Characteristic of Premixed Hydrogen/Air Tubular Flames in Microcombustor: Effects of Stimulated and Unstimulated Inlet Conditions on Flame Dynamics

The present study investigates the characteristics and periodic behavior of H2/air tubular flames in a 1 mm diameter microtube under exciting and nonexciting inlet conditions. Under unstimulated inlet conditions, increasing inlet velocity positively impacted flow stratification and self-sustaining of the tubular flames, leading to higher maximum temperatures within the flame kernel due to reduced flow temperature gradients near the wall. Conversely, under stimulated conditions, varying excitation amplitudes resulted in two flame propagation modes: flame with semi-repetitive extinction/ignition (FSREI) and pulsating flame, observed across different exciting amplitude ranges. It was found that the formation of recirculation fields generated by negative propagation speed temporarily stored the released heat of combustion and prevented it from extinguishing in the flowing phase. From the kinetics point of view, the maximum reaction rate during the pulsating mode belongs to H + O2 = HO2, while competition between H + O2 = HO2 and H2 + OH = H2O + H occurred in the FSREI mode. Results revealed that in the pulsating mode, fluctuations in mass fractions of the heavier species are more considerable near the outlets. However, radical mass fraction fluctuations were significant near the inlet slot in pulsating mode.