Spectral emissivity and temperature of heated surfaces based on spectrometry and digital thermal imaging – Validation with thermocouple temperature measurements

Abstract

The spectral emissivity of an object is required to measure its temperature remotely, based on thermal imaging from a high-speed digital camera. However, it is often difficult to obtain the spectral emissivity of objects under transient conditions, as is the case with burning fuels; hence, the accuracy of the temperature measurement is compromised. This manuscript describes a method of temperature measurement of objects with simultaneous use of a spectrometer and a high-speed camera. The method was demonstrated by viewing the bead of an R-type thermocouple heated by a premixed natural gas flame to temperatures in the range of 1450–1530 K, and by comparing such remote temperature measurements with those obtained based on the thermocouple’s voltage output. The spectral emissivity of the heated thermocouple was acquired from spectroscopic analysis of its output radiation, in the wavelength range of 500–1000 nm. It was shown that the thermocouple approached graybody behavior at the upper end of this band. It was also shown that spectral emissivity of R-type thermocouple is close to the emissivity of S-type thermocouple, reported in the literature in the wavelength range of 500–700 nm and, also, close to the emissivity of pure platinum. This suggests that the effect of rhodium on the platinum emissivity is negligible. The temperature of the R-type thermocouple was measured at four different flame conditions (by varying the fuel/air equivalence ratio) using the spectrometer and the high-speed camera, and it was compared with the thermocouple temperature measurements. The flame was either steady or unsteady; the latter condition was implemented to evaluate the transient response of the measurements. In both cases, the comparison showed that the results of the three different temperature measurement methods have very high consistency and were within 11 K in the explored temperature range. Hence, this method is suitable for temperature measurements of objects of varying temperature and emissivity, such as solid fuel particles during their combustion.