One of the essential objectives in fire safety engineering is to safeguard firefighters during operations. A smoke explosion is an unforeseen deflagration that can occur in a compartment fire endangering firefighter safety. In addition to the classical compartment fire development stages, smoke explosions can occur for underventilated conditions while the ventilation openings remain the same. This thesis is focused on the smoke explosion phenomenon to obtain a better understanding of its process. A better understanding of the smoke explosion phenomenon raises firefighters' awareness of its causes and potential consequences.

This study starts with a comprehensive review of the previous compartment fire experiments that led to smoke explosions. The effects of the fuel location and the ventilation opening size on the mass loss rate and heat release rate are analysed based on the previous experiments. A novel ventilation factor is introduced for the compartment with two vertical front openings. It is demonstrated that the overall compartment fire behaviour (excluding the possible occurrence of explosions) is nearly independent of the location or the porosity of timber crib fuel for underventilated conditions but depends on the ventilation factor. A preliminary explanation for the occurrence of the smoke explosions is provided, and it was found that the effect of crib porosity on the occurrence of smoke explosions needs further investigation.

As part of the study, the flammability limits of the pyrolysate gases produced from thermally decomposed medium density fibreboard (MDF) and plywood are quantified. Finding the flammability limits of the pyrolysate gases is believed to assist in studying the smoke explosion phenomena as the smoke explosion is known to be caused by the gradual transformation of the oxygen and smoke gas mixture to reach a flammable range. An experimental procedure is developed to generate pyrolysis from decomposing thermally thick fuels and measure the flammability of these pyrolysis gases. The flammability characteristics provided fundamental knowledge for understanding the smoke explosion phenomenon in the compartment fires.

This study investigates the burning history of medium-density fibre (MDF) cribs in an underventilated compartment with two openings. Such compartment fires are explored before by other researchers and demonstrated that would lead to smoke explosions. A total of 19 compartment fire experiments were completed, five of which led to smoke explosions. A gas conditioning system is designed for the smoke explosion experiments, which includes a flame ionisation detector (FID) for hydrocarbon measurements and an enhanced Phi-meter for equivalence ratio measurements. In addition to these measurements, heat release rate, mass loss rate, temperatures, pressures, and O2, CO2 and CO gas concentrations with the compartment are measured for each experiment.

The smoke explosion occurs in a compartment fire when the changes of pyrolysate gases and oxygen concentrations in the compartment make the mixture forms a flammable mixture, i.e., falling within the flammable region of the flammability diagram. The increase of oxygen and pyrolysate gas concentrations to form a flammable mixture occurs following the transition of flaming combustion to smouldering. The averaged equivalence ratio of the compartment immediately before the occurrence of smoke explosions were found between 1.5 to 2.0.

The development of fire in severely underventilated compartment fires are described and classified to three different scenarios. These scenarios are: a. flaming combustion from ignition to burnout with no smoke explosion, b. flaming combustion transitioning to smouldering combustion and smouldering combustion continues until the burnout with no smoke explosion, and c. flaming combustion transitioning to smouldering combustion and smoke explosion occurring after the transition to smouldering combustion.

The questions raised by the external examiners of the thesis and the authors response to those questions are appended to the thesis.