Wind Effect On Smoke Movement In Compartments - Literature review Example

Literature Review This section contains a literature review of previous studies that examined wind effect on fire movement in smoke compartments. Related studies are also reviewed. A bulk of the surveyed literature comprises secondary sources spanning 4 decades of researches on smoke movement. The targeted documents for analysis comprise those that explore the wind effect on smoke movement in compartments. A number of other primary sources have been reviewed that explored wind effects, stack action and buoyancy pressures and their influences on smoke movement in compartments with multiple holes. The premise of this paper is on findings by Huang et al (2008) that examined fire growth in reduced-scale compartment under windy conditions. Survey of literature in this section examines investigations by Huang to guide investigations into the wind effect on smoke movement in compartments. Factors influencing Smoke Movement in Compartments In an experimental investigation of fire growth in reduced-scale compartment, under varied approaching external wind conditions, Huang et al (2009) established that external wind imposes two opposing effects. First, it promotes combustion inside a department as a result raising the temperature. Second, it blows away and attenuates the combustible gases within the compartment to decrease the temperature and to quicken its extinction. Similar findings were made in early researches by Tamura (1969), which conducted a computer study on movement of smoke through the use of a mathematical model of a 20-storey building. The results showed the relative influence of a number of factors apart from wind to play a role in smoke movement in compartments. As evidenced by earlier researches by Tamura (1969), there has been noteworthy interest in analytical and practical problems on the effects of wind on movement of smoke in compartments. The study by Huang et al (2009) used fire tunnel experiments within a scaled down compartment in the presence of external wind in an effort to investigate the processes of fire growth among compartments in windy conditions. In the study, the researchers set the wind velocity to 0.0, 1.5 and 3.0m/s. The location of the fire source was also changed among three locations: downwind corner, upwind corner and the centre. The findings established that temperature rose and fire burnt out faster in windy conditions. Chen, Liu and Chow (2009) also established that ambient wind has a range of influences on the steady temperature of smoke. The critical speed of wind is vulnerable to heat loss of the compartment walls. Similar findings were made by Webb (2006), who found that there is no temperature variation between the exterior and the interior of the compartment. The researcher observed that air flows into the compartment from the higher openings and leave through the lower openings, implying that the direction of air flow is downward bound. McGrattan & Hostikka (2010) made similar findings and concluded that in cases where there is fire within the department, then due to buoyancy, smoke will rise. This implies that the ambient wind acts against thermal buoyancy. In determining the effects of temperature and wind action, Chen, Liu and Choe (2009) made critical suggestions that indoor temperature should be assumed to be uniform while ventilation should be neglected since it is small. The researchers found that the temperature of smoke is related to the fire power, the rate of the air flow and the heat loss from the compartment (Chen, Liu and Choe 2009). The study by Chen, Liu and Choe (2009), Webb (2006) and McGrattan & Hostikka (2010) found that opposing wind, thermal buoyancy and wind action compete inside the fire compartment, where the stronger one determines the direction of the smoke movement. Other researches on wind effects on smoke movement, under different fire conditions, were explored by Chen, Liu and Chow (2009). The researchers analysed the wind effects on movement of buoyant smoke control and motion. The analysed fire compartment in their study was an enclosure that had a set of dual openings on the opposite walls. The windward one was at the lower elevation close to the floor while the leeward one was at the upper elevation close to the ceiling. Chen, Liu and Chow (2009) established that in high rise buildings. The behaviour of compartment fire and smoke movement is influenced significantly by the ambient wind. A study by Cai and Chow (2011) made similar findings and commented that smoke moves due to several factors such as the difference of buoyancy between the ambient air and hot smoke, hot gas expansion, wind effect and air flow controlled through mechanical air handling equipment that is referred to as stack effect. The researchers observed that study of smoke movement is essential. Significant findings by Huang et al (2009) observed that external wind has two opposing effects. External wind promotes combustion in the compartment and hence raises the temperature. External wind also blows away and reduces the combustible gases inside the compartment. It hence decreases temperature and hastens its extinction. Fire depends on the position of the fuel, approaching velocity as well as the geometry of the compartment and the opening. In situation where the wind velocity is high, the flame widens (Friedman 1992). Smoke production A major hazardous