Increased surface temperature effects on wall heat transfer during unsteady flame quenching

Abstract

The coupling of thermal and chemical processes is significant during laminar flame quenching. A direct thermal response of a quenching flame is the heat transfer to the wall. The unsteady heat transfer during premixed laminar flame quenching was measured in a constant volume chamber over a range of wall temperatures from 298 K to 423 K and a range of equivalence ratios from 0.8<<1.2 for both methane and propane. For all of the measurements, the maximum heat flux could be correlated to the heat release rate in the steady flame prior to quenching. The fraction of the heat release rate attributed to heat transfer was independent of the equivalence ratio but dependent on the wall temperature. The experimental results were independent of buoyancy and catalytic effects and of whether the wall was locally heated or the entire bomb was globally heated. Calculations of the heat transfer were made for both one dimensional and two dimensional flame quenching using finite difference methods with chemistry specified as a single reaction step. Comparisons of the numerical results with the experimental data emphasized the sensitivity of the heat flux to the specifications of the reaction mechanism parameters. In particular, it was found that the single step mechanism and simplified chemical transport models could not predict the dependence of the wall heat transfer on the wall temperature. It was concluded that during quenching low activation energy recombination reactions may contribute significantly to the deviation between the single step thermal reaction mechanism and the observed experimental results.