and to provide excellent optical access. Modularity is required to support the various diagnostic techniques and to enable different fuelinjection schemes such as a ramp fuel injector or a cavity flame-holder. Various laser diagnostics (SPIV, PLIF, focused schlieren, and TDLAS/T) require wallto-wall optical access with maximum access in the flow direction. TDLAS/T also requires an accommodation for wedged windows while the other techniques use flat windows with parallel faces. The CARS (coherent anti-stokes Raman scattering) technique requires specially designed walls featuring small slots and a stand-off window mount to prevent failing the windows with the high intensity laser beams. For hydrogen fueled tests, an unswept ramp fuel injector has typically been used in this facility. However, hydrocarbon fuels require longer residence times for combustion and therefore a cavity flame holder will replace the unswept ramp for that portion of NCHCCP testing. The modular design objective is achieved with a cage support structure that can accommodate any number of potential flowpath walls. Figure 3 shows an exploded view of the combustor section. The cage support structure in the center houses the various walls that serve to form the flowpath. Each wall seals with an o-ring on the outside of the cage. Adjacent test-section components such as the isolator and extender attach to the top and bottom of the cage and are similarly sealed with o-rings. The cage construction approach allows particular walls to be replaced without disassembling the entire testsection. For instance CARS walls can be inserted in place of the large windows without removing the fuel injector or extender section. The window frames have been designed to minimize window cracking and frame obstructions while also allowing for easy cleaning. The frames accommodate flat or wedged windows and the glass is sealed with high temperature ceramic paper gaskets. In addition to optical access, the combustor and extender sections have been equipped with internal wall thermocouples, external wall thermocouples, low-frequency pressure taps, and high-frequency pressure taps. The location of the measurement instrumentation on the fuel injection wall is presented in Fig. 4. Figure 3. Combustor Exploded View Figure 4. Fuel Injection Wall Measurement Instrumentation All parts of the test section must be able to withstand high heat loads and maintain an air-tight seal. The combustor and extender sections feature extensive internal water cooling to minimize thermal distortions and protect the o-rings. In addition, the fuel injection wall in all sections has a thermal barrier zirconia coating. 3

4 Figure 5. Supersonic Combustion Tunnel Solid Model Isometric View Figure 5 shows an isometric view of the solid model for Configuration C. Large windows in the isolator, combustor, and TDLAT section provide excellent optical access throughout the flowpath. The unswept ramp fuel injector and diverging wall can be seen on the bottom wall in the combustor section. The hardware for initial investigation of Configuration C has been fabricated. Although windows will not be placed in the isolator or TDLAT sections during initial testing, it is expected that all flow diagnostics will eventually be applied to this flowpath configuration for both ramjet (subsonic) and scramjet (supersonic) modes of combustion. Experimental Approach The objectives of this research will be accomplished through the application of the experimental technique Stereoscopic Particle Image Velocimtry (SPIV). An illustration of the SPIV experimental setup on a solid model of the supersonic combustion tunnel is shown in Figure 6. The implementation of the SPIV technique to measure the 3D flow-field of a scramjet combustor was developed by Chad Smith. 3 First, the flow is seeded with small particles in order to track the flow. Two pulses from a Nd:YAG laser are converted to planar sheets with a small time delay Δt. The pulsed laser sheets illuminate the injected seeding particles within the measurement volume and the high-speed CCD cameras capture the particle shift Δx. Two camera views from different perspectives are able to measure all three components of velocity. A calibration procedure using a target of dots with precise diameter and spacing is necessary to determine the image-to-world mapping function. Computer correlation software uses the time and particle shift information to calculate velocity vectors at the measurement plane. This is performed by dividing the measurement area into smaller interrogation sub-regions. At each sub-region, a statistical cross-correlation method determines the most likely displacement of that group of particles. SPIV is particularly useful when applied to highspeed combustion because it is non-intrusive and it is used to make 3D spatially resolved, instantaneous and time averaged measurements. In addition, flow parameters such as turbulence intensity and vorticity can be determined. These measurements not only help to understand the complex flow structures but are used to compare against and validate computer models. A Spectra Physics PIV-400 dual-cavity Nd:YAG laser served as the light source and operates at 10 Hz. Typical experimental operating laser power ranged from mj/pulse depending on the optical setup. Two LaVision ImageProX2M CCD cameras were used for imaging. The cameras have a resolution of 1600x1200 pixels with a pixel size of 7.4x7.4 square microns. The CCD size is 12.2x9 mm 2. Nikon AF Micron-Nikkor lens with 105mm and 60mm focal lengths couple with the CCD cameras depending on the field of view and focal distance. Narrow bandpass filters centered at 532 nm and 10 nm width blocked all other wavelengths of light for the purpose of maximizing the signal-tonoise ratio. Figure 6 SPIV Experimental Setup 4

