Optimization of planar LIF/Mie imaging for droplet sizing characterization of dilute sprays

Abstract

Abstract The aim of this work is to investigate the planar LIF/Mie droplet sizing technique from an experimental and theoretical point of view. This technique is a good alternative compared to point measurements (e.g. Phase Doppler Anemometry) or integral approaches (e.g. Laser Diffraction). It allows measurement of the Sauter Mean Diameter over a wide field, providing the spray topology and droplet size in a limited amount of time. Nevertheless, its implementation remains challenging due to the fact that the usual assumptions underlying the technique are not fully valid in practice. To overcome these limitations, an innovative experimental set-up has been developed including the use of a telecentric lens and a TwinCam beam splitter device. The benefit of the key optical elements introduced in the set-up will be discussed and quantified in the paper. In particular, it is shown that the telecentric lens removes angular dependency of the Lorenz-Mie optical signals. This is demonstrated using an ethanol hollow cone spray (seeded with rhodamine 6G) generated from a simplex injector at atmospheric pressure and ambient room temperature. A calibration procedure, based on Phase Doppler Anemometry measurements, is used to convert the LIF/Mie into Sauter Mean Diameter maps. The optical depth of the spray has been measured, exhibiting values less than 0.4, which corresponds to a dilute cloud of droplets where multiple light scattering is not an issue. These effects have been quantified using Monte Carlo simulations. Finally, a study of the injection parameters has been undertaken, showing the robustness and efficiency of the planar LIF/Mie droplet sizing method to obtain two-dimensional maps of the Sauter Mean Diameter.