Flame Stability in Underexpanded Natural Gas Jets

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Abstract
—It is known that, for nozzle diameters greater than some critical value, free jet flames remain stable at all driving pressures. For releases from orifices smaller than this limiting diameter, subsonic jet flames become unstable when the exit velocity is increased above some blow-out stability limit. In these unstable jet flames, it has been postulated that flow conditions suitable for stable flame formation may be obtained by increasing the driving pressure to produce a supercritical (choked and underexpanded) jet. This paper presents an experimental study of the flame blow-out stability limits in high pressure natural gas jets. The critical diameter for unconditionally stable flames is determined and the postulate of high pressure restabilisation confirmed. Also, previous work on the probabilities of subsonic jet light-up resulting from a localised ignition has been extended to supercritical jets. It is shown that any ignition at or beyond that axial location associated with the lower flammable limit cannot result in the establishment of a stable flame.