Water vapor on microwave-enhanced spark ignition of CH4-air flame: Experimental and numerical analysis

To enrich the understanding of Microwave Assisted spark Ignition (MAI) under water dilution, this work investigated the early flame development, plasma jet formation, and energy deposition during the MAI through the ignition experiments of methane-air under 0 %∼25 % water contents (rH2O) and 0.6∼1.0 equivalence ratios (φ). Furthermore, a two-temperature well-mixed reactor together with a detailed mechanism of plasma-assisted methane combustion was run herein to solve the energy conservation equations for both electron and gas, and to provide the molar fraction evolutions of electron and excited species. The experimental results showed that MAI enhanced the early flame development under water vapor dilution by inducing a plasma jet to directionally accelerate the local flame front, and extended the rH2O limit from 20 % to 22.5 % at φ=1. At a given φ, as the rH2O increased from 0 to 20 %, the energy absorption during the first microwave pulse was almost unaffected, however, the absorbed energy and thus the characteristic plasma length induced by the second pulse decreased. This deteriorating trend of energy absorption was mitigated when the φ was increased from 0.6 to 1.0. As a result, the MAI performance, which mainly depended on the interaction between the flame front and the plasma, was more prominent when both the φ and rH2O were low or when both were high. The simulation well reproduced the trend of energy absorption and MAI performance varying with the rH2O and φ, and showed that the addition of water on the one hand accelerated the electron number decay by lowering the gas and electron temperature, and on the other hand it delayed the ignition which in turn prolonged the energy absorption time. In this sense, the stoichiometric mixture or a stronger electric field input is advantageous to maintain electron concentration or accelerate energy absorption, and is therefore conducive to improving the performance of MAI at a high rH2O. These findings provide new insights into MAI and valuable guidance for the design and application of this technology.