Models for droplet heating and evaporation: application to simulation of the autoignition process in diesel engines

A review of liquid and gas phase models for fuel droplet heating and evaporation, suitable for implementation into CFD codes with a view to modelling the processes in Diesel engines, is presented. To describe properties of ‘hot’ Diesel fuel sprays, new sub-models for spray break-up, droplet heating and evaporation and Shell autoignition were implemented into the KIVA II CFD code. This customised version of the KIVA II code was used to simulate autoignition in Diesel engines. The results of simulation were shown to be consistent with in-house experimental data referring to autoignition timing at in-cylinder pressures from 5 to 9 MPa and an injection pressure of 160 MPa. The autoignition delay time is shown to be sensitive to the choice of liquid phase models for droplet heating, but not sensitive to the choice of gas phase models. It is recommended that the effective thermal conductivity liquid phase model and the gas phase model taking into account the effects of finite thickness of the thermal boundary layer are used for the simulation of the ignition process in Diesel engines. Some recent non-traditional developments of the models referring to droplet heating and evaporation are discussed. These are the model based on the coupled solution for the liquid and gas phases, the kinetic model of evaporation, and the dynamic decomposition technique for the solution of ordinary differential equations describing droplet heating and evaporation and the ignition of the fuel vapour/air mixture.