Fire Safety Science Digital Archive

Three main regimes of behaviour can be identified for fully developed crib fires in compartments. In one of these crib porosity controls and in another fuel surface; these correspond to similar regimes for cribs in the open. The third regime is the well known ventilation controlled regime. Nilsson's data have accordingly been analysed in terms of these regimes and approximate criteria established for the boundaries between them. It is shown from these that in the recent G.I.B.lnternational programme the experiments with larger spacing between the crib sticks were representative of fires not significantly influenced by fuel porosity. The transition between ventilation controlled and fuel surface controlled regimes may occur at values of the fire load per unit ventilation area, lower than previously estimated. In view of the increase in interest in fuels other than wood on which most if not all empirical burning rates are based, a theoretical model of the rate of burning of window controlled compartment fires is formulated. With known and plausible physical properties inserted it shows that the ratio of burning rate R to the ventilation parameter AwHw^1/2, where Aw is window area and Hw is window height, is not strictly constant as often assumed but can increase for low values of AwHw^1/2/At where At is the internal surface area of compartment. This is in qualitative accordance with Nilsson's and the C.I.B data. It is shown that R/AwH^1/2 is, approximately the product of two quantities, one dependent on the fuel and the other on a heat transfer coupling between the fuel bed and its surroundings.