In the present work, a study of different numerical heat transfer models is presented used for Homogeneous Charge Compression Ignition (HCCI) internal combustion engine simulations. Since the heat loss through the walls of an engine is an important parameter during engine optimization, as it influences power, efficiency and emissions, accurate modeling techniques need to be available. In this work, the predictive capability of different Computational Fluid Dynamics (CFD) models has been assessed, by using data obtained from experiments on a Cooperative Fuel Research (CFR) engine, a simple single cylinder pancake engine, which has been probed with local heat flux sensors into the combustion chamber walls. The open-source software OpenFOAM R was used to perform simulations of this engine, under both motored and HCCI operation, with a specific focus on the performance of different heat flux models. Due to the simple engine geometry, more numerically demanding heat flux modeling methods could be used, maintaining an acceptable computation time. This allowed a full comparison between the equilibrium wall models as in standard use, an improved empirical heat flux correlation and a numerically intensive low Reynolds formulation. The numerical results considering all aspects of the heat flux-both its progress in time as well as quantitative aspects such as the peak heat flux or the total heat loss-have then been compared to an extensive experimental database. This allowed a full analysis of the performance of the different methods. It was found that the low Reynolds formulation described the physical behavior near the wall the best, while predicting acceptable results concerning the heat flux through the engine walls. The best heat flux prediction was however obtained with an improved empirical model, which additionally has a much shorter computation time. This is crucial when moving on to heat flux simulations of more complex production-type engines. Lastly, the equilibrium models were never capable of accurately predicting the wall heat flux.

The present work details a study of the heat flux through the walls of an internal combustion engine. The determination of this heat flux is an important aspect in engine optimization, as it influences the power, efficiency and the emissions of the engine. Therefore, a set of simulation tools in the OpenFOAM® software has been developed, that allows the calculation of the heat transfer through engine walls for ICEs. Normal practice in these types of engine simulations is to apply a wall function model to calculate the heat flux, rather than resolving the complete thermo-viscous boundary layer, and perform simulations of the closed engine cycle. When dealing with a complex engine, this methodology will reduce the overall computational cost. It however increases the need to rely on assumptions on both the initial flow field and the behavior in the near-wall region. As the engine studied in the present work, a Cooperative Fuel Research (CFR) engine, is a simple single cylinder pancake engine, it was possible to implement more complex and numerically demanding methodologies, while still maintaining an acceptable computation time. Both closed and full cycle simulations were therefore performed, for which the heat flux was calculated by both implementing various wall function models and by resolving the complete thermo-viscous boundary layer. The results obtained from the different kind of simulations were then compared to experimental heat flux data, which was measured using a thermopile type heat flux sensor in different locations in the CFR engine. By comparing the results from the different types of simulations, a performance evaluation of the used methodology could be carried out. It was found that the heat flux obtained by resolving the thermo-viscous layer was accurate compared to experiments, while the wall functions were not able to correctly capture the heat flux. Full cycle simulations resulted in a slightly improved result, especially when resolving the boundary layer, but due to the increased computational cost, this method does not seem beneficial.

The present work details a study of the 3D-modeling of dual-fuel engines, with a specific focus on medium speed marine engines. These operate under the fumigated approach, which means that the second fuel, which replaces most of the diesel fuel, is added in the intake duct. This type of engine requires the least amount of modifications to the current medium speed diesel engine and is therefore an ideal retrofit solution to tackle global warming and local air quality issues. In this work, the operation of such an engine has been modeled, with special care for the combustion modeling. Namely, it comprises both auto-ignition of the diesel pilot, and flame propagation in the premixed fuel-air mixture. A tabulated kinetics approach, currently being used to model diesel operation, has been extended to include this flame propagation. Both a Coherent Flame Model (CFM) and a Flame Surface Wrinkling Model (FSWM) have been implemented to handle this premixed combustion mode. Coupling a flame propagation model to the tabulated chemistry should provide a practical way of capturing both the complex dual-fuel combustion process as the chemistry information, while maintaining an acceptable computation time. Additionally, a tabulated laminar flame speed method has also been implemented. Experimental results were obtained on the operation of a medium-speed engine in diesel, natural gas/diesel dual-fuel and methanol/diesel dual-fuel mode, under varying operating loads and speeds. Results have been compared to the ones obtained from the simulations performed in OpenFOAM with the dedicated combustion modeling technique. It was found that while both FSWM and CFM capture some physical trends, they are currently not able of capturing the dual-fuel combustion process in total.Improvements with regards to the ignition and mixing-controlled combustion modeling are necessary, since the current approach is not able of fully capturing the dual-fuel phenomena. This work however provides insight in the complex combustion process and serves as a basis for further developments. It can also be used as an initial engine development tool for fast calculation and optimization of dual-fuel operation.

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In this work, the possibility to perform a cold-flow simulation as a way to improve the accuracy of the starting conditions for a combustion simulation is examined. Specifically, a dual-fuel marine engine running on methanol/ diesel and natural gas/diesel fueling conditions is investigated. Dual-fuel engines can provide a short-term solution to cope with the more stringent emission legislations in the maritime sector. Both natural gas and methanol appear to be interesting alternative fuels that can be used as main fuel in these dual-fuel engines. Nevertheless, it is observed that combustion problems occur at part load using these alternative fuels. Therefore, different methods to increase the combustion efficiency at part load are investigated. Numerical simulations prove to be very suitable hereto, as they are an efficient way to study the effect of different parameters on the combustion characteristics. These simulations often describe the engine with a limited engine geometry neglecting the inlet and exhaust duct. This gives rise to the need to assume certain starting conditions such as the turbulence coming from the intake valve and the homogeneity of the air/fuel mixture entering the combustion chamber. Hence this work presents the execution of a cold-flow simulation taking into account the whole engine geometry that can provide more realistic initialization values for combustion simulations. FIGURE 7 Inlet and outlet mass flow of the NG simulation.