Luca Maddalena

Integrated Analysis for the Design of Tps Based on Variable Transpiration Cooling for Hypersonic Cruise Vehicles

The thermal management of hypersonic air-breathing vehicles presents formidable challenges. Reusable thermal protection systems (TPS) are one of the key technologies that have to be improved in order to use hypersonic vehicles as practical, long-range transportation systems. Both the aerodynamic and the material performances are strongly related to the near-wall effects. The viscous dissipation within the hypersonic boundary layer, coupled with the high dynamic pressure flight trajectories, generates surface temperatures for which the strength and the environmental durability of the material can be widely exceeded. In this type of environment, active cooling systems have to be considered in order to afford long duration flights in hypersonic regime. Transpiration cooling represents a promising technique in terms of temperature reduction and coolant mass saving. In order to explore the potential of this technique, it is important to understand the physics that characterize the boundary layer and its interaction with the vehicle’s surface. The integrated analysis of the hypersonic boundary layer coupled with the thermal response of a porous medium is performed here for a flat plate and a 2-D blunt body configuration. A constant value of the transversal wall velocity is used to simulate uniform transpiration. A saw-tooth wall velocity distribution is used to simulate the variable transpiration strategy. An equal amount of coolant usage has been imposed in order to compare the cooling effectiveness in the two cases. The uniform transpiration allows a reduction of 49% on the stagnation point heat flux in comparison with the case without transpiration. The variable transpiration reduces the stagnation point heat flux by an additional 7% with respect to the uniform transpiration case. The heat fluxes derived from the solution of the hypersonic boundary layer as well as the imposed wall temperature are used to perform an integrated analysis that includes the porous material. The test cases analyzed emphasize the importance of evaluating the influence of the material’s thermo-physical properties at the initial design stage. For the flight conditions considered in this analysis a combination of low material porosity and high thermal conductivity are necessary to generate the required injection strategy. The integrated analysis is essential for the purpose of establishing the optimum transpiration strategy needed to maintain the surface temperatures in the required range. The change in the transpiration distribution along the vehicle surfaces (variable transpiration) allows to selectively cool down the structure in the regions where the higher heat fluxes are located (i.e. nose, leading edges) and diminishes the amount of required coolant fluid. The transpiration for the blunt body can be limited to the regions where the local wall heat flux is greater than or equal to approximately 20% of the stagnation point heat flux. This strategy allows the reduction of the total amount of coolant by 62% for the uniform transpiration and by 58% for the variable transpiration.

Integrated Analysis of Reusable Thermal Protection Systems Based on Variable-Transpiration Cooling

Reusable thermal protection systems are one of the key technologies that have to be improved to use hypersonic vehicles as practical, long-range, transportation systems. The proposed concept of variable-transpiration cooling is investigated in this work by coupling the hypersonic boundary-layer solution with the thermal response of a porous material. The simulations of the hypersonic boundary layers are obtained using an in-house-developed reduced-order model capable of handling generic injection velocity profiles at the porous wall and the high-fidelity computational-fluid-dynamics code Langley Aerothermodynamic Upwind Relaxation Algorithm. The material thermal response is included adopting a one-dimensional model for the porous medium. The integrated analysis is performed for a flat plate and a two-dimensional blunt-body configuration. A sawtooth wall velocity profile was chosen to represent the variable-transpiration strategy. The uniform transpiration on the blunt body allows for a reduction of the st...

Design and Application of Filtered Rayleigh Scattering Experiments for Mixing Studies of New Strut Injectors for Scramjets

The description of the setup and experimental methodology used to quantify the local mixing of a helium-air mixture with the non-intrusive laser based Filtered Rayleigh Scattering (FRS) technique in a Mach 2.5 vortical flow is presented. Of particular interest is the effect of the imposed vortex dynamics on the degree of mixedness of the two constituent mixture components in the region of the mutually interacting co-rotating vortices generated by a strut injector specifically designed for the study of multiple vortex interactions in supersonic flow. The application of the FRS technique in such highly complex vortical flows, however, is not straightforward. Thus, detailed preliminary experimental characterization of all subsystems of the measurement is required along with a methodical experimental design procedure in order to achieve consistently successful measurements that are meaningful and relevant. All aspects of the experimental design and methodology will be presented, beginning with the necessary theoretical background of the FRS procedure and concluding with the preliminary results and supporting evidence showing the proper design of the mixing experiments.

Teflon probing for the flow characterization of the 1.6MW arc-heated wind tunnel of the University of Texas at Arlington

From the beginning of space exploration, protecting the vehicle’s structural integrity from the high-enthalpy flow encountered during atmospheric reentry, has been a primary concern. Particularly in the last decade, the necessity to extend the flight duration in the hypersonic regime has prompted a continuous development of new heat-shields. In this scenario, the testing of Thermal Protection Systems (TPS) plays an essential role in understanding the thermal response of candidate TPS materials when prolonged exposures to highenthalpy flows are considered. In order to reproduce the aero-thermodynamic environment present in real flight conditions the Mach number, flight duration, altitude, surface temperature, gas-surface interaction effects are some of the parameters to be replicated. Existing ground testing facilities are not able to simultaneously reproduce all the aforementioned parameters. Furthermore, testing facilities introduce flow disturbances and contaminations (such as non-uniformities deriving from the arc stabilization techniques and debris due to the erosion of the material parts exposed to the hot flow within the test facility) with respect to the free-stream flow in real flight conditions. The flow characterization of the arc-heated wind tunnel of the University of Texas at Arlington is investigated in this work. The Teflon material has been selected for this analysis because of its well-documented ablation properties. The stagnation point heat flux retrieved by the analysis of the ablated surface is coupled with Pitot pressure measurements to calculate the total enthalpy. Several locations downstream of the nozzle exit have been surveyed to determine the values of the stagnation point heat flux. The analysis of the ablated surface of Teflon is used to identify possible flow non-uniformities.