Experimental investigation of the flame structure of dilute sprays issuing into a hot and low-oxygen coflow

Abstract

The combustion of liquid fuels in a hot and low-oxygen environment is commonly encountered in a range of practical situations. To enable investigation of the fundamental combustion processes relating to such applications, liquid fuels were injected into the reaction zone as dilute sprays in this study. Droplets of ethanol, n-heptane, and n-heptane/toluene blends were produced via an ultrasonic nebuliser, and were carried by air through a central jet to a hot coflow of combustion products. The resulting flames were then analysed using four simultaneous laser diagnostic techniques. Planar laser-induced fluorescence (PLIF) was implemented to perform imaging of key intermediate species, including hydroxyl (OH) and formaldehyde (CH₂O), while the Mie scattering technique was used to detect the location of droplets. The sooting behaviour of these flames was also investigated, via the laser-induced incandescence (LII) technique. The existence of distinct inner and outer reaction zones is a key feature of all of the flames studied, and this “double flame structure” was found to be related to partial premixing of air and fuel, as well as penetration of droplets into the inner reaction zone. A change in the stabilisation of the inner flame front was observed with variations in fuel type, with a greater likelihood of ignition kernels in the case of the n-heptane and n-heptane/toluene flames, whereas the equivalent ethanol flame displays a bifurcation structure. Variations in the jet Reynolds number and liquid fuel loading were also found to have a notable impact on the distribution and evaporation of droplets, which was in turn found to affect the formation of the double flame structure. Due to the complex coupling between turbulence, chemistry and droplet evaporation in these flames, the accurate prediction of such results a prioi is not within the limits of current modelling capabilities. These findings provide a valuable insight to enable future advancements in spray combustion modelling and the design of practical combustion devices.