Models for droplet transient heating: a comparative analysis

Results of recent studies of the effect of the temperature gradient inside fuel droplets on droplet evaporation, break-up and the ignition of fuel vapour/air mixture are reviewed. This is investigated by comparing the 'effective thermal conductivity' and the 'infinite thermal conductivity' models, both of which have been implemented into a zero-dimensional code. The predictions of the code are validated against available experimental data. It is pointed out that in the absence of break-up, the influence of the temperature gradient inside droplets on droplet evaporation under realistic diesel engine conditions is generally small (a few percent). In the presence of the break-up process, however, the temperature gradient inside the droplets can lead to a significant decrease in evaporation time. This is attributed to the fact that the effect of the temperature gradient inside droplets leads to a substantial increase in droplet surface temperature at the initial stages of its heating. Even in the absence of break-up, the effect of the temperature gradient inside the droplets leads to a noticeable decrease in the total ignition delay. In the presence of break-up this effect is enhanced substantially, leading to more than halving of the total ignition delay.