Global behaviors of enclosure fire and facade flame heights in normal and reduced atmospheric pressures at two altitudes

摘要

Enclosure fire safety design and regulations are commonly specified for normal pressure conditions at sea level. There is need however to extend the design requirements for conditions in reduced pressure atmosphere such as city at high altitude, in which the corresponding fundamental behaviors should be clarified first. Experiments are designed and carried out in this work at two different altitudes (Hefei city: 50 m, 1 atm; Lhasa city: 3650 m, 0.64 atm) in a 0.4 m cubic fire enclosure with various window dimensions and an attached 1 m (wide) × 2.2 m (high) facade with and without an opposite facing wall. Gas temperature profiles inside the enclosure and facade flame heights are measured to find out their behavioral changes for these two pressures and to develop global correlations. It is found that, for a given fuel supply rate the gas temperature inside the enclosure is lower while the outside flame is higher in the reduced pressure atmosphere, owing to the lower fuel consumption inside the enclosure and air entrain-ment into the flame. The flame height (without a facing wall) normalized by a characteristic window length scale ?_1 can be well correlated with the dimensionless excess heat release rate, but being a bit higher due to lower entrainment or larger fluctuations at lower pressure, but can be accounted for globally by a correction factor of 0.8. In addition, the characteristic length scale ?_3 representing the horizontal projection of the flame is higher, and thus, the critical distance of the facing wall from the enclosure, at which interaction of the facade flame and the opposite facing wall starts, is larger at lower pressure. The relative higher facade flame height and the larger critical fagade-to-facing-wall distance suggest that hazards of enclosure facade fires at low pressure (high altitude) are higher and it need more conservative regulations than those at standard pressure. By accounting for their behavioral changes, global models have been correlated for gas temperature inside the enclosure as well as facade flame height without and with an opposite facing wall applicable for these two atmospheric pressures. The present results and global correlations in both normal-and reduced pressure atmosphere provide fundaments of guideline for regulations extension to high altitude and are a significant supplement and improvement over previous results in literatures.