Emissivity Influence on Thermocouple Correction

Abstract

The reading of a thermocouple is not equal to the gas temperature because of the complicated heat transfer process among the thermocouple, the gas, and the environment. So the corrections for thermocouple measurements are necessary to obtain the real gas temperature. The current correction methods using multiple thermocouples assume that the thermocouples have the same surface emissivity. However, the emissivity of a thermocouple depends on its surface condition, and the emissivity of the thermocouples normally are different. CFD simulations are carried out to study the influence of emissivity difference on the correction accuracy. For the extrapolation method, the correction accuracy could be improved or weakened depending on the emissivity variation of the thermocouples. For the equation methods by De, the correction accuracy is poor for the low speed flow and good for the high speed flow. However, the correction accuracy for the high speed flow is weakened if the emissivity of the thermocouples have large difference. For the equation method by Brohez, the overall correction accuracy is good but the accuracy degrades greatly with the large emissivity difference of the thermocouples.