Volume expansion pressures and global heat transfer in compartment fires

Abstract

Volume expansion pressures generated by fire must be considered in designing smoke control systems. These pressures depend primarily on the heat deposition rate by the fire in the gas phase as well as enthalpy and mass exchanges with the surroundings through openings in the enclosure. This paper describes systematic experiments with fires in enclosures, vented through apertures near the floor which exchanged insignificant enthalpy flux with the outside. The heat deposition rate, expressed as a fraction of the total convective heat release rate in freeburn, γ c , was determined as a function of time in t-square fires (in which the heat release rate increases with the square of time) based on measurements of volume expansion pressures in the enclosures. Full scale experiments using several different fire growth rates were augmented with extensive experiments in one-quarter scale models, following verification of the scaling method. The major experimental phase simulated fires in rooms with floor areas of 10 m×10 m, 15 m×15 m and 15 m×29 m and ceiling heights of 2.4, 3.7, 4.9 and 10 m, using both methane (clean burning) and propylene (very smokey) burners. Heat deposition fractions were found to correlate well according to γ c =fn( t 2c * ), where t 2c * is a nondimensional time proportional to t/(t gc 2/5 H 4/5 ) for a fixed environment ( t being the time from ignition, t gc the growth time to a fixed freeburn convective heat release rate, and H the ceiling height above the fire source). Values of γ c were near unity at small t 2c * , decreasing to approximately 0.2 at the highest values of t 2c * investigated.