Temperature and mixture fraction measurements in gases by laser-induced electrostrictive gratings

Abstract

We applied time-resolved light scattering from electrostrictive gratings to measure nonintrusively gas compositions and temperatures. The infrared radiation of a pulsed Nd:YAG laser generates a spatially periodic density grating that oscillates in time, and the beam of a cw laser is used to read it out. The oscillation period of the signal depends on the sound velocity in the medium. If the gas composition is known, the measurement of the sound velocity allows the temperature to be deduced. We evaluated this novel technique in a tube furnace for temperatures in air up to about 1400 K. Although the measured temperatures agree within the error bars with the thermocouple readings, there is some evidence that the temperatures determined by the laser-induced grating technique are systematically too high. For singleshot temperature measurements in air, we determined a statistical error of 70 K at a furnace temperature of 1400 K.

On the other hand, if the temperature is known, concentrations in isothermal binary mixtures can be determined by the laser-induced grating technique. We performed measurements in methane-nitrogen and hydrogen-nitrogen mixtures. The limits for the minimum detectable variation of the gas composition of 0.5 mole fraction mixtures were found to be 2% and 0.6%, respectively. One advantage of the proposed method is the simple evaluation process that allows for on-line measurements of temperature or gas compositions.