Numerical simulation of laser heating effect in optical temperature measurement based on Kubelka-Munk theory

The fluorescence intensity ratio (FIR) technique is widely used in many fields as one of the sensing methods of phosphor thermometry. In the gas turbine field, the rare earth ions are often doped to thermal barrier coating (TBC). When excited by laser pulses TBC glows phosphorescent, and it can be utilized for thermometry. The laser energy absorbed by the TBC is not only used to generate the luminescence but also introduces heat into the TBC and causes a temperature rise further increasing optical temperature measurement error. Therefore, it's essential to predict the temperature rise and the temperature measurement error, and take measures to reduce the error due to the laser heating effect. However, there are few researches concerning this problem, and most of them focus on the relationship between the laser energy and the point temperature measured by the FIR technique. Here we show a method for evaluating the effect of laser heating on temperature fields and optical temperature measurement errors by establishing thermal equivalent model and discrete model of body luminescence, considering laser and phosphorescence scattering in coatings made of YSZ based on Kubelka-Munk theory. We found that continuous pulsed laser heating caused severe temperature rise and significant temperature measurement error. Furthermore, the temperature rise can be decreased to the level of a single pulsed laser heating by decreasing the frequency of laser. In addition, the main influencing factors of the temperature measurement error are the density of laser energy and coating thickness.