Combustion, Effects of Enclosure Temperatures on Fire Growth, Fire Development by Flowing through the Openings - Coursework Example

1. Ventilation and Combustion Fire is a phenomenon that occurs as a result of natural chemical and physical reactions. The interactions between fire and the combustible substances the environment is not in a linear relationship. The quantitative approximation of the different processes involved in combustion is usually complicated. In an enclosed fire, there are processes of involving massive move of heat towards and away from the combustible substances as well as the environment. This study tends to introduce the most prevalent of these processes; this section includes a general qualitative description of the chemical and physical phenomena associated with fire. The discussion is simplified into a general exposition of the process of combustion, covering a qualitative description of the fire development in an closed environment and its effects on the enclosure. 1.1. General Description of the Combustion Process Combustion process is a complicated issue. It covers multiple subjects including fluid mechanics, chemical kinetics among others. Fire safety engineering has a section that deals with fire fundamentals and discusses the topic of combustion (Klaene & Sanders 2008, p.37). This study uses a practical observation in a burning candle to illustrate the natural processes that takes place in combustion both in a closed environment and in the open. Michael Faraday in his one of his researches in the 19TH Century, explained that the use of candles is the most appropriate to introduce the study of natural reactions in a combustion scenario (Jarosinski & Veyssiere 2009, p.47). He illustrated that various sources of ignition such as a match or a friction spark builds heat on the candle wick and then begins to melt the solid wax. The wax undergoes vaporization, and the air around the burning move away through diffusion. It moves towards the region that oxygen occupies. This process oxidizes the gases in a complicated sequence of chemical reactions, all over the area where oxidized fuel mixture is flammable. At the point where the flame from the candle is stable it spreads heat to the wax in its solid state. The wax then melts. Because the wax vaporizes and disappears from the wick, it moves upward on the wick then vaporizes. This results into a stable process of combustions (Snow 2001, p.37). The flame supports processes involving the flow of energy and masses of combustible substances. 1.1.1 The flow of energy The movement of energy takes place through the processes of radiation, conduction and convection. In gasses, the dominant process of heat energy flow is radiation. Radiation usually involves the soot particles; products of combustion which radiate heat in every direction. The main mode of heat transfer is the radiation that takes place towards solid. It melts the solid but at some point; convection also comes in (Kalantre 2001, p38). Heat moves upwards and away from the combustion zone, in a process called the convective heat flux. As the flame grows larger and more luminous, the melting process becomes faster. The heat energy that reaches the solid through radiation is not adequate to vaporize the wax but can only melt it. The wick plays the role of transporting the wax after melting. It transports the melted wax upwards to where the hot gasses are. This is the place with the combined radiation, convection and conduction processes that supply heat energy in order to vaporize the wax that have melted. 1.1.2. The mass transfer Heat transfer and the transformation phases are also demonstrated in details by the burning of a candle. During melting, heat transforms combustible substances from solid to liquid state. Burke (2007, p.41) explains that the massive balance demands that the mass of combustible gases and wax that moves away from the wick through vaporization process be replaced (Robbins et al 2013, p.53). To that effect, the liquid moves upwards through the wick by the complex process of capillary action. The moment it reaches the top, the transfer of heat from the flame makes it vaporize. The gases then disappear from the wick in another complicated process called diffusion. The most interior sections of the flame carry inadequate supply of oxygen that cannot fully support the combustion. It is only enough to partially support some chemical reactions (Wang et al 2009, p. 37). The result of this incomplete process is soot among other substances that comes from incomplete combustion processes (Purkiss 2006, p.71). These products then travel up in the flame through the convective movement of heat and then react with oxygen. At the top of the flame, almost every bit of the combustible fuel has burnt to generate water and of course, carbon dioxide. To observe the efficiency of the combustion, we look at the smoke to ensure they have disappeared from the top flame on the candle. 