Heat and mass transfer considerations in the use of electrically heated thermocouples of iridium versus an iridium rhodium alloy in atmospheric pressure flames

Some thin (diameters 51, 25 and 13 μm) thermocouples of iridium and an alloy of 60% iridium 40% rhodium have been used with and without electrical heating to measure temperatures in flat laboratory flames of H₂, O₂ and N₂ up to 2400 K. It was found that they suffered from a decrease in diameter with use, which limited their lifetime. Otherwise, it proved possible to determine local temperatures to within 25 K. The emissivity of these thermocouples was measured from observations made in a vacuum and found to depend on temperature very much as predicted by electromagnetic theory. In addition, measurements were made of heat transfer coefficients between the flame and thermocouple and these agreed well with standard correlations. The variation of heat transfer coefficient with wire diameter was used to measure the thermal conductivity of the burnt gases of several flames. These experimental values agreed to within 10% with theoretical thermal conductivities computed from Chapman-Enskog theory. Radial temperature profiles revealed the extent to which these flames can be considered one-dimensional and free from edge effects. For instance, it appears that conditions on the axis of a fuel-rich H₂/O₂/N₂ flame burnt on a Padley-Sugden burner are free from edge effects for a distance of about two flame diameters downstream of the reaction zone. The axial temperature profiles demonstrated that, when uncoated, these thermocouples undergo catalytic heating, particularly in the reaction zones of fuel-rich flames, the extent of which is controlled by the rate of diffusion of hydrogen atoms to the surface of the thermocouple wires. Measurements of the magnitude of this catalytic heating indicated that the thermal accommodation coefficient (i.e., the fraction of the energy released by heterogeneous recombination of radicals which is retained by the surface) is equal to unity.