Instantaneous Imaging Diagnostics for Measuring Particle Sizes and Spatial Distributions Over Extended Regions in Two-Phase Flows.
Two new optimal diagnostics capable of instantaneously measuring the size and spatial distribution of many particles in two-phase flows of moderate number densities (10 ^3-10^4 per cm ^3) have been developed and characterized. They utilize a broadband laser source to illuminate a planar section of the flowfield, and commercially available, uncooled, CCD video cameras to image the scattering normal to the illuminated plane and record the spatial distribution of the particles. Individual particle image intensities and/or polarization ratios are related to particle size by comparison with Lorenz-Mie scattering theory. Simple image processing algorithms can be used to provide nearly real-time measurements of size and spatial distribution. Important advantages over existing techniques include the ability to size many particles simultaneously while retaining spatial distribution information, simplified data reduction, and the elimination of velocity bias from the measured distributions. Experimental results for monodisperse droplet streams are presented to demonstrate the particle sizing capability and identify sources of uncertainty. A detailed analysis of both random and systematic error sources is given, and suggestions for improving the accuracy of the technique are presented. The results indicate that individual particles in the size range of 10-100 mu m can be sized with an accuracy of +/- 20% using the current experimental setup based on scattered energy. Accuracies of +/-10% should be possible after implementation of suggested improvements. Measurements of the instantaneous polarization were found to be unsuitable for individual particle size determination, but could be useful for determining size distribution statistics based on a large number of images. The techniques are ideally suited for measurements of homogeneous, spherical particles immersed in a homogeneous medium, pseudo-plane wave illumination, and single particle scattering. A discussion of the sensitivity of the results to departures from these ideals is included, and appropriate application regimes are given. A FORTRAN code capable of integrating the scattered fields over arbitrary solid angles and arbitrary spectral profiles, including variable optical properties, has been compiled and verified against other published results from theoretical and experimental investigations. Comments on the accuracy of other published codes and common sources of misinterpretation are included.
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- Engineering: Mechanical; Engineering: Aerospace; Physics: Optics