Adiabatic surface temperature as thermal/structural parameter in fire modeling: Thermal analysis for different wall conductivities

The new useful concept of “Adiabatic Surface Temperature” or more commonly known as AST, introduced by Wickström et al. in 2007, is investigated in this study. Adiabatic surface temperature can be used for bridging the gap between fire models and temperature models; for example, it offers the opportunity to transfer both thermal information of the gas and the net heat flux to the solid phase model, obtained by CFD analysis. In this study two numerical analyses are carried out in order to evaluate the effect of wall thermal conductivity and of convective heat transfer coefficient on the adiabatic surface temperature as thermal/ structural parameter in fire modeling. First one CFD analysis simulating a fire scenario, “conjugate heat transfer”, with a square beam exposed to hot surface, is carried out to calculate AST, convective heat transfer coefficient and temperature field in the beam. In the second one, a conductive analysis is carried out on “standalone beam” imposing a third type boundary condition on its boundaries assuming the AST, evaluated in the conjugate analysis, as external temperature. Different convective heat transfer coefficients are imposed on the beam walls; the beam is of concrete or steel. Results are presented in terms of net heat flux on beam surfaces, convective heat transfer coefficients and temperature profiles on the beam walls, temperature fields for the two, CFD and conductive, analyses and the relative temperature and net heat flux percent errors. Results underline that convective heat transfer coefficient profiles and adiabatic surface temperatures on the bottom and lateral beam walls are independent of the wall thermal conductivity value, whereas the net heat flux values increase as wall thermal conductivity increases, fixed the emissivity.