Numerical and theoretical investigation of ethanol/air flame instability

A series of two dimensional (2D) detailed numerical simulations of premixed cylindrical expanding ethanol/air flames in a constant volume encloser under elevated pressures and equivalence ratios are performed to investigate cellular flame instability. The results demonstrate that at pressure of 10 atm and temperature of 358 K, ethanol/air flame cellular instability increases non-monotonically with equivalence ratio (ϕ) from 0.8 to 1.6 and has the most intense instability at ϕ = 1.2. The trend is similar in both theory and simulation, while the latter overpredicts the critical flame radius compared with the theory. At the equivalence ratio of 1.2, the flame instability increases monotonically with pressure from 2 to 20 atm. The 2D simulation results are theoretically analysed by Peclet number, logarithmic growth rate of disturbance, flame thickness and critical flame radius for the onset of flame instability. It is found that hydrodynamic (DL) instability is insensitive to ϕ. In contrast, thermal-diffusion (TD) instability is overwhelming and changes dramatically with increasing ϕ mainly due to molecular diffusion. Therefore, the destabilising effect of logarithmic growth rate on the ethanol/air flame surface with increasing ϕ is actually due to the weakening stable effect of TD instability. The almost constant critical Peclet number and drastically reduced flame thickness lead to a great decrease in the critical flame radius with pressure rising both in theory and 2D simulations. It is concluded that the onset of instability advances with higher pressure and the acceptable quantitative comparison in terms of critical flame radius was observed between simulation and theory.