Concepts, Theories and Studies Regarding Compartment Fires under the Influence of Wind - Literature review Example

Literature Review Name: Tutor: Course: Date: CHAPTER TWO LITERATURE REVIEW 2.0 Overview The aim of this chapter is to review literature on concepts, theories and studies regarding compartment fires under the influence of wind. Compartment fires are examined in terms of temperature, wind characteristics, wind-induced conditions, and impact of ventilation, visibility and heat release rate of compartment fires in high-rise buildings. The conceptual approach relates to four constructs; average temperature, visibility, temperature, average temperature and heat transfer rate that are deemed to influence the movement and spread of compartment fire in high-rise buildings under the influence of different wind velocities. This literature search makes use of various secondary sources such as journals, books, articles and databases that explore wind movement and its impact of fire spread in high-rise buildings. The key areas addressed are compartment size, type of fuel used, condition of ventilation and speeds of wind. By reviewing this part of the literature, the reader is able to understand fire behavior in a compartment under different models and conditions. 2.1 Development stages in compartment fires Fire begins when a naked flame comes into contact with fuel in presence of oxygen creating exothermic chemical reactions [19]. The components that influence the spread of the flame are air supply, incident heat, and surface to mass ratio, orientation and fuel composition [18]. It then spreads once the material is ignited until it becomes fully involved with subsequent heat generation and rise in temperature. However, it is essential to recognize the various fire stages in compartments, potential changes arising from unplanned ventilation and fire progression that firefighters can predict [7]. Fire behavior in buildings is affected by behavior of materials under fire such as energy release rates and mass loss, fire development stages and behavior of fully developed fire such as duration, temperature and ventilation roles. Fire development goes through four stages of incipient, growth, full development and lastly decays as shown in figure 1 below. (Source: [7]) Figure 1: Fire development stages in a compartment Smoke results from effects of fire on fuel and thus a zone model is important to estimate smoke layer properties that affect steady state conditions being investigated. Hazards posed by smoke are high temperatures, light obscurity or visibility and gas species concentration [29]. On the other hand, increase in heat release rate in ventilation that is controlled when the fire is burning can be changed by altering the supply of oxygen to the fire. Escape in the face of fire is the most crucial safety aspect in a building [12]. For example, entry of firefighters into the building may increase flow of oxygen since the access point is a ventilation opening. Shortening evacuation time is not influenced by increasing the walking speed when stagnation happens in the floors [29]. Moreover, elevated temperature may cause failure to window glazing while firefighters can perform tactical ventilation to prevent fire from progressing into the growth stages. Due to limited oxygen or consumption of available fuel, a compartment fire may end up in the decay stage if no intervention is provided. On the contrary, limited oxygen supply causes fire movement to the decay stage even when heat release rate decreases. A significant threat ensues when temperature continues to rise even when oxygen concentration has dropped due to presence of flammable gaseous products and high concentration of pyrolized hot fuel [7]. Fire developmental stages are incipient, growth, flashover/fully developed, and decay stages. In the incipient stage, the cooler air within the room and plume of hot gases to give rise flames from the fire. As it nears the ceiling, the hot gases become more defined, increases in volume and spreads horizontally across the ceiling [7]. In the growth stage, adequate oxygen increases the heat release of fire. Convection from the plume generates radiant heat increasing the compartment lining temperatures and can continue to ignite other fuel in the compartment (p.9). The flashover stage is a point of transition to full development of fire with temperatures reaching 500-6000C and floor heat flux attaining 15-20kW/m2 [7]. In this case, burning gases pushes out compartment openings at higher velocities. Fully developed stage has unburned gases that burn as they leave the ceiling and give rise to flames showing through windows and doors. The fire spread involves the heat transfer to other fuel packages such as structural materials, compartment linings and contents. In the decay stage, limited oxygen and availability of fuel may not support sufficient heat release and moves the fire to the decay state. However, it may worsen if there are flammable gaseous products and hot fuel (p.9). 