Detailed investigation of flame transmission from a vessel to a discharge duct

The present paper deals with a problem of explosion initiated in a vessel and vented through a duct. On the basis of numerical simulation (CFD) and visualization by means of high speed camera it completes and discusses the existing results and hypotheses (especially those presented in the work of Ponizy and Leyer [1]) concerning the process of enhancement of vessel pressure rise during such an explosion. In particular, numerical simulation indicates that a secondary explosion in the duct, known as a “burn-up”, which is responsible for higher reduced explosion pressures during ducted venting, has its source in a highly turbulent zone generated at the duct entrance by the flame itself independently of the shape of the vessel/duct passage. Camera images confirm the hypothesis that the flame penetrating into the duct is strongly torn in this turbulent zone and mixed with fresh gases. On the other hand, these images show that the reverse flow created by the burn-up returns back to the vessel some amount of rapidly burning gases, which contributes to the vessel pressure increase in the same degree that the blockage of the outflow from the vessel and the intensification of combustion of the unburned mixture previously left in the vessel.Experimental results evidence also a strong dependence of the flame behaviour in the vessel on the vessel length and the duct diameter. They demonstrate that the final effect of the burn-up action on maximum explosion pressures reached in the vessel depends on the whole process of flame propagation in the vessel, which in turn is closely related to the vessel and duct dimensions. In particular, in some configurations, an apparition of the “tulip flame” combined with inertia of the gas column in the duct may in a spectacular manner reduce the effect of the burn-up action.