Thermochemical heat release of laminar stagnation flames of fuel and oxygen

Heat transfer is a complex phenomenon that can involve conduction, convection, radiation, condensation, and boiling. In the case of heat transfer by flames produced by pure oxygen or oxygen enriched air combustion, a mechanism called thermochemical heat release (TCHR) can be held responsible for up to 60% of the total heat transfer rate. In these very hot flames chemical equilibrium is reached before full conversion into products is achieved. TCHR is the result of recombination reactions in the thermal boundary layer. In this paper a method is described for the numerical calculation of the effect of TCHR which can be applied to model TCHR for fuels of an almost arbitrarily complex composition. In this method the flame chemistry is decoupled from the chemistry in the thermal boundary layer. An equilibrium calculation is used to determine the chemical composition after the flame. This mixture is then used as input for the stagnation layer calculations, for which a simple CH4 mechanism suffices. It is shown under which conditions this method can be applied, the effect of strain rate is studied, and the method is demonstrated by calculating a TCHR multiplication factor for a number of different fuels. A polynomial fit for the TCHR-factor is presented as function of C/H-ratio, equivalence ratio, equivalent temperature of a reference mixture and stagnation plane temperature. The fit gives accurate results for the TCHR contribution to the total heat transfer for most fuels. Finally, the importance of hydrogen recombination chemistry on the TCHR is indicated.