Temperature measurements in a string of three closely spaced droplets before the start of puffing/micro-explosion: Experimental results and modelling

The results of experimental and theoretical investigation of the mutual effects of three composite Diesel fuel/water droplets, one behind the other, on their puffing/micro-explosion are presented. The analysis is focused not only on finding the time instant when puffing/micro-explosion starts, but also on the investigation of time evolution of temperature at the water-fuel interface before the development of puffing/micro-explosion. The experimentally observed temperatures at the water-fuel interface are shown to increase almost linearly with time for the lead, middle and downstream droplets. Assuming that puffing/micro-explosion starts when the temperature at this interface reaches the water nucleation temperature, the values of the latter temperature as a function of the heating rate were found from the experimental data. The results are shown to be consistent with the earlier found correlation for this temperature for all three droplets. Time to puffing/micro-explosion is shown to decrease with increasing gas temperature; this time for the lead droplet is always shorter than that of the middle and downstream droplets, and the difference between them decreases as the distance between droplets increases. The experimental results are interpreted in terms of the previously developed model based on the assumption that the water sub-droplet is located exactly in the centre of the Diesel fuel droplet and that this process is triggered when the temperature at the water/fuel interface attains the water nucleation temperature. The effect of interaction between lead, middle and downstream droplets is considered via modifications to the Nusselt ( Nu ) and Sherwood (Sh) numbers for these droplets due to the interaction between them. (c) 2021 Elsevier Ltd. All rights reserved.