Mechanical Engineering - Theses
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ItemNo Preview AvailableAssessing the impact of wall heat transfer on knocking combustion of hydrogen using DNS and LESDou, Xinbei ( 2023-06)Hydrogen has recently received significant attention due to its potential as a clean energy source. However, under some operating conditions, hydrogen is prone to auto-ignition, leading to engine knock and damage to engine components. In order to understand the underlying mechanism of hydrogen knocking combustion, this thesis employs numerical simulations to study the knock mechanism of different intensities. A particular focus is placed on the end-gas auto-ignition behaviour and its impact on pressure oscillations and wall heat transfer. 1D simulations with detailed chemistry are performed to investigate two types of hydrogen end-gas auto-ignition in a confined space: auto-ignition initiated by a hot spot and auto-ignition initiated within a homogeneous mixture. The former configuration explores deflagrative auto-ignition with pressure oscillations of several bars, while the latter investigates developing and developed detonation with pressure oscillations of hundreds of bars. The results show that the hot-spot-induced auto-ignition is mainly influenced by hot spot locations, with negligible impacts from the near-wall temperature gradients. Pressure oscillations are mainly influenced by flame annihilation, and the wall heat flux is mainly influenced by the flame head-on-quenching event, providing insights into understanding the trace knock phenomenon. For auto-ignition initiated within a homogeneous mixture, near-wall temperature gradients have a significant impact on the number of auto-ignition events, their locations, and combustion modes. Especially under lean conditions, with increased near-wall temperature stratification, the auto-ignition location can switch from the near-wall to the near-flame region, and its mode can switch from the developing to the developed detonation. By analysing the temperature and pressure development in the end gas, the generation of auto-ignition is found to have a close relation to pressure fluctuations. Furthermore, near-wall temperature gradients also have an influence on the wall heat flux and pressure oscillations, mainly through their impact on auto-ignition behaviour. For auto-ignition initiated within a homogeneous mixture, pressure oscillations are dominated by the auto-ignition event, and the wall heat flux is influenced by both flame head-on-quenching and the pressure wave hitting the wall. The conclusions in this part help explore the mechanisms of conventional and superknock. In addition to the 1D simulations, this research also explores the occurrence of hydrogen knock using Large Eddy Simulations (LESs) in the Cooperative Research Fuel (CFR) engine, featuring conventional engine knock. Initially, the study evaluates the performance of four commonly-used wall heat transfer models, namely, the Angelberger, Han and Reitz, O'Rourke and Amsden, and GruMO-UniMORE models. The study concludes that while the commonly-used wall heat transfer models perform well in normal combustion, they generate cycles that fall outside the experimental range in knocking combustion, with an increasing number of such out-of-experimental-range (OOR) cycles as the knock intensity increases. Among different wall heat transfer models, the Han and Reitz model is the most suitable model for hydrogen knock modelling as it balances simplicity and accuracy and simulates fewer out-of-range cycles under all conditions. Furthermore, the wall heat transfer characteristics are explored using the Han and Reitz model. For knocking combustion, auto-ignition is prone to occur at locations where the near-wall temperature stratification is most disturbed, which is near the knock meter cavity. The near-wall temperature gradient grows in the burnt area, acting as a barrier and protecting the wall from high wall heat flux. On the contrary, the pressure wave generated by auto-ignition has a significant impact on the wall heat transfer. It influences the wall heat flux through its impact on the flow velocity. As knock intensity increases, the impact of the pressure wave persists for a long time, leading to a higher peak heat transfer rate compared with low-intensity cases.