Optimisation of Low NOx Hydrogen Micromix Combustor

The growing environmental concerns have prompted a search for alternative fuel options, leading to the exploration of hydrogen as a promising alternative aviation fuel. Hydrogen's clean energy attributes, with emissions limited to H2O and NOx, make it an attractive candidate for sustainable aviation. However, its integration into gas turbines necessitates the modification of conventional combustors to accommodate hydrogen combustion. The concept of micromix combustion has emerged as a promising and dynamic solution to address the challenges of safe hydrogen combustion. Aachen University has conducted extensive studies and experiments about micromix combustors in the last decades, and many researchers have also shown great interest in it. The results of these investigations have demonstrated the feasibility and potential of the micromix combustor concept as a viable and effective option for hydrogen combustion. This project is dedicated to enhancing the performance of the micromix combustor, with a specific focus on improving its NOx reduction capability through geometric modifications of the burner. The optimization process of the micromix combustor is carried out in this thesis through comprehensive numerical simulations utilising ANSYS FLUENT. Initially, a comprehensive literature review was conducted to assess and evaluate various numerical settings relevant to hydrogen combustion. Different numerical models were compared, and the most suitable settings were selected for subsequent investigations. Specifically, the turbulence model, combustion model, and kinetic mechanisms were thoroughly examined, and suggestions for the numerical model’s selection were made for micromix hydrogen combustion. Following that, the geometric design of the micromix combustor was studied, involving the modification of parameters such as mixing distance, air gate height, and hydrogen injection pipe diameter. Simulation studies of the modified models were conducted. The results convincingly demonstrated the significant influence of geometric modifications on combustion performance. NOx emissions produced during combustion were successfully reduced by changing the internal geometry of the micromix combustor, and conclusions were drawn regarding the tendency of NOx emission variation with changes in geometric parameters. Beyond single micromix combustor elements, the combustion of multi-injection elements has also been studied. The burner was enlarged to reduce the number of elements; in this process, the effects of air gate width variation on flame size and NOx emission were studied. Additionally, a new MIC design was proposed to reduce NOx formation and eliminate the risk of flame merging between adjacent micromix injection elements. In this project, applying hydrogen/ammonia blended fuel to the micromix combustor was attempted. The numerical simulation study was conducted for different fuel compositions, and the results demonstrated that the addition of ammonia into hydrogen could lower the combustion temperature and slow the burning velocity. Overall, this thesis has demonstrated the promising potential for enhancing the NOx reduction capability of the micromix combustor through physical modifications. The optimization of the micromix combustor was accomplished by altering the burner's geometry, and the feasibility of utilizing different fuel types in the micromix combustion concept was explored. These findings contribute valuable insights towards advancing cleaner and more efficient combustion technologies.