Boundary layer flashback limits and flame dynamics of turbulent premixed hydrogen-air flames

Accurate measurements and predictions of boundary layer flashback limits are important in the development of hydrogen-fired propulsion and combustion systems. Despite considerable advances, sub-millimeter-range flame-wall interactions and related flashback dynamics particularly under fully developed turbulent flow conditions involving high Reynolds numbers are still not well understood. To address this issue, here we use a jet-stabilized conical flame to investigate wall heat flux-dependent flashback limits and flame front evolutions of unconfined premixed hydrogen flames with Reynolds numbers between 7,600 and 23,200. The injector cooling methods (heat flux density) and equivalence ratio (flame temperature) are revealed to be central to the determination of the head-on quenching distance and associated wall heat transfer occurring at local regions containing highly concentrated energy. The quenching distance is directly measured and found to be in the range of 0.2 ∼ 1.1 mm, an order of magnitude identical to that of the characteristic thickness of premixed hydrogen flames. When the inlet temperature is elevated, the slope of the flashback limit defined in the 〈u¯nozzle,〉bi-parameter domain is increased nonlinearly and markedly shifted toward a higher nozzle velocity condition, rendering the single nozzle system more vulnerable to boundary layer flashback. Mechanistically, several key triggers are observed to induce turbulent boundary layer flashback of fully-premixed hydrogen flames: (i) significantly deformed non-axisymmetric flame fronts with cusp formation, (ii) intermittent flame propagation into the nozzle upstream and the formation of isolated regions (hot spots), and (iii) large-scale front merging eventually leading to a sudden collapse of the nominally conical structure. Novelty and significance statement: The novelty of the present investigation is detailed measurements and characterizations of heat-flux-controlled flame-wall interactions and the flashback limits/dynamics of premixed hydrogen flames under relatively high Reynolds number flow conditions. Based on transient measurements synchronized with OH PLIF-based flame front visualization, we identify, for the first time, several key flashback precursors, including the emergence of symmetry-broken structures with cusp and hot spot formations, along with sporadic flame holding and immediate recovery, followed by a sudden increase in the nozzle tip temperature – contributing synergistically to large-scale front merging and complete flashback. © 2024 The Combustion Institute