5 B C A thickness of 2.5 mm. This is achieved through the use of 4 quartz lens. Figure 8 shows the optical setup. The beam starts as a 6 mm circular spot and spreads at the first -19 mm cylindrical plano-concave lens. The second 150 mm cylindrical plano-concave lens is placed 131 mm away in order to create a constant width. The third 250 mm cylindrical plano-convex lens focuses the beam thickness down to 2.5 mm. The last -100 mm cylindrical plano-concave lens is placed 150 mm from the third in order to create a constant thickness. The precisely dimensioned sheet can now be delivered to the measurement area. Fine adjustment is available on all optics to ensure the sheet is delivered to the measurement plane with no vertical or horizontal tilt. Figure 7. Fuel Seeder Schematic The fuel seeder, shown in Figure 7, is a fluidized bed aerosol generator. They work by flowing gas through a bed of dry particles. The particles sit on the porous plug in which hydrogen gas flows through causing the particles to suspend and travel vertically in the fluidizer. Key features of the fuel seeder are the shearing nozzle and pickup tube. The shearing nozzle accelerates the hydrogen to Mach 3. The high speed flow causes a pressure gradient which serves to pull the particles up into the pickup tube. At this point the high speed flow impacts the particles with a shearing force causing breakup of particle agglomeration. Seen in Figure 7, path C is a hydrogen by-pass loop which was necessary at the high fuel mass flow rates. The needle valve is adjusted such that the proper fuel mass flow can be achieved. This is verified and measured at the outlet with a mass flow sensor. The flow rate through the fluidizer is controlled remotely via a metering valve. This allows for adjustment of seeding density during an experiment. The particles are spherical, 0.25 μm diameter silicon dioxide with a density of 1800 kg/m 3. There is a trade off with particle diameter between scattering characteristics and flow tracking. According to Melling, particle diameters less than 1 μm accurately track the flow and follow turbulent fluctuations on the order of 10 khz. 4 The laser is converted to a sheet with a constant width of approximately 30 mm and Figure 8. Optical Train Setup Figure 9 demonstrates how the laser is delivered from the PIV control room into the combustion tunnel room. The laser is delivered through a small hole in the wall and redirected with a series of mirrors until it is delivered through the optical train. From there the sheet is directed vertical and then horizontal along the measurement plane. Figure 9. Laser Delivery Path 5

