Calibrations for planar laser nephelometry in densely seeded two-phase flows

Abstract

Nephelometry is a laser based imaging technique, which uses the signal of light scattered off particles to provide information about the local number density of these particles. In densely seeded flows of practical interest, such as pulverised coal flames, particle loadings can become very high, to the point where the incident laser beam is severely attenuated. Therefore, measurements in flows are hampered by these practical difficulties, and there are limited data at these conditions. Laser attenuation experiments were conducted in suspensions of spherical particles in water at various concentrations. Results enabled the formulation of a calibration for the effects of diffuse scatter and laser sheet extinction. A model for the distribution of light through a densely seeded, light-scattering medium is developed and is compared to experimental results. It is demonstrated that the scattered signal may be considered proportional to the local particle concentration multiplied by the incident laser power. The incident laser power varies as a function of the attenuation by obscurement. The current laser sheet extinction model may be implemented for both collimated and diverging light, in addition to nonhomogenously seeded scattering media. This correction for planar nephelometry images thus extends the technique to provide pseudo-quantitative data for instantaneous particle concentration measurements.