Global warming, primarily driven by CO2 emissions from fossil fuel combustion, necessitates a shift to sustainable energy sources. Hydrogen, being carbon-free and renewable, is essential for achieving net-zero emissions and addressing the global energy crisis. The combustion of hydrogen, particularly in a lean premixed condition, offers significant benefits: controlling flame speeds, reducing exhaust gas temperatures, and lowering nitrogen oxide (NOx) emissions. However, these flames are susceptible to thermodiffusive and hydrodynamic instabilities, which may induce self-acceleration and significantly impact the turbulent burning velocity across various combustion systems, thereby elevating fire and explosion risks. Identifying the regimes of cellular instability and self-acceleration could enhance combustion modelling, a critical tool in the design of combustion systems and in assessing fire and explosion hazards. To address these challenges, a full investigation and understanding on self-acceleration characteristics of hydrogen-air flames under various conditions was conducted, with a particular focus on flame speed and flame surface area. This research employed advanced experimental techniques, including Schlieren imaging, Particle Image Velocimetry (PIV), a 3D swinging laser sheet system, and Direct Numerical Simulation (DNS) supported by the Advanced Flow Simulator for Turbulence Research (ASTR).
Using Schlieren imaging system, the onset of instability was identified by the critical stretch rate, where the flame speed deviates rapidly from its previous response to stretch. Notably, the critical Peclet number (Pecl) increased with higher equivalence ratios and temperatures, indicating a more stable flame. Conversely, Pecl decreased with increased initial pressure due to the associated decrease in the flame speed Markstein number (Mab). Correlations of Pecl and Kacl were developed as a function of Mab with increasing pressure, facilitating the estimation of the severity of large-scale atmospheric hydrogen flames. Comprehensive quantitative data on the self-acceleration of unstable laminar hydrogen-air flames was obtained, revealing self-similarity after instability onset. The previously assumed acceleration exponent α = 1.5 was found to be invalid, with derived α values ranging from 1.125 to 1.39. Higher acceleration exponents were observed in the lean condition, while lower exponents were found in the rich condition. A modified theoretical expression for the constant (A) was proposed and validated against experimentally derived results, highlighting a global pulsating acceleration pattern during the acceleration phase after flame instability.
Particle Image Velocimetry (PIV) was utilized to explore the disturbance of unstable laminar hydrogen-air outwardly propagating spherical flames. It was found that self-acceleration of gas velocities ahead of the front, and shared the same acceleration exponents as the flame front. The power spectral density (PSD) displayed by the flow ahead of the flame front exhibited similarity to flame front fluctuations, attributed to wrinkled flame front-driven gas disturbance. Higher local gas velocities were observed just ahead of the tips of the cellular structures, compared to other regions along the flame front, particularly for the extra lean conditions.
3D laser sheet measurements were employed to quantify the flame surface area of hydrogen flames. For planar flames, the parameters ϵ, representing the deviation of the Lewis number from a critical value, and ω2, derived from classical linear stability analysis to represent thermal-diffusive effects, both exhibit a distinct linear correlation with the enhancement in flame surface area observed in planar flames. This suggests that the 3D swinging laser sheet system is an effective method for investigating flame surface area. For spherical flames, the stretch factor I_0 (the ratio of the increase in flame burning velocity to the enhancement in flame surface area) exceeds 1 when the equivalence ratio (ϕ) is 0.3 (lean condition), particularly under high-pressure conditions.
The morphological characteristics and acceleration behaviour of cellular flames were investigated using Direct Numerical Simulation (DNS) with simplified chemical kinetics. The DNS results indicate that the self-acceleration capacity of thermodiffusively unstable flames (ϕ = 0.4) is significantly higher compared to thermodiffusively stable cases (ϕ = 0.6, 0.8). The stretch factor I_0 for thermodiffusively unstable flames exceeds unity, whereas for thermodiffusively stable flames, it remains approximately equal to one. This suggests that, in thermodiffusively stable flames, the primary contribution to acceleration arises from increased flame surface area. In contrast, thermodiffusively unstable flames exhibit additional mechanisms contributing to self-acceleration. Further analysis of local normal strain rates revealed elevated values at the tips of the finger-like protrusions within the cellular flame structures, indicative of rapid expansion at these points. The significant variations in normal strain rates in thermodiffusively unstable flames are attributed to enhanced flame instabilities or localised effects. Species distribution analysis showed that active radicals, such as OH, O, and H, are highly concentrated at these protrusion tips, referred to as ‘leading points’. In thermodiffusively unstable flames, the high mass diffusivity of hydrogen results in the formation of more ‘leading points’, altering the dynamics of flame expansion. This leads to elevated local normal strain rates and increased concentrations of active species like hydroxide radical (OH) , oxygen radical (O) , hydrogen radical (H) at these locations. Consumption rates of hydrogen and production rates of water are strongly influenced by curvature, with positive curvature regions enhancing localised combustion due to hydrogen's high diffusivity under lean conditions.
