Investigation Of The Coupling Effect Between Hydrodynamic Darrieus-Landau Instability And Turbulence

With the increasing demand of energy conservation and emission reduction,turbulent premixed combustion is widely used in various advanced combustion technologies because of its clean and efficient characteristics.However,in practical applications,the turbulent premixed combustion often deviates from the predetermined conditions by the influence of hydrodynamic instability(Darrieus-Landau,DL instability),resulting in low heat efficiency,emission deterioration,this makes the development of advanced combustion technology more challenging.Therefore,quantifying the coupling effect between DL instability and turbulence is a key issue in the turbulent premixed combustion research.To explore this problem,this dissertation carried out a series of experiments and simulation studies on the Bunsen turbulent premixed flame.following conclusions have been obtained:(1)Methane-air premixed flames from four Bunsen-type burners were investigated at Reynolds numbers ranging from 1000 to 8500.Results showed that DL instability is dominant in the flame propagation process.The burner size and the internal structure will affect the disturbance energy distribution of the inflow,thereby changing the role of DL instability.Wavelet analysis was performed to obtain the proportion of effective disturbances(P_ed),the P_ed can not only predict whether DL instability exists in the working condition,but also can characterize the effect of DL instability.(2)Methane-air premixed flames from five Bunsen-type burners were investigated at Re numbers ranging from 1000 to 8500.Results showed that as the intensity of turbulence increases,DL instability begins to be suppressed by turbulence.The suppression of turbulence is mainly through two paths.One path is that turbulence affects the inflow of the burner exit thereby affecting the strength of the DL instability;the intensity of the DL instability affected by this approach can be characterized by the proportion of effective disturbances.The other path is that turbulence affects the DL instability by affecting the flame wrinkles.In these experiments,the first path dominates,so the coupling effect of DL instability and turbulence on the flame speed during the change of inflow disturbances could be formulated by P_ed.Finally,a P_ed-based correlation was proposed to describe the dependence of the turbulent flame speed on the burner size.(3)After the P_ed-based correlation was proposed,this dissertation verified the P_ed-based correlation through a large amount of experiments(nozzle diameters:4 mm-12 mm,Re number:1000-8500,equivalent ratio:0.7-1.3).Results showed that the change of equivalence ratio will change the P_ed and laminar combustion velocity,thereby affecting the effect of DL instability on flame propagation.The results shown that the P_ed-based correlation can predict the turbulent flame speed at various burner sizes,various equivalence ratios,and various Reynolds numbers.(4)Numerical simulation was carried out to simulate the process that turbulence suppresses DL instability by wrinkling flamelets.The DL-stable or DL-unstable turbulent flames can be simulated by nonlinear large eddy simulation coupled with partially-stirred-reactor(Pa SR)methods.The correspondences between flame surface curvature and tangential strain rate of different conditions were analyzed.The results show that turbulence and DL instability have an obvious competitive relationship at the flame surface,which proves that there are two interaction paths between turbulence and DL instability.The simulation results show that with the increase of turbulence intensity,strong stretching will affect the flame surface wrinkles caused by DL instability,thereby suppressing the effect of DL instability.In summary,this dissertation studies the effect of DL instability on the turbulent premixed flames.In weak turbulence,DL instability is dominant.As the intensity of turbulence increases,turbulence affects the inflow disturbances and the flame wrinkles,thereby affecting the role of DL instability.The influence of turbulence on the inflow disturbances can be measured by the proportion of effective disturbances.Because the former pathway is dominant in the interaction between turbulence and DL instability,so the P_ed-based correlation can be able to predict the turbulent combustion speed under different working conditions.