THOR - Numerical investigation of passive concepts for leading edge cooling

Kurzfassung

Within the European FP7 research project THOR, two different passive cooling concepts have been studied for a generic leading edge made of a thermal protection material. A ceramic composite of standard carbon fibres in silicon carbide is used as a reference TPS material. The first concept focuses on the thermal conductivity of the TPS. The anisotropic conductivity of the composite material at high temperatures is increased by a factor of about 2 to 3 compared to the reference material by introducing carbon-based fibres with very high thermal conductivity. The second concept allows for radiative heat transfer inside the model by placing a cavity in the insulation material below the ceramic surface. The reference model, both conceptual modifications and their combination have been tested experimentally in DLR’s arc-heated facility L3K. In preparation of the thermally coupled numerical analysis of the air flow and structure response, an extensive sensitivity analysis of the flow field has been conducted with the DLR TAU code to find the major influence parameters for the prediction of the surface heat flux impinging on the generic leading edge model. Thermal non equilibrium and Argon as component of the air is negligible. The influences of the model placement in the wind tunnel and specification of the flow conditions are small taking into account the uncertainty of the respective measurements. In the symmetry plane of the model the three dimensional flow field near the leading edge is well approximated by a two dimensional analysis. Further downstream along the flat plate part of the TPS small deviations are observed. It is shown that the assumption about the catalytic behaviour of the TPS material dominates the predicted heat flow. The measured temperatures on the reference configuration (standard TPS and no cavity) are lying between the numerical results under the assumptions of a non catalytic and a fully catalytic surface. The temperatures in the stagnation region are well predicted under a non catalytic assumption whereas along the flat TPS a very high level of oxygen recombination has to be assumed to reach the experimental temperature level. The finite rate catalysis models for a silicon carbide available in TAU do not predict such behaviour. Nevertheless, a similar temperature level on the reference configuration is required for quantitative comparison with the experiments because the heat flux radiated from the TPS surface is strongly depended on the temperature (Stefan-Boltzmann law). Therefore, a tailored temperature depended catalytic behaviour is prescribed to conduct the numerical simulations of the passive concepts, which fits the experimental temperature level for the reference configuration with a peak temperature of about 1800K. The drop of the peak temperature observed in the experiments is well predicted by the coupled numerical model for both passive concepts as well as for their combination. The heat additionally removed from the stagnation region increases the temperature level at the beginning of the flat plate directly downstream of the curved leading edge. The heat is partly removed from the model via increased radiation at the higher temperatures in that region. This effect is observed for all passive concepts in the experiments. The coupled numerical simulation predicts a similar size of the increased temperatures in case of the second concept (radiation in cavity) but a much smaller influence of the highly conductive fibres in that region. A possible reason for the deviation can be the unknown thermal conductivity of the composite material with highly conductive fibres at temperatures higher than 1200K.