Abstract

The model of gaseous fuel and air mixing, developed by the authors2–3, is applied here to calculate maximum mixing times of propane and air. The degree of mixing is determined using the mass fraction of fuel while the rate of mixing is determined from the rate of this mass fraction. The values of both these parameters are local, i.e., measured within an infinitesimal element of fluid. An Eulerian representation is used. The model is based on the assumption that both fuel and air behaves like single chemical species. It is further assumed that pressure is low and only fuel and air are present within the fluid element. Under non-reacting conditions, the model is valid anywhere in the combustor. Under reacting conditions, the model is valid within those combustor regions where the fuel-air mixture is not flammable. The results of this analysis show that mixing times of propane and air are most reduced by high gradients of temperature and velocity, as long as these gradients act in phase. To a lesser degree, high gradients of pressure also help reduce mixing times. High initial pressure and temperature increase mixing time. Mixing with air penetration into the fuel flow is slower than with propane dispersion into the surrounding air. In general, the exact mixing time has to be determined numerically. Nevertheless, the analytical solutions included here provide maximum mixing times of propane and air under most conditions. These results provide important guidelines for the development of high intensity, high efficiency and low emission combustors.

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