Large eddy simulation of dual-fuel combustion under ICE conditions
Abstract: The present thesis aims at studying n-heptane/methanol dual-fuel combustion under internal combustion engine conditions and strives to improve the understanding of its ignition, combustion, and pollutant emission mechanisms. Large-eddy simulation (LES) coupled with Eulerian stochastic fields (ESF) approach is employed to simulate single/dual-fuel combustion in a constant-volume vessel to mimic the single/dual-fuel combustion in conventional/dual-fuel premixed engines. The experimental configuration from Engine Combustion Network (ECN) is considered as the baseline case in the simulations. The main works are summarized in two parts: model development and studies of the fundamental physics involved in dual-fuel combustion.First, the ESF approach with a novel modified method is proposed, implemented, and evaluated. Results show that the modified ESF method removes the numerical error in the element mass conservation and shows capability in predicting both premixed and non-premixed flames relevant to dual-fuel combustion. Second, LES of n-heptane single-fuel and n-heptane/methanol dual-fuel combustion is carried out and validated against ECN Spray-H experiments. A good agreement is obtained in terms of flow, combustion, and emissions characteristics. Finally, a parameter study is performed to investigate the effects of the dual-fuel strategies, including the primary fuel concentration, the ambient temperature, and the pilot fuel injection timing. It is concluded that: 1) The ambient methanol is found to have an effect of suppressing the two-stage ignition and heat release of n-heptane, this is more significant under high ambient methanol concentration conditions. 2) The effects of methanol on the n-heptane ignition and NOx formation are strongly dependent on the ambient temperatures. The retardation of the n-heptane high temperature ignition is more remarkable under low ambient temperatures. The NOx and soot in the dual-fuel case is lower than that of the single-fuel case in moderately high initial temperatures, while an opposite trend is observed in higher temperatures. 3) A late injection may lead to an overlap of the ambient methanol auto-ignition and the delivery of n-heptane. This overlap results in high soot and NOx formation.
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