A Study Of Flame Instabilities In Nonpremixed Reacting Plumes By Direct Numerical Simulation

Emissions of pollutants from the combustion of conventional fossil fuels in energy utilization applications have caused major environmental concerns and have been regarded as an important contributor to global climate change. There is no doubt that clean energy solutions may play an increasingly important role in future energy supply. Hydrogen as a promising energy carrier is getting more and more attentions from researchers. The benefits of hydrogen call for further studies of the combustion instability and flame dynamics. This study is aimed to investigating the flame dynamics and instabilities of nonpremixed hydrogen flame using fully three-dimensional Direct Numerical Simulations (DNS) based on detailed chemistry mechanism and transport, which have been carried out using parallel computation using 768 processors.This thesis includes four major chapters. The first chapter is devoted to the hydrodynamic instability of hydrogen-air nonpremixed flames; The second is devoted to the effect of preferential diffusion; The third one is devoted to the thermo-acoustic instability, and the fourth one is devoted to the evaluation of gradient model in turbulent transportBoth two- and three-dimensional DNS have been performed to gain insight into the hydrodynamic instability of hydrogen-air impinging flames. Results show that buoyancy is the prominent factor for flow instabilities associated with outer vortices. The inlet disturbance accounts for the asymmetric behavior of flame structure and the formation of inner vortices is because of the shear instability. The asymmetric behavior, which is covered up in two-dimensional DNS, is apparent in three-dimensional DNS. In terms of computational costs, the adoption of two-dimensional simulations is an attractive alternative as means to reduce the computational time. However, more realistic and accurate predictions can only achieved in three-dimensional simulationsThe nonpremixed hydrogen-air flame is simulated using three-dimensional DNS. The instantaneous results of flame structures show that the preferential diffusive-thermal instability induced by preferential diffusion has a significant impact on the flame structure and can attenuate the effect of intrinsic hydrodynamic instability associated with buoyancy and velocity shear. Besides, the flame compositional structure is affected by preferential diffusion, which is attributed to the presence of the highly diffusive H2 as well as the highly diffusive radical H.Further, a comparative study has been carried out to investigate the thermo-acoustic instabilities in nonpremixed flames using experimental facilities, numerical simulation and theoretical analyses. It can be observed that the flame surface breaks in forced reacting plumes and two flame fronts are developed consequently because of the thermo-acoustic instability coupled with the original hydrodynamic instability. Flame pinch-off could occur easily in the cases with the strong perturbations of low frequencies. This suggests that the nonpremixed flame system exhibits a low-pass characteristic, which is sensitive to the low frequency perturbations.Lastly, the averaged results calculated from DNS are used to provide the statistical information on turbulent scalar transport and evaluate the closure modelling in nonpremixed flames. The analysis suggests that the phenomenon of counter-gradient diffusion of both the conserved and non-conserved scalars can be detected for both the unity Lewis number case and the case considering preferential diffusion. The gradient model for scalar closure is found to be incapable of accurately predicting the scalar transport in nonpremixed hydrogen flames. The counter-gradient transport is closely related to the pressure-driven convective behavior due to the heat release in the region between the burnt products and unburnt hydrogen, which is subject to the pressure variation across the turbulent flame. Consequently, the turbulent scalar transport in nonpremixed hydrogen flames is characterized by gradient diffusion and counter-gradient diffusion.