DESIGN AND TESTING OF A DUAL-MODE SCRAMJET FOR OPTICAL MEASUREMENT TECHNIQUES

Transcription

1 DESIGN AND TESTING OF A DUAL-MODE SCRAMJET FOR OPTICAL MEASUREMENT TECHNIQUES Author: Brian Advisor: Chris Goyne and Jim McDaniel University of Virginia Abstract The following research paper presents an up to date status of the design and testing of a Dual-Mode Scramjet model which is experimentally operated at UVa s Aerospace Research Laboratory. The demand for application of advanced optical measurement techniques has facilitated the complete re-design of all test sections. The details of the design, analysis, and capabilities of the DMSJ are the focus of the paper. Finite Element Analysis results are presented for thermal stresses of various parts which provided confidence in design modifications. Discussion of future work, in particular the application of the experimental Stereoscopic Particle Image Velocimetry (SPIV) technique, will be presented. Lastly, initial pressure and temperature measurements are reported. The successful operation of all test sections has allowed an ongoing effort to build a comprehensive experimental database. 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 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. Figure 1. TBCC Concept 2 1

2 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, and extender sections as the DMSJ in Figure 1. 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 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 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. 2

3 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. Figure 5. Supersonic Combustion Tunnel Solid Model Isometric View 3

4 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. Analysis Initial design of the test-section components was based on past experience and engineering best practices. However, a major advance in the design is the large windows that allow full optical access to the combustor and this unfortunately led to significant hardware failures. Upon initial testing of the combustor, the windows experienced high stresses and failed due to thermal deflection of the frames. Permanent thermal distortion of the extender section and thermal failure of the top o-ring was also encountered. This is the point of the process that the Finite Element Analysis (FEA) emerged as a critical tool. A very detailed finite element model of the entire testsection was constructed to help guide improvements and eliminate hardware failures. 7 Figure 6 and 7 show results of the FEA model for wall temperatures and thermal deformation. 8 Figure 6. Wall Temperatures 8 Figure 7. Thermal Deformation 8 Thermal boundary conditions from initial CFD studies were used in the FEA and the analysis was validated against the limited experimental thermal data available. These efforts revealed shortcomings in the initial design and suggested relatively inexpensive approaches to solve them. Figure 6 shows the wall temperatures for Configuration A and B. Configuration A suffered failing o-rings between the combustor and extender sections. Serpentine cooling loops were added to the combustor cage to provide additional protection to o-rings. There is a significant reduction of the wall temperatures for 4

5 configuration B which includes the cooling modifications. Figure 7 shows significant thermal distortion of the extender in which a gap of.03 in. is created between the combustor and extender. Thermal distortions of the extender was eliminated by adding interior cooling lines in locations identified as critical by the FEA. The cooling lines reduce the thermal distortions and closed the gap between the combustor and extender. Titanium was identified as a better material choice for the frames that were experiencing high thermal loads. The FEA showed that titanium frames, along with increased clearance around the glass, would reduce the window stresses significantly. 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 9. Double-pulsed Laser CCD Sensor Image frame 1 Δx Image frame 2 Particle Images Optics Train Laser Sheet Imaging Optics Measurement Volume Correlation Software Velocity Vectors Particle Seeded Flow Figure 9 SPIV Experimental Setup Figure 8 Window Equivalent Stresses 9 Figure 8 shows high stresses in the window due to the thermal deformation of the stainless steel window frame. The stresses on the window on the order of the rupture stress of the fused silica which is consistent with the failing windows upon operation. The titanium window frames reduce thermal deformations which puts the window in a lower state or stress below the rupture limit. Since implementation of these design modifications, the experimental effort has progressed quickly. Window and o-ring failures have been eliminated completely and design of additional hardware components has progressed with additional confidence provided by the FEA. 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 captures the particle shift Δx. Computer correlation software uses the time and particle shift information to calculate velocity vectors at the measurement plane. SPIV is particularly useful when applied to high-speed combustion because it is non-intrusive and it is used to make 3D spatially resolved, instantaneous and time averaged measurements. A preliminary SPIV vector measurement is shown in Figure 10, which shows the velocity vector field at the exit of the wind tunnel. The yellow colors indicate a higher velocity. 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. 5

6 Figure 12 Axial Wall Temperature Distribution 7 Figure 10 SPIV Vector Field Measurement 6 Results In addition to velocity measurements, high and low frequency pressure taps will measure the axial pressure distribution within the DMSJ. A typical averaged axial pressure distribution inside the UVA supersonic combustion tunnel is shown in Figure 11. Results are shown for the cases of fuel-off, mixing and reacting at φ=0.17, and reacting at φ=0.34. Axial distances are normalized by the normal height of the ramp and are referenced to the point of fuel injection while pressures are normalized by the pressure at the most upstream axial station at the exit of the facility nozzle (37 kpa). Figure 11 Axial Pressure Distribution 7 For the fuel-off case, the presence of the ramp is seen by the pressure spike at x/h=-6 which is a result of a shock wave formed during compression. The flow then expands at the base of the ramp which is followed by a series of reflected waves. At x/h=20 the pressure begins to rise due to the atmospheric backpressure. When fuel is injected, the mixing pressure distribution differs only slightly after the point of injection due to the presence of the fuel jet and change in flow properties due to fuel-air mixing. Fig. 12 shows a cooling effect of the hydrogen on the fuel injector insert. In addition, the rise in temperature at x/h=50 is where the fuel is ignited due to a termination shock. For the reacting case, the fuel is ignited which causes a significant rise in pressure starting at the base of the ramp and continuing throughout the scramjet flowpath. This pressure distribution indicates that the flow is in scram mode and the pressure rise is not present upstream of the ramp fuelinjector. The flow upstream of the ramp is supersonic and the flow downstream is a mixture of subsonic and supersonic flow. The pressure then rises monotonically to match the pressure at the exit. In Figure 12, wall pressures rise significantly downstream of fuel injection due to the presence of a flame. As the fuel equivalence ratio (φ) increases the pressure inside the DMSJ combustor increases. Additionally, Fig. 12 shows that the wall temperatures increase as the equivalence ratio increases which is expected. 6

7 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 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. 7 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 is 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 reflections, and particles coating on windows. Injecting particles into the free-stream also presents a new problem to be solved. 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. 7

8 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 , ) Le, D.B, Experimental Study of a Dual-Mode Scramjet Isolator, Journal of Propulsion and Power. Vol. 24, No. 5, September-October ) Lin, Kuo-Cheng et al. Study on the Operability of Cavity Flameholders inside a Scramjet Combustor,: AIAA Paper , ) Data acquired by Chad Smith and processed by Brian. Data processed with LaVison software. 7) 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. 8) 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