This study investigates the Conditionally Averaged Structure (CAS) of reactive scalars using 3D Direct Numerical Simulation (DNS) datasets of ultra-lean and near-stoichiometric premixed \(\textrm{H}_2\) -air turbulent flames, with effective Lewis numbers \(Le\approx 0.5\) and \(0.8\) , and Karlovitz numbers \(Ka\sim\mathcal{O}(100)\) and \(Ka\sim\mathcal{O}(1000)\) , respectively. The CAS unravels how turbulent mixing progressively layers surfaces with curvatures from negative to positive in the neighborhood of the zero curvature location of the reference scalar isosurface, triggering differential and preferential diffusion, amplifying specific chemical pathways. These effects enhance density-weighted local displacement speed, \(\widetilde{S_d}\) , even at zero curvature for ultra-lean, high \(Ka\) , \(\textrm{H}_2\) flames. Irrespective of the reactant equivalence ratio and the chosen isoscalar surface, \(\widetilde{S_d}\) is enhanced at large negative curvatures from localized flame-flame interactions and is explained by the existing interaction model. CAS at zero-curvature captures the reactant side negatively curved self-interactions and product side positively curved structures. The positive curvature generates enhanced heat release driven by differential diffusion. Investigation of species concentration, their production rates, and detailed chemical pathways within the CAS thus reveals the mechanism underpinning the enhancement of heat release rate at zero curvature locations. The curvature organization within the ultra-lean flame CAS promotes enhanced \(\textrm{H}_2\) diffusion from the negatively curved regions to the positively curved ones downstream. This enhances reactive radical (H, O, OH) production, and their back diffusion to increase \(\textrm{H}_2\) consumption rates, heat release rate and consequently \(\widetilde{S_d}\) , compared to laminar flames, all at zero-curvature locations of these intensely turbulent flames.