feature of smoke that may not be obvious is that of elevated temperatures. It increases the temperatures associated with the movement of smoke. Cummings (1998) observed that Smoke is generated by incomplete combustion of gases from flaming or non-flaming combustion. A study by Cummings (1998) found that the rate at which smoke is generated is a function of the quantity and type of material involved in the fire, the rate of combustion and the rate at which air is restrained to the hot smoke layer or the plume during the expansions and movement of smoke between compartments. The later parameter of air entrainment will depend greatly on the geometry of the surrounding spaces and geometry. Smoke Movement A study by Cummings (1998) investigated the movement of smoke in corridors in order to quantify parameters that have an effect on the movement of smoke. The study indicated that it may be possible to use upper layer temperature data outside a fire compartment to predict the initial smoke wave velocity inside adjoining corridors. To estimate the amount of smoke present in any specific are from the origin of fire and the speed in which it propagates. Cummings (1998) observed that it is critical to understand the movement of smoke. Two major factors play a critical role in movement of smoke. These include the buoyancy and the mobility of the smoke, which is due to the fact that it often consists of hot gases that are less dense compared to the surrounding air (Kashef, Bénichou and Lougheed 2003). Second, normal movement of air inside the compartment, which may have nothing to do with the fire and which however can carry smoke throughout the compartment (Harrison and Spearpoint 2006). An earlier study by Klote (2003) examined smoke movements in compartments. Klote (2003) noted that the key driving forces that cause propagation of smoke include buoyancy, expansion, heating, wind, air conditioning and ventilating. In fire situation, smoke movement is generally caused by a combination of several driving forces, namely stack effects, buoyancy and wind effects. Similar findings were found by Chung (2007) who concluded that a range of aspects influence movement in compartment buildings, such as wind effects alongside other factors such as stack effect, buoyancy force and mechanical ventilation. Chung (2007) established that wind effect plays a critical role in natural smoke flow in tall buildings. The wind tunnel test conducted by Chung (2007) provided vital data for evaluation of smoke movement in Taiwanese tall buildings. Concerning wind pressure around compartments, a number of useful conclusions were reached, including findings that smaller vent size may offer high velocity of smoke that will provide higher pressure to conquer wind pressure. Chung (2007) observed that wind pressure increases with the height of the buildings. Smoke Motion Direction Chen, Liu and Chow (2009) investigated the effects of wind on smoke motion direction. The researchers also sought to examine the effects of wind on temperature of ventilation-controlled fires dual vent compartments. The researchers established that in the cases of opposing wind, the force of the wind as well as thermal buoyancy compete in the fire compartment. The researchers commented that stronger wind will dominate the direction of the motion of smoke. Further, a critical wind speed can as a result be defined. When the ambient speed of wind overcomes that of the critical value, wind drives smoke to push downwards, or else smoke will travel upwards as it is pushed by thermal buoyancy (Klote 2012). Early researches that dwelt on computer analysis of smoke movement in compartments found that wind effect on buildings depend on the speed of wind and the direction of wind, that was further compounded by the variable nature of wind (Tamura 1969). Concerning wind velocity, Tamura (1969) observed that the factor increased with height so that value of negative pressure inside the building in the upper levels would be greater than those at the lower level. In effect, this results to upward movement of air inside the compartment. Stack effects, wind effects and Buoyancy When outside a compartment is cold, there is usually an upward movement of smoke inside the building shafts such as elevators, mechanical shafts or mail chutes. Klote called this effect a stack effect. The air within the building has a buoyant force since it is less dense and warmer than the extern air. Such a buoyant force causes air to rise inside the shafts of the building. The importance of normal stack effect is higher than the outside temperatures. On the other hand, when the outside air is warmer than the building air, a downward air flow will frequently exist in shafts. Such a downward air flow is referred as reverse stack effect (Vaari and Hietaniemi 2000). A study by Klote (2003) found that smoke movement in compartment fires can be dominated by stack effect. In buildings with normal stack effect, the active air currents can propagate smoke to considerable distances from the place of origin. If the fire is under neutral plane, smoke propagates with the building air into the shafts and upwards. The upward movement of the smoke is enhanced by buoyancy forces of the smoke and the temperature (Klote 2003). Discussions on the effect of wind on smoke movement have also been explored by Porch et al (2000). These findings are significant for theoretical analysis of smoke movement