The present paper developed and validated a numerical procedure for the calculation of turbulent combustive flows in converging and diverging ducts. Through simulation of the heat transfer processes, the amounts of production and spread of NOx pollutants were measured. Also, this paper reported the results of a numerical investigation of the effects of the fuel Molecular Weight on NOx emissions in converging and diverging ducts. The fuels which were examined were methane, naphtha, and kerosene. The differential equations in the Von-Misses coordinate system were transformed to a cross-stream coordinate system in order to concentrate more grid lines near wall boundaries. A marching integration solution procedure employing the TDMA (Tri diagonal Matrix Algorithm) was used to solve the discretized equations. In this system, by using a general variable, mass, momentum, energy, and chemical species conservation equations were written in a general form. The nature of the governing differential equation was parabolic. The flow behavior in the vicinity of the walls was determined utilizing the wall function method. The turbulence model was the Prandtl Mixing Length model. Modeling of the combustion process, was done with Arrhenius and Eddy Dissipation method. Thermal mechanism was used for modeling the formation of the nitrogen oxides. Finite difference method and Genmix numerical code were used for numerical solution of equations. Our results indicated the important influence of the limiting diverging angle of diffuser on the coefficient of pressure recovery. Also, the converging and diverging effects of duct on its output behavior were indicated. Moreover, due to its intense dependence on the maximum temperature in the domain, the NOx pollutant amount attained also maximum level. Heavier fuel molecules, increased temperature and resulted in the increase of NOx mass fraction.

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