Wind Effect on Smoke Movement in Compartments and Factors Affecting Compartment Fire - Literature review Example

CHAPTER 2: LITERATURE REVIEW 1. INTRODUCTION The aim of this chapter is to present a discussion on the effects of wind on the movement of smoke in compartments. Since this is a literature review, the discussion will be based on the views and works of different researchers (Creswell, 2007, p. 45; Dellinger, 2005, p. 27). This section will address the important points of existing knowledge which will include fundamental findings, as well as methodological and theoretical contributions made by different people on the topic (Cooper, 1998, p. 78). The following review is organized by eight parts. Each part corresponds to a section. Section two of the literature review is about compartment fire. Section three will cover the stages of fire development while Section four covers the factors affecting compartment fire. The effects of wind on compartment fires are covered in Section five while Section six discusses the spread of flames due to wind. Details on fire dynamics will be discussed in Section seven while Section eight and nine cover Huang’s experiment and the summary respectively. 2. COMPARTMENT FIRE In compartment fires, the whole essence of growth of fires is embraced. In this case, the compartment restricts the supply of air to the fire. These factors (one factor or many factors?) are the ones that determine the spread and growth of fire, the time and the maximum rate of burning. Compartment fires are affected by both thermal and oxygen limiting feedback processes (Carvel & ‎Beard, 2005, p. 150). In the design of fire safety, or the process of investigating fire in buildings, it is important to understand these effects together with the characteristics of fire growth with fuel. It is important to have the ability to express necessary physics in computational mathematics because this helps to direct the focus on major elements in a given situation. In fire investigation this process helps in developing the qualitative description of the fire process, which will be used for the approximate events time lines in the late stage. In design analysis, fire growth behavior is crucial issues of life safety. The duration of the fire in a compartment and its temperature determines the structural fire protection needs. Current descriptive regulations do not address the design analysis which can be improved by performance-based approach (Chen, Liu, Zhang, Deng & Huang, 2008, p. 912). 3. STAGES OF FIRE DEVELOPMENT Ignition When a compartment fire is not controlled and it has enough fuel and ventilation it is possible for the fire within the compartment to grow from a level of localized burning to a full room fire. The ignition process triggers the burning of the burning of fuels and this causes the production of a lot of heat which eventually results in the production of extremely hot temperatures within the compartment (Grimwood, 2008, p. 113). The ignition stage is followed by the post flash over stage that has the stage of full development and the decay stage. Ignition provides the fire spark that sets the available fuel on fire. Considering the fundamentals of fire behavior, ignition must have oxygen, fuel and heat. Growth The growth or development of fire applies from ignition to the flash over stage. Therefore, its average temperature can be about 600C at its highest. There is a popular correlation developed by McCaffrey, Quintiere and Harkleroad for the growth of fire called the MQH correlation (Quintiere, 1998, p. 100). The growth stage of fire may be defined in two different ways. In this stage the fire is small and it is yet to significantly impact the internal environment of the compartment which includes toxicity, visibility and heat. When combustion starts, the growth of fire depends largely on the configuration as well as the characteristics of fuel if the fire is fuel controlled. The air within the compartment gives enough oxygen for the fire to grow. In this stage of fire development adjacent fuel is warmed up by radiant heat and the process of pyrolysis continues. A plume consisting of hot gases moves up from the fire and combines with the cool air inside the room (Utiskul, 2003, p. 34). This energy transfer starts to increase the room temperature. As the plume approaches the ceiling the hot gases start spreading in a horizontal manner across the ceiling. Transition beyond this stage cannot be defined in strict terms. However, in the flames close to the ceiling, the hot gas layer tends to become more openly defined and its volume increases. If the fire gets enough oxygen the fire can grow very quickly. At this level the fire is very dangerous to the health and lives of the people around. Flash-over The flash over stage is the stage between the growth stage and the fully developed fire. At the flash over there is a quick change to state where there is full surface involvement of all the materials that are combustible in the compartment. Flash over conditions