A Different Approach to Vent Flow Calculations in Fire Compartments using the Critical Flow Condition

Abstract

In enclosure fires, density-driven vent flow through an opening to the fire compartment is directly dependent on the state of the fire and the evacuation of smoke and hot gases. If a fire is strongly under-ventilated, there may be heavy production of flammable gases. If a sudden opening occurs, e.g., a window breaks or a fireman opens a door to the fire compartment, fresh air enters the compartment and mixes with hot gases, thus creating a flammable mixture that might ignite and create a backdraft. In this article, we consider the critical flow approach to solve the classical hydraulic equations of density-driven flows in order to determine the gravity controlled inflow in a shipping container full of hot unburnt gases. One-third of the container’s height is covered by the horizontal opening. For the initial condition, i.e., just before opening the hatch, zero velocity is prescribed everywhere. When the hatch is opened, the incoming air flows down to the container floor and the hot gas flows out. The interface in between them (the neutral plane) can move up like a free surface in internal flows, making it possible to use the techniques of open channel hydraulics devised by Pedersen [1]. In this article the critical flow condition, known from classical hydraulics, is used providing a new equation for the vent flow problem. Two flow correction coefficients are considered at the opening, taking into account the uneven distribution of velocity (α) and the effect of mixing and entrainment (C). The value of these coefficients is evaluated using computational fluid dynamics simulations and physical model results performed for the same geometry. Together, these two coefficients form the flow correction coefficient used in practical formulas for vent flow in fire protection engineering. These are known to have a little different values for different geometries and flow situations. The resulting flow coefficient varies slowly with the density difference, shows a small variation with geometry and compares well with previously published data.