Phase-doppler interferometry with probe-to-droplet size ratios less than unity. I. Trajectory errors.

Phase-Doppler interferometry in which a probe volume that is much smaller than the droplets being measured has been shown to work well when coupled with a phase-ratio and intensity-validation scheme that is capable of eliminating trajectory-dependent scattering errors. With ray-tracing and geometric-optics models, the type and magnitude of trajectory errors were demonstrated quantitatively through stochastic trajectory calculations. Measurements with monodispersed water droplet streams and glass beads were performed to validate the model calculations and to characterize the probe volume. Scattered-light intensity has also been shown to provide a robust means of determining the probe cross-sectional area, which is critical for making accurate mass flux measurements.

Phase-Doppler interferometry with probe-to-droplet size ratios less than unity. II. Application of the technique.

Practical limitations associated with the use of small probe volumes with respect to the droplet size that is being measured by the phase-Doppler interferometry technique are discussed. An intensity-validation scheme and corresponding probe volume correction factor have been developed that reject trajectory errors and account for the rejections in calculation of the probe cross-sectional area. The intensity-validation scheme also provides a tractable method of setting the photomultiplier tube gain and laser power. Volume flux measurements in dilute sprays have shown a significant improvement over those made by standard phase-Doppler interferometry techniques at small beam waist/droplet size ratios.