Unresolved fluxes in turbulent diffusion flames are investigated by introducing the specific volume to analyze the effects of density variations. Unresolved fluxes are found to be related to scalar correlations involving this specific quantity. The algebraic models proposed for the turbulent scalar and momentum fluxes allow to recover and generalize well-known previously established relations and highlight the possible occurrence of non-gradient diffusion. These correlations are evaluated from the consideration of strained laminar diffusion flames and chemical equilibrium conditions. Finally, the calculations performed confirm that such non-premixed flames may exhibit a strong production of turbulence near stoichiometric conditions.