are explained in various ways. Generally, the temperature of the ceiling within the compartment must get to between 500 and 600 o C. (Shackelford, 2008, p. 43). The heat flux, also described as the measure of transfer of heat to the compartment floor must get to 15-20 kW/m2. At flash-over, burning gases tend to push open any opening within the compartment. Such an opening may be the door connecting one room with the next room at a very high velocity. However, flash over does not occur all the time. Before the fire gets to the flash over stage it must have enough fuel and oxygen. If the materials ignited at the beginning lack enough energy or the heat of combustion and it does not release it as fast as required flash over cannot be reached (Purkiss, 2007, p. 213). An example is small can of trash ignited at the center of a room. If the fire consumes the oxygen in the compartment the rate of heat release will go down and the fire inside the compartment will fail to attain the flashover stage. Fires that do not attain enough heat release rate for flash over to take place because of limited ventilation is greatly hazardous because increased ventilation can cause a flash over induced by the ventilation. Flashover Potential Indicators It is important to recognize flash-over and to understand the mechanisms under which this phenomenon of fire behavior is formed. It is however more important o posses the ability to recognize major indicators and to predict flash over probability. Flash over is the close to simultaneous ignition of a larger part of the directly exposed combustible matter in a compartment (Liua & Chowb, 2009, p. 2522). When some organic materials are heated they go through thermal decomposition with the release of flammable gases. Flash over is attained when the larger section of the exposed surface is a given space is heated to its auto-ignition temperature to the point that it releases flammable gases (Babrauskas & ‎Grayson, 1990, p. 67). The incident heat flux at floor level for flash-over to occur should be 1.8 Btu/ft²*s (20 kW/m²), and the temperature should be 500 °C (930 °F) or 1,100 °F. A good example of flash over is the ignition of furniture in a room. The fire developing from the furniture can form a hot smoke layer that goes on to spread across the ceiling inside the room. The depth of the hot buoyant smoke increases because it is enclosed by the walls. This layer radiates heat that reaches the surfaces of the combustible materials that are directly exposed within the room and this result in the production of flammable gases through pyrolysis. As the temperature of the produced gases rises the gases ignite all through (Klinoff, 2011, p. 108). There are a number of flash over. For instance, a rich flash over takes place when the flammable gases ignite at the upper part of the flammability range. This can take place in compartments where the fire died due to inadequate oxygen. The source of ignition could be a smoldering object or embers being stirred up by air. The flash over may be delayed when the grey smoke with low temperature ignites after collecting outside the room of origin. This causes a volatile situation and in case the ignition takes place at the ideal mixture it could result into an explosion of violent gases in smoke. This is called a smoke explosion or a fire gas ignition although this depends on how severe the combustion process is. Fully Developed Fire The release of energy at the post-flash over stage is at the maximum. However, this release is limited because of ventilation. Gases that remain unburned collect at the ceiling level and burn very frequently as they exit the compartment and the resultant effect is the flame appearing through windows and doors. The average temperature for the gases in the compartment in a fully developed fire falls between 700 oC and1200 o C (1292o-2192o F). It is important to bear in mind that the compartment in which the fire began may attain the stage of full development while the rest of the compartments are yet to get involved (Karlsson & ‎Quintiere, 2002, p. 116). Flames and hot gases emanating from the compartment with the fire transmit heat to other fuel packages such as compartment linings, structural materials and contents and the result of this can be the spread of the fire. Conditions could have wide variations when the fire is fully developed in a particular compartment, a growth stage fire in another compartment, and an incipient fire in another compartment. A ‘fully developed fire’ is used in reference to conditions in a particular compartment and this does not mean that the entire building is involved. Decay Stage A compartment fire can get into the decay stage with the consumption of available fuel or because of limited amounts of oxygen. A fuel package without enough oxygen or lacks enough release of heat to push a compartment to the flash over stage can pass through the fire stage development but will not get to the rest of the fuel packages. On a larger scale with no intervention a whole