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Article Excerpt
INTRODUCTION
Fires in high-rise buildings create unique challenges for the design of fire safety systems. In the event of a fire in a high-rise structure, the heated buoyant gases produced by the fire will naturally rise and migrate to upper floors through any internal opening in the building. Combustion gases can accumulate in increasing concentrations on the upper floors, creating hazardous, often deadly, conditions for occupants who become isolated on upper floors. One method of minimizing the amount of smoke that can enter upper floors is the use of a floor pressurization system that is designed to force the smoke to remain in areas away from habitable spaces. Such a system could be triggered by a smoke alarm or detection of water flow from a sprinkler head. The air-handling equipment would then activate and pressurize floors above the fire floor and force the smoke to remain in vertical passages such as elevator shafts as it moves to upper floors.
Obviously the volume of fresh air that must be supplied by such a system is governed by numerous factors, including the construction of the building, the intensity of the fire, environmental conditions, and the construction of vertical shafts such as the elevator shafts and stairwells. This paper describes a mathematical model that takes into consideration all of the major factors that influence the vertical movement of smoke during a fire in a high-rise building. The model is used to evaluate the feasibility of a floor pressurization system that is designed to create safe conditions so that occupants on upper floors have a high probability of experiencing a smoke-free environment during the fire. The model is used to quantify the volume and location of fresh air supply that is needed for realistic building designs, typical construction practices, and practical fire conditions.
The movement of smoke to upper floors in a high-rise is dominated by flow through vertical shafts like elevator shafts and, to a lesser degree, through stairwells. In fact, smoke spread via vertical shafts in buildings has been estimated to be about 95% of the total vertical flow during a high-rise structural fire [1, p. 147]. Vertical flow of smoke over several stories through openings in floor construction is minimal, particularly in high-rise buildings. Therefore, any smoke control technique should concentrate on limiting smoke from entering habitable areas from the large, vertical conduits such as elevator shafts, which are the largest, unrestricted passages open through the entire height of the building. Stairwells are also in contention for the vertical transport of smoke, but for the purpose of this paper, stairwells are assumed to be well sealed and stairwell doors remain closed throughout the fire event. Therefore, the emphasis here is on smoke transport through a simulated elevator shaft.
The pressure distribution inside a building has long been known to decrease with height such that the pressure inside a vertical shaft like an elevator shaft and the pressure on individual floors will reach equal values at a location called the neutral pressure plane (NPP). The concept of the NPP is important because it is a quantitative measure of how smoke can be distributed throughout a building during a fire. For floors below the NPP, pressure differences between the shaft and the floor drive smoke into the shaft, leaving lower floors relatively smoke free. However, above the NPP, the pressure difference is reversed and smoke leaves the shaft in increasing amounts on upper floors and contaminates the habitable spaces. This pattern of smoke-heavy upper floors and relatively smoke-free lower floors has been predicted through analysis (2), (3) and has been experienced in actual fire conditions in tall buildings (4). Any smoke control technique that uses air-handling equipment to pressurize areas within the building must consider the location of the NPP and have knowledge of how the local floor pressure is affected by the presence of air added to each floor. Pressurization of floors above the NPP will improve fire safety by selectively increasing the floor pressure where the danger of smoke ingress normally has the greatest potential to contaminate the floors. On the other hand, pressurization of floors below the NPP will not significantly improve smoke conditions on lower floors, because the pressure distribution in the building already favors air moving into the shaft, which leaves those floors relatively smoke free.
SMOKE MODEL
The network smoke movement model used in this paper is called COSMO for COntrol of SMOke in a high-rise structure. It is based on a complete analysis of the basic principles of conservation of mass, momentum, and energy. By considering all of the factors that influence the movement of smoke, the program represents an improvement on other well-established programs that have traditionally been used by fire protection engineers to quantify the movement of smoke during a structural fire.
