Computational fluid dynamics studies on spray assisted combustion of alternative energy carriers

Abstract

This dissertation belongs to the research fields of computational combustion physics and chemistry, with a primary focus on the modeling and simulation of diesel spray assisted combustion of alternative energy carriers. Computational fluid dynamics and combustion modeling are utilized within an open-source software framework, to investigate complex fluid flow phenomena involving turbulence, phase change, and chemical reactions under engine-relevant conditions. The dissertation comprises four research articles, investigating the combustion characteristics of various alternative fuels (namely methane, methanol, and hydrogen) when ignited by pilot diesel spray. To cope with the ongoing worldwide decarbonization strategies, utilization of alternative fuels together with researching advanced combustion concepts are sought after. In that regard, spray assisted dual-fuel (DF) combustion is amongst the low temperature combustion (LTC) technologies which relies primarily on low reactivity fuel (LRF), or blend of different fuels, to deliver the main energy in the combustion system. The premixed LRF-air charge is then ignited by a directly injected high reactivity fuel (HRF) such as diesel spray. Due to the adopted lean-burn concept of the premixed charge, LTC is attained with better fuel economy and cleaner emissions. Moreover, early introduction of the LRF into the combustion chamber increases the mixture homogeneity, and thereby mitigates soot formation. Presently, the physicochemical characteristics of DF combustion processes are not well understood for alternative LRFs. Such aspects are further explored in this dissertation. Regarding the numerical framework, large-eddy simulation is utilized for turbulence modeling, Lagrangian-Eulerian coupling for liquid transport and phase change, and the direct integration of finite-rate chemistry for combustion modeling. The Spray A target conditions from the Engine Combustion Network are considered a baseline for simulations, whereas the mixture composition is modified to account for LRF(s) in the spray assisted DF configuration. Throughout the dissertation, the performed simulations (0D, 1D and 3D), the post-processing and analyses are all retained within an open-source software environment. While Publication I revisited the numerical framework and modeling assumptions for evaporating sprays, Publication II investigated the ignition characteristics of methanol, compared with methane, when ignited by a pilot diesel spray. The challenges identified therein, for methanol ignition under DF configuration, were further addressed in Publication III. There, hydrogen enrichment to the premixed charge was proposed as a chemical remedy to facilitate methanol ignition, while extending the operational window. Finally, in Publication IV the focus was shifted to beyond the HRF spray autoignition, wherein local numerical microscopy was conducted to investigate the combustion mode development between autoignition and premixed flame initiation and deflagration. The main findings of the dissertation are as follows. In Publication I, the fluid dynamical setup is validated and the sensitivity of the modeling assumptions is assessed. In Publication II, for DF spray assisted ignition of methanol, the operational window to achieve smooth ignition is observed to be narrow. In Publication III, hydrogen enrichment as LRF in the previous setup is shown to extend the operational limits, by advancing ignition delay and mitigating ambient reactivity. In Publication IV, we present world's first numerical evidence on emerging deflagration fronts in dual-fuel spray assisted combustion for diesel-methane blends utilizing embedded fully resolved numerical simulations. The present dissertation pioneers high-fidelity numerical investigations on alternative fuel spray-assisted combustion problems.