Fast flames in a vented duct

A theoretical study of steady flame propagation in pre-mixed fuel-air inside a partially confined duct containing closely spaced obstacles has shown that the venting of the burnt products has a dramatic influence on the overall flame speed. An important effect of closely spaced obstacles is to trap large pockets of unburnt material behind the leading edge of the flame. The subsequent burning of these pockets injects large quantities of burnt gas into the flow field. If this material is vented behind the flame then the overall flame speed is likely to be slow. However, if the flow field behind the flame is sufficiently restricted, a significant proportion of this material is vented in the direction of the flame motion. It advects the leading edge of the flame forwards leading to very high steady-state flame speeds. For this to occur a simple flow model predicts that the depth of the trapped layer between the obstacles must approach a critical value which depends on the expansion ratio across the flame and the overall duct parameters. For obstacles larger than this critical value the model predicts that no steady-state flame is possible. If no steady solution exists then the flame could continue to accelerate, generating damaging overpressures. Experiments have been performed to investigate flame propagation in a rectangular duct (2m ×5cm×5cm) filled with propane-air. Comparison of the experimental results with the predicted effects of obstacle size and the degree of confinement is favourable.