Accuracy of Particle Flux and Volume Fraction Measurement by Shadow Doppler Velocimetry
This paper reports on the accuracy of measurement of volume fraction and flux of a dispersed phase by Shadow Doppler Velocimetry (SDV). The SDV is an imaging technique, based on a conventional laser Doppler velocimeter and a linear photodiode array, for simultaneous particle- size and velocity measurement. The SDV also provides a measurement of the spatial position of a particle as it travels through the probe volume; as a result, it is not necessary to establish the dependence of the cross-sectional area of the probe volume on particle size for flux and volume fraction measurements. Instead, this area is defined by the operator of the instrument and is independent of the irradiance distribution of the incident laser beams, unlike — for example — phase Doppler anemometry (PDA). The accuracy of the flux measurement has been assessed from integration of flux and volume fraction measurements across a fully developed 20x20 mm turbulent water channel flow flowing at 0.16 m/s, laden with a known fraction of quasi-neutrally buoyant polyethylene spheres of nominal mean diameter of 100 μm, and the random uncertainty in the derived particle volume fraction was no more than 20% for particle volume fractions up to 0.005% and typically 10%. The integration procedure contributed a random uncertainty of less than 12%, the determination of particle volume a systematic uncertainty of 6% and the determination of the area of the sampling-space a further 8% systematic uncertainty, whereas the random uncertainty in determining the area of the sampling-space was estimated to be approximately 10%. For particle volume fraction larger than 0.005% and up to 0.05%, signal identification was affected by turbidity and multiple occupancy effects and thus the SDV measurement was compensated by the signal error rate, resulting in a particle volume fraction accurate to better than 40%.
KeywordsShadow Doppler Velocimetry Flux and volume fraction measurement Irregular particles
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