Structure of flame quenched by cold surfaces

Abstract

The region of interaction of a cooled metallic heat sink with a premixed lean flat ethylene/air flame is investigated. The detailed distribution of burning velocity is measured, by means of a refinement of the particle track method. Two dimensional distributions of temperature and composition in this region are obtained from thermocouples and microprobe sampling, followed by gas-chromatographic analysis. The burning velocity-heat loss relation is found to depend on the orientation of the flame front to the heat sink. Heat release rates deduced by substituting temperature and flow data into conservation equations reveal some unexpec­ted features close to the heat sink; in particular the zone of heat release becomes narrower and the maximum rate of heat release does not decrease appreciably towards the heat sink, in spite of a considerable reduction in temperature. These features are interpreted in terms of the composition pattern, which suggests that the narrowing is due to quenching of the second stage of reaction, while the reaction rate effect is caused by diffusion of hydrogen atoms to the surface, which acts as a sink for them. It is shown that a reaction such as H + O 2 → Surface HO 2 would account for all the features observed; conclusions are drawn regarding the status of quenching theory and the role of flame-surface interactions in flame structure studies.