A new method for simultaneous measurement of droplet or particle surface area and velocity

A light-emitting-diode (LED) based extinction tomography system is extended to obtain simultaneous velocity measurements alongside the surface area concentration in aerosols. Line-of-sight images are taken to obtain transmittance measurements through an aerosol, which can be deconvoluted to obtain instantaneous tomographic measurements of surface area per unit volume using the Beer-Lambert law. These images are taken with a long exposure time so that particle streak velocimetry (PSV) can be used to infer the particle velocities. An analysis of the behaviour of Gaussian particles reveals that increasing the exposure time affects the surface area measurement in a predictable way. A ‘correction factor’ is rigorously derived to preserve the total surface area when the exposure time is increased, leading to a new image processing approach which enables the computation of aerosol surface area from image streaks that correlate to velocity. An analogous approach to the surface area deconvolution is developed for the line-of-site velocity measurements to obtain the local velocity in the ROI. The technique is applied to a pharmaceutical flow through a dry powder inhaler and throat model, where the surface area and velocity measurements are combined to obtain a ‘surface area flux’ for the first time.