화학공학소재연구정보센터
Combustion and Flame, Vol.109, No.1-2, 43-64, 1997
A coherent flame model of premixed turbulent combustion in a counterflow geometry
Premixed combustion in turbulent counterflowing streams is studied theoretically by adopting a coherent flame model. With this model, a flame surface density <(Sigma)over bar>-described by a transport equation-is used to calculate the mean chemical reaction rate relative to the distribution of a Reynolds mean progress variable (c) over bar. The value of a model; constant beta, which is connected to flame surface stretch and annihilation, is selected to match numerical solutions with experiments at different nozzle exit conditions giving various values of the mean progress variables at the stagnation point (c) over bar(0). The (k) over tilde-<(epsilon)over tilde> equations are used to predict the turbulence strain rate for the <(Sigma)over bar> balance equation and the turbulent viscosity mu(t) = C-mu<(rho)over bar>(k) over tilde(2)/<(epsilon)over tilde> for second-moment terms. In this turbulence model, the term representing correlation of pressure fluctuations and dilatation is found to be important to turbulence production in the reaction zone. It is found that two different regions exist in the counterflow field: an outer region, which is far from the stagnation point and without chemical reaction, and an inner sublayer, which is close to the stagnation point, which experiences a large bulk strain rate and is located in the combustion zone. In order to provide an appropriate theoretical description of the experimental conditions, the inner region is allowed to have a finite thickness, thus imposing a finite displacement on the outer solution, due to thermal expansion in the sublayer. A composite similarity solution is obtained by matching the boundary conditions of the inner and outer solutions at the cold flame edge. The effective mean bulk strain rate is increased slightly by the displacement effect. Theoretical predictions are found to be in good agreement with experimental data. The constant beta is found to be approximately proportional to the mean bulk strain rate.