An optical study of pre-ignition heat release

Abstract
The distributions of temperature and velocity are measured by optical methods in an ethylene–air mixture flowing against a uniformly heated surface, in order to determine the profile of heat release rate before ignition. The objective is to deduce the variation of heat release rate with temperature (in a manner analogous to the thermal analysis of flame structure but in absence of the large diffusive flux of active species associated with flames) so as to compare the rate laws with those applying in flames and to investigate their use for predicting ignition in other flow systems. Various practical igniter systems are investigated using cine-interferometry and particle tracking. The results manifest a constant activation energy of 8.8 kcal mol-1 (37 kJ mol-1) and differ entirely from the heat release rate laws in equivalent premixed flat flames. The back-diffusion of radicals in the latter gives rise to much higher rates of heat release at low temperatures and no constant 'activation energy' based on concentrations of the primary reactants. The possibility of using such results to predict ignition, on the criterion that beyond this condition temperature would rise with time, without a steady state solution, is discussed. For unidimensional systems a simple analytical solution is proposed in an Appendix by Dr. J. Adler. This yield interesting and plausible predictions of ignition temperature as a function of flow velocity and initial temperature using present results. Computer solutions would be required for more complex system.

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