Measurement of temperature and velocity of thermally sprayed particles using thermal radiation.

Abstract

Methods to measure the surface temperature and the velocity of thermally sprayed particles were developed and applied to plasma-sprayed Mo, Ni-Cr alloy, and alumina particles in the air.In the temperature measurement, radiation from particles within a cylindrical measurement volume is measured and its spectrum is analyzed by a monochromator. Then the spectrum is compared with the spectra of blackbody radiation to determine the surface temperature of the particles through the least squares method.Spatial filtering technique using a mask with four thin parallel slits was adopted in the velocity measurement. When a single particle travels in front of the plates, radiation passing through the spacings between the plates is detected by a photo-multiplier, which gives a signal with four peaks. Then the velocity v of the particle is given by υ=d/t, where d is the distance between the neighboring slits and t the period between the peaks. It was also found that the mean velocity of a high-density particle stream can be determined by analyzing the power spectrum of the signal by a method such as FFT.As the results of those measurements, the change in the temperature and the velocity of sprayed particles with the distance from the exit of a plasma-torch was obtained. Where as the metal powders exhibited similar history of cooling and deceleration with the distance, alumina particles were found to be accelerated to a much higher velocity close to 300 m/s but decelerated at a much faster rate. The temperature of alumina particles could be measured for only a limited region due to the weak radiation. Also the effects of particle size on the distribution of both the temperature and the velocity of sprayed Mo particles were examined.