structure can get to full development and as the consumption of fuel goes on the fire progresses into the decay stage. However, the fire may get into the decay stage through another more problematic way (Utiskul, 2003, p. 180). When the compartment’s ventilation profile fails to provide enough oxygen the fire can then enter the decay stage. The rate of heat release goes down as the concentration of oxygen drops. While the temperature goes with the rate of release of heat the temperature in the decay stage of fire can remain high for a time especially if the building is well insulated and energy efficient. This represents a big threat as solid fuel packages go on pyrolyzing and the compartment with the fire may have high concentrations of hot pyrolized fuel as well as flammable products in gas form from incomplete combustion (Dellinger & Leech, 2007, p. 23) 4. FACTORS EFFECTING COMPARTMENT FIRE The severity of a compartment fire may be influenced by various factors such as density, distribution, fire load type, the geometry and size of the compartment, thermal properties of compartment boundary and ventilation properties of the compartment. a) Fuel The type of the fire load affects compartment fires because it determines the amount of flammable substances and the quantity of heat generated from the combustion of the material. The fire load determines how big and severe or rapid the fire will burn. This in turn determines the type of destruction (Klinoff, 2011, p. 123). b) Fire load density Fire load density helps to determine the severity and duration of fire. It is the determining factor of how much fuel will be consumed and how long the fire will last. A fire with a higher density has higher chances of being severe and long lasting. c) Compartment size and geometry The size and geometry of the compartment either restricts or promotes fire spread and growth. A small compartment means that the amount of oxygen will be limited and the fire will not have a lot of space for growth. A bigger compartment on the other hand provides enough space for air and growth of the fire meaning the fire can be bigger and longer lasting (Huang, Ooka, Liu, Zhang, Deng, Kato, 2009, p. 314). d) Ventilation conditions of the compartment Ventilation conditions determine the amount of oxygen available for combustion. Good ventilation brings in more oxygen supply which causes fire to burn strongly and more rapidly. Restricted ventilation means lesser oxygen hence a weak and less fierce fire (Utiskul, 2003, p. 77). 5. EFFECT OF WIND ON THE COMPARTMENT FIRE Fire Attributes Affected by the Wind. Wind exhibits a strong effect on the behavior of fire especially because of its fanning effect on fire. It affects various fire attributes including temperature, speed and direction. Wind changes the direction and intensity of fire and therefore the rate of burning. Very unstable atmospheric conditions have the greatest effects on the attributes of wind mentioned (Liua & Chowb, 2009. Wind has an effect on the intensity and rate of spread of the fire. High winds flowing in the direction of fire can cause a rapid movement in the head of the fire (Liua & Chowb, 2009, p. 80) FLAME SPREAD DUE TO WIND The spread of fire is affected by certain major factors including the speed of wind, moisture, slope, fuel loading, the temperature of the atmosphere and the fuel surface area to volume ratio. a) Wind speed (heat convection by low wind speed heats the unburnt fuel around the fire significantly and is therefore favorable for fire spread, however, with increase of wind speed, the convection brings more heats away from the fire and hence decrease the fire spread. This is basic effect of wind on the fire spread) The speed of the wind correlates positively with the speed of fire spreads. When blown by wind, the blazes become more radiant and cause the fuel ahead of the fire to make the heat more radiative. There is a change of heat from convection to advection. hot air has the capacity to accelerate the drying and heating of fuel to burning point. There is a complex relationship between the length of the flame and the speed of wind (Shackelford, 2008, p. 90). The length of the flame correlates positively with the speed of wind when the speed is low or less than 3 m/s. Since the wind provides a lot of oxygen, the length of the flame becomes lower with increase in the speed of wind at more than 3m/s since the flame is inclined forward. b) Slope Relationship with wind Slope affects the speed of fire spread in a similar manner to the speed of wind. When fire burns on sloppy ground it gives more radiative heat to the fuel in front of it and this increases the advective heating. The fire spreads faster on a slope but this may not be the case on horizontal grounds. When the slope is big then the spread of the fire will be higher. The slope increases the length of the flame because it is easier to heat the fuel due to high provision of oxygen (Babrauskas & ‎Grayson, 1990, p. 82). This acts an accelerating factor for the burning