Computational Analysis of Two-Phase Mixing Inside a Twin-Fluid, Fuel-Flexible Atomizer

We have developed a twin-fluid atomizer for combustion that creates a two-phase mixture of fuel and atomizing air upstream of the injector exit where a high-pressure region is established. The static pressure decreases rapidly as the fuel-air mixture exits from the injector, which causes air bubbles in the mixture to expand and breakup the surrounding liquid. This type of fuel injector can effectively atomize various biofuels including highly viscous straight vegetable oil and glycerol. While the combustion benefits have been demonstrated in our prior studies, an understanding of the underlying flow field and mechanism of the two-phase mixture formation process within the injector remains elusive. In this study, a computational fluid dynamic (CFD) model is developed to investigate the two-phase mixing and how it is affected by the operating conditions, particularly the atomizing air to liquid ratio (ALR) by mass. The axisymmetric isothermal CFD model, based on the mixture model for two-phase flows and Reynolds averaged Navier-Stokes equations, utilizes air and water as the working fluids. Both fluids are treated as incompressible, with constant fluid properties. The analysis reveals the flow field within the injector and successfully replicates the upstream penetration of the atomizing air into the liquid supply tube observed experimentally. The penetration depth increases with increase in the ALR, which again agrees with the experimental results.