Upward vs downward injection of droplets for the optimization of a radiative shield

A numerical and experimental study of a spray has been carried out at the laboratory scale, focusing on the comparison between downward and upward injection situations. The simulation has been carried out thanks to a dedicated numerical code addressing the two-phase flow with an Eulerian–Lagrangian approach, and treating the radiative transfer with a Monte Carlo method. The experimental setup involves a FTIR spectrometer and an IR camera, both characterizing simultaneously the spectral attenuation through the spray when irradiated by a blackbody. Numerical results and experimental data both show a better ability of the spray to attenuate radiation when injected upward. The residence time of droplets is highly increased when they are injected upward. This results in an increased attenuation ability of the spray. A gain by a factor 3 or more is possible regarding the attenuation of radiation if water is injected upward. This observation has to be considered with care however because of a possible poor stability of the spray. The influence of an air flow affecting the spray dynamics has also been investigated numerically, tests being done with a 2 m/s air flow. The droplet motion is strongly altered and the radiation attenuation is affected. In the upward injection case the deformation of the spray may be so strong in its upper part that the attenuation tends to zero because droplets are carried away.