Computational Liquid Dynamics and Combustion Territory Design - Research Paper Example

Computational fluid dynamics and fire zone model Many researcher have been directed a lot effort on fire protection where a lot of effort has been directed on development of software and modelling techniques. The models which are in common use in the provision of solutions to basic fire equations are the computational fluid dynamics (CFD) and the fire model (Quintiere, 1980; Kim, 1993; Wakatsuki, 2001). The use of zone model is in common use because it is associated with lower cost in addition to being less demanding when used in computation. The National Institute of Standards and Technology (NIST) was the pioneer of the FDS simulator that uses Large Eddy Simulations (LES). This involve the use of primary LES technique where there is tabulation of large data in which a big portion of the energy system is conveyed with need to having direct resolution so as to enable the representation of process to the required accuracy. With the use of small scale eddies there is additional benefit of reduction in computational needs and this results to overall improvement. Due to the fact that in LES modelling does not require the averaged parameters to obtain transit solution is an easy task (Bullen, 1978; Emmons, 1997; Wakatsuki, 2001). A scrutiny of results which have been obtained from models have clearly indicated that there is a very close match which is an indicator of the reliability of the models when employed in the prediction of the velocity and temperature in well defined situations on condition that an appropriate grid resolutions with the necessary specifications the boundary conditions. A sketcher case can be used in conducting of demonstration as this will enable important aspects to be applied in modelling validation studies. In the CFD modelling k-episilon technique is utilized and consequently LES technique which finds application in FDS model generates a temporal resolution that is important in the evaluation of entrainment. This is a clear pointer that with the time averaged technique there can be a substantial impact on the total amount of air entrained in the fire. The development of FDS based model has a mathematical approach that is commonly applied in CDF models where there is emphasis on flow reduction in the heat generated by the fire. In developing a basic model for FDS mathematics was the instrument used and approach which is commonly used in CFD models that emphasizes on the reduction of heat flow emanating from the fire. With the sub model for turbulence an emphasis is placed on the flow taking place in fires with its importance being clearly seen in practical engineering where turbulence is being experienced. When a choice of a sub model is being made there is need to consider the degree of accuracy desired (Karlsson and Quintiere 2000). More often the large Eddy simulation is most applicable method in FDS (Yang, D & Hu, L, 2010; Merci , B & Vandevelde, P. 2007). With a fine grid resolution modelling of turbulent flow can be done without the need of a sub-grid approximation (referred to as Direct Numerical Simulation (DNS). The DNS finds application in compartment fires in addition to it being used for research purpose. By applying eddy simulation all fluctuations larger than the mesh size can be simulated. The uncertainty in the estimation of small eddies low as eddies in this case are of uniform character (Novozhilov, 2001). combustion sub models the are capable treating the conversion process of fuel and oxygen to combustion products and heat energy and as a result it is possible to use FDS in simulation of fire and the associated effect on the surrounding environment as well as simplifying the fluid flow. This case involves the use of two models: the mixture fraction combustion model as well as the finite model (Bullen ML, Thomas PH, 1978). The finite rate reaction model is seen to be the most appropriate for use when checking the resolutions of DNS calculations in the resolving of the diffusion of the gas species. The fig 1: shows the relationship between limiting oxygen volume fraction and gas temperature used in the extinction model in FDS to determine whether burning can take place. In the extinction model there is spontaneous reaction when the process of mixing oxygen and fuel in the cells as a consequence of the reactants falling in the Burn zone as indicated in figure 1. In the case where the combination takes place in the No Burn zone, mixing of fuel and oxygen occurs with nor reaction being realized (Quintiere, 2004; Bishop. et al. 1992) and this is described as a null reaction. In this model (instantaneous reaction model) the major products of combustion produced are CO2 H2O, CO and soot the proportion of the products being proportion with the rate at which the reaction is taking place. Therefore , the amount of products produced are given putting into consideration the level of consumption of fuel (Thomas, 1981; Epstein, 1988). There are numerous experiments which have been done with the aim making an assessment of the minor and major carbonaceous species and the of soot present at various time intervals in the upper layer of the in an ISO 9705 room (Drystlale et al, 1985; Merci , B & Vandevelde, 2007 ). Fire models like the NIST fire dynamics simulators are usually made use of by fire engineers, law enforcement agents, regulatory bodies as well as fire researchers in the process of design and when analysing safety measures for fire instances like the cases of post fire forensic applications and in the determination of the level of fuel consumption during a fire incident. The importance of a thorough understanding of compartment fire behaviour cannot be overemphