Non-ignition of an ejected flame through a gap greater than the quenching distance

Abstract

When a gap is greater than the quenching distance, the flame from an explosion inside a chamber can pass through the gap, but it will not always ignite an outside combustible mixture. There is a non-ignition gap distance, greater than the quenching distance, below which ignition will not occur, and above which it will. The present paper presents experimental determinations of non-ignition gap distances for different values of several parameters (gap width, number of gaps, and chamber volume) for various rectangular gaps with a wide range of lengths. A rectangular gap has three dimensions. One of these is here referred to as the gap distance, the others being gap width and gap length; diagramme definitions of the terms are given. The gap distance for non-ignition varies with the values chosen for the other two dimensions. It increases as the gap width decreases. But when gap length is varied for gap widths over 4.0 cm, the non-ignition gap distance first increases with increasing gap length, and then levels off. A second increase may occur or not, depending on other parameters. In particular, any given sectional gap area (fixed values for gap width and gap distance) may be subdivided, giving rise to multi-layered gaps. Experiment establishes that such subdivision is available as a method of preventing ignition of an outside combustible mixture, since ignition may occur through a single gap of given area and not through a subdivided multi-layered gap of the same area. At multi-layered gaps, one gap flame is far in advance of the others. Other gap flames are either soon extinguished within the gap, or advance a short distance without emerging. It seems permissible to consider the resultant as one flame for analysis. The magnitude of the non-ignition gap distance for multi-layered gaps is proportional to ut, u being the ejection velocity of the flame from the gap, and t the time taken for the flame to pass from the jet origin to the gap exit.