Observations on the delay time of backdraught in the absence of a pilot source

Most backdraught studies to date have used gaseous fuels and piloted ignition scenarios. In such circumstances the delay between the opening of the compartment door and the onset of backdraught is entirely dependent on the time taken for the gravity current to extend from the open doorway to the location of the ignition source, typically on the opposite wall in experimental studies. Few studies have investigated backdraught in the absence of a pilot source. A series of reduced scale backdraught tests have been carried out using solid polypropylene and polyethylene fuel sources, without a pilot source being present. The observed backdraught delay times are considerably longer than the times suggested in the literature, based on the velocity of the gravity current. The delay times are shorter at higher temperatures, but vary with the chemistry of the fuel as well as the temperature and geometry of the compartment. A simple theory of auto- ignition chemistry is presented to explain the observations.