1.2. Fire development in an enclosure Inside an enclosed environment, fire growth takes place in a variety of ways, depending on the geometrical dimensions and the size of the enclosure. Essentially, it depends on the amount of ventilation in the enclosure, the type of combustible substances amount of fuel and the surface area of the enclosure (Grimwood 2005, p.29). A typical fire in an enclosure develops through various events as per the description of the ignition processes. After the ignition stage, the fire develops and generates massive flows of heat energy as the flame spreads and occupies wider areas. In the initial phases, the enclosed environment does not have any effect on the flame. The flames are still under the control of the combustible substances (Mayfield & Hopkin 2011, p43). Apart from the release of heat energy, combustion also produces a number of toxic and nontoxic gases and solids. The production of heat energy and other products of combustion is an extremely complicated matter. The fire safety engineers usually have to depend on the size of the enclosure and its surface area in order to approximate the amount of heat energy in the enclosure (Spratlan 2011, p.54). 1.3. Stages of Fire Growth in Enclosures This section of the study gives a general idea of how fire grows in enclosures. This usually takes place in a sequence of stages and can happen under the influence of various environmental factors (Richman 2008, p63). This study mainly focuses on the enclosure temperatures, mass flows of combustible gasses and the variations of pressure across the openings of the enclosures. 2. Effects of Enclosure Temperatures on Fire Growth Fire development in enclosures is considered in terms of the temperature variations in the compartment. The discussion of fire development is divided into a sequence of stages. The stages show an ideal variation of compartment temperature with time. The stages for the situations where there is no fire control attempt, the stages include ignition, growth, flashover and fully developed flame stage (Schottke 2012, p.52). 2.1 Stages of Fire development 2.1.1. Ignition: +. Ignition can be described as a process that generates an exothermic reaction whose properties is a great increase in temperature beyond the normal threshold (Dunn 2010, p.43). It originates from either piloted ignitions by matches and friction sparks or through impulsive ignition by the building up of heat in the combustible substances. The resultant combustion processes can either be smoldering combustion or flaming combustion. 2.1.2. Growth: The rate of growth of fire depends on the type of combustion, fuel, the environmental interaction and access to oxygen. The fire description depends on the rate of release of heat energy and the generation of combustion gases. Smoldering fire can generate dangerous toxic gases at low rate of energy release (Svennson 2002, p.39). 2.1.2. Flashover: Flashover is the intermediary stage between from the fire growth period to the fully developed stage of fire development within an enclosure. Fire safety engineering considers flashover as the separating point between two phases of a compartment fire. These phenomena may be caused by various mechanisms owing to the fuel properties, fuel position and orientation, enclosure geometrical dimensions and conditions of the upper layer. Flashover is a phenomenon with a close association to thermal instability (Daskal 2010, p.21). 2.2. Fully developed fire: This is the stage where the energy produced in compartment is at its highest limit and has very limited accessibility to oxygen. This is referred to as ventilation-controlled combustion, unlike the fuel-controlled combustion (Särdqvist 2002, p59). This is because the oxygen required in the combustion is thought of to be entering through the open spaces. In a ventilation-controlled fire, the gases that have not been burnt can gather at ceiling. As they leave through the open spaces, they burn and cause fires to pass the openings. The average temperature of the gasses in the enclosed compartments during the fully developed stage is usually high, between 700°C to 1200°C (Chitty 2003, p.51). 