Three dimensional shapes of hydrogen-air flames within millimetric Hele Shaw cells

Premixed hydrogen -air laminar flames with equivalence ratios ranging from 0.32 to 2.68 adopt different shapes during propagation in a gap (from 1.5 mm to 4.2 mm) between two plates. Thermodiffusive and Darrieus- Landau instabilities are visible for lean mixture while Darrieus-Landau is visible at higher equivalent ratio. Using for the first time simultaneous schlieren visualisation in this configuration, we were able to make two important observations: (i) either symmetric or asymmetric shapes are observed and quantified within the gap as a function of equivalence ratio and gap thickness; (ii) 3D flame surface for unstable laminar flame was estimated. These results validate (or invalidate) the quasi -2D assumptions made in Hele-Shaw cells. Linear stability analysis allowed us to find an a priori criteria differentiating symmetric and asymmetric cases. Finally, curvature effect on flame speed enhancement relative to surface amplification is estimated for unstable hydrogen -air laminar flames. This experimental 3D amplification factor provides means to take into account instabilities effect on large scale turbulent models.