Experimental and numerical study of thermocouple-induced perturbations of the methane flame structure

In flame temperature measurements by a thermocouple, it is usually assumed that, due to its small size, the thermocouple produces negligible perturbations of the flame structure. Our studies show, however, that this assumption may be incorrect. The temperature of a premixed atmospheric methane/oxygen/argon flame measured by several thermocouples was found to be systematically higher than the theoretical temperature at small distances from the burner (in the region with a high temperature gradient). The external flow of the flame over a thermocouple was simulated using the full set of unsteady Navier–Stokes equations to explain the discrepancy between experimental and theoretical data. An approximate allowance for the heat release due to chemical reactions was made by adding a source term to the energy equation to provide a given temperature distribution in the unperturbed isobaric flame. The observed discrepancy was found to be related to deceleration of the flow in the vicinity of the thermocouple, resulting in additional heat release due to chemical reactions in the flow. In addition, significant additional heating of the thermocouple was observed, when it was placed in the zone with maximum concentrations of H and OH radicals.