Numerical modeling of spray-assisted dual- and tri- fuel combustion processes

Abstract

This dissertation is related to the research areas of computational physics and numerical modeling of combustion. The study intends to determine the ignition characteristics of dual-fuel (DF) and tri-fuel (TF) sprays when a high-reactivity fuel spray (n-dodecane) is mixed with a low-reactivity fuel (methane/hydrogen/or its blends) and an oxidizer/EGR in a hot ambient environment. In particular, engine-relevant operating conditions are investigated. A better understanding of the ignition phenomena could lead to improved ignition control, improved thermal efficiency, and lower emissions. By utilizing computational fluid dynamics, numerical combustion modeling and high-performance computing, detailed investigations of the three-dimensional physics and chemistry of such reacting flows can be studied. The present dissertation is based on three journal publications. Large-Eddy Simulation (LES) and quasi-DNS approaches are used in combination with finite rate chemistry and OpenFOAM for CFD simulations. In Publication I, Yao and Polimi reduced mechanisms were studied to determine the effect of temperature on DF spray ignition and methane's inhibition of n-dodecane chemistry. Publication II discusses the use of the tri-fuel (TF) strategy with diesel spray-assisted ignition in mixtures of methane and hydrogen. The ignition characteristics and heat release rate of TF sprays have been investigated in engine-relevant conditions. In Publication III, the DF ignition studies are extended to better understand the combustion progression after ignition. In general Publication I- Publication II are related to understanding spray-assisted ignition phenomena while Publication III focuses on understanding the evolution of ignition fronts to deflagration using a simplified approach based on a three-dimensional reacting shear layer. The main conclusions of this dissertation are as follows: 1) Methane inhibits n-dodecane spray IDT at low temperatures, especially in the simulation of DF sprays at various ambient temperatures relevant to the engine conditions. Moreover, this behavior has been numerically confirmed to be similar with both Yao and Polimi reduced chemical mechanisms. 2) As hydrogen is added to the ambient methane in the same DF setup, it becomes a TF setup, wherein n-dodecane's ignition characteristics are delayed even further than in the DF setup. Moreover, the high-temperature combustion heat release mode in TF appears more pronounced than in the low-temperature combustion mode, in comparison with methane-diesel combustion in DF. 3) The numerical simulation of shear layer-driven dual-fuel combustion processes allows for the numerical evidence of the emergence of deflagration fronts in dual-fuel combustion within a short time interval, 0.2 to 0.4 IDT after the ignition.