Study on gas-droplet heat and mass transfers in oscillating flows

Abstract

A numerical study on the dynamics of evaporating droplets in oscillatory flows is performed by using an Eulerian-Eulerian model including two-way couplings. The diluted liquid fuel-gas two-phase flow is assumed to be laminar. The droplet is assumed to be spherical during its lifetime and its thermal conductivity is also assumed to be infinite. Heat and mass transfer of this two-phase flow is characterized by analyzing the effects of acoustic fields on the two-phase temperatures, droplet diameter and concentration, spray evaporation rate and vaporizing species mass fraction. Results show that acoustic forcing can substantially influence the dynamics of the two-phase flow. The presence of droplet clustering as a consequence of acoustic forcing affects the two-phase flow in two aspects. The first is the oscillation of the two-phase flow parameter which roots in the periodic variation of droplet concentration. The second is the enhancement of the droplet evaporation rate at all conditions. The maximal relative increase in spray evaporation rate can be up to 72.9% in the parameter range studied. The mechanism for the enhancement of droplet evaporation rate is the optimal distribution of the heat in the liquid phase whose direct cause is droplet clustering. The enhancement of evaporation rate is highly dependent on the acoustic oscillation amplitude; however, there is no significant relationship between the acoustic oscillation frequency and the evaporation rate. Additionally, the oscillation amplitudes of the two-phase flow parameters are found to decrease with the growth of acoustic forcing frequency. It is made clear that the evaporation of droplets has a negligible effect on the occurrence of droplet clustering despite its marked influence on the two-phase flow through the reduction of the droplet relaxation timescale. Increasing the droplet initial concentration is also found to be beneficial for the enhancement of droplet evaporation rate.