An isolated fuel drop in a hot oxidizing atmosphere is a source of fuel vapour which is surrounding the droplet surface within a limited distance called “flame reaction zone” or flame radius. The reaction zone forms a flame envelope around each droplet where the fuel vapour defuses into the oxidant and reacts with it. In other words, the reaction zone is the “sink” where both the fuel substance and oxygen are consumed and turned into thermal energy [1]. Understanding the single drop combustion is important in the overall spray combustion mechanism.  the “sensitivity” of the air-fuel mixture to explode increases as the fuel drop size decreases. Recent publications show strong relationship between particle size and soot formation . Large particles do not completely burn, and thus they turn into particulate matters (soot) to be later exhausted into the air.

The mass loss rate is dependent on the fluid properties and it increases as temperature and pressure increases. It defines the speed in which liquid fuel is turning into the gaseous phase. The temperature profile at the fluid drop surface in the case of non-flamed evaporation is different from that involved burning. In the first case, temperature decreases by the heat absorption of the fluid drop; in the second case, temperature gradually increases due to the chemical reactions during the combustion process.

Smaller droplets evaporate faster and mix better with the oxidant than larger drops, due to their smaller weight. Rapid mixture formation is essential in IC engines especially those with direct injection (DI) strategy as the time between injection and ignition spark is extremely short. Increasing the fuel velocity within the combustion chamber by increasing the pressure is found to develop the evaporation rate; the turbulent air adds more velocity components to the fuel spray at different directions. This is not only improving the atomisation quality, but also helps in a better heat transfer through the chamber walls and a better mixing with air, by the formation of vortexes [2].

Evaporation rate of Kerosene Jet fuel

The temperature and pressure profiles inside the combustion engine cavity are certainly gradient, and during the short life time of the spray, most of the compression (and heating up) is happening during the compression stage. From the figure above, it can be seen that larg droplets will have no chance to completely evaporate before ignition even with as high temperature as 1000C. Moreover, larger drops will not be able to reach the boiling point within the available time at such a high speed.

Under a temperature of 500C particles with 15 µm diameter will take around 0.7 ms to completely evaporate. For the same droplet at 300C, around 1.6 ms is the estimated evaporation time ;  which shows the importance of the ambient temperature on the evaporation process .

_______________________________________________________________________________________________________________

References:

[1] Annamalai K. and Puri I.K. (2007), “Combustion Science and Engineering”, CRC series in Computational Mechanics and Applied Analysis, Taylor & Francis

[2] Van Basshuysen R. (2009), “Gasoline Engine with Direct Injection”,1st edition (English), Vieweg+Teubner, Wiesbaden Germany.

[3] Ghassemi H.; Baek S. W.; and Khan Q. S. (2006), “Experimental Study On Evaporation Of Kerosene Droplets AT Elevated Pressures And Temperatures”, Combust. Sci. and Tech. (Taylor & Francis), Vol. 178,pp. 1669–1684.