Abstract
The present study introduces a novel experimental configuration employing optical OH-PLIF imaging to directly determine the stretch factor (I0) in laminar premixed hydrogen flames as they transition from a quasi-stable to a thermodiffusively unstable regime. The setup consists of a rod-anchored V-shaped flame stabilised in a laminar flow of premixed reactants. In the near field of the anchoring rod, the mildly strained flame remains quasi-stable, characterised by a smooth surface and a well-defined inclination angle (0s) relative to the main flow. This region defines the first, stable-branch of the V-flame, with a corresponding burning rate Ss. Further downstream, the flame abruptly transitions into a distinct regime dominated by pronounced thermodiffusive (TD) instabilities, as evidenced by cellular structures and a strongly wrinkled flame surface. The distance between this transition point and the anchor decreases with increasing equivalence ratio. This second TD-unstable branch is characterised by a marked increase in the mean flame-surface angle (0u) relative to the flow direction, allowing direct evaluation of the increase in global flame speed, Su/Ss, between the stable and TD-unstable branches. Notably, it is assumed that this ratio represents the normalised flame consumption speed Sc/SL. Determination of I0 additionally requires the ratio of increase in surface-area due to the thermodiffusive instabilities (TDI). Three complementary methods are employed to evaluate the flame-surface area of the TD-unstable branches (A), comparing it to a smooth reference area (A0), yielding consistent trends in A/A0 across the range of equivalence ratios studied. The resulting I0 estimates, while subject to uncertainties primarily in A, decrease monotonically with increasing equivalence ratio, from approximately 1.1-1.3 at cent = 0.35 to 0.8-0.9 at cent = 0.4, which is consistent with theoretical predictions. Additional numerical simulations in a reduced two-dimensional representation of the experimental configuration show the same transition behaviour and yield qualitatively consistent results. Novelty and significance statement: This work introduces a novel method for directly measuring the stretch factor I0 from OH-PLIF imaging of rod-anchored lean premixed laminar hydrogen-air V-flames. The method exploits the abrupt transition from a quasi-stable regime with a nearly flat flame surface to a thermodiffusively unstable regime with highly wrinkled flame surfaces. In contrast to jet burners or spherical expanding flames, the proposed configuration enables the onset of thermodiffusive instabilities to be investigated largely independently of configuration-induced effects on the flame dynamics, such as imposed curvature and flame-branch interactions. The well-characterised laminar flow field allows the transition to be reproduced systematically with minimal external perturbations and clearly identified over a relevant range of fuel-lean operating conditions. Complementary numerical simulations in a reduced two-dimensional representation of the experimental configuration reproduce the same qualitative transition behaviour and support the experimental findings. Together, the experimental and numerical results provide a valuable basis for theory development and validation of numerical models for flames prone to thermodiffusive instabilities.