Assessment of a hydrogen-fueled swirling trapped-vortex combustor using large-eddy simulation

In recent years, the study of hydrogen combustion has gained significant interest due to its exceptional lean blow-out (LBO) performance, high efficiencies, and almost negligible greenhouse gas emissions. The present study combines the advantages of hydrogen combustion with the beneficial characteristics of trapped-vortex combustion (TVC) and swirling flows. Large-eddy simulation (LES) is conducted to evaluate the performance of a swirling trapped-vortex combustor. LES sub-gird terms are described using k-equation subgrid model, while, turbulence-combustion interaction is modelled employing the PaSR (Partially Stirred Reactor) closure with a detailed 23-step chemical reaction mechanism for hydrogen/air. A radial vane swirler is incorporated to supply an annular mainstream and generate a wake region in front of the cavity leading edge to enhance cavity flow entrainment. To evaluate the performance of a hydrogen-fueled TVC under non-swirl and swirling conditions with varying swirl numbers (0.1-0.6), several flow and combustion characteristics are analyzed. These includes temperature distribution, vortical structure, combustion efficiency, NO emission, and flame shape. The results show that a large recirculation zone dominates the cavity for the all swirl numbers resulting in a strong fuel-air mixing and high combustion efficiency. It is demonstrated that the swirling motion effectively mitigates the negative impacts of pressure fluctuations arising from the interaction of combustion with cavity oscillations. A stable flame is observed inside the cavity and in the shear layer at the cavity lip in all cases, merging to form a distinctive C-shaped flame. Increasing the swirl number causes the second part of the flame to move to the wake region before the leading edge, enhancing combustion stability and efficiency. Combustion efficiency exceeds 98.5% for all cases, reaching 99.8% for a swirl number of 0.6. The temperature distribution is nearly uniform inside the cavity, with an improved distribution at the TVC's outlet for higher swirl numbers. Under non-swirling conditions, the corrected NO concentration is below 5 ppm. Increasing the swirl number escalates the outlet NO emission, nevertheless, a maximum NO concentration of 19 ppm is observed for the swirling cases. Comparing these findings with existing data suggests that the swirling TVC offers advantages over other hydrogen-fired TVC's.