S. Gulli

Characterization of Complex Porous Structures for Reusable Thermal Protection Systems: Porosity Measurements

This work is focused on the nonintrusive characterization of the local and average porosity of a prototype carbon–carbon nose, representative of a reusable thermal protection system based on transpiration cooling. A study based on the x-ray computed tomography scan of the specimen has been carried out with the purpose of defining the most important guidelines for the permeability tests, which are the minimum area to be probed with a hot-film anemometer and the correct distance of the mass flux sensor from the wall. The former has been calculated from the average porosity calculation, whereas the latter has been retrieved from the statistical analysis of the dimensions, and the distribution of the void structures inside the porous network coupled to the theory of fluid flow through perforated plates. Several longitudinal and transversal sectioning planes with respect to the symmetry axis of the carbon mask have been analyzed to calculate the internal porosity from the two-dimensional images, whereas the th...

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.

Investigation of Transpiration Cooling Effectiveness for Air- Breathing Hypersonic Vehicles

The thermal management of air-breathing vehicles presents formidable challenges. The high dynamic pressure flight trajectories, the necessity of reducing the aerodynamic drag, the extended flight duration time and the need for a reusable Thermal Protection System (TPS) are stringent requirements. The work presented in this paper is focused on transpiration cooling and investigates the effects of fluid injection into the hypersonic laminar boundary layers. In particular, a simulation model, which is composed of a coupled solution of Self-Similar Method (SSM) and a Difference-Differential Method (DDM), is introduced to study the transpiration cooling along a flat plate. The reduced order code is intended to assess the boundary layer characteristics and will serve as a research tool for the design and analysis of future experimental investigations in the UTA’s 2MW arc-heated facility that has been modified and is currently in use for TPS studies. The DDM solves a system of coupled algebraic and Partial Differential Equations (PDE) for the case of Pr=1 and Le=1. Self-Similar solutions are considered in order to compare the code results for the case without transpiration.

Variable transpiration cooling effectiveness in laminar and turbulent flows for hypersonic vehicles

Reusable thermal-protection systems with active cooling, such as transpiration, are among the promising technologies for thermal management of hypersonic vehicles designed as practical, long-range transportation systems. This paper numerically investigates the effectiveness and efficiency of a variable-velocity transpiration technology for fully laminar and fully turbulent hypersonic flows over a two-dimensional blunt leading-edge geometry. For both flow types, variable transpiration based on a sawtooth velocity distribution is compared to a uniform-velocity transpiration approach. An equal amount of coolant has been imposed to compare the cooling effectiveness between two strategies. The results numerically demonstrate the significant reduction in stagnation-point heating and coolant mass savings achievable with the variable-transpiration strategy, which is observed both in laminar and turbulent flows. The transpiration cooling efficiency is shown to be higher in laminar flow compared to turbulent flow d...

Characterization of Complex Porous Structures for Reusable Thermal Protection Systems: Effective-Permeability Measurements

Reusable thermal protection systems are one of the key technologies that have to be improved to enable long-duration hypersonic flights. Transpiration cooling has been demonstrated to be one of the most promising active cooling techniques in terms of coolant mass requirements and disturbance of the external flow. Previous numerical studies, conducted by the authors on the conjugate boundary-layer/material-response analysis, along with the current manufacturing capability of manipulating the natural properties of porous materials (e.g., porosity, permeability, and thermal conductivity) have demonstrated the cooling potential when variable transpiration is considered. In this work, a methodology for the nonintrusive characterization of the local effective permeability of a complex carbon–carbon porous structure is proposed. The concept of effective permeability, conceived as the local blowing capability of a porous structure with respect to a selected coolant fluid, is also discussed. Specifically, the cool...

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...

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.

Variable Transpiration Cooling: A New Solution for the Thermal Management of Hypersonic Vehicles

The overall aerodynamic performance of every flying vehicle is strongly dependent on near-wall effects. In hypersonic flows, the viscous effects near the wall have an even greater importance from the standpoint of thermal loads (i.e., heat flux and temperature distributions) and aerodynamic performance (i.e. L/D). Based on these considerations, it is important to understand the physics that characterize the boundary layer and its interaction with the vehicles surface to simulate its behavior for different surface parameters such as the type of material, surface manufacturing, surface coating, wall geometry, mass exchanges, etc. The work presented in this paper is focused on the mass exchanges at the surface, and investigates the cooling effectiveness of variable fluid injection into the hypersonic laminar boundary layer on a flat plate. A reduced order model that captures the relevant physics has been developed and implemented in a code that solves the Navier-Stokes equations written for stationary, no-reacting hypersonic boundary layer neglecting the radiative thermal exchange. The code uses a coupled solution of Self-Similar Method (SSM) and DifferenceDifferential Method (DDM) for a flat plate in the case of Pr=1 and Le=1. The variable transpiration is obtained choosing selected distributions for the coolant (air) velocity at the wall. The analysis of the minimization of the wall heat flux and of the coolant’s mass flow rate is performed. The comparison between the computationally inexpensive reduced order code and the CFD code GASP shows similar qualitative and quantitative results on the heat fluxes and shear stresses prediction.

Improvements on Infrared-Camera Calibration for Accurate Surface Temperature Measurements in Arcjet Facilities

Ground testing of thermal protection systems play an essential role for understanding the response of candidate materials for thermal shields. Nonintrusive surface temperature measurements are necessary to map the temperature distribution for the correct interpretation of the results obtained in wind-tunnel facilities. An accurate calibration of the infrared camera to be used for a planned experimental campaign on variable transpiration cooling is described in detail. A prototype carbon–carbon cone has been used for both the calibration procedure and experiments. For this application, standard calibration procedures, based on the combined use of blackbody radiators for the calibration of the camera sensor together with tests for emissivity characterization, would not allow for the determination of the accurate surface temperature distribution. This is due to the diversified surface finish and localized defect characteristics of a typical component. The calibration procedure described here, which is based ...