COMBUSTION AND FLAME 28,301-317 (1977) 301 Heat and Mass Transfer Considerations in the Use of Electrically Heated Thermocouples of Iridium versus an Iridium/Rhodium Alloy in Atmospheric Pressure Flames A. N. HAYHURST* and D. B. KITTELSONt Department of Chemical Engineering, Cambridge University, Cambridge, England Some thin (diameters 51, 25 and 13 /~m) 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, 02 and N 2 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 thermo- couples 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. Measure- ments 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. INTRODUCTION This paper describes the use of thermocouples of iridium versus an alloy of 60% iridium and 40% rhodium for temperature measurements in lab- oratory flames of hydrogen, oxygen and nitrogen with temperatures up to 2400 K. It was originally hoped that these measurements would be suffi- * Present address: Department of Chemical Engineering and Fuel Technology, Sheffield University, Mappin Street, Sheffield, SI 3JD, U.K. t Present address: Department of Mechanical Engineering, University of Minnesota, Minneapolis, Minn. 55455, U.S.A. ciently accurate to replace the sodium D-line technique for determining the temperatures of flames used in some ionization studies [1]. How- ever, catalytic heating of the thermocouples made this somewhat impossible, particularly in the regions of interest near the reaction zone of a flame. Despite this, the measurements made are of interest for several reasons. First, they illustrate some limitations of Ir versus 60% Ir/Rh thermo- couples, which are the only ones currently avail- able for use in oxidizing atmospheres in the 1800-2400 K temperature range. Second, the experimental method uses electrical heating of the thermocouple to compensate for radiative Copyright © 1977 by The Combustion Institute Published by Elsevier North-Holland, Inc.