3. Fire development by Flowing through the Openings The other way through which fire safety Engineers divide the compartment fires into a series of stages is by observing the massive flows in and out of the enclosure openings during the combustion process (Annable & Jackman 2011, p. 23). The massive movement consequently depends on the amount of pressure in the compartments and the pressure differences across the openings. The weight of the column of the compartment air increases as we move downwards to the ground level. With an assumption that the temperature of the air inside the lower level of the enclosure compartment is the same as the outside temperature, the pressure in the lower level will grow in proportion to the outside pressure. The stages different stages are listed below: 3.1. Stages of fire Growth 3.1.1. Stage 1: In this stage, the pressure inside the compartment is greater than the pressure outside. This is because of the growth of the hot gases. Hot gases in this stage usually have a larger volume than cold gases. If the opening is lower than the ceiling, then the cold gases will be pushed outside through the opening as a result of the hot gas growth (Hadjisophocleous & Richardson 2005, p.175). Owing to that, the difference in pressures across the opening is a positive difference depending on the compartment. It means that there will be no floe inwards through the open spaces but only outward movement of cold gases. 3.1.2. Stage 2: This stage does not last longer than a few seconds. Fire safety Engineers often ignore this stage. It reaches when the layer of smoke has just arrived at the top of the opening. At this stage, the hot gases have begun flowing outwards. The inner pressure is still greater than the external pressure. Both hot and cold gases move outwards through the open spaces. This stage does not have any mass inflow into the compartment. 3.1.3. Stage 3: In the third stage, the hot gases move outwards through the upper parts of the opening. The mass balance here requires that the cold gases of the same mass moves in through the lower sections of the opening. This stage goes on for a significant duration. This stage does not end until either the smoke fills the entire room or flash over takes place. 3.1.4. Stage 4: This is a stage that is often referred to as the “well-mixed” stage. The compartment is usually full of smoke, which fire engineers generally assume is well mixed. They assume that it has some uniform average temperature. This stage has a close link to the fully developed fire stage of growth. In common situations, flashover takes place between the third and the forth stages. 3.2. Factors Influencing Fire Development in Enclosure Factors that control the development of fire in closed compartments can be divided into two classes depending on their sources. The first category is the factors that have to do with the enclosure, and those that are related to the fuel. These factors include: - The size of the source of ignition and its location -The type of fuel, amount of fuel, position of the fuel, orientation, spacing and surface area of the fuel containers. -Geometrical dimension of the enclosure -the size of the openings of compartments and their locations -The material features of the boundaries of the enclosure 3.2.1. Ignition source: The source of an ignition can consist of a friction spark, with extremely low energy carriage, a heated surface (or a large ignition pilot flame). The source of energy for ignition can either be chemical, mechanical or mechanical. According to Brennan (2009, p.23), the speed of the fire development on the fuel depends on the amount of energy of the source. For example, a friction spark may only cause smoldering combustion. The same applies to a glowing cigarette. Smoldering combustion lasts for a long time before flaming takes place (Ramachandran & Charters 2011, p52). Even when the flaming occurs, it generates low heat and large quantities of toxic gases. A pilot flame does generate flaming combustion. The result of this is the spread of flame and the growth of fire. For a pilot flame located at the low ends of a window curtain, it may create quick upward spread of flame and the development of fire. The flame would on the other hand, cause slower fire development if it is placed at the curtain top. In this case it may cause slow downward spread of flame. 3.2.2. Fuel: One of the main factors determining the rate of fire development in a closed compartment is the type of the combustible material, and of course the amount. In constructions fires, the combustible materials are usually solids, such as furniture. In certain industrial environments, the combustible fuel source may consist of highly inflammable liquids. De Souza (2002, p.34) argues that it is also possible to find plastics and wooden furniture, which cause fire growth to be faster and can last longer. Plastic materials can cause fire to burn for shorter time but can grow very rapidly. A heavy load of fire does not entirely mean that the fire will cause more hazards. Rather, a rapid development of fire is usually of more hazardous impacts to the lives of human beings. The location of the fuel containers