Flame detection via active plasma probing

Combustion diagnostics of highly diluted air fuel mixtures are of great importance for reducing the carbon footprint of various types of combustion systems. Flame detection techniques, such as ion sensing and optical diagnostics, have been reported for diagnosing combustion status. In this paper, a flame front detection technique based on active plasma probing is introduced and analyzed. Unlike the conventional ion sensing used in internal combustion engines, a separate electrode gap is used to detect the flame front arrival. Further, the voltage potential across the electrodes of the spark plug probe is modulated actively to be slightly below the breakdown threshold prior to flame arrival. At the arrival of the flame front, the ions in the flame tend to decrease the breakdown voltage threshold and trigger a breakdown event. An optically accessible constant volume combustion vessel is employed to investigate the efficacy of such active plasma probing for the detection of the flame front under quiescent and flow conditions. Three types of fuels are used in the tests, including methane, propane, and DME, with an air fuel ratio sweep (from stoichiometric to extremely lean) for each fuel. Efforts are made to characterize the probing criteria of minimum, yet adequate voltage to succeed in the detection of the arrival and departure of the flame front most sensitively and reliably, for all three types of fuels under various mixture strengths. For comparison, conventional ion current measurements are conducted under identical background conditions to benchmark the efficacy of flame detection. Results show that the active plasma probing can promptly detect the arrival and departure of the flame front, robustly regardless of the fuel type and excess air ratios. The precise control of the detecting voltage is key for reliable flame detection with high sensitivity.