Clarifying the mechanism of flashover from the view of unburned fuel volatiles and secondary fuels

Abstract

Several definitions of flashover exist. According to thermal theory, flashover occurs because of thermal instability in the upper hot layer of a fire compartment. The upper layer is particularly important. Practically and visually, flashover is identified by the exiting of a flame from a doorway. The ignition of unburned fuel volatiles in a fire compartment is highlighted. This definition of flashover may imply a premixed flame and the necessity of sufficient fuel volatiles. Alternatively, flashover is identified as the ignition of secondary fuels. This definition concerns secondary fuels. The aforementioned definitions are based on different physical and chemical processes that are involved in flashover and accentuate different items, such as hot layer, unburned volatiles and secondary fuels. The practical relevance of the flashover theory and the practical methods to identify flashover have not been elucidated adequately. This study utilizes an experimental compartment in which ignited fuel exists without or with a secondary fuel, to clarify the necessity of sufficient fuel volatiles and secondary fuels for flashover. The length of the compartment was one-third of the length of an ISO 9705 test chamber. The fuel was gasoline and iso-propanol, which was poured in pans with diameters from 19 to 30 cm. In the beginning of the experiments, only the first fuel was ignited. The secondary fuel, if presented, received heat and released volatiles. If the criterion of premixed flame applies, then the time to flashover in the experiments with a secondary fuel should differ from that in the experiments without a secondary fuel. Throughout the experiments, mass loss rate of fuels was recorded. The results indicated that the times to flashover without a secondary fuel approximately equaled with a secondary fuel. Therefore, only thermal instability is responsible for flashover.