The objectives of this paper are (i) to establish analytically some important features of the scalar diffusion process in turbulence and (ii) to derive a closure model for this process in terms of the scalar probability density function (pdf). The present analysis shows that in isotropic turbulence the conditional scalar dissipation [χ(ψ)=D<∂φ/∂xi∂φ/∂xi‖φ=ψ≳], its derivative (∂χ/∂ψ), and the conditional scalar diffusion [Θ(ψ)=D<∂2φ/∂xi ∂xi‖φ=ψ≳] are zero at the extreme values of scalar concentration. Models for conditional‐dissipation ratio (χ/es) and conditional‐diffusion ratio (Θ/es) are derived from the observation [Girimaji, Combust. Sci. Technol. 78, 177 (1991); NASA Contract. Rep. CR 4446 (1992)] that the scalar pdf can be characterized by the β pdf at all stages of non‐premixed mixing. The conditional‐dissipation model is compared with the DNS data of Eswaran and Pope [Phys. Fluids 31, 506 (1988)] and the mapping‐closure‐based model [O’Brien and Jiang, Phys. Fluids A 3, 3121 (1991)]. The applicatio...
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