Carbon monoxide and smoke production downstream of a compartment for underventilated fires

Abstract

The carbon monoxide, carbon dioxide and smoke yields measured downstream of a small scale compartment (volume 0.125m3 – 0.375m3) were studied for underventilated fires of propane. The flow rate of propane was increased gradually and species were collected under a hood. The heat release rate (HRR) of the combustion products was also measured (by oxygen depletion) and was found to initially increase as the flow rate of propane was increased (overventilated burning inside the compartment). Before external burning started, an intermediate plateau in the measured HRR was observed, corresponding to the Heat Release Rate =1500AH1/2 , where A and H are area and height of the opening respectively. The same behaviour was observed for all openings and remarkably all compartment geometries employed in this work. Further experiments indicated that the occurrence and extent of that plateau depends on the temperature inside the compartment and thus the growth rate of fire. Species production during this plateau period was investigated, as combustion was underventilated during this phase. It was observed that carbon monoxide (CO) and smoke yields increased during this period. The CO yield increased by a factor of 5, whereas the smoke yield by a factor of 3. Moreover, comparison and differences are discussed between the values of the ratio of carbon monoxide to smoke yield from our study and from the literature. The present results for the increased amounts of carbon monoxide and smoke are applicable if, during the fire growth, underventilated conditions develop without external burning. Current engineering calculations for smoke and carbon monoxide ca not predict the high concentrations of carbon monoxide and smoke measured in such a scenario. Whether these conditions can be developed will depend on whether the gas temperatures at the opening of the enclosure are able to ignite the unburned gases issuing from the enclosure. This in turn, depends on the fire growth rate, i.e. for a fast increase in the fuel supply rate the gas temperatures in the enclosure are lower than for a slower increase in the fuel supply rate due to (transient) heat losses to the walls of the enclosure and as result outside burning starts much later.