A JOINT PDF MODEL FOR TURBULENT SPRAY EVAPORATION/COMBUSTION

摘要

A new joint-probability density function (PDF) method is proposed for modeling turbulent gas-spray flows, with special effort in considering the influence of gas turbulence on droplet dispersion, evaporation, and combustion. By taking the droplet velocities, temperature, diameter, and droplet-seen gas velocity, temperature, and mass concentration as stochastic variables, a joint-PDF evolution equation for the properties of droplets and gas eddies seen by droplets is derived. To close the turbulent dispersion model of droplets, the Langevin equation proposed by Simonin for simulating the gas velocity seen by particles, is taken to account for the crossing-trajectory effect and the continuity effect. The turbulent heat and species transport were modeled accordingly by a Langevin equation model with isotropic drift coefficients. The PDF model combined with the second-order moment (SOM) model of gas turbulence was used to simulate well-specified spray evaporation in a sudden-expansion chamber. The predicted droplet mean velocities, velocity fluctuations, mean diameter, root-mean-square (rms) diameter, and droplet axial mass flux profiles are in good agreement with the measurement results. The PDF model can give good predicted droplet properties even in the recirculation zone, where the droplet concentration is low and the stochastic particle trajectory (SPT) method always fails to give good results. The proposed method provides a sound basis for simulating turbulent spray combustion.