Scalar transport in a turbulent jet
نویسنده
چکیده
Communicated by D.B. Spalding) A model equation for the scalar dissipation rate, based on the Two Scale Direct Interaction Approximation (TSDIA) of Yoshizawa [1J was solved and applied to a turbulent round jet in conjunction with turbulence modelling based on the eddy viscosity and diffusivity. The model coefficients were adjusted by using a similarity analysis for the round jet. This led to an improvement in the prediction of concentration fluctuations on the axis of a jet with respect to results obtained with the equal length scales model. The turbulent Schmidt number, no longer assigned an ad-hoc constant value, displays experimentally observed behaviour in the jet. Introduction In the modelling of a turbulent diffusion flame the mean concentration and concentration fluctuation fields are of importance. These scalar variables are the first two moments of a probability density function which is needed to calculate the mean density and temperature [2]. In the equation for the mixture fraction, which is invariant during combustion and is equal to the mean concentration in the equivalent isothermal flow, the turbulent flux term must be modelled. On dimensional grounds an eddy diffusivity coefficient should contain the scalar dissipation, which stands for the destruction of scalar fluctuations. Using eddy viscosity and diffusivity models, such as the k-c model of Jones and Launder [3] mostly the equality of integral velocity and scalar length scales [4] is invoked to circumvent the explicit modelling of the scalar dissipation eg. This implies a constant Schmidt number which is at variance with experiments in the round jet [5]. Furthermore, with the equal length scales model, the concentration fluctuations are not well predicted in a turbulent round variable density jet while the concentration is accurately predicted [2].
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