CFD modelling and investigation of two-stroke dual-fuel marine engines with high pressure gas admission

Student thesis: Doctoral Thesis

Abstract

Due to their durability, cost-effectiveness and high efficiency, the large two-stroke marine engines are widely used by the merchant ships. However, as the conventional two-stroke diesel engines suffer from the high pollutants emissions, the dual fuel versions burning natural gas and pilot fuel to initiate combustion is an alternative, which can considerably reduce the engine environmental footprint. The application of the high pressure and direct injection of the natural gas can remarkably benefit the emissions. In order to understand in-depth the full-cycle operating processes in a two-stroke dual fuel marine engine with high-pressure gas direct injection, the related CFD models were customized and developed by the use of ANSYS Fluent, and validated by employing available experimental data. Subsequently, the parametric investigation of the dual fuel injection was conducted and the recommended sets of design parameters are identified. Furthermore, the internal processes in the whole cycle of the engine dual fuel and the diesel operating mode were analysed and compared.The spray process of the liquid/pilot fuel was modelled and validated by the available experimental data, taking into account the variable Thermophysical liquid fuel properties with the ambient conditions. Aiming to develop the models for the high-pressure gas injection, the conserving-equation sources approach was developed, considering the effects of the barrel-shaped shocks patterns near the nozzle exit. The derived CFD models were validated by the published measured penetrations of nitrogen injection under two pressure ratios values. As the diffusion flame dominates in the high-pressure direct injection (HPDI) gas combustion, the non-premixed dual fuel combustion model was developed, in which the pilot fuel combustion was treated as the ignition kernel.Based on the measurements in the rapid compression and expansion machine (RCEM), the derived heat release rate (HRR) and the NO emission was used to validate the CFD results. By comparing the results of the two investigated non-premixed combustion models, the steady flamelet diffusion model was recommended, where the reaction rates of Hanson and Salimian (1984) for the extended Zeldvich mechanism were applied.In order to determine the injection and geometric parameters of dual fuel operation in the marine engine S60ME, the parametric research of HPDI, combustion processes was conducted with the aim to maintain the power level and reduce the NO and CO2 emissions. The investigated parameters included the dual fuel injection timing, the gas injection duration, the lateral angle of gas nozzle, the holes number of gas injector, and the different inclination angle for each gas hole.Based on the results of the conducted parametric study, the dual fuel design parameters for the marine engine S60ME were recommended. By using the developed dual fuel combustion models, the whole-cycle processes in the large two-stroke marine dual fuel engine were investigated, by comparing the diesel model operating mode. The results indicated that the NO and CO2 emissions for the dual fuel mode were lower than that of the diesel mode by 31% and 21% respectively. The diffusion flame for the diesel mode was located downstream the liquid vapour plumes, whilst the dual fuel mode exhibited the high-temperature flame in the vicinity of the stoichiometric surface of the gas plumes. Moreover, the diesel mode achieved the higher flame temperature than the dual fuel mode. Due to the lower carbon dioxide (CO2) for the duel fuel combustion, the scavenging efficiency for the dual fuel mode was 4.2% higher than that of the diesel mode.
Date of Award1 Oct 2017
LanguageEnglish
Awarding Institution
  • University Of Strathclyde
SponsorsUniversity of Strathclyde
SupervisorGerasimos Theotokatos (Supervisor) & Dracos Vassalos (Supervisor)

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