Long duration air-breathing hypersonic flight vehicles experience very high temperatures primarily due to stagnation, shock waves and skin friction due to viscous dissipation. Extreme heat flux (of the order of 1-10MW/m2) is experienced by the combustor wall due to exothermic reaction involving heat addition to produce thrust. Substantial research has been done in the area of high temperature materials and thermal management techniques applicable to scramjet combustor. One of the concepts for advanced structural thermal protection system that promises the highest magnitude of thermal load management1, is an actively cooled thermal protection system. Considering this possibility and with the emerging multifunctional cellular materials, it is possible to tailor the structure in such a way that it can be used for cooling as well as load bearing member for the airframe with reduced mass.
The present study aims to study the effect of active cooling on the heat transfer characteristics of a representative scramjet combustor panel made of cellular structure. The configuration comprises of high temperature materials with ceramic coating with open cell configuration for cooling. An endothermic fuel which undergoes heat absorbing chemical reactions supported by energy extracted from heated air/surfaces is passed through cooling channels along the length of the panel under specified conditions. The effect of coolant flow on the heat transfer characteristics is observed. The comparison of actively cooled panel with flow and no-flow conditions will elucidate the enhancement in heat transfer effectiveness in long duration hypersonic flight conditions. A steady state thermal analysis is performed using analytical method and compared with numerical simulation.
An analytical model is developed which computes convective heat flux at the wall by reference temperature method and through-the-thickness temperature by energy balance. Empirical relations are used for determining heat transfer coefficient of the coolant. Critical parameters like temperature gradient and interface temperature are determined. Numerical analysis has been performed using finite element software package. The structure is modelled using finite element mesh having 10 node solid element. Thermal load in terms of convective heat flux along initial temperature condition and back wall temperature condition is specified and solved for nodal temperature and results are post-processed to obtain the temperature gradient and the interface temperature. Also, parametric variation has been carried out to determine the effect of coolant flow rates, cooling channel dimensions and panel thickness on cooling performance
It has been observed that the effective thermal conductivity (k), effective specific heat (Cp) and the heat transfer coefficient (h) of the coolant are the key parameters influencing the overall heat transfer characteristics. Active cooling is seen to increase heat transfer rates due to convective cooling which reduces the thermal gradient and the interface temperature allowing survivability of the structure without melting under the specified aero-thermal environment. Channel dimensions and the flow rates have significant influence on the heat transfer characteristics. Future work aims to optimize the panel mass with fully coupled aero-thermo-mechanical analysis, details of which will be discussed in the paper.
Keywords: Active cooling, scramjet, cellular materials, effective properties, high temperature materials.
1 H.N.Kelly and M.L.Blosser, Active cooling from the sixties to NASP, Current Technology for Thermal Protection Systems, NASA CP-3175:189-249, 1992