2.2 Wind characteristics of compartment fires Compartment fire is fire burning inside an enclosed space such as room within a building. The controlling factors of a compartment fire are heat from the burning fuel and flow of oxygen into the fire [24]. Wind has great influence on fire behavior by increasing hot smoke and fire growth in the building. Consequently, window failure in a fire compartment in the presence of windy conditions leads to rapid and significant increase in heat release. Scott [24] suggests that wind characteristics constitute two components; direction of travel and speed at reference height above the ground. It is difficult to analyze the data on the direction and speed of wind because it varies every day, hour, minute and second. Wind flow brands and patterns propagate the fire out of the room of origin [22]. In free burning cases using n-heptane fuel, rise in wind speed decreases the burning time. However, using the same amount of fuel in compartment fires, burning time will even be shorter implying that there are two opposite wind effects with compromising results [2]. In a short duration of elevated wind speed, the wind is not significant to affect fire behavior because the fire requires a minute to respond to elevated wind speeds. For fire modeling applications, an appropriate time averaging period is 1-2 minutes of average wind speed. In the case of wind direction, degrees clockwise from upslope or respect to the north are used to compute relative spread direction [24]. This means that under the action of wind, it is difficult to extract smoke and fire spread because wind velocity increases from Zero to high values at certain elevations. 2.3 Effect of wind on compartment fires Compartment fires under dominant influence of wind especially in high-rise buildings require better fire management and safety guidelines design. Compartment fires that are not controlled without or with the presence of ambient wind follows a typical path of fire behavior. Chen et al. [2] observes that the fire after ignition will get through the growth phase to the flashover then to the fully develop phase. Here, it continues for some time before burning itself out at the decay phase. Buoyancy forces drive air drawn from outside the compartment through an opening that is shared with escaping hot smoke and gases [2]. A fully developed compartment fire will have an air flow rate that is strongly dependent on opening geometry and least on room temperature. Buildings are affected by turbulence caused by winds within the boundary layer which is the region between 100m and 3000m above the earth’s surface as illustrated in figure 2 below. The boundary layer is evidenced by intense vertical mixing which tends to leave wind speed and direction nearly constant with height [17]. Krishna et al [13] observes that for each terrain category, ground roughness is influenced by variation of wind speed with height. Winds tend to blow with higher speeds in smoother terrains and lesser speed in rougher terrains. Beyond gradient heights, wind speeds are equal in all terrains but take some distance over the terrain for speed profile to fully develop. (Source: [13]) Figure 2: Boundary layer for different terrain conditions Wind causes suction or pressure normal to the building or structural surface. When blowing horizontally to the ground, wind moves at high speeds. While the wind surface of a building will have positive pressure, negative pressure is experienced in the remaining surfaces as shown in figure 3 below. (Source: [13]) Figure 3: Wind flow and pressure differences in buildings In a compartment, the wind creates the corner (backflow) and centerline (main flow) areas that can influence the behavior of the fire [2]. Fire is ejected and blown downwards from the compartment in the center fire case. However, as the wind speed increases, the flame that is spilling out will extend horizontally. Backflow impels the fire upwards in upwind and downwind situations. In windy cases, the external flame puffs but in fire cases of wind speed approaching 3m/s, the entire opening is occupied by fire while in a lower wind speed of 1.5m/s, the fire only occupies the upper part of the window [2]. Moreover, Chen et al. [2] found that compartment fire using 250ml fuel did not reach the post-flashover stage as it did the 500ml fuel. External flames ejecting from compartment windows have certain thermal behaviors that are affected by ambient wind. As explained earlier, ambient wind has two contradictory effects; it dilutes and removes combustible gases but also supplies more oxygen causing severity to the fire. In many urban areas, average wind speed is in the range of 3-8m/s at 32m above sea level and gains a maximum speed of at least 40m/s [3]. Wind speed is even higher at the top of high-rise buildings. If there are two opposing winds, the thermal buoyancy and wind force compete in the compartment of fire. Despite the stronger wind dominating the smoke motion direction, ambient wind speeds in excess of the critical values drives the smoke upwards that could have moved up by thermal buoyancy. Moreover, wind speeds rising steadily from a high value to zero or vice versa, steady smoke temperature leaps to another state and is affected compartment walls’ heat loss [3]. In strong windy conditions, the burnout time and temperature rise are drastically reduced. External plume inclines to the downward side when wind is approaching in high velocities of over 3m/s and increases the flame which risk spreading the fire to adjacent buildings [9]. Adequate ventilation in flats requires both mechanical and natural ventilation. In the later, the temperature difference and cross-ventilation is maximized through window design based on prevailing wind direction, orientation and adequate layout. Inadequate cross-ventilation may demand use of mechanical ventilation [1]. However, mechanical extraction systems are not sensitive to wind but guarantees fixed extract volumes of smoke [6]. The best cross-ventilation flats are those located on the windward direction where room and windows minimize odor spread and maximize cross-ventilation potential. 