INFLUENCE OF INTERNAL REFRACTIVE-INDEX GRADIENTS ON SIZE MEASUREMENTSOF SPHERICALLY SYMMETRICAL PARTICLES BY PHASE DOPPLER ANEMOMETRY

A model based on geometric optics for predicting the response of inter ferometric (phase Doppler) instruments for size measurements of partic les with radially symmetric but inhomogeneous internal refractive inde x profiles is developed. The model and results are important for appli cations in which heat or mass transfer from the particles or droplets is significant, for example, in liquid-fuel combustion. To quantify th e magnitude of potential bias errors introduced by the classical assum ption of uniform internal properties on phase Doppler measurements, we compute calibration curves for a sequence of times during the evapora tion of a decane droplet immersed in an environment of T = 2000 K and p = 10 bars. The results reveal considerable effects on the relation b etween phase difference and droplet diameter caused by the refractive index gradients present. The model provides an important tool to asses s sizing uncertainties that can be expected when applying conventional (based on uniform properties) phase Doppler calibration curves in spr ay combustion and similar processes.