Three-dimensional dynamics of unstable lean premixed hydrogen-air flames: Intrinsic instabilities and morphological characteristics

The 3D swinging laser sheet technique was employed to study the development and morphological characteristics of premixed hydrogen-air unstable flames in a spherical explosion vessel. Pressure dependencies for laminar flame propagation were sought to exploit the role of the Darrieus-Landau (DL) and Thermal-diffusive (TD) instabilities in the unstable self-accelerating flame regime. A sufficiently low Markstein number, as a consequence of the increased pressure, leads to more cracking and smaller cells over the flame surface. The degree of wrinkling on the flame surface is proportional to the increase in flame burning velocity, a relationship that holds true for low pressures but is not applicable under high pressures. External turbulence can significantly alter the extent of flame surface wrinkling even at low root mean square velocities, producing a more wrinkled flame surface compared to intrinsic cellularity, and distinctly affecting flame dynamics. The increased wrinkling and flame speed due to external turbulence can be attributed to the synergistic effects between thermo-diffusive instabilities and turbulence, resulting in higher fuel consumption rates per flame surface area and the formation of finger-like structures that enhance flame displacement speed in curved segments. The parameters, $ϵ$, deviation of the Lewis number from a critical value, and ${\omega }_2$, obtained through classical linear stability analysis, display a clear linear relationship with the ratio of the wrinkled surface area observed in planar flames. This study enhances the understanding of hydrogen flame instabilities, which is crucial for preventing explosions in hydrogen storage and utilization, and provides valuable insights into flame dynamics, supporting the design of safer and more efficient hydrogen-fueled engines and turbines.