Optimization of a simplified sub-model for NO emission prediction by CFD in large 4-stroke marine diesel engines

Abstract

A simplified sub-model for NO emission prediction at pressurized conditions has been put forth at Åbo Akademi University [7,9] including NO formation via the thermal NO path (3 reactions) and via the nitrous oxide intermediate paths (2 + 5 reactions). CFD simulations carried out with the sub-model for marine and off-road diesel engines showed, however, that it significantly – by an order of magnitude – over-predicted NO emission as compared to measurements. The objective of this work was to find the reasons to the discrepancy and to suggest and incorporate improvements. By detailed investigations, a number of programming technical errors and chemical kinetic shortcomings were identified. The improved sub-model and its sub-parts were then tested for CFD simulation of a medium-speed, four-stroke, direct-injection marine diesel engine for different loads and fuels. The importance of NO reduction by soot and hydrocarbons was also investigated. All the sub-models correctly predicted the trend of increasing NO emission with increasing load. In absolute amounts, NO emission was over-predicted by a factor of 2 to 4, if no fitting of rate constants was allowed. Including NO reduction by soot and hydrocarbons, decreased NO emission by ca 4–25% for the cases studied.