A High-Pressure Droplet Model for Spray Simulations

Droplet vaporization models that are currently employed in simulating sprays are based on a quasisteady, low-pressure formulation. These models do not adequately represent many high-pressure effects, such as nonideal gas behavior, solubility of gases into liquid, pressure dependence of gas- and liquid-phase thermophysical properties, and transient liquid transport in the droplet interior. In the present study, a high-pressure quasisteady droplet vaporization model is developed for use in comprehensive spray simulations for which more rigorous vaporization models, such as those based on unsteady formulations, are beyond the present computational capabilities. Except for the gas-phase quasisteady assumption that is retained in the model, the model incorporates all high-pressure effects. The applicability of the model for predicting droplet vaporization in diesel and gas turbine combustion environments is evaluated by comparing its predictions with the available experimental data and with those from a more comprehensive transient model. Results indicate a fairly good agreement between the quasisteady (QS) and transient (TS) models for a wide range of pressures at low ambient temperatures, and for pressure up to the fuel critical pressure at high ambient temperatures. The QS model generally underpredicts the vaporization rate during the earlier part of droplet lifetime and overpredicts during the later part of lifetime compared to those using the TS model, and the difference becomes increasingly more significant at higher ambient pressure and temperature. The differences can be attributed to the quasisteady gas-phase average temperature and composition assumption for the QS model that reduces and increases the gas-phase heat and mass fluxes at the droplet surface during the earlier and later part of lifetime, respectively. The applicability of the QS model is quantified in terms of the maximum pressure as a function of ambient temperature.