reaction and this enhances the flames on a vertical manner. c) Temperature of the atmosphere Burning is favored by a high atmospheric temperature. The speed of spread of fire and the length of the flame correlate positively with the temperature. With increase in the temperature of the atmosphere, the temperature of the fuel also increases and this dries the fuel further (Yeoh, ‎Kwok Kit Yuen, 2009, p. 89). d) Fuel Moisture Fuel moisture correlates negatively with the speed of spread of fire and the length of the flame. If the moisture in the fuel is high, then the time taken by the fuel temperature to get to burning point will be higher and therefore more heat will be needed to dry the fuel and this means that fire will spread more slowly. When the moisture in the fuel arrives at a certain level, the heat in the fire is never sufficient to dry the fuel and therefore the temperature of the fuel cannot reach the burning point. This means that the burning must stop and the fire ends (Purkiss, 2007, p. 34). e) Fuel loading There is a positive correlation between fuel loading and the speed at which fire spreads and the length of the flame. The more the fuel loading means more heat is given out in burning and this causes the fire to spread faster. Bigger amounts of combustible gases are released as well and the flame stretches higher (Huang et al., 2009, p. 314). f) Fuel surface area to volume ratio Dense fuel ensures that there is good burning continuity but the supply of air is not enough because there are only small intervals in the flame. Otherwise the continuity in the fuel is not great except the large amount of air supply (Carvel & ‎Beard, 2005, p. 78). Measuring Fire Heat is a type of energy in which the molecules vibrate and it is able to initiate and support chemical changes as well as change of state. Heat is the energy that alters the temperature of any object. When heat is added the temperature rises but when it is removed the temperature drops. The units for measuring heat energy are joules although it may also be measured in calories. Temperature is described as the measure of the level of molecular activity of any material compared to a point of reference. The unit for measuring temperature is degrees Farenheit or degrees Celsius. Ice melts at 32 º F, water boils at 212 º F, ice melts at 0 º C, while water boils at 100 º C. The heat release rate is the rate with which energy is released from fire. This is otherwise called power (Utiskul, 2003, p. 76). The rate of release of heat is measured in units of Watts (W). This is the SI unit equivalent to one joule per second. Based on fire size the heat release rate is also measured in kilowatts which is equal to 1000 Watts or megawatts which is equivalent to 1,000 000 Watts. Heat flux is the heat energy rate transferred per surface unit area in kW/m2. Heat Transfer Heat transfer is a significant factor in ignition, growth, spread, decay and dying of a fire. Heat always moves from the hotter to the cooler object (Babrauskas & ‎Grayson, 1990, p. 108). When an object receives heat energy, its temperature increases and the object loses the heat energy has its temperature dropping. Conduction: Conduction refers to the transfer of heat between contacting solids or within solids. Fig.1: Transfer of heat Fire Phenomena Fire development depends on several factors such as fuel properties, mechanical or natural ventilation, fuel quantity, the geometry of compartment including the height and volume of the ceiling, fire location, and ambient conditions such as wind and temperature among others (Karlsson & ‎Quintiere, 2002, p. 116). Traditional Development of fire As can be seen (below) the traditional fire development curve displays the time history of a fire with limited fuel amounts. In other words, oxygen is not a li9miting factor to the fire. As more fuel burns the level of energy increases until all the available fuel starts to get consumed. This is the stage of full development. With the burning of the fuel, the level of energy starts to decay. The important thing is that there is oxygen availability to mix with heated gases in the fuel which enables the completion of the fire triangle and energy is generated (James & Quintiere 1998, p. 99). Fig. 2: Traditional fire development The Behavior of fire in a Structure The behavior of fire is a structure curve shows the time history of a fire with limited ventilation. The fire begins in a structure with close windows and doors. During the early part of the fire growth stage there is enough oxygen supply to mix with hot gases causing flaming combustion (Klinoff, 2011, p. 71). As depletion of oxygen in the structure increases the fire begins to decay, the amount of heat released from the fire drops and consequently there is a drop in temperature. In case a vent opens for example when the firemen enter through the door there is entry of oxygen. Oxygen mixes with hot gases within the structure and the level of energy starts rising. Ventilation changes can cause rapid increase in the growth