Microwave-induced plasma on spherical flame dynamics of spark-ignited hydrogen-air under water dilution

•Performance of microwave-assisted spark ignition based on carbon-free fuel (hydrogen) is investigated.•Wrinkles induced by microwave pulses are found to accelerate the cracking and cellularization of the lean hydrogen flame.•Effect of water dilution on the microwave plasma and flame dynamics are quantitatively analyzed. Microwave-assisted spark ignition (MAI) offers a commercially feasible way to enhance conventional spark ignition performance under extreme conditions. While previous studies suggested microwave pulses had no obvious effect on self-sustained flame from the perspective of energy deposition, the hydrodynamic effect of microwave plasma remained under-explored. This study experimentally investigated the effect of the microwave-induced plasma on lean hydrogen flame dynamics (Lewis number ∼ 0.35), examining the influence of water content (rH2O) in this process under different pulse repetition frequency (PRF) and pressures. Using a linear high-speed shadow imaging system coupled with electrical diagnostics, we synchronously recorded the flame and plasma morphology evolution while monitoring spark and microwave energy. Results revealed that microwave pulses during spark ignition generated wrinkles and perturbances accelerating the cracking and cellularization of the subsequent self-sustained flame. This effect intensified with increasing PRF from 1 to 10 kHz, particularly at high rH2O. The overall microwave impact on early flame development was more pronounced at high rH2O, attributed to stronger interactions between the microwave plasma jet and flame front due to reduced distance. Interestingly, electrical diagnostics showed an inversed relationship between total absorbed microwave energy and rH2O, highlighting the crucial role of the first microwave pulse at 1 kHz PRF. This study demonstrates that early microwave pulses can influence hydrogen flame dynamics even in the self-sustained stage, offering new insights into MAI mechanisms. These findings open avenues for research into active control of self-sustained flame dynamics, potentially leading to improved ignition strategies in challenging combustion environments.