Numerical study of spray combustion of a biodiesel surrogate fuel using the LES-FGM approach

In this paper, we report on Large Eddy Simulations (LES) of spray combustion of a Karanja Methyl Es-ter (KME) biodiesel surrogate. The surrogate fuel considered herein is a blend of n-dodecane and methyl butanoate. In terms of combustion modelling, we employ a specific implementation of the Flamelet Gen-erated Manifold (FGM) approach, according to which the energy equation is cast in terms of the sensi-ble enthalpy. This approach allows for the direct computation of the temperature and the temperature-dependent properties of the mixture, which results in significant computational savings. Concerning the numerical methodology, we employ the Eulerian approach for the gaseous phase, coupled with La-grangian particle tracking for the motion of the fuel droplets. Our numerical setup follows closely the well-known "Spray A" operating conditions. Three different cases are considered herein: in the first two cases we employ the thermophysical properties of the liquid surrogate fuel and two different values of the oxygen concentration (15% and 21% respectively). In the third case, the so-called "hybrid" one, we employ the properties of the liquid KME itself; these are considerably different than those of the liquid surrogate fuel. Therefore the hybrid case is in principle more representative of the spray combustion of the actual KME. According to our numerical results, the ignition of the surrogate fuel consists of the same stages as those observed in n-dodecane flames. In the first case (15% O 2 ), the flame topology remains the same as in the combustion of (pure) n-dodecane. The Ignition Delay Time (IDT) and combustion temper-ature also remains practically unchanged while the Flame Lift-off Length (FLOL) increases by 12%. When the O 2 concentration increases from 15% and 21%, the IDT increases by approximately 17% and the com-bustion temperature by approximately 11%, while the FLOL decreases by roughly 9%. On the other hand, the liquid fuel properties have a significant impact on the combustion process and flame topology. In particular, in the combustion of KME there is an increased number of ignition kernels on the periphery of the jet and a large decrease of the FLOL, by approximately 50%. Moreover, our simulations predict that with KME there is production of acetylene in low-temperature zones, which can impact the levels of soot formation.(c) 2022 The Combustion Institute. Published by Elsevier Inc. All rights reserved.