OCR Text |
Show separately. In tensor form a/axi[pUiffiNO-(uef("i,NO)(affiNO/a\)] - SNO WNO where i = 1,2,3, and p is the average density, U. is the average 1 velocity in i direction, J1 ff is the effective viscosity which is a e . h combination of the laminar and turbulent viscosities, CTi , NO IS t e Prandtl number in the i direction for turbulent NO diffusion based (14) on the eddy-diffusivity assumption. SNO is the mean turbulent net rate of chemical production of NO and W NO is the molecular weight of NO. This equation can be solved to give NO distlibutions without significant effects on the flow field and temperat~re calculations: Methods of modelling mean turbulent reaction rates can be based on either (i) moment methods (26), or, (ii) probability density function (pdf) techniques (28). The pdf method has proven very useful in the theoretical descliption of turbulent flows (28). In previous studies by us (29), a single-variable pdf in terms of a normalised temperature representing the reaction progress was used to predict the NO emissions in a cylindrical furnace. Despite the encouraging results obtained using a single-point pdf method, the closure of the molecular mixing has been a problem in turbulent-combustion modelling. This problem is aggravated by an increased number of fluctuating variables involved in the prediction of NO concentration profiles . Thermal NO formation d pend critically on th m an -radi 1 concentration with allowance for non- quili riUIn t nl . Th prompt-NO concentration field in turn d p nds riti Uy on th CH-radical concentration and the temperature. (18) |