Active Plasma Probing for Lean Burn Flame Detection 2023-01-0293

Combustion diagnostics of highly diluted mixtures are essential for the estimation of the combustion quality, and control of combustion timing in advanced combustion systems. In this paper, a novel fast response flame detection technique based on active plasma is introduced and investigated. Different from the conventional ion current sensing used in internal combustion engines, a separate electrode gap is used in the detecting probing. Further, the detecting voltage across the electrode gap is modulated actively using a multi-coil system to be slightly below the breakdown threshold before flame arrival. Once the flame front arrives at the probe, the ions on the flame front tend to decrease the breakdown voltage threshold and trigger a breakdown event. Simultaneous electrical and optical measurements are employed to investigate the flame detecting efficacy via active plasma probing under both quiescent and flow conditions. The RT-FPGA system provides flexible, prompt, and precise control for the detecting frequency to analyze the overall flame propagation process. Two types of fuels are used in the study, including methane, and DME, with an air-fuel ratio sweep from stoichiometric to extremely lean. Efforts are made to characterize the criteria of minimum, yet adequate voltage to succeed in the detection of the flame front arrival. For comparison, conventional ion current measurements are conducted under identical testing conditions. Results show that the active plasma probing has benefits in detecting the flame arrival robustly for lean combustion under both quiescent and turbulent flow conditions. Under lean combustion, the intensity of ion current signal is significantly suppressed and signal rise time is prolonged due to the lower flame temperature. The active plasma probing provides a detecting response approximately one hundred times faster compared with the conventional ion current sensing, which shows potential for more effective real-time combustion control.