Measurements of particle flame temperatures using three-color optical pyrometry

Abstract A three-color near-infrared optical pyrometer, with wavelengths centered at 998, 810, and 640 nm, was used to monitor the combustion of polymer particles. Individual spherical poly(styrene) particles, 47–355 μm in diameter, burned in air at 1050–1400 K gas temperatures, surrounded by sooting, diffusion envelope flames. The pyrometric results were interpreted in view of two models for soot radiation: (a) A conventional model, which assumes that the flame is optically thin and isothermal and thus, the spectral emissivity is inversely proportional to the wavelength. With this method the calculated flame temperatures are averages, biased to areas with high temperature and/or soot concentration. (b) An alternative model, in which the envelope flames are assumed to be again optically thin but nonisothermal in the radial direction. The theoretical development of the latter model is included herein. For nonisothermal flames the spectral emissivity was shown to be nearly independent of the wavelength. This model in conjunction with three-color pyrometry may provide a way of estimating the highest temperature of soot in the flame, as well as the temperature gradient across the flame. Temperatures calculated this way were higher by 200–230 K than those calculated using the conventional model. Experimental results suggest that the agreement among the three individual temperatures obtained from three-color pyrometry depends on which of the above models for soot radiation is used. Based on the agreement between temperatures, the degree of isothermality of the flame may be determined and thus, indications about the controlling processes during combustion (oxygen diffusion or volatile combustion) may be obtained. However, additional work is needed, involving simpler, one-dimensional flame configurations to confirm this model.