Theory and modelling of dual-fuel combustion

Abstract

Dual-fuel combustion is an attractive strategy for utilising lean mixtures of alternative fuels such as natural gas in internal combustion engines. In pilot ignited dual-fuel, a pilot injection of a more reactive fuel such as diesel provides the source of ignition for the fuel-lean mixture, resulting in high thermal efficiency and low emissions. The dual-fuel combustion process involves competition between deflagration, diffusion and autoignition combustion modes, presenting a challenge for established turbulent combustion models commonly tailored to model a single combustion mode. The present study addresses three key challenges that arise in dual-fuel engines, in addition to the challenges of the better-understood single fuel diesel engine: (a) the chemical interaction of the fuels on the ignition process; (b) the effect of the inhomogeneous and reactive conditions on the flame structure; (c) and the effect of both fuels on flame propagation speed. Chemical Explosive Mode Analysis of dual-fuel combustion reveals a complex process in which the majority of the inhomogeneous mixture is consumed by deflagration. Thus, the description of the effect of dual-fuel conditions on the propagation process is critical to the understanding and modelling of dual-fuel combustion. Flame propagation speed is analysed in detail using both laminar and turbulent conditions. The results reveal that the flame speed depends on chemical contribution of both fuels, and also depends significantly on the release of heat and pre-ignition chemical species ahead of the flame. A new model is developed to accurately capture these effects. The effects of dual-fuel composition on the ignition process are studied through laminar and turbulent simulations using methane as surrogate for natural gas. In order to isolate the thermal and chemical contributions of methane, the ignition process is compared with a simulation in which methane is treated as inert. Methane is known to retard ignition of higher-hydrocarbons. However, the analysis provided in this thesis reveals that the combination of the chemical effects of methane with the molecular transport makes an additional contribution to the retardation of ignition. A combination of different approaches is identified for modelling the premixed and nonpremixed phases of dual-fuel combustion. Two different modelling approaches are developed: a hybrid mixture fraction Conditional Moment Closure/G-equation approach; and a mixture fraction-progress variable Double Conditional Moment Closure (DCMC). The predictions are assessed by comparison to previous experimental data for a dual-fuel Rapid Compression-Expansion Machine. The hybrid model can adequately describe ignition and the transition to premixed flame propagation, and ignition of the premixed end gas. However, the hybrid approach is sensitive to the criteria used to couple the models, and these criteria are avoided with the DCMC approach. The DCMC model is applied as a tabulated flamelet solution assuming statistical homogeneity in space and time. The novel application of DCMC in dual-fuel showed good prediction of heat release rate compared with the experimental data and highlights the potential of the model to predict dual-fuel combustion.

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Identifiers

ORCID for Edward Richardson: orcid.org/0000-0002-7631-0377

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