On the effects of the fuel injection phase on heat release and soot formation in counterflow flames

Total rates of heat release, (omega) over dot(T,tot), and soot emission, (omega) over dot(Ys,tot), are studied in ethanol-N-2/air and n-butanol-N-2/air counterflow flames. A gas flame is first established as a base case and fractions of the gaseous fuel are then replaced by droplets and nitrogen, keeping the total fuel mass flux constant. Several values of the liquid to total fuel mass ratio, phi(l), are employed, covering the entire range from a gas flame to a spray flame (0 < phi(l) < 1). Different initial droplet radii, R-0, are considered, as well as both low and close to extinction strain rates. Results show qualitative similarities for both fuels, even though quantitative differences are observed. In general, ethanol flames release more heat and less soot than n-butanol flames. For low strain rates, (phi(l) = 1 leads to lower soot emissions than for the reference gas flame, independently of Ro. Also, (omega) over dot(T,tot) is higher for R-0 = 25 and 40 mu m. Close to extinction, increasing phi(l) notoriously improved (omega) over dot(T,tot )without considerably raising (omega) over dot(Ys,tot) for R-0 = 25 mu m. Also, for R-0 = 40 mu m, there are some particular values of phi(l) for which similar situations occur. These results show that the fuel injection phase plays an important role in optimizing combustion processes. (C) 2022 Elsevier Ltd. All rights reserved.