Detailed Numerical Simulation And Theoretical Study On Cellular Instability Of Ethanol Premixed Flame

Flame instability plays a role on local flame shape,structure,self-acceleration of the laminar flame and thus has an influence on heat release rates,pollutant emission and even detonation.At first,this thesis employed a series of two dimensional(2D)detailed numerical simulations(DNS)of premixed cylindrical expanding ethanol/air flames in an encloser under different pressures and equivalence ratios to investigate cellular flame instability.A simplified ethanol mechanism and detailed transport parameters are adopted.It is found that the flame instability increases monotonically with increasing pressure from 2 atm to 20 atm,where the most intense instability condition is at φ= 1.2.Linear theoretical analysis predicts hydrodynamic(DL)instability and thermal-diffusion(TD)instability and compares the critical flame radius with numerical simulation.DL instability is not sensitive to equivalence ratio and always has an unstable influence.However,TD instability varies significantly with the increase of equivalence ratio due to the dominant role of molecular diffusion.From lean to rich,the increase in total growth rate is actually due to the weakening of the stability of TD instability.The theoretical critical flame radius Rc is greatly affected by flame thickness δ1.The numerical simulation results are in good agreement with the theoretical analysis regarding to the critical flame radius.Second,the effects of hydrogen-doped ethanol flames were carried out under fuel-lean,fuel-rich and stoichiometric conditions to study the inherent instability.The local curvature probability density function and the wrinkle coefficient are analyzed.It is found that for fuel-lean mixture,curvature correlates with heat release rate positively,while for fuel-rich mixture it has a negative relationship due to the preferential diffusion effect of hydrogen.In addition,for the former,the instability is stronger than the latter indicating the instability is suppressed for fuel-rich condition.For stoichiometric condition,the instability is strongest since the flame speed is the highest and the flame thickness is thinnest.Finally,turbulent premixed flame of syngas fuel under weak and strong turbulence intensities are investigated.A corresponding laminar case with small initial disturbance is compared.It is found that the initial turbulence intensity has a great influence on the flame structure and the flame speed.Compared with laminar case,the surface wrinkling is rather random and disturbed by large and small turbulence scales while the cells are symmetric and large for laminar flame.