The influence of combustion-induced particle concentration variations in laser-Doppler anemometry

The probability density distribution of Doppler signals in a laser Doppler anemometer may not be identical to the probability density distribution of velocity. Possible differences in the two quantities, arising from combustion processes, are examined. Three mechanisms by which combustion can cause spatial and temporal variations of particle concentration in laser Doppler anemometry are identified. It is shown that temperature gradients, chemical reaction and correlation of fluctuations of volumetric particle concentration with velocity are potential sources of bias errors. A discussion of the influence of fluctuating velocities suggests that in many practically interesting flows the bias errors calculated on the bias of an implied maximum correlation between the measured and vector velocity are overestimated.