6 Results Preliminary SPIV results for both fuel conditions at x/h=6 and x/h=12 are presented. In addition, a comparison of the SPIV velocity fields for both fuel conditions at the extender exit are compared to LES/RANS CFD results. A sample raw data set containing image pairs from both cameras is shown below in Fig 10. the freestream region of approximately 700 m/s. Despite a small low speed region the flow is predomanatly supersonic and the velocities contiue to increase in y which is outside of the measurement area. Alternatively, the flow field at x/h=6 and φ=.49, is characterized by a high speed jet in the center of the fuel plume surrounded by lower speed flow. The vorticies are again a predominant flow feature in the cross-plane. It is evident that the fuel plume has spread slightly further for the high equivalence ratio case. Figure 10. SPIV Raw Images The images have units of particle intensity and show a high seeding density and high signal to noise ratio. It is also evident that only the fuel plume is seeded and part of the tunnel area is not seeded. The image pairs from both cameras are necessary for a single 3- Component instantaneous velocity measurement. Average quantities are therefore calculated from a large number of those instantaneous velocity measurements. Initial studies on high-speed turbulent flows report that approximately 60 instantaneous vectors are needed for a converged average velocity measurement. That quantity is based on a 95% confidence interval assuming a 5% uncertainty on velocity and a turbulence intensity of 20%. 5 Using the same parameters, RMS velocity will be converged after 770 vectors. Based on these qualifications, the following 3C average velocity fields are presented in Figure The contours are colored by velocity magnitude with crossplane x and y velocity vectors in black. At x/h=6 and φ=.18 the fuel plume is relatively small and has not spread significantly. The effect of the vorticies is dominant and a kidney shaped velocity field is present. There is a low speed region close the the fuel injection wall with increasing velocity into Figure 11. x/h=6 φ=0.18 Velocity Magnitude Figure 12. x/h=6 φ=0.49 Velocity Magnitude At x/h=12 which is further downstream of the fuel injector, the fuel plume has obviously spread more than at x/h=6. The effect of the vortices on the velocity contour is less evident and the kidney shaped 6

7 distribution is no longer. This is most likely a result of the turbulent combustion process which has developed further along in the flowpath. There still exists a low speed flow in the region of the vortices but is approximately 4 times the magnitude at x/h=6. Again, the flow is predominantly supersonic with increasing velocity in y which transitions to the freestream velocity of approximately 800 m/s. At x/h=12 and φ=.49, the flow field is similar to that at x/h=6. The main difference is that the high-speed region has spread and moved away from the wall which is partly a result of the divergence. The high speed core resulting from the fuel jet has slowed compared to x/h=6 since the fluid has had more time to adjust to the surrounding flow features. At the exit of the extender, seeding of both the fuel and freestream air is possible due to the lack of windows. Freestream seeding is not possible in the combustor section due to particles coasting on the windows, which precludes imaging. As a result, velocity vectors can be calculated over the entire measurement area. The plots are colored by velocity magnitude. For the means of comparison, both SPIV measurements and CFD calculations are presented at the exit for both fuel conditions in Figures Figure 15. Extender Exit φ=0.18 SPIV Figure 13. x/h=12 φ=0.18 Velocity Magnitude Figure 16. Extender Exit φ=0.18 CFD 6 Figure 14. x/h=12 φ=0.49 Velocity Magnitude For the low phi case, φ=0.18, there is a horseshoe shaped velocity contour. The low speed region is again near the fuel injection wall and the effect of the vortices is still evident. Peak velocities in the freestream are approximately 600 m/s. The SPIV measured flowfield is qualitatively very similar to the CFD. The largest discrepancy is due to the asymmetry of the measured velocities. This is to be expected as no experimental facility is free from physical asymmetries in the flowpath. The CFD 7