can lead to remarkable impacts on the growth of fire. In cases where the combustible materials are positioned such that they are burning away from walls, cool air enters the plume from every direction (Smith 2012, p.49). When they are placed nearer to a wall, there is a limited inflow of cold air. This causes both higher temperatures and bigger fires because combustion must happen over a longer distance. The spacing of the combustible fuel and the fuel packages orientation also of important. The spacing inside the enclosed fire compartment considerably determines the speed of spread of fire between the combustible materials and the walls. Flames that burn upwards usually expands on fuel surface with vertical orientation, much faster than sideway expansion of fuel surface along horizontally oriented (Carvel & Beard 2005, p.61). Lining materials that are combustible, and are used on the compartment walls as curtains or ceiling can lead to very fast expansion. If an initial fire was caused of the lining material on one end of the compartment room, the flame expands with the movement of gases (Kerber 2011, p29). This causes very rapid expansion of fire. In a compartment where the ceiling is non-combustible, the flame expands in a horizontal orientation across the combustible material. This process is very slow and requires the lining material to absorb much heat before the spread of the flame begins. The former if referred to as concurrent-flow flame spread while the latter is the opposed-flow flame spread. 3.2.3. Enclosure geometry: The level of hot smoke in the compartment and the upper of the compartments will radiate toward the combustion fuel and increase the rate of burning. Other combustible materials inside the room will also absorb heat (Dudley 2011, p.31). The thickness and the temperature of the hot layer and that of the upper bounding surfaces will have great impacts on the fire development. References Annable, K. & Jackman, L. 2011. Water Mist Fire Protection in Offices, IHS BRE Press, London, UK. Brennan, C. 2009. You Want Me To Do What? The Physiology and Psychology of Firefighting, Fire Engineering, London, UK. Burke, R. 2007. Fire Protection. CRC Press, London, UK. Carvel, R. & Beard, N. A. 2005. The Handbook Of Tunnel Fire Safety, Thomas Telford, London, UK. Chitty, R. 2003. Fire Safety Engineering. IHS BRE Press, London, UK. Daskal, B. S. 2010. Average Joe’ Firefighting Operations, Fire Engineering, London, UK. De Souza, E. 2002. Mine Ventilation: Proceedings of the North American/Ninth US Mine Ventilation Symposium, Kingston/Ontario/Canada/8-12 June 2002, Taylor & Francis, New York. Dudley, R. 2011. Automatic Fire Detection and Alarm Systems, IHS BRE Press, USA. Dunn, V. 2010. Collapse of Burning Buildings, Second Edition, Fire Engineering, UK. Grimwood, P. 2005. Firefighting Flow Rate: Barnett (NZ) – Grimwood (UK) Formulae, p33-37. Hadjisophocleous, G.V. & Richardson, J.K. 2005. Water flow demands for firefighting. Fire Technology 41, p. 173-191. Jarosinski, J. & Veyssiere, B. 2009. Combustion Phenomena, CRC Press, London, UK. Kalantre, Y. V. 2001. A Study of Effects of Fire on Ventilation System Performance, University of Maryland, College Park, USA. Kerber, S. 2011. Impact of ventilation on fire behavior in legacy and contemporary residential construction, p. 56-57. Klaene, B. & Sanders, R. 2008. Structural Firefighting Strategy and Tactics (2nded.). Jones & Bartlett, Sudbury, MA. Mayfield, C. & Hopkin, D. 2011. Design Fires for Use in Fire Safety Engineering, IHS BRE Press, UK. Purkiss, J. 2006. Fire Safety Engineering Design of Structures, Second Edition. CRC Press, UK. Ramachandran, G. & Charters, D. 2011. Quantitative Risk Assessment in Fire Safety, Routledge, UK. Richman, H. 2008. Engine Company Fireground Operations, Third Edition, Jones and Bartlett, Sudbury, MA. Robbins, S. P., DeCenzo, D. A., and Wolter, R.M. 2013. Supervision Today, Seventh Edition, Prentice-Hall, Inc., USA. Särdqvist, S. 2002. Water and other extinguishing agents.: RäddningsVerket, Karlstad, Sweden. Schottke, D. 2012. Fundamentals of Fire Fighter Skills, Jones & Bartlett Publishers, Burlington, MA, USA. Smith, J. P. 2012. Strategic and Tactical Considerations on the Fireground, Third Edition, Prentice-Hall, Inc, USA. Snow, D. A. 2001. Plant Engineer's Reference Book, Butterworth-Heinemann Co, US. Spratlan, K. 2011. Firefighter Obesity: A Public Safety Risk. Fire Engineering, UK. Svennson, S. 2002.The operational problem of fire control (Report LUTVDG/TVBB-1025-SE), Lund University, Department of Fire Safety Engineering, Sweden. Wang, Y., Burgess, I., Wald, F. & Gillie, M. 2009. Performance-Based Fire Engineering of Structures. CRC Press, UK. Williams, C. 2009. Automatic Fire Sprinkler Systems. IHS BRE Press, UK. Read More