Support : Institut Carnot "Energies du futur"
This project focuses on Direct Numerical Simulation of a two-phase flow : a liquid jet sheared on both sides by a high-speed gas. The computation is performed with the flow solver YALES2 developed at CORIA, France. The interface tracking is ensured by using an Accurate Conservative Level-Set described in (Desjardins et al., J. Comp. Phys., 2008). Two configurations are considered : a plane liquid sheet, and a coaxial jet.
The plane liquid sheet is found in aircraft engines, where a liquid fuel (kerosene) is sheared by a high-speed gas (air). The strong shear between both phases leads to a spray, enabling the mixing, and thus, the combustion. The liquid jet can be subject to a flapping motion under some specific conditions, influencing the size of the droplets produced. Understanding the flapping mechanism is therefore of interest to improve combustion’s efficiency.
The following animation displays a direct numerical simulation of a plane liquid sheet, sheared on both sides by a faster gas. The velocity ratio between phases is 6, and the density ratio between phases is 10. Contours of liquid-gas interface are plotted in grey, along with contours of Q-criterion, colored by spanwise vorticity. For readability, only a half of those Q-criterion is displayed.
Close to the injection, an interfacial instability is developping alternatively on both interfaces, behind which a vortical detachment is observable. Those vortices, with modifying the pressure field around the liquid sheet, are responsible for the flapping motion.
Other computations are performed on a coaxial configuration. This configuration is the one found in rocket engines, where a liquid comburant (oxygen) is sheared by an annular gas (hydrogen). This configuration is experimentally studied at LEGI (see animation).
The following animation displays the contours of interface (in grey) and contours of Q-criterion. For this coaxial configuration, the flapping motion is found to be rather a consequence of interfacial waves’ pairing, than a pressure balance induced by gaseous vortical structures.