| A new model is presented to construct a likely form for the joint scalar probability density function, as opposed to assuming its shape, with application to the moment equation solution of turbulent (non-premixed) combustion. The model approximates the thermo-chemical pdf in a general turbulent reacting flow by the scalar pdf that decayed from a binary field in homogeneous turbulence, parameterized by an appropriate set of lower moments. Full resolution, complex chemistry, scalar decay simulations are achieved with the computationally affordable Linear Eddy Model. The constructed pdf, in contrast to the assumed shape approach, incorporates the underlying physics of advection by all turbulent scales, molecular diffusion and chemical reaction. The constructed pdf has no adjustable constants or calibration factors, and its tabulation allows the expensive chemical kinetics to be decoupled from the moment equation solution. In an a-priori experimental comparison using a two variable chemical mechanism, the constructed pdf demonstrates qualitative superiority over a common assumed shape pdf. The model shows improved higher moment hydroxyl radical predictions in application to the steady-state solution of turbulent, round jet flames in moderate chemical non-equilibrium. In a further general chemistry a-priori pdf test for hydrogen and methane jet flames, constructed pdfs display consistent and substantial improvement over a sophisticated, conventional, assumed shape pdf. Beside the ability to incorporate density weighting, the constructed pdf is parameterized by a small set of lower moments, and in particular, first moments, promising tractable and accurate implementation in a moment equation solver. |
