A COHERENT FLAME MODEL OF PREMIXED TURBULENT COMBUSTION IN A COUNTERFLOW GEOMETRY

Premixed combustion in turbulent counterflowing streams is studied the oretically by adopting a coherent flame model. With this model, a flam e surface density <(Sigma)over bar>-described by a transport equation- is used to calculate the mean chemical reaction rate relative to the d istribution of a Reynolds mean progress variable (c) over bar. The val ue of a model; constant beta, which is connected to flame surface stre tch and annihilation, is selected to match numerical solutions with ex periments 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 re action zone. It is found that two different regions exist in the count erflow field: an outer region, which is far from the stagnation point and without chemical reaction, and an inner sublayer, which is close t o 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 reg ion is allowed to have a finite thickness, thus imposing a finite disp lacement on the outer solution, due to thermal expansion in the sublay er. A composite similarity solution is obtained by matching the bounda ry conditions of the inner and outer solutions at the cold flame edge. The effective mean bulk strain rate is increased slightly by the disp lacement effect. Theoretical predictions are found to be in good agree ment with experimental data. The constant beta is found to be approxim ately proportional to the mean bulk strain rate. (C) 1997 by The Combu stion Institute.