Numerical simulation of a channel two-phase mixing layer flow

A major critical aspect of the injection systems of modern low NOx combustors is the formation of the fuel liquid film and its subsequent break-up in the presence of air blast co-flows. The relevant phenomena of atomization, mixing and vaporization, as well as the amount of emissions, strongly depend on film formation and break-up. With the aim of both characterizing the nature of the instability leading to ligament formation in sheared liquid-gas layers and determining the droplet size distribution occurring in the subsequent ligament break-up process, many authors have employed numerical simulations as design tools. In this work we perform numerical simulations of a two-phase flow mixing layer configuration, and we study the nature of the instabilites leading to primary instability. We discretize the full Navier-Stokes equations for two dimensional incompressible flow constituted by two immiscible fuids driven by an axial pressure gradient in a channel. The fow is integrated in a portion of a channel, with suitable inflow-outflow boundary conditions. A Volume Of Fluid (VOF) method is employed in order to track the interface; surface tension is implemented by means of the CSF method. The density and viscosity ratios are fixed with reference to typical fuel-gas configurations of injection systems of aeronautical combustors and the effects of Weber and Reynolds numbers, as well as of the depth of the liquid layer, are investigated. The calculations show good agreement with a full three dimensional linear stability analysis independentely developed for the problem.