The most important improvement over other accepted models is the inclusion of a complete heat transfer and momentum balance on the smoke as it passes up the vertical shaft. Many existing models avoid the complexity of a detailed heat transfer analysis and assume that temperature in all fluid elements is either constant or each zone has a known temperature distribution [1, pp. 103-108; 5, p. 121]. By doing so, these programs avoid the solution of the conservation of energy, which is a critical foundation that must be satisfied before a valid solution to the smoke movement program can be determined. Even though an assumed temperature distribution greatly simplifies the smoke movement analysis, it may violate the conservation of energy and lead to erroneous answers. Furthermore, the temperature distribution throughout the building and within the shaft is the primary factor that drives the vertical motion of the smoke. Any error in the temperature distribution of the smoke in the shaft can lead to incorrect trends in smoke movement.
Another improvement of the COSMO smoke movement model is the manner in which the model treats the motion of the smoke in the shaft. Many other existing models simplify the calculations by replacing a detailed momentum analysis on the smoke layer with an assumption of a pressure distribution based on a static, constant density fluid [5, Ch. 8]. This type of assumption does not accurately predict the pressure distribution when it is applied to an element of an ideal gas that is moving through an elevator shaft.
COSMO calculates smoke properties by using the ideal equation of state and knowledge of thermodynamic properties of air as a function of temperature. The conservation of mass accounts for air entering the shaft from the lower floors, which dilutes the smoke in the shaft, and smoke leaving the shaft on upper floors, which contaminates habitable spaces. The conservation of momentum applied to an elemental slice of the shaft considers inertia forces, pressure forces, buoyancy forces that draw the smoke upward, gravitational forces that pull the smoke downward, and frictional forces at the surface of the shaft that retard the motion of the smoke.
The conservation of energy applied to the differential element of smoke in the shaft includes both radiation and convection from the hot layer of smoke. The convective heat transfer rate between the hot gases and the cooler surface of the shaft utilizes traditional free and forced heat transfer coefficients for flow in a vertical duct. The radiation model assumes emission and absorption within a transparent smoke layer as it transfers heat to the surface of the shaft. The simplified geometry of the shaft and adjacent floors utilized in formulating the smoke movement software is shown schematically in Figure 1.
[FIGURE 1 OMITTED]
The equations of energy, momentum, and mass are combined with property relationships and the resulting system of equations is solved for the local pressure, temperature, density, velocity, and flow rate of the smoke as it moves through the vertical shaft. The calculated pressure in the shaft along with the calculated pressure on the floors as a function of vertical height permit the determination of the mass and volume flow rates of the smoke as it enters or leaves the shaft. The local pressure distribution in the shaft, floor, and atmosphere are then used to locate the NPP where the reversal of flow into and out the shaft occurs.
Smoke enters the shaft at the fire floor, where the local pressure is above the local atmospheric pressure due to the expansion of the hot combustion gases. The pressure rise on the fire floor forces gases through the gaps around the doors and the openings in the surface of the shaft. Once the light, hot gases enter the shaft, they rise and ultimately leave the building through the vent at the top of the shaft. As the hot combustion gases move upward through the shaft, they leave the shaft in an amount depending upon the local difference between the shaft and floor pressures. Fresh air can enter the shaft and dilute the smoke if the local floor pressure exceeds the pressure in the shaft. The direction of the gas movement and the quantity of gases transferred to and from the shaft are obviously dependent on an accurate prediction of the shaft pressure as a function of the height within the structure. The emphasis in this paper is not on the basic equations used in the model but on the application of the model to illustrate how buildings react to movement of smoke and how air-handling units can serve as a smoke management system and improve fire safety in a high-rise structure. The mathematical foundation behind the model is detailed in (6).
When used in the design of a fire safety plan, COSMO can show how factors such as building construction, fire conditions, heat transfer from the smoke in the elevator shaft, and operation of a floor pressurization system can influence the location of the NPP and the pressure distributions in both the shaft and the floors. The program is therefore able to qualitatively predict the movement of smoke throughout a realistically designed high-rise structure. The model is also capable of predicting the location of the NPP, which is an important aspect when designing a life safety program for a tall building.
MODEL LIMITATIONS
The movement and control of smoke during a fire have been studied for a number of years, and the number of papers that report on smoke movement investigations is fairly extensive (7-33). Summaries of the findings of smoke movement papers appear in (34-37), and some of the more recent investigations have concentrated on the design of smoke management systems (38-42). All studies and analyses of smoke movement are complicated by the fact that smoke is influenced by many factors and moves in response to extremely small pressure differences. Therefore, any attempt to predict the path that smoke takes throughout a...
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