8 slightly under predicts the size of the high-speed region in the freestream. Figure 17. Extender Exit φ=0.49 SPIV Figure 18. Extender Exit φ=0.49 CFD 6 For the high phi case, φ=0.49, the flowfield is characterized by a high-speed core of approximately 750 m/s surrounded by symmetrically decreasing velocities as the walls are approached. The influence of the ramp is less than for the low phi case but is still clear. The CFD over-predicts the size and magnitude of the high speed core. Again there are some slight asymmetries for the measured field but not as significant as the low phi case. Overall, the size, shape, and magnitude of the average velocity magnitude at the exit of the combustor for both fuel conditions agree very well. Importance /Significance The expansive list of advanced laser diagnostic would not be possible without the redesign of the UVa supersonic combustion tunnel. The hardware is unique and is the only dual-mode scramjet model capable of handling high temperatures, continuous testing, multiple fuel injection schemes, and full optical access in the combustor. Significant engineering work was necessary to accommodate all of the requirements such as modularity. A specific challenge was the ability to fit sufficient water cooling in the metal structures while maintaining optical access. In addition, the challenge of designing windows and frames which survive the thermal distortions has been a breakthrough. Initial testing of the hardware combined with the FEA model have enabled engineering design modifications which have proven successful through the continued success in operation. The successful design of this unique DMSJ will allow measurements of: static pressure and temperature, species concentration (N2, O2, H2, CO, CO2, H2O, hydrocarbons and OH (qualitative)), scalar correlations, three-component velocity, threecomponent turbulence intensity (RMS), density, Reynolds stresses, and mass and energy fluxes. 8 This will create the most comprehensive experimental database to date and serve as an integral tool for CFD modeling. Studies have been conducted on a subset of the current experimental hardware by Smith. These prior measurements demonstrate proof of concept and only report velocity fields of scramjet mode operating with no isolator. The current research will be the first to examine the complex flow inside a true dual-mode scramjet with an isolator. The addition of an isolator allows the DMSJ to operate in dual-mode and pushes a shock-train into the isolator. The shocktrain significantly changes combustion characteristics and SPIV measurements will help to determine these effects. The effects of heat release on fuel/air mixing and chemical kinetics is not well understood. SPIV implemented on a DMSJ is an innovative measurement technique and only one successful study has been reported. 3 The technique is difficult to apply due to the DMSJ high-speed flow, hightemperature environment, small field of view, laser 8

9 reflections, and particles coating on windows. Injecting particles into the freestream also presents a new problem to be solved because particles coat the windows and render them useless for SPIV measurements. Lastly, this research will support the UVa Hy-V Scramjet Flight Experiment. This experiment consists of a scramjet model mounted to a Terrier improved-orion sounding rocket. When the rocket has reached the desired Mach number, and before apogee, a shroud is opened which exposes the DMSJ flow path. The flight experiment is especially useful as a comparison to the ground test experiments with the same flow conditions. The flight experiment is heavily instrumented and the proposed DMSJ ground test measurements will help define the isolator and combustor. The flight experiment will encounter more realistic flow instabilities which are able to be controlled and tested in the wind tunnel facility. SPIV measurements in the wind tunnel will extend the knowledge of the flow conditions inside the Hy-V Scramjet. Overall, continued fundamental research and development is necessary to the success of scramjet technology and the viability of a RALV. 9

10 References 1) NASA Fundamental Aeronautics Program Hypersonics Highly Reliable Reusable System. NASA - Aeronautics Research Mission Directorate. Web. 03 Feb < 2) McDaniel, James et al. US National Center for Hypersonic Combined Cycle Propulsion: An Overview, AIAA Paper , ) Smith, Chad, Three-component Velocimtry in a Scramjet Combustor, AIAA Paper , ) Melling, A., Tracer Particles and Seeding for Particle Image Velocimetry. Measurement, Science and Technology, Vol.8 No.12, Dec. 1997, pp ) Kirik, Justin. An Investigation of a Scramjet Cavity Flameholder using Stereoscopic Particle Image Velocimetry. 6) Fulton et al., "Large-Eddy/Reynolds-averaged Navier-Stokes Simulation of a Dual-Mode Scramjet Combustor". AIAA Journal of Propulsion and Power. 7) Lin, Kuo-Cheng et al. Study on the Operability of Cavity Flameholders inside a Scramjet Combustor,: AIAA Paper , ) Rockwell, B, Et al. (2012). Close Collaborative Experimental and Computational Study of a Dual-Mode Scramjet Combustor. AIAA Aerospace Sciences Meeting. January 9-12, 2012, Nashville, TN. 9) Rockwell, B. Dual-Mode Regime Update. University of Virginia. Aerospace Research Laboratory, Charlottesville, VA. 2 May ) Rockwell, B. Window Frame Analysis. University of Virginia. Aerospace Research Laboratory, Charlottesville, VA. 7 December