Investigation of fuel dilution in ethanol spray MILD combustion

Abstract

MILD combustion is a promising technique for low heating value fuels. In this paper, spray MILD combustion of diluted liquid fuel is numerically studied. The modelling framework is based on the RANS approach, the EDC combustion model with a skeletal chemical mechanism, and the Lagrangian tracking of droplets. The validated numerical model is used to simulate the combustion of ethanol diluted with water is studied for different oxidizer temperatures and O2 concentrations under MILD condition. Results indicate that a low-level fuel dilution up to 5 percent has tendency toward expansion of MILD region, but higher degrees fuel dilution within the range of 5–20 percent results in a reduction of MILD combustion regions due to domination of physical effect contribution of addition of water to ethanol fuel compared to its chemical effect. Fuel water addition also leads to an increase in OH concentration due to H2O dissociation through O + H2O → 2OH, which assist the oxidation of fuel driven species, CO in particular. Changing the oxidizer stream temperature can lead to considerable changes in the flame structure: the typical flame shape is converted to a bifurcation style by reducing hot co-flow stream temperature from 1400 K to 1200 K. Moreover, for liquid fuel dilution, preheating and inlet air dilution do not lead to monotonic strengthening the MILD zone in spite of gaseous fuels.