Mass pyrolysis rates and excess pyrolysate in fully developed enclosure fires

Abstract

Several critical aspects of fully developed enclosure fires need significant investigation and understanding of the underlying physics. Fire resistance of wall, ceiling and floor building elements depends on the maximum heat fluxes and the heat flux history from a fire and on the structural and thermal properties of the building elements of an enclosure. Maximum heat fluxes usually occur after the enclosure is fully involved in fire. In this regime, called post-flashover, combustion can be fuel (well-ventilated fires) or ventilation controlled (under-ventilated fires). For ventilation-controlled fires, the gas temperature is determined from the consumption of the incoming air into the enclosure and the energy exchanges of the combustion products with the enclosure boundaries. Although heat losses to the walls affect the development of gas temperature, quasi-steady-state conditions can be reached wherein conduction heat losses are small in comparison with the heat release rate in the enclosure. This situation applies for thick walls. In contrast for thin walls, steady-state conditions are developed early while radiation and convection heat losses are significant. This work deals with the late stages of fully involved fires in enclosures. The enclosure geometry (rectangular (cubic-like) and corridor), the fuel area and the opening size affect the air inflow, the burning rate and the excess pyrolysate issuing out of the enclosure. The excess pyrolysate, burns outside forming a fire plume on the external façade wall. For larger openings, the air inflow increases, the excess pyrolysate decreases and the burning rate reaches a maximum value that depends on the stoichiometric air to fuel ratio by mass. Further increase in the size of the opening leads to well-ventilated fire and finally, to free burning fire behaviour. The present analysis and results for these phenomena also explain Kawagoe's equation of mass loss rate m ̇ T =0.1 A o H o (kg/s) for pyrolysis in under-ventilated fires.