Ignition and extinction of strained nonpremixed cool flames at elevated pressures

Cool flames, governed by low-temperature chemistry, are closely related to engine knock. Since the low-temperature chemical kinetics is promoted at elevated pressures, the ignition and extinction of nonpremixed cool flame at elevated pressures were experimentally and computationally investigated herein in the counterflow. Specifically, the hysteretic ignition and extinction behavior of the nonpremixed cool flame was for the first time observed and quantified. S-curve analysis was conducted to demonstrate the thermal and chemical structure of the cool flame and to elucidate the dominant chemical pathways during the ignition and extinction processes. The dominant low-temperature chemical reactions shift from those responsible for radical runaway to exothermic reactions that sustain the cool flame. Increasing the ambient pressure and/or the oxygen concentration in the oxidizer stream promote the heat release from the cool flame, and hence, result in an extended hysteresis temperature window between ignition and extinction. It is further noted that while the observed cool flame ignition temperatures were well predicted by computation, significant discrepancies existed for the extinction temperatures based on the well-adopted reaction mechanism used. Possible reasons were discussed to facilitate further cool flame studies and the development of the low-temperature chemistry.