Effect of Microwave Pulses on the Morphology and Development of Spark-Ignited Flame Kernel

Xiaobei Cheng,Zhaowu Wang,Jyh-Yuan Chen,Zhaowen Wang,Huimin Wu,Xinhua Zhang

doi:10.3390/en14196205

Abstract

Microwave-assisted spark ignition (MAI) is a promising way to enhance the ignition performance of engines under lean conditions. To understand the effect of microwave-induced flow during MAI, the development and morphology of spark-ignited methane-air flame kernel under various microwave pulse parameters are experimentally studied. Experiments are conducted in a constant volume combustion chamber, and flame development is recorded through a high-speed shadowgraph method. Flame area and deformation index are adopted to evaluate the flame characteristic. Results show that increasing the microwave pulse energy from 0 to 150 mJ exhibits a threshold process for expanding the flame kernel area under 0.2 MPa ambient pressure. When the pulse energy is below the threshold of 90 mJ, the microwave enhancing efficiency is much lower than that beyond the threshold. Increasing microwave pulse repetition frequency (PRF) changes the flow on flame surface and raises the absorption efficiency for microwave energy, and thus helps to improve the MAI performance under higher pressures. Hence, 1 kHz pulses cause more obvious flame deformation than those with higher PRF pulses under 0.2 MPa, while this tendency is reversed as the ambient pressure increases to 0.6 MPa. Besides, microwave pulses of different repetition frequencies lead to different flame kernel morphology, implying the various regimes behind the interaction between a microwave and spark kernel.

Highlights

Introduction iationsLean burn, an efficient method to reduce the NOx emissions while improving the fuel consumption of spark ignition engines [1,2], has drawn significant attention from the automobile and aerospace industry
The addition of microwave generates a bright spot at the center of flame kernel, and the bright spot becomes brighter and the flame area increases as the microwave pulse energy increases
The effects of microwave pulses on the spark-ignited flame kernel development are experimentally investigated based on a self-developed constant volume combustion chamber system

Summary

Introduction

An efficient method to reduce the NOx emissions while improving the fuel consumption of spark ignition engines [1,2], has drawn significant attention from the automobile and aerospace industry. An inevitable challenge for this low-NOx-CO2 combustion strategy is to ensure the ignition stability and the relight capacity in aero-engines [3,4] Considering that both the ignition kernel and flame front contain large number of charged particles, it is possible to couple energy into the ignition kernel and accelerate the flame development through an additional electric field. The electric field has been verified to be a promising method to increase the flame speed and have significant influence on reducing polluted emissions [5,6,7], and the ion wind induced by the electric field on a Licensee MDPI, Basel, Switzerland.

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