2.4 Wind-induced internal flows Wind-induced action affects airflow patterns in that flames that move out of an opening can easily spread to other floor levels. For airflow to occur, both flow path and a pressure difference are necessary [23]. This means that resistance to airflow and the magnitude of the pressure difference provided by the flow path governs the airflow rate. Wind also affects generation of high smoke quantities in the flashover phase. Zhigang [29] notes that pressure distribution around the building affects wind-induced air flows into it especially through the openings. At the windward side, there is overpressure while on the parallel sides and the leeward side the wind makes an under pressure. Dynamic pressure is proportional to the pressure of wind flows away and into the surface, hence; wind-induced flow pressure is provided (p.12) as; Pw = Cp1/2ρuv2ref Where; Vref in (m/s): reference height wind speed; ρu: external air density (kg/m3); Cp: pressure coefficient and Pw is wind induced pressure in Pascals. The pressure difference will be determined by pressure inside and outside the wall of a compartment building. A more negative pressure above the vent is triggered by wind action to remove more smoke. However, wind-induced action in areas with adjacent buildings should be demonstrated not to provide undesirable effects of bringing down smoke [4]. In a wind-induced case of air flowing at 20m/s, internal fires cause whirls in the halls of compartments. When such strong winds blow towards a vertical surface, the smoke is pushed downwards resulting in negative coefficients of wind pressures on the leeward side [4]. When buildings are next to each other, incident wind fields cause turbulent effects implying that wind-induced action is the driving force for natural ventilation. Changing the behavior of natural ventilation is by increasing the number of high-rise buildings close to each other so as to modify the prevailing winds especially in urban areas [29]. 2.5 Fire attributes affected by wind Fire attributes that increase the intensity of fire are availability of oxygen and sufficient fuel. First, more oxygen will accelerate burning causing increase in fire temperature. Second, hot gases are cooled or diluted making the fire weaker and decreasing in temperature. When flames lean on the leeward side, the spilled-out flames take a horizontal trajectory that reflect the main air stream entering the compartment [14]. In free burning, rise in wind speeds decreases the burning time. Fire is made severe by approaching winds [2]. Although the wind removes combustible diluting gases and heat, it promotes the severity of fire by supplying more oxygen. With different fuel amounts, compartment behavior of fire differs because lower fuel supply limits the fire from reaching post-flashover phase compared to higher fuel quantities. Regarding the spread of fire, Long-fei et al. [14] argues that air temperature decreases as the distance from the fire surface increases. More importantly, temperature of lower and upper gas layers, floor/wall/ceiling temperatures and the visible gas and smoke concentration are some other attributes that can be affected by wind. In absence of adjacent buildings, the wind gives an upper pull by taking away smoke [4]. Surfaces with wind-blown flames are responsive and represent small pool fires [21]. Moreover, a significant pressure can occur or increase causing windows and walls to fail. The air flow in a compartment fire influence heat release rate because the indoor temperature is often higher than the outdoor temperature [3]. Ambient wind has great influence on smoke movement and compartment fire behavior. In a two-vent compartment, the stronger wind dominates the direction of smoke in motion. As soon as the ambient wind exceeds the critical value, smoke is driven downwards which would have been under influence of thermal buoyancy forces. Steady temperature of smoke is also affected by ambient wind [3]. The steady state temperature easily jumps from one state to another when wind speeds decrease from high to zero or zero to high values slowly. Consequently, the heat loss in the compartment walls affects the critical wind speed. Although little has been said about the effect of wind on burn duration or heat release rate, there is a greater possibility that winds reduces the burn duration and increases the peak heat release rate [22]. 2.6 Impact of ventilation on compartment fires The size of vents in an enclosure determines the air quantities entering the enclosure and influences the burning rate. A fully developed fire is affected by the distribution, amount and the form of compartment ventilation. Winds and temperature differences are essential in natural ventilation of buildings [2]. Similarly, burning inside and outside the compartment generates buoyancy forces that create the flow in and out of the enclosure. Ventilation in compartments is characterized by; Ventilation Factor; A√H Where; H is the opening height (m): and A is the opening area (m2). A compartment may have cross ventilation when there are openings on two opposite walls. The burning rate potentially increases, especially if there is a wind blowing. This is because for larger fire size, temperatures are higher than those in