A droplet evaporation model for high temperature and pressure spray applications.

摘要

A zero-dimensional single droplet evaporation model, suitable for the high temperatures and high pressures encountered in diesel engines, has been developed, based on an extensive review of previous work on droplet evaporation. The new model includes the dependence of gas and liquid thermodynamic and transport properties on pressure, temperature, and composition. High pressure thermodynamic equilibrium at the droplet interface is calculated using the Lee-Kesler equation of state using a computationally efficient approach. The reduction of heat and mass transfer due to surface blowing (Stefan flow) is included, and heat transfer in the liquid is calculated using the effective conductivity model which accounts for internal circulation. The single droplet evaporation model has been implemented and tested into the KIVA multidimensional simulation which models gas flows, sprays, and chemical reactions in internal combustion engines. The new model has been tested against experimental data for single droplets and sprays. The range of ambient temperatures and pressures for single droplets was 373-773 K and 0.1-10.3 MPa, while for sprays it was 573-773 K and 2.2-2.9 MPa. The overall agreement was satisfactory, although it is suspected that the blowing effect is overpredicted by the model. Computational studies performed with both the single droplet and spray models revealed that accurate estimation of properties is critical for the prediction of the evaporation rate, although high pressure effects were found to be secondary. The high pressure effect on equilibrium composition at the droplet interface was found to increase evaporation rates. Stefan flow was found to reduce the evaporation rate significantly for the entire range of ambient temperatures and pressures considered. The effective conductivity model, which includes the effect of internal circulation, increased the evaporation rate in transient sprays. Parametric studies on the effect of ambient temperature and pressure, droplet size, injection velocity, and fuel volatility were performed for single droplets and sprays.