The flamelet-based chemistry tabulation technique is a popular reduced-order model for non-premixed turbulent flames. In this approach, the one-dimensional flamelet equations are solved, and thermo-chemical quantities are tabulated with respect to the mixture fraction and either its scalar dissipation rate or a progress variable. In generating the individual flamelets to populate the chemical table, the correct choice of the species Lewis numbers plays an important role. Experimental observations have shown that, in turbulent non-premixed jet flames, the effect of turbulent transport on the flame structure becomes gradually dominant over molecular mixing with (*i*) increasing axial distance from the burner exit plane, and (*ii*) increasing jet Reynolds number. In the current work, this transition is characterized and a priori models for the effective species Lewis numbers in turbulent non-premixed flames are assessed.

First, a flamelet-based methodology is proposed to extract these effective Lewis numbers from data sets of turbulent non-premixed flames. This methodology is then applied to the Sandia flames B, C, D, and E. The effective Lewis numbers are found to transition from their laminar values, close to the burner exit plane, to unity further downstream. Previously-suggested models for the effective Lewis numbers, based on the local Reynolds and Karlovitz numbers, are then assessed. To overcome the limitations associated with the experimental data, a campaign of Direct Numerical Simulations (DNS) of Sandia flame B (Re_{jet}≈8200) is carried out. A baseline grid is carefully designed, and grid independence is assessed through simulations using refined grids in the axial, radial and azimuthal directions. Radiation and differential diffusion effects are systematically isolated by considering radiating and unity Lewis number cases, respectively. The DNS database is then validated using experimental data. Finally, effective Lewis numbers are extracted from the DNS data, and models based on the local Reynolds and Karlovitz numbers are tested.