Partially Premixed Combustion in Counterflow Flame and Dual Fuel Compression Ignition Engine
The overall objective of this research is to examine strategies for reducing NOx and soot emissions in diesel engine. The thesis has two parts. In the first part, the effect of unsaturation or the presence of a double bond in the fuel molecular structure on NOx and soot formation is investigated. Simulations have been performed for partially premixed flames burning n-heptane and 1-heptene fuels in a counterflow configuration and a constant volume diesel combustion vessel to examine the effect of unsaturation at different level of partial premixing and strain rate. A validated detailed kinetic model with 198 species and 4932 reactions has been used in the counterflow flame simulations. Results indicate that the presence of unsaturated bond leads to increased formation of acetylene and propargyl through scission reactions, resulting in higher prompt NO, PAH and soot in 1-heptene flames than in n-heptane flames. Since these results are obtained in laminar flames, the study is extended to examine the effect of double bond in spray flames at diesel engine conditions. 3-D simulations are performed using CFD code (CONVERGE) to examine the structure and emission characteristics of n-heptane and 1-heptene spray flames in a constant-volume combustion vessel. The directed relation graph methodology is used to develop a reduced mechanism (207 species and 4094 reactions) starting from the detailed mechanism (482 species and 19072 reactions). Results indicate that the combustion under diesel engine conditions is characterized by a double-flame structure with a rich premixed reaction zone (RPZ) near the flame stabilization region and a non-premixed reaction zone (NPZ) further downstream. Most of NOx is formed via thermal NO route in the NPZ, while PAH species are mainly formed in the RPZ. The presence of a double bond results in scission reactions, leading to higher temperature and consequently higher NO in 1-heptene flame than that in n-heptane flame. It also leads to a significantly higher PAH species, implying increased soot emission in 1-heptene flame than that in n-heptane flame. Reaction path analysis indicated that this is due to significantly higher amounts of 1,3-butadiene and allene formed from scission reactions due to the presence of double bond. In the second part of this research, a dual-fuel strategy for reducing emissions in a diesel engine has been examined. N-heptane and methane have been used as surrogates for diesel (pilot fuel) and natural gas (main fuel), respectively. The physical and chemical processes of dual-fuel combustion are simulated using CONVERGE and a reduced reaction mechanism (42 species, 168 reactions). The mechanism is validated against the experimental data for ignition and flame speed at engine relevant conditions. In engine simulations, methane is premixed with air during the intake, and then ignited by the n-heptane pilot injection. The heat release for the single-fuel case involves a hybrid combustion mode, characterized by rich premixed combustion and diffusion combustion, whereas for the dual-fuel combustion cases it also involves lean premixed combustion with a propagating flame. In addition, simulations focus on the effect of injection timing and the amount of n-heptane injection on the ignition, combustion and emissions in diesel engine. The minimum amount of n-heptane in terms of fractional energy required to ignite the methane/air mixture is analyzed at medium and high engine load conditions. The optimum injection timing is also determined considering engine thermal efficiency and soot/NOx emissions by sweeping through a range of start of injections (SOI) for each engine load and n-heptane injection quantity. The effects of SOI and the amount of n-heptane on emissions are analyzed. Results indicate high UHC emissions due to unburned methane in the crevice region at medium load, and high CO emissions in the n-heptane spray region at high load. The present results can provide guidaselines for the dual-fuel engine development.
NOx and PAH emissions
counterflow partially premixed flame
two stage ignition