Ignition and extinction of low molecular weight esters in nonpremixed flows

Abstract

An experimental and kinetic modeling study is carried out to characterize combustion of low molecular weight esters in nonpremixed, nonuniform flows. An improved understanding of the combustion characteristics of low molecular weight esters will provide insights on combustion of high molecular weight esters and biodiesel. The fuels tested are methyl butanoate, methyl crotonate, ethyl propionate, biodiesel, and diesel. Two types of configuration – the condensed fuel configuration and the prevaporized fuel configuration – are employed. The condensed fuel configuration is particularly useful for studies on those liquid fuels that have high boiling points, for example biodiesel and diesel, where prevaporization, without thermal breakdown of the fuel, is difficult to achieve. In the condensed fuel configuration, an oxidizer, made up of a mixture of oxygen and nitrogen, flows over the vaporizing surface of a pool of liquid fuel. A stagnation-point boundary layer flow is established over the surface of the liquid pool. The flame is stabilized in the boundary layer. In the prevaporized fuel configuration, the flame is established in the mixing layer formed between two streams. One stream is a mixture of oxygen and nitrogen and the other is a mixture of prevaporized fuel and nitrogen. Critical conditions of extinction and ignition are measured. The results show that the critical conditions of extinction of diesel and biodiesel are nearly the same. Experimental data show that in general flames burning the esters are more difficult to extinguish in comparison to those for biodiesel. At the same value of a characteristic flow time, the ignition temperature for biodiesel is lower than that for diesel. The ignition temperatures for biodiesel are lower than those for the methyl esters tested here. Critical conditions of extinction and ignition for methyl butanoate were calculated using a detailed chemical kinetic mechanism. The results agreed well with the experimental data. The asymptotic structure of a methyl butanoate flame is found to be similar to that for many hydrocarbon flames. This will facilitate analytical modeling, of structures of ester flames, using rate-ratio asymptotic techniques, developed previously for hydrocarbon flames.