in windy conditions. Klote and Nelson (1997) observed that wind action is a critical feature in smoke movement. Hence, fire spread in tall and short buildings is different. Tamura (1969) also defined stack action as the principal mechanism through which smoke move across different elevations of compartments. Tamura (1969) demonstrated that with fire on the lower floors during cold season, the concentration of smoke in stairwell, elevators and shafts and on the upper floors reached critical levels in short durations. The study found that smoke moves vertically through air ducts and elevator shafts, and to a less extent, the stairwells. The vertical air movement and the rates of smoke propagation that is caused by wind action are substantially less compared to that of the stack action. Tamura (1969) also observed that large opening in the outside walls of fire floor at lower levels result in a greater rate of vertical smoke movement that is caused by stack action. However, with stack action alone, large opening in the windward wall increases the rate of vertical smoke movement. Wind Action and Stack Action Combined When the conditions of wind action and stack action are combined, the resulting air flow is different. The pattern of the air flow is caused by stack action that is modified by the impacts of wind action. Tamura (1969) established that between the conditions of wind action and stack action, stack action alone results in the greater rate of air flow and hence the greatest potential for smoke spread through different flows or compartments in the event of fire. High temperature smoke from the fire contains buoyancy force because of its reduced density. Much larger pressure differences in pressure are possible for tall fire compartments where the distance between the neutral planes is greater (Klote 2003). Aside from buoyancy, energy released by fire may cause the movement of smoke to expand. In fire compartments that have only one opening, the air in the compartment will flow out of the fire compartment (Ball 1999). A study by Chen et al (2011) investigated cross-ventilation compartment fire under windy conditions. The study found that ambient wind had two contradictory effects on compartment fire. These include promoting the severity of the fire by supplying more oxygen and cooling of the fire through removal of heat and dilution of the combustible gases. Chen et al (2011) observed that wind effect has significant impact in larger fires than smaller fires. Strong ambient wind plays an important role in fire spread since wind velocity increases from zero to elevations that are higher. Chen et al (2011) also observed that strong winds impact fire spread as well as the behaviour of the movement of smoke in buildings, where mechanical ventilation routines may not take out smoke sufficiently under the action of the wind. Chen et al (2011) established that the behaviour of compartment fires varies depending on the amounts of fuel. The researchers also found that the wind creates the backflow and the main flow areas within a compartment. In the case of centre fires, fire is blown downward and dispelled from the compartment. The dispelled fire spreads further horizontally with the increase of the speed of wind. In the downwind and upwind cases, the fire is dispelled upwards through backflow (Hadjisophocleous 1999). The research by Chen et al’s (2011) also established that in general, the approach makes the fire more severe. Here, the wind has two contradictory effects on compartment fire through removal of heat and dilution of combustible gases. The researchers also observed that external flames puff during windy conditions. In situations of large fires with high wind speed, the flame takes the entire opening except for the upper part of the window in situations of lower wind speed (Chen et al 2011). For compartment fires with more than two openings, a number of experimental studies have explored the effects of wind on smoke movement. A recent study by Kumar and Naveen (2007) investigated a study on two similar sized openings positioned on the opposite walls on the development of fire. The study concluded that temperatures in cross ventilation conditions are greater than temperatures in single-ventilation conditions for large fires. Principles of Smoke Movement In discussing the relationship of elements influencing the movement of fire in buildings, Klote and Nelson (1997) attempted to identify the principles of smoke movement. According to the researchers, smoke behaves differently in tall buildings compared to short buildings. In the case of lower buildings, the movement of smoke is influenced by such factors as fire pressures, convective movement and heat (Klote and Nelson 1997). The researchers further observed that expulsion or smoke from compartments reflect on this principle. Concerning tall buildings, similar factors are complicated by the stack effect – the vertical natural wind movement through buildings due to the differences in densities and temperatures inside and outside air. The researchers observed that the stack effect can be a critical factor in the movement of smoke. Further findings revealed that the dominant factors responsible for smoke movement in tall buildings comprise stack effect, influences of external forces of wind and the forced air movement inside the building (Klote and Nelson 1997). 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