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Development of CFD Tools for Large Marine Diesel Engine Applications

Periodic Report Summary - MARINECFD (Development of CFD tools for large marine diesel engine applications)

The design of more powerful, fuel-efficient and environmentally friendly propulsion systems is currently one of the main goals of engine researchers and manufacturers worldwide. State of the art computational fluid dynamics (CFD) methods can be a valuable tool to gain insight into in-cylinder mixing and combustion phenomena and investigate strategies for minimising harmful pollutants from internal combustion engines.

The research performed under the MARINECFD programme consisted of physical model development and detailed CFD studies of internal combustion engine aerothermochemistry, using state of the art techniques. Emphasis was placed on large marine diesel engine applications. The new models were validated against advanced experiments, both in constant-volume combustion chambers and marine diesel engines.

More specifically, the following four areas were studied in the course of MARINECFD:
1. modelling of fuel spray atomisation in large marine diesel engines. Due to the large size of injectors and the use of heavy fuel oil (HFO) in marine applications, the governing physics of spray breakup was affected. In this context, a new model was developed, to account for the different thermophysical properties of HFO, and was subsequently evaluated within the framework of a number of spray atomisation models.
2. modelling of fuel evaporation. Existing evaporation models were further developed to account for multi-component fuels, as well as to improve the heat transfer predictions from the gas to the liquid phase.
3. heat transfer modelling. Engine heat transfer modelling often mistreated thermal radiation, which could account for up to 50 % of the total heat losses. In MARINECFD, heat transfer models were implemented to allow for a proper treatment of heat losses to engine walls.
4. computation of water addition techniques. Computational studies of water addition techniques in the combustion chamber were performed to represent nitric oxides' (NOX) reduction from large marine diesel engines. Further model development for fuel-water droplet collisions and liquid films was underway by the time of this report.

In order to account for the thermophysical properties of the typically used HFO, a new fuel model was developed and implemented in the CFD code KIVA. In addition, the performance of two spray atomisation models was evaluated in constant-volume combustion chamber cases, under conditions relevant to marine diesel engines. A multi-component evaporation model was also developed, including an improved treatment of the heat transfer from the gas to the liquid droplets, resulting in increased evaporation rates.

With respect to NOX emissions' reduction, a computational investigation of two water addition techniques was performed, namely air fumigation and direct water injection. The investigation showed that, in the framework of water addition techniques, optimising fuel and water injection parameters could lead to significant NOX reductions with only minor effects on the engine fuel consumption. To improve combustion predictions further model development, in terms of modelling fuel-water droplet collisions, was undertaken.

Computational study of direct water injection in a large two-stroke marine diesel engine consisted of colour-coded contours of temperature at a horizontal plane, including the injectors, for different locations of water injectors at 8 oCA aTDC. The injected water mass was equal to 40 % of the fuel mass.

Finally, detailed heat transfer model development, focussing on an improved model for wall heat transfer, was performed and applied for the first time to large marine diesel engine simulations.

The progress of the MARINECFD project was presented on several occasions, including international conferences and visits to research centres and industrial stakeholders worldwide. Several peer-reviewed publications demonstrated and elucidated the main achievements of the research work performed. The proposed innovations provided tools and methodologies for optimising combustion and reducing the environmental impact of marine diesel engines, thus meeting international emissions regulations while maintaining their high efficiency.