Velocity Match Between Plasma and Flame in Microwave-Assisted Spark Ignition

As a prospective ignition technique, microwave-assisted spark ignition (MAI) can accelerate flame and thus satisfy the demand for effective ignition. However, its acceleration mechanism on flame velocity is still unclear, which hinders its application and optimizations. Therefore, this research focuses on understanding the flame acceleration mechanism behind MAI. In this paper, MAI is achieved through radiating a microwave pulse after spark ignition. The flame velocity and the precise absorbed energy under different combustion conditions are investigated with flame kernel's Schlieren images and microwave energy diagnostics, respectively. The results show that MAI accelerates flame primarily by the interaction between microwave plasma and flame front, rather than the free electrons within the flame. Specifically, microwave accelerating flame takes two processes: energy absorption, and velocity match. Energy absorption refers to the microwave energy absorbed by residual electrons after ignition to generate and sustain microwave plasma, while velocity match involves the outward plasma with faster velocity propelling the slower flame front. Moreover, the plasma velocity shows a positive relationship with absorbed microwave energy. The velocity match between microwave plasma and flame is important to successful flame acceleration. Besides, the efficiency of energy absorption and flame acceleration varies with plasma type (glow or incandescent). Glow plasma is characterized by effective flame acceleration but relatively low absorption efficiency, and incandescent plasma is in reverse. The maximum absorption efficiency can be elevated to 70 % with incandescent plasma occurring. Ultimately, the plasma velocity declines with its traveled distance, which might impair the flame acceleration.