The air flow to an underventilated compartment fire often depends on the flow velocities in the gravity wave of cold air that feeds the fire with oxygen. This problem has been studied in laboratory experiments and by CFD simulations. The main problem seems to be whether mixing and entrainment between the two layers of hot and cold air has a profound effect on the flow velocities. In this article, an analytical gravity wave model that can calculate the velocities in a simple gravity wave is presented. This model uses the equations of stratified flow hydraulics and the translatory wave solution of the flow equations. It is found that the velocities of the model compare very well to the velocities reported from laboratory tests and numerical simulations. Numerical simulations of stratified flow in a CFD model are discussed with respect to model construction. It is concluded that the densimetric Froude number is the main parameter for the velocity calculations and the length/height ratio is important for the friction forces.
This paper is an attempt to integrate theoretical Computational Fluid Dynamics (CFD) calculations with practical fire-fighting tactics commonly used when arriving at the scene of an underventilated fire. The paper shows that CFD has a great potential in improving understanding and creating better effectiveness in the estimation of fire-fighting tactics. If burning has occurred in a lack of oxygen for a long time, excessive pyrolysis products may have accumulated in the fire compartment. If air is suddenly introduced in the compartment a backdraft may occur. The CFD code used for the simulations is fire dynamics simulator (FDS). In this paper, we focus on the conditions that can lead to backdraft, and not the deflagration or rapid combustion in itself. Therefore, the simulations focus on the gravity current and the mixing process between cold fresh air and hot smoke gases by considering a uniform temperature inside the building as initial condition. The different tactics studied include natural ventilation, positive pressure ventilation (PPV) and dilution by water mist. Their effectiveness is observed comparing them with a reference scenario, where no action is taken. The main objective of natural ventilation is to find the fire source, and the venting is more effective with several openings. Tactics involving PPV are very effective in evacuating the unburnt gases, but increases the mixing, and consequently the probability of backdraft during the early stage of operation. On the other hand, the addition of water mist can reduce the danger of backdraft by reducing the concentration of unreacted combustible gases below the critical fuel volume fraction (CFVF), where ignition cannot occur. If the dilution level is insufficient the danger of backdraft is increased, mainly because the process of gases evacuation is longer due to cooling, which reduces the density difference between hot and cold gases. During a fire-fighting operation, the choice of tactic depends mainly on whether there are people left in the building or not, but also on the fire-fightersÕ knowledge of the buildingÕs geometry and the fire conditions. If the situation shows signs of strongly underventilated conditions, the danger of backdraft has to be considered and the most appropriate mitigation tactics must be applied. 12 Nomenclature C airThe heat capacity of air (approximately 1 kJ/(kg K)) CFVF Critical fuel volume fraction CMC Critical mixture composition D(X) Dilution level, corresponds to a scenario with X % mass concentration GML Gas measurements line LFL Lower flammability limit M o Initial mass of hot gases M vap Mass of vapor added to obtain the desired dilution M CH4 Mass of unburnt gases (methane) Q fan Flow rate of the PPV fan tSimulation time T Temperature T final Temperature of hot gases after dilution T in Temperature of hot gases inside the compartment T out Temperature of cold gases outside the compartment (ambient) UFL Upper flammability limit v fan Velocity at the inflow boundary, due to PPV fan V gas Volume of hot gases in the en...
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 (a) 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.
Abstract. The phenomenon of backdraft is closely linked to the formation of a flammable region due to the mixing process between the unburned gases accumulated in the compartment and the fresh air entering the compartment through a recently created opening. The flow of incoming fresh air is called the gravity current. Gravity current prior to backdraft has already been studied, Fleischmann (1993, Backdraft phenomena, NIST-GCR-94-646. University of California, Berkeley) and Fleischmann (1999, Numerical and experimental gravity currents related to backdrafts, Fire Safety Journal); Weng et al. (2002, Exp Fluids 33:398-404), but all simulations and experiments found in the current literature are systematically based on a perfectly regular volume, usually parallelipedic in shape, without any piece of furniture or equipment in the compartment. Yet, various obstacles are normally found in real compartments and the question is whether they affect the gravity current velocity and the level of mixing between fresh and vitiated gases. In the work reported here, gravity current prior to backdraft in compartment with obstacles is investigated by means of three-dimensional CFD numerical simulations. These simulations use as a reference case the backdraft experiment test carried out by Gojkovic (2000, Initial Backdraft. Department of Fire Safety Engineering, Lunds Tekniska Ho¨gskola Universitet, Report 3121). The Froude number, the transit time and the ignition time are obtained from the computations and compared to the tests in order to validate the model.
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