A computational study of preferential diffusion and scalar transport in nonpremixed hydrogen-air flame

The nonpremixed hydrogen-air reacting flow is simulated using three-dimensional direct numerical simulation coupled with flamelet generated manifolds based on detailed chemical kinetics. From the comparisons between one computational case taking into account preferential diffusion and another case with unity Lewis number assumption, the instantaneous results show that the flow is more vortical in the absence of preferential diffusion. This indicates that preferential diffusion may smooth the flame under certain circumstances when coupled with the intrinsic hydrodynamic instability. The flame compositional structures are also influenced by preferential diffusion in a significant manner. Further, the statistical information suggests that turbulent scalar flux is affected by preferential diffusion. The phenomenon of counter-gradient diffusion of both the conserved and non-conserved scalars can be detected for the two cases. The gradient model for scalar closure is found to be incapable of accurately predicting the scalar transport in nonpremixed hydrogen flames

A comparative study of instabilities in forced reacting plumes of nonpremixed flames

A comparative study has been performed to investigate the flow instabilities and their interaction for nonpremixed methane-air flames using experimental results, numerical simulations and theoretical analyses. The effects of buoyancy and the strong external perturbations on the vortex dynamics and instabilities of forced reacting plumes are studied. Results suggest that the flame surface breaks in forced reacting plumes and two flame fronts are formed eventually due to the convective instability coupled with buoyancy-driven instability. Flame pinch-off could occur in a short time for the cases with the strong perturbations of low frequencies. This indicates that the nonpremixed flame system exhibits a low-pass characteristic, which is sensitive to the low frequency perturbations. In addition, the buoyancy instability can be observed from the comparisons, which is of an absolute unstable nature. Despite the fact that theoretical analyses are derived from the single transport equation for mixture fraction with a number of assumptions, the results are still in good agreement with those obtained from the experiments and numerical simulations

Large-eddy simulation of ow and combustion dynamics in a lean partially premixed swirling combustor

A lean partially premixed swirling combustor was studied by resolving the complete flow path from the swirl vanes to the chamber outlet with large-eddy simulation (LES). The flow and combustion dynamics for non-reacting and reacting situations was analysed, where the intrinsic effects of swirl vanes and counter flows on the vortex formation, vorticity distribution for non-reacting cases were examined. A modified flame index was introduced to identify the flame regime during the partially premixed combustion. The combustion instability phenomenon was examined by applying Fourier spectra analysis. Several scalar variables were monitored to investigate the combustion dynamics at different operating conditions. The effects of swirl number, equivalence ratio and nitrogen dilution on combustion dynamics and NOx emissions were found to be significant.This work is supported by the UK EPSRC through Grant EP/K036750/1 and the National Natural Science Foundation of China through Grant No. 51376107. The computation is supported by the Tsinghua National Laboratory for Information Science and TechnologyPeer ReviewedPostprint (author's final draft