DOPPLER OPTICAL PROBE FOR DROP SIZE, VELOCITY, AND FLUX MEASUREMENTS IN ASSISTED ATOMIZATION

Abstract

For research as well as for process control, reliable drop size, velocity, and flux measurements are desirable in particular in dense, high-speed flows. A new optical probe has been recently manufactured by A2 Photonic Sensors company that combines an accurate phase detection capability (its latency length is small, about 6 µm) with the collection of a Doppler signal that provides the absolute velocity of an incoming gas-liquid interface. In this article, raw signals acquired over diverse flow conditions in terms of gas velocity and liquid concentration are analyzed. A dedicated signal processing routine is then proposed and optimized. The latter provides statistics on drop velocity and size. It also gives access to local liquid concentration and liquid flux. This Doppler probe combined with its processing has been tested in sprays produced from assisted atomization over a wide range of flow conditions. Transverse profiles of spray characteristics are presented for gas injection velocities ranging from 32 m/s to 283 m/s, for drops Sauter mean diameters D₃₂ varying from 37 µm to 275 µm, and for number densities-as estimated from liquid concentration and D₃₂-comprised between 1 #/mm³ and 218 #/mm³. The Doppler probe happens to be able to consistently detect chords as small as 4 µm, and to ensure a significant (up to 70%) fraction of direct velocity measurements. Besides, the injected liquid flow rate is recovered from the spatial integration of local liquid fluxes within 8% for gas velocities up to 50 m/s and within 17% for gas velocities above 90 m/s. Hence, the new Doppler probe combined with the proposed processing provides reliable statistics on drop velocity, size, and flux, and is a valuable tool for investigating dense, high-velocity, and fine sprays.