Computational Study and Optimization of Flow and Combustion Processes in Marine Engines Operating with Heavy Fuel Oil

Abstract

Heavy Fuel Oil (HFO) is the predominant marine fuel. Its future use will be affected by the global 0.50% sulfur cap entering into force in 2020. It is widely accepted nowadays that Computational Fluid Dynamics (CFD) studies can substantially contribute to understanding and optimizing engine aerothermochemistry, provided that the key in-cylinder processes, namely, spray break-up, evaporation, fuel-air mixing, ignition and combustion, are properly accounted for in the frame of CFD modeling. The present thesis constitutes an extensive computational CFD study of non-reactive and reactive HFO spray physics in the context of marine engines, including applications of optimizing HFO injection in large two-stroke marine engines. The study is supported by experiments. In this frame, a new integrated model for calculating the thermophysical properties of marine HFO has been developed in the present work. The model considers HFO as an equivalent one-component heavy petroleum fraction with undefined composition, and requires as input four values of fuel bulk properties, commonly measured at fuel bunkering. Thus, the model accounts for any HFO quality stored onboard a vessel. The model predicts a large set of fuel properties relevant for engine CFD studies, including temperature dependence. Further, model validation is performed by means of measurements of a number of properties of different HFO qualities. Next, the new model is applied to calculate the thermophysical properties of seven widely used marine heavy fuel grades as prescribed by ISO 8217:2010. Also, the model is tested with CFD simulations of non-reactive HFO spray flow in a large constant volume chamber, and the results are compared against recent experiments. Here, spray modeling is based on a proper adaptation of the cascade atomization and drop break-up (CAB) model. All computational results are in very good agreement with experiments. Moreover, the new detailed model of HFO thermophysical properties is utilized for extensive CFD studies of HFO ignition and combustion in a large spray combustion chamber (SCC) and in a large marine engine; results are compared against existing and recent experiments, for two HFO qualities. A new kinetic model accounting for HFO aromaticity is developed and used for ignition modeling. Computational results are reported for reactive spray flow in the SCC, and are in good agreement with experimental data. The effects of HFO preheating on spray development are demonstrated. Finally, simulation results in a large marine engine are in good agreement with experiments in terms of pressure, heat release rate and pollutant emissions. Overall, the present modeling is shown to be appropriate for detailed CFD studies of HFO spray flow and combustion in marine engines. In a final step, multi-objective optimization is deployed to investigate the effects of a three-pulse HFO injection strategy (pilot-, main- and post-injection) on Specific Fuel Oil Consumption (SFOC) and the final concentration of nitric oxides (NOX) and soot of a large two-stroke marine engine. An optimization procedure including unconstrained and constrained problems is formulated, by coupling the present modified KIVA-3 based CFD code with an optimization tool based on Evolutionary Algorithms (EAs). For both problems, significant reductions in pollutant emissions are attained, in comparison to a reference case characterized by HFO continuous injection; for the constrained problem, the reductions reach 15% and 11% for NOX and soot, respectively.