LDV measurements of drop velocity in diesel-type sprays

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LDV measurements of drop velocity in diesel-type sprays

Wu, K.J. / Santavicca, D.A. / Bracco, F.V. / Coghe, A.

Axial and radial components of the drop velocity were measured by laser Dopplr velocimetry (LDV) within n-hexane-into-nigroten sprays from single-hole cylindrical nozzles at room temperature. The gas-to-liquid density ratio, the injection velocity, and the nozzle length and diameter were varied. The LDV system included an argon-ion laser, dual beam LDV optics with frequency shifting and 90-deg collection, a counter processor, and minicomputer data acquisition. The total error in the measured mean and fluctuation drop velocities was less than 10% from the centerline to the half-radius (half-the-width at half-the-depth) and larger and more uncertain beyond it. Velocity bias was the most difficult error to quantify. It is found that beyond 300 nozzle diameters from the nozzle so much ambient gas has been entrained by the drops that the susequent structure of diameters from the nozzle so much ambient gas has been entrained by the drops that the subsequent structure of the spray is dominated by the entrained ambient gas, and the fully developed incompressible jet structure and drop-gas equilibrium are being approached. this conclusion is supported by all the measured drop velocity parameters: jet half radius, centerline velocity decay, axial mean velocity distribution, axial and radial velocity fluctuation distributions, and independence of drop velocity on drop size. Large uncertainties about the magnitude of errors at the outer edges of jets using both laser Doppler velocimetry and hot-wire anemometry suggest that this region is still poorly characterized even for imcompressible jets.