Investigation of Flashback Limits and Detection Strategies for Jet-Stabilized Hydrogen Flames

Abstract In this study, we examine the flashback boundaries of a piloted single-jet hydrogen burner (POShyDON) for different equivalence ratios. Through the use of instantaneous OH* chemiluminescence imaging, it was observed that flashback events are initiated by a propagation of the flame into the mixing tube in the boundary layer region. To identify a reliable flashback detection method, essential to ensure the safe operation of pure hydrogen flames, the simultaneous signals of multiple sensors are compared. Three temperature sensors are used: one for the flow temperature in the mixing tube, one for the material temperature in the mixing tube, and one for the material temperature at the dump plate. Findings indicate that the thermocouple in the mixing tube provides faster and more reliable measurements of flashbacks than the ones measuring material temperatures. Moreover, we examine static pressure in the mixing tube, which is capable of identifying flashbacks but is affected by the ignition process and high-frequency oscillations. Analogous findings are observed with pressure fluctuation measurements. However, by combining the pressure sensors located before and after the flame, a reliable criterion for detecting flashbacks can be established. Two optical sensors mounted in the mixing tube are also shown to reliably detect flashback. However, the optical access requirement poses a challenge for gas turbine applications. Finally, we discuss a novel flashback sensor that relies on measuring the temperature dependent resistance of the material at the rim of the mixing tube. Also, this sensor is shown to effectively detect flashback precursors, but lacks the ability to detect repeated flashback events. This newly tested detection method offers a compelling solution for flashback detection, being non-invasive, cost-effective and fast.