of fire and this may potentially cause a flash over condition which is the fully developed stage of a compartment fire (Shackelford, 2008, p. 140). Fig. 3: Behavior of fire in a structure 6. HUANG’S EXPERIMENT The Huang’s experiment was done by Huang Hong and others in order to explain the process of growth of fire in compartments subjected to external wind conditions. The fire tunnel experiments were done in a compartment with a reduced scale. The velocity of the approaching wind from outside was set at 0.0, 1.5 and 3.0m/s. The place where the source of the fire was placed kept changing from the center, the upwind corner and the downwind corner. The studied focused on the effect wind has on a situation where there is through ventilation (Huang, et al., 2009, p. 312). Measurements were taken fro the temperatures of the air and the surfaces of the walls within the compartment and the flames coming out through the opening. Records were taken for the fuel mass loss rate and the influx of heat from the opening. Temperature increases very fast and the time for burn-out is cut-down when conditions are windy. It was discovered that external wind can either raise fire temperature by promoting combustion, or reduce the temperature by blowing out the gases within the compartment. When the velocity of the approaching wind is high there is a great inclination of the external plume to the downward side and this causes the flame to grow bigger and this increases the risk of spread of the fire to nearby buildings (Huang et al., 2009, p. 315). 7. SUMMARY In compartment fires, the whole essence of growth of fires is embraced. In this case, the compartment stands for confined spaces which regulate the supply of air to the fire and its thermal environment. These factors are the ones that determine the spread and growth of fire, the time it takes and the maximum rate of burning. The stages of fire development include pre-ignition, ignition, growth, flash over, full development and decay. Pre-ignition comes before ignition. The ignition process triggers the burning of the burning of fuels and this causes the production of a lot of heat which eventually results in the production of extremely hot temperatures within the compartment. The growth or development of fire applies from ignition to the flash over stage. In this stage the fire is small and it is yet to significantly impact the internal environment of the compartment which includes toxicity, visibility and heat. The flash over stage is the stage between the growth stage and the fully developed fire. The decay stage is the stage in which the fire dies out due to lack of fuel or oxygen. Compartment fires are affected by fire load type, fire load density, and compartment size and geometry and ventilation conditions of the compartment. Wind has various effects on compartment fires. It affects various fire attributes including temperature, speed and direction. Wind also causes the flame to spread because of its speed, slope, altering atmospheric temperature, altering the moisture of the fuel, influencing fuel loading and the fuel surface area to volume ratio. Fire dynamics is the study of the interaction of fire science, chemistry, mechanical engineering and chemistry in fluid mechanics and heat transfer to cause an influence on the behavior of fire. The fire dynamics discussed in this section include measuring fire, heat transfer, and traditional development of fire and behavior of fire in a structure. Finally the section discusses Huang’s experiment. Bibliography Babrauskas, V. & ‎Grayson, S.J. 1990. Heat Release in Fires. Taylor & Francis. Carvel, R. & ‎Beard, A. 2005. The Handbook of Tunnel Fire Safety. Cengage. Chen, H., Liu, N., Zhang, L., Deng, Z. and Huang, H., 2008. Experimental Study on Cross-ventilation Compartment Fire in the Wind Environment. Fire Safety Science 9: 907-918. Cooper, H. (1998). 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Huang, H., Ooka, R., Liu, N., Zhang, L., Deng, Z., Kato, S. 2009. Experimental study of fire growth in a reduced-scale compartment under different approaching external wind conditions. Fire Safety Journal 44(2009)311–321. James G. & Quintiere 1998. Principles of Fire Behavior. Cengage Learning. Karlsson, B. & ‎Quintiere, J. 2002. Enclosure Fire Dynamics. Routledge. Klinoff, R. 2011. Introduction to Fire Protection. Cengage Learning. Liua, A. & Chowb, W. 2009. Wind effects on smoke motion and temperature of ventilation-controlled fire in a two-vent compartment. Building and Environment. Volume 44, Issue 12, December 2009, Pages 2521–2526 Purkiss, J. 2007. Fire Safety Engineering Design of Structures, Second Edition. Elsevier. Shackelford, R. 2008. Fire Behavior and Combustion Processes. Delmar Cengage Learning. Utiskul, Y. 2003. Extensive Study of Wall-vent Compartment Fire Behavior Under Limited Ventilation. University of Maryland, College Park. Yeoh, G.H., ‎Kwok Kit Yuen 2009. Computational Fluid Dynamics in Fire Engineering: Theory, Modeling and Practice. Butterworth-Heinemann. Read More