Effects of Scale and Orientation of Probe Region on the Results of Quantitative Temperature Measurement using LITGS for Non-uniform Measurement Objects

Abstract

Laser Induced Thermal Grating Spectroscopy (LITGS) is an anticipated technique for the quantitative temperature measurement especially for high pressure environment with high accuracy. In general, higher spatial resolution is required for flame measurement. However, the detailed discussion on the spatial resolution in LITGS has not been sufficient. To understand the spatial resolution in LITGS, quantitative temperature measurements to non-reacting jet with two different configurations, i.e., (a) non-reacting jet with quasi-one-dimensional temperature gradient and (b) non-reacting jet from a micro nozzle, were conducted. Non-reacting acetone/air premixture was employed for the measurement object. For the experiment (a), the direction of the probe volume in LITGS was aligned in the perpendicular and the parallel directions to the temperature gradient, i.e., the former has no temperature gradient in the probe volume, but the latter has temperature gradient in it. In addition, the size of probe volume was adjusted by the adjustment of crossing angle and the beam separation in the pump beams of LITGS. As a result, the measured temperature using LITGS showed good agreement with the temperature measured by thermocouple when there was no temperature gradient in the probe volume. On the other hand, the difference of measured temperature was observed when there was temperature gradient in the probe volume. In the experiment (b), temperature could be measured even though the size of the measurement object was smaller than that of the probe volume although the signal intensity was decreased. However, the signal-to-noise ratio decreased, and the uncertainty of the measured temperature increased compared to the case that the scale of measurement target was sufficiently large compared to the probe volume.