Liquid droplets and gas interactions in two-phase flow

Abstract

The work focuses on the interactions of the two phases (liquid and gas) in droplet flows. Most studies of sprays do not resolve the liquid phase nor the near field and droplets are treated as point sources of mass, momentum, energy and species. In the present work, two- and three-dimensional Direct Numerical Simulations of fully resolved droplet arrays are analysed. Simulations of droplets arrays in inert and reacting environments are performed and evaporation rates and fuel vapour mixing in laminar and turbulent flows are assessed. The novel model developed in this work combines the one- and two-fluid formulations for multiphase flows. The energy transport equation is solved based on a one-fluid formulation while the species, velocities and pressure equations are solved with a two-fluid formulation. In addition, a Level Set technique is combined with the Ghost Fluid method in a mass conserving approach in order to track the liquid interfaces. The numerical algorithm was parallelised in order to satisfy the computational demand of the simulations. The validation tests performed show that the model implemented is able to capture the dynamic behavior of droplet interactions and heat and mass transfer across interfaces. The effects of turbulence and droplet density on droplet evaporation rates in reacting flows is investigated for n-heptane and kerosene droplet arrays. The evaporation rates are compared to existing models commonly used in Large Eddy Simulations and Reynolds-averaged Navier-Stokes computations. A shell around the droplet approach is proposed in order to estimate the gas properties used in these models. It is noted that this approach allows the models to capture transients and provides predictions of the evaporation rates with errors around 2%. The gas phase mixing is assessed by examining the distribution of scalar dissipation. Novel multi-conditional models are proposed that use mixture fraction, distance to previous droplet and zone of location as the conditioning variables for the scalar dissipation. The scalar dissipation is found to be well predicted in terms of magnitude and distribution. The accurate representation of the mean scalar dissipation is achieved. The b-PDF description of the mixture fraction seems to capture well the global behaviour for a laminar environment and for time averaged results in the turbulent cases.