The effect of electron ambipolar diffusion on the ion current signals in a premixed methane flame

Ion sensing technology has the potential to be an in-cylinder combustion diagnostic solution. However, the large-scale application of this technology still be difficult in the last decades due to the absence of fundamental level research on the ion current formatting mechanism. In this study, a multi-physics analysis of the ion formatting process in a premixed methane flame is conducted, and the effect of electron diffusion process on the ion current signal is explicitly studied. Through the incorporation of the flame plasma physics, ionization reaction mechanisms and ion-electric field interactions, a numerical flame ionization model is constructed. By comparing the flame schlieren imaging data and the modelling results, the ion current signal formatting mechanism is elaborated, and the effect of electron/ion motions is studied. The results show that the ion current signal amplitude is in good agreement with the flux of the charged species. More importantly, the model reveals that the ambipolar diffusion of the electrons play a dominate role in the ion current formatting process, and it will significantly affect the ion current waveform. Due to this diffusion process, electrons cannot transport outside of the flame front zone, and its distribution will be decided by the heavy ions in the flame. Consequently, a serial of experimental results about the ion current signal can be analyzed based on the constructed multi-physics model, and the findings in this work could be crucial for the development of the future ion sensing system.