Abstract
A method of in situ particle size and velocity measurement in parti-cle laden flow by forward-scatter dual-beam Laser Doppler Velocimeter (LDV) has been investigated both analytically and experimentally. The analysis is based on Mie's electromagnetic scattering theory for spherical particles to relate the collected scattered light power for the LDV optical arrangement to the particle diameter. The measured de-pendence of the pedestal height in the Doppler burst signal to the particle diameter agrees well with calculations from Mie's theory. It should be emphasized that the in-creasing dependence assumption of collected scattered power on the square of particle diameter is not satisfied for the LDV collection optics geometry and the calibration with numerical calculations based on Mie's theory is indespensable. Although the mask has to be attached on the collecting lens to block the unscattered beams, it may be noticed that the monotonic dependence of collected scattered power on the particle size can not be sustained for too small collection opening. The selective data sampling scheme to set the threshold of the certain Doppler cycle number for each burst signal is proved to re-duce significantly the error resulting from particle trajectry through nonuniform incident intensity distribution in the measurement volume, however, overall measurement error at most approximately 30% is evaluated.
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