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254 F I R E R E S E A R C H I. Physical Aspects of Fires Weinberg, F . J . and Wilson, J . R. with appendix by J . Adler (Impenal College, London, England) "An Optical Study of Preignition Heat Release," Proc Roy. Soc, Ser A 314, 175 (1970) Section: I Subjects: Ignition, heat release, preignition, Optical densities. Temperatures during Ignition, Ethylene-air, Ignition temperatures. Velocity fields. Thermal igmtion Authors' Abstract The distributions of temperature and velocity are measured by optical methods m an ethylene-air mixture flowing against a uniformly heated surface m order to deternune the profile of heat release rate before igmtion The objective is to deduce the variation of heat-release rate with temperature (m a manner analogous to the thermal analysis of flame structure but in the 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 m other flow systems Various practical igmter systems are investigated using cine-inter- ferometry and particle tracking The results mamfest a constant activation energy of 8.8 kcal mol-' (37 kJ mol"') and differ entirely from the heat release rate laws m 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 pos- sibility of using such results to predict igmtion, on the criterion that beyond this condition temperature would rise with time, without a steady state solution, is discussed For umdimensional systems a simple analytical solution is proposed in an Appendix by Dr J Adler This yields interesting and plausible predictions of Igmtion temperature as a function of flow velocity and initial temperature, using the present results Computer solutions would be required for more complex systems J . Meteorological Aspects of Fires
