Transcritical evaporation and micro-explosion of ethanol-diesel droplets under diesel engine-like conditions

摘要

A transcritical multi-component evaporation and micro-explosion sub-model is suggested to investigate the micro-explosion of blended ethanol-diesel droplets under diesel engine-like conditions. A homogeneous theory is used to qualitatively predict the bubble generation, and the subsequent bubble growth leads to the final explosion. The evaporation rate under subcritical state is calculated by a multi-component diffusion sub-model along with an isothermal-isobaric flash. The finite thermal conduction and mass diffusion equations are solved inside the multi-component droplets. The transcritical transition is defined by the time instant that the droplet surface attains the mixture critical temperature, then the finite regression rate is controlled by the thermal diffusion, and the critical mixing surface moves continuously inward. The suggested model is validated against the experimental data. Based on the numerical results, the heat transfer rate in ambient and the superheat limit are found to be the predominated factors on the micro-explosion under low pressures. However, with accelerating pressure, the net result of competition among the evaporation rate, heating rate, transcritical transition, as well as the bubble growth rate determines the micro-explosion for small micron droplets ( 24 mu m). When the ambient temperature is higher than 1000 K, the enhanced regression rate caused by the transcritical transition makes the droplet lifetime too short to explode. Finally, the suggested sub-model is implemented into the OpenFOAM code. The spray of blended ethanol-diesel under diesel engine-like conditions is simulated, and the occurrence of micro-explosion is confirmed and found to show large effects on the spray characteristics.