single ventilation in the cross ventilation condition (p.909). In adjustable mechanical ventilation rates, of a compartment measuring 4m length, 3m width and 2.8m width, Peacock et al. [20] using CFAST model showed that the room average temperature had good overall agreement with experimental results but was inconsistent with CO concentration. Natural flows happen through doors and windows in horizontal flows and also through compartment floors and ceiling in vertical flows (p.26). When a building orientation is properly arranged to any prevailing wind, the performance of natural ventilation is substantially improved. Roughness of the terrain affect the outcome of wind comfort as high wind speed can result in dangerous or uncomfortable conditions for inhabitants. Resulting flows through natural vents controls the spread and growth of fire in the compartment by governing the combustion products and the flow of air [29]. 2.7 Heat Release Rate (HRR) during compartment fires Fire risk assessment in high-rise buildings considers heat release rate and fire load as essential inputs in fire design [5]. Heat Release Rate (HRR) is used to investigate combustion characteristics of fire in the growth and post-flash-over stages. This is because HRR provides information on hazard assessment, possible fire environment, smoke production rate, and fire size [5]. Fire development is dependent on material properties of enclosure boundaries, location and size of compartment openings, arrangement and properties of fire loads and the ignition source. An important representation of design fire is the heat released when time is varied [5]. Shi et al. [25] shows that heat release rate can be determined through an equation: Q = ʎm ΔH Where; Q: Heat release rate (HRR) ʎ: Combustion efficiency ΔH: Heat of combustion m: Fuel mass loss rate in g/s This shows that airflow velocity is equivalent to 1/3 power of heat release rate. In a typical office building, a fully developed fire burning on plastics and wood would have the following characteristics; Table 1: Properties of fire in typical office building Property Description Effective heat of combustion Order of 16kJ/g Radiant heat flux 150-200kW/m2 Latent heat of vaporization 5-8kJ/g Mass burning rate/unit surface area 20-40g/m2-s Energy release rate/unit surface area 320-640kW/m2 Heat release rate must be known to estimate the fire hazard but fire development and control is unique in the growth and post-flashover stages. In a fully developed fire, the compartment’s exposed surfaces gets involved in the fire. The fire becomes ventilation controlled as combustion air is reduces and combustible gases emitted. Potential damage to the building ensues when the fire load is high creating longer fire durations. In the post-flashover stage, structural stability is essential since fire resistance performances of structural elements are estimated using time-temperature curves. The heat of smoke and flames causes fuel vaporization at mass flow rate ṁF. However, fuel may not necessarily burn in the compartment despite all the fuel eventually burning. Depending on the air supply rate, all oxygen could be burned or all fuel burned. Heat release rate within the enclosure is promoted by what is burning inside [21]. Therefore, the heat release rate within the compartment is given by; Where; ṁF: mass flow rate of compartment air; Δhair: heat of combustion/unit mass of air. The effective constant is about 3kJ/g of air. φ: boundary of rich-fuel and lean-fuel combustion regimes. Excess air begins off the transition to ventilation-controlled fire in the initial fire feed of compartment fire [21]. Consequently, fire moves to vents withdraws during the event of fuel consumption and before complete burnout, the fire is extinguished. Ventilation induced flashover causes rapid progression of fire. When the heat release rate for ventilation controlled fire, smoke vapor and fuel gas are below ignition temperature, they produce a pattern shown in the figure below. (Source: [16]) Figure 4: HRR for ventilation induced compartment fire 2.8 Burning rates of fuel Intensity of fire gradually reduces with limited supply of oxygen and so is the drop in temperature. Heat release rate and mass loss rate come in as concepts crucial at the initial stages of fire. A hot layer of fire forms near the ceiling because as fire gases burn, they rise upwards towards the ceiling. To predict the burn rate characteristics of fire in a compartment, it is essential to analyze severity of burning, burning time and the post-flashover effect it will have on the structures. The fuel mass loss rate is when the condensed fuel decomposes and transforms into hot gases due re-radiation from surrounding hot boundaries. The formula below provides the mass loss burning rates; Mass loss burning rate = Rate of unburned soot and fuel gases + Burning rate Where; mass loss burning rate is denoted by ṁF, Rate of unburned soot and fuel gases as ṁa, and burning rate as ṁb. Therefore; ṁF = ṁa + ṁb Moreover, equivalent ratio () is another parameter that describes burning and mass loss rate, when no provisions are made regarding unburned gas fuels. Equivalent ration is given by;  = ṁF / ṁ0 · s [25]. When there is a value in global equivalent ratio that is greater than 1, a distinct burning regime of under-ventilated or fuel-rich circumstance is experienced 1(>1). This is because inadequate oxygen for burning cannot support complete oxidation of fuel. Conversely, where 1( Read More

