Role of formation and transportation of hydroxyl radicals for enhanced late soot oxidation in a low emissions heavy-duty diesel engine.

Paper i proceeding, 2010

In previous research enhancement of late soot oxidation by increased mixing rates has been identified as an important measure to decrease engine-out soot emissions. Additionally, the availability of oxidants, especially hydroxyl radicals (OH) in the mixing zones, is considered as essential for an effective fuel and soot burn-out process.

The objective of this study was to combine experimental and computational methods to identify the role of OH for late soot oxidation. Experimental data from a high pressure/high temperature spray chamber and a heavy-duty single cylinder engine were analysed using combustion CFD. The formation, lifetime and transportation of OH were studied together with other important in-cylinder parameters. The soot formation and oxidation processes were analysed for a dataset of engine results with equal NOx emissions but having a large variation of engine-out soot emissions.

The spray chamber experimental set-up includes a flame divider designed to enable studies of the OH history in the leading edge of the flame vortex. The engine combustion CFD results could be compared with CFD analysis of the spray chamber results regarding the behaviour of OH.

From this study, it is suggested that the fluctuating interface between the soot leading edge and the important soot oxidant specie OH is influenced by interactions between local turbulent flame movements and the availability of OH. The soot and OH interface was observed to develop smoothly in a strongly re-directed side of a flame as well as on the free side of the same flame. According to experimental observations and analysis of CFD source terms of soot formation and oxidation rates, both the OH radical pool and the soot oxidation reach farther downstream in the flame than the soot formation. The study gave an increased understanding of limiting factors for enhanced late soot oxidation during diesel engine operation.