Heat flux characterisation in hot jet and flame/wall interaction by IHCP resolution coupled with infrared measurements

Abstract

The aim of this work is to develop a method dedicated to the identification of heat fluxes between turbulent flames and 3D solids in order to improve the conception of aeronautic structures designed to endure and sustain fire events. Transient temperature modelling inside the impinged structure can only be realistic if the convective and radiative heat fluxes used as boundary conditions of the numerical model are accurately known. To do so, these two different heat transfer contributions are to be separated from the total heat flux information, obtained by solving an IHCP (Inverse Heat Conduction Problem). The inverse technique that was implemented for this study is based on adding back-face infrared measurements data as observation equation, thus closing this ill-posed problem. Infrared camera provides an accurate temperature map, allowing high resolution heat flux mapping without perturbing the impingement zone. The number of unknowns initially equal to the camera resolution is reduced by solving the problem in the cosine space thanks to Discrete Cosine Transform. As for the resolution of this multidimensional transient IHCP, Beck's inverse algorithm has been coupled to future time-steps stabilisation method. Flame to wall heat transfer knowledge is improved thanks to the study of a dedicated test bench composed of a propane / air burner on the one hand, and a 14kW air heater on the other hand. To take into account the spatial non-uniformity of the adiabatic wall temperature used in Nusselt calculation, a new adiabatic wall temperature computation is presented and compared to Nusselt calculations based on constant reference temperature. In the scope of this study, we present the comparison between propane/air flame and hot round isothermal air jet impinging perpendicularly on a titanium plate.