ificant threat ensues when temperature continues to rise even when oxygen concentration has dropped due to presence of flammable gaseous products and high concentration of pyrolized hot fuel [7]. Fire developmental stages are incipient, growth, flashover/fully developed, and decay stages. In the incipient stage, the cooler air within the room and plume of hot gases to give rise flames from the fire. As it nears the ceiling, the hot gases become more defined, increases in volume and spreads horizontally across the ceiling [7].

In the growth stage, adequate oxygen increases the heat release of fire. Convection from the plume generates radiant heat increasing the compartment lining temperatures and can continue to ignite other fuel in the compartment (p.9). The flashover stage is a point of transition to full development of fire with temperatures reaching 500-6000C and floor heat flux attaining 15-20kW/m2 [7]. In this case, burning gases pushes out compartment openings at higher velocities. Fully developed stage has unburned gases that burn as they leave the ceiling and give rise to flames showing through windows and doors.

The fire spread involves the heat transfer to other fuel packages such as structural materials, compartment linings and contents. In the decay stage, limited oxygen and availability of fuel may not support sufficient heat release and moves the fire to the decay state. However, it may worsen if there are flammable gaseous products and hot fuel (p.9). 2.2 Wind characteristics of compartment fires Compartment fire is fire burning inside an enclosed space such as room within a building. The controlling factors of a compartment fire are heat from the burning fuel and flow of oxygen into the fire [24].

Wind has great influence on fire behavior by increasing hot smoke and fire growth in the building. Consequently, window failure in a fire compartment in the presence of windy conditions leads to rapid and significant increase in heat release. Scott [24] suggests that wind characteristics constitute two components; direction of travel and speed at reference height above the ground. It is difficult to analyze the data on the direction and speed of wind because it varies every day, hour, minute and second.

Wind flow brands and patterns propagate the fire out of the room of origin [22]. In free burning cases using n-heptane fuel, rise in wind speed decreases the burning time. However, using the same amount of fuel in compartment fires, burning time will even be shorter implying that there are two opposite wind effects with compromising results [2]. In a short duration of elevated wind speed, the wind is not significant to affect fire behavior because the fire requires a minute to respond to elevated wind speeds.

For fire modeling applications, an appropriate time averaging period is 1-2 minutes of average wind speed. In the case of wind direction, degrees clockwise from upslope or respect to the north are used to compute relative spread direction [24]. This means that under the action of wind, it is difficult to extract smoke and fire spread because wind velocity increases from Zero to high values at certain elevations. 2.3 Effect of wind on compartment fires Compartment fires under dominant influence of wind especially in high-rise buildings require better fire management and safety guidelines design.

Compartment fires that are not controlled without or with the presence of ambient wind follows a typical path of fire behavior. Chen et al. [2] observes that the fire after ignition will get through the growth phase to the flashover then to the fully develop phase. Here, it continues for some time before burning itself out at the decay phase. Buoyancy forces drive air drawn from outside the compartment through an opening that is shared with escaping hot smoke and gases [2]. A fully developed compartment fire will have an air flow rate that is strongly dependent on opening geometry and least on room temperature.

Buildings are affected by turbulence caused by winds within the boundary layer which is the region between 100m and 3000m above the earth’s surface as illustrated in figure 2 below.

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