The role of precessing vortex core in two combustion regimes: Numerical simulation studies

Large Eddy simulation (LES) with finite rate chemistry was used to investigate the combustion dynamics in a lab-scale PRECCINSTA combustion chamber. Transient three dimensional numerical simulations were carried out at two different thermal powers (10 kW and 35 kW) with a fixed equivalence ratio of 0.7. The predicted results were compared with the experimental data and good agreements were found between them. In the cold flow field under both conditions, a precessing vortex core (PVC) in the inner shear layer (ISL) existing between the swirling jet and the inner recirculation zone (IRZ). However, two different flow and combustion dynamics were observed when combustion occurred. At thermal power of 10 kW, there was a V-shaped flame and the combustion of the flame was stable. The PVC disappeared and the vortices arrangement was symmetrical in the ISL. However, at 35 kW, there was a M-shaped flame with a PVC in the ISL and combustion instability triggered. In depth analysis of the characteristics of flow, temperature and heat release field, we found that the flame surface was wrinkled periodically by the PVC which enhanced the mixing between the cold fresh gas and hot burned products. Then, the mixture was ignited locally and heat release was rapid in the middle of the combustion chamber. These effects were directly related to the periodic vortices motion which was induced by PVC. It was confirmed that the influence of PVC on flame surface and heat release is an important factor for triggering the combustion instability at thermal power of 35 kW. The zone division based on different roles of flow/flame and thermoacoustic coupling was also discussed to illustrate the combustion instabilities caused by PVC.