Extending the range of back-lit imaging in two-phase flows using an interrogation-window based method

This paper introduces a novel image processing methodology to enable an extension of the range of applicability of the back-lit imaging method to provide the simultaneous measurement of size and velocity in turbulent flows with a high concentration of objects having various shapes and sizes. The new image processing methodology applies an interrogation-window method similar to that used in particle image velocimetry, alongside interface identification as used in backlight imaging. The matching of droplets or particles of arbitrary shape is achieved by dividing the image into several small windows and cross-correlating objects in each window to obtain a coefficient of dissimilarity, extracted using a variety of metrics of droplet or particle morphology. To demonstrate its utility this technique is applied to measurement of velocities from (i) a highly turbulent spray flow and (ii) a swirling particle-laden flow as applicable to dry powder inhalation systems. Results are validated against phase Doppler anemometry data and a sensitivity analysis of the controlling parameters of the image processing methodology demonstrate that the technique is robust for a variety of input conditions. A key advantage of this approach lies in complementing the diagnostics capability of PDA methods to span a broader range of particle sizes irrelevant of shape and particle concentration.