This dissertation belongs to the field of computational physics and chemistry with a research focus on reacting fuel sprays in internal combustion engine context. Computational fluid dynamics (CFD) and chemical kinetics modeling methods are utilized to simulate turbulent reacting fluid flows in engine conditions. The dissertation comprises four journal publications targeting the modeling of dual-fuel pilot ignition system, commonly used in natural gas engines. In such an ignition concept, high-reactivity fuel (e.g. diesel) is shortly injected into a mixture of low-reactivity natural gas and air during the compression stroke. Diesel fuel autoignites and releases enough energy to initiate a premixed natural gas-air flame. Hence, the diesel spray acts similar to a large spark.
The use of natural gas as a primary fuel in engines is a contemporary topic of interest from the industrial and academic point of views. Utilization of a lean natural gas-air mixture together with modern low-temperature combustion techniques may enable reductions in emission levels. However, methane, the main component of natural gas, is a harmful greenhouse gas. Incomplete combustion due to e.g. unsuccessful ignition leads to direct methane emissions, degraded thermal efficiency and increase in fuel consumption. To avoid such complications, advanced ignition system designs have been proposed, including the dual-fuel pilot ignition.
The focus of the present work is on the analysis of mixture formation and autoignition characteristics in engine conditions. The present study is the first numerical investigation on dual-fuel pilot spray ignition problem by large-eddy simulation (LES) and finite-rate chemistry. A major effort was put on creating a highly efficient finite-rate chemistry solver by utilizing two open source libraries OpenFOAM and pyJack. The developed solver framework enabled the use of high-resolution (62.5 micrometers) grid and complex chemical mechanisms (e.g. 97 species and 997 reactions) in the simulations. The investigated spray setup corresponds to the Engine Combustion Network (ECN) Spray A configuration. ECN provides an open-access data repository and a forum for international experimental and numerical collaboration, enabling an extensive validation of numerical models in terms of a single-fuel diesel spray.
The dual-fuel simulations offer the following novel accomplishments: 1) The first published LES study on interactive physics and chemistry of the pilot spray ignition process. 2) The applied zero-, one- and three-dimensional simulations complement one another providing profound understanding on why methane prolongs diesel ignition. 3) Low- and high-temperature combustion characteristics are assessed, indicating the importance of low-temperature reactions. 4) Consistent with experiments, the connection between spray over-leaning and retarded ignition process is explained and quantified.
|Publication status||Published - 2019|
|MoE publication type||G5 Doctoral dissertation (article)|
- dual-fuel, pilot ignition, ECN, spray A, LES