Dynamics of lean laminar premixed spherical hydrogen/air flames at varying initial temperatures

Laminar flame propagation in hydrogen/air mixtures remains a key subject for clean combustion technologies, particularly under lean conditions where intrinsic flame instabilities are prominent. This study presents an experimental investigation of laminar, premixed, expanding hydrogen/air flames over a broad range of equivalence ratios ($\varphi$ = 0.3–1.4) at atmospheric pressure and varying initial temperatures ($T_u$ = 303–453 K). Particular focus was placed on lean and ultra-lean conditions, where thermo-diffusive instabilities are most pronounced. The early flame development was characterized, and the influence of ignition energy was shown to become negligible beyond a flame radius of 5 mm. The onset of intrinsic instabilities was mapped as a function of equivalence ratio and initial temperature. Unstretched flame speeds and burnt Markstein lengths were determined using a nonlinear extrapolation method, particularly in the scarcely documented ultra-lean regime ($\varphi <0.4$). Results show good agreement with literature for $0.8<\varphi <1.4$, while the presented model underpredict laminar flame speed for $0.3<\varphi <0.6$. The Markstein length decreases with $\varphi$, highlighting increased sensitivity to preferential diffusion effects and thermo-diffusive instabilities. Elevated initial temperatures enhance laminar flame speed and promote thermo-diffusive stability, as reflected by increasing Markstein lengths and delayed onset of instabilities. These trends are consistent with theoretical predictions involving the Lewis and Zel’dovich numbers.