CONDITIONS OF “PLUGHOLING” IN CASE OF SMOKE EXHAUST SYSTEM OF NATURAL TYPE

Efficiency of smoke exhaust system of natural type may be reduced due to the phenomenon of “plugholing”, which lies in the fact that the clean air out under ceiling smoke layer due to the action of lift forces passes through the smoke exhaust opening. The conditions for the emergence of “plugholing” are investigated in the case of a natural impulse to smoke. It is shown that in a case of a one-dimensional approach “plugholing” occurs when the Froude number is 2 and the mean bulk temperature of under ceiling layer 2 times greater than the temperature of cold air. A numerical study of thermal dynamics picture of fire in the model room is made with model combustible load by using a three-dimensional field model. We consider the room in the form of a parallelepiped with dimensions of 30
25
15 m. Turbine oil is combustible material. Combustion power is constant over time and is 1.257 MW. Hole size for smoke in the ceiling is 1.2
1.2 m or 3
3 m. The dependences on the time of mass flow of the gas mixture coming from inside out through the smoke exhaust opening are obtained. It is shown that the flow rate obtained using a three-dimensional mathematical model, is significantly smaller than when using a one-dimensional approach. It is found that proposed in the literature formulas for calculating critical flow mixture of combustion products and air through the smoke exhaust opening, when “plugholing” begins, are considered invalid for the present fire conditions.