Numerical Simulation of Combustion under Marine Engine Like Conditions

This thesis presents the numerical study of combustion under marine engine like conditions. The thesis is divided into two main parts. In the first part, combustion is studied in a large two-stroke marine engine with conventional diesel fuel. In the second part, two different dual-fuel combustion modes of diesel-methane i.e. non-premixed combustion and premixed combustion are studied. All numerical models are validated with the experimental data.

First, in a simplified geometry of the marine engine, conjugate heat transfer (CHT) calculations are applied to simultaneously solve the in-cylinder gas phase dynamics and the temperature field within the liner of the engine. The effects of different initial temperatures across the liner and the effects of the amount of water vapor in the air on the sulfuric acid formation and condensation in a large two-stroke marine engine are studied. An initial temperature is calculated based on heat transfer modeling and it is observed that the sulfuric acid vapor formation is more sensitive to the variation of the water vapor amount than the sulfuric acid condensation. In the next step, the effects of the turbulence modelling on the simulation of the full cycle of the engine including scavenging process, combustion, and emission formation is studied in a real geometry of a large two-stroke marine research engine. The Unsteady Reynolds Averaged Navier-Stokes (URANS) and Large Eddy Simulation (LES) turbulence models are utilized for modeling of in-cylinder turbulent flow. The accuracy of the tangential velocity and swirl flow in the top of the cylinder where the fuel is injected is crucial to predict the air-fuel mixing correctly. It is found that URANS predicts a solid body rotation for the tangential velocity in this region. However, LES predicts a tangential velocity that is uniformly distributed in the radial direction that is consistent with experimental results. Furthermore, during the scavenging process, LES is able to predict the Burgers vortex upstream of the cylinder near the scavenging ports. Also, LES predicts a higher angular momentum inside the cylinder in comparison with URANS. During the combustion process, the LES model shows a moderately better performance in capturing the experimental pressure and heat release rate profiles than URANS. However, the predicted gas temperature at the liner wall is approximately 45% higher for URANS than LES during the expansion stroke, which is attributed to a higher predicted turbulent viscosity in the URANS case. A higher temperature of gas beside the liner wall may decrease the sulfuric acid formation and increase the heat transfer. The higher predicted swirl by LES than that in URANS leads to an earlier and stronger interaction between the flame and the spray, decreasing the oxidation of the emissions. The second cycle LES simulation shows that the solutions after the scavenging process are in-sensitive to the initial conditions and the main governing parameters are boundary conditions and injection characteristics.

Next, two different dual-fuel combustion modes of non-premixed combustion and premixed combustion are studied. The non-premixed combustion is simulated and validated with the experimental data of a large two-stroke marine research engine under low and high engine loads. Based on the results, further methane jets penetration in the low load case leads to better air-fuel mixing and a higher combustion intensity than that in the high load. Effects of the pilot diesel fuel injection timing on combustion and emission formation and the governing mechanisms are also investigated in detail. Results indicate that the intense combustion of the accumulated methane expands the methane flame towards the piston when the pilot injection timing is retarded. The NO formation is lower in the high load case due to the lower combustion intensity. Also, retarding the pilot injection timing decreases the NO formation. Furthermore, the effect of the direction of pilot diesel injection is investigated which shows a significant effect on the methane start of combustion and intensity as well as flame propagation direction which leads to different heat transfer trends from the combustion chamber walls.

Premixed combustion is analysed in a constant volume ombustion chamber (CVCC) and validated with experimental data. Results show that by simulation of methane-air mixing, the numerical model is able to capture the ignition delay time (IDT) within a maximum relative difference of 7% to the measurements. A higher relative difference of 38% is obtained when methane gas injection is omitted and the methane-air and temperature are assumed homogeneous. Therefore, it is concluded that the simulation of methane-air mixing process is crucial in this type of combustion due to the presence of inhomogeneities in both methane fuel and temperature distribution after mixing. Creating the idealized inhomogeneities for separately investigation of methane and temperature inhomogeneities shows that the inhomogeneity in the temperature has a more profound influence on the IDT than the inhomogeneity in the methane distribution. Furthermore, the effects of the number of pilot fuel nozzle holes on the auto-ignition are studied. The auto-ignition process in two cases with 4 nozzle holes is investigated and compared with the base case with 8 nozzle holes. Considering the same amount of pilot fuel, the injection rate is assumed to be double in one of the cases, while in the other case, the injection duration is doubled. Results show that a reduction of the nozzle hole numbers can improve the pilot diesel ignition in the case with 4 nozzle holes and double injection duration compared to the base case with 8 nozzle holes. However, combustion in the case with 4 nozzle holes and a double injection rate is incomplete due to flame impingement on the walls.