Ignition of gases at hot surfaces under nonsteady-state conditions

Abstract

The ignition of gases at a suddenly heated circular rod has been investigated by measuring the temperature field and its development with time with a Mach-Zehnder interferometer. Except near the ignition limits, the ignition was found to set in abruptly, with a considerable delay (up to 60 msec) after the rod had been heated up. During this period, the temperature distribution in the gas was found to be determined essentially by heat conduction, heat generation by the combustion reaction being negligible. Applying the concept of steady-state heat balance, an approximate relation was derived for the ignition delay, which is in accordance with the measured data and which agrees closely with expressions given for the condition of ignition at a hot body in steady-state cross flow. This relation explains the ignition delay, as determined mainly by the initial heat-conduction process in the gas. Experiments have been performed in mixtures of pentane, propane, and hydrogen, with air at various equivalence ratios, and at gas pressure between 0.5 and 6 atm. Ignition delay has been measured in stoichiometric pentane-air mixtures at rod temperatures between 700° and 1200°C, and with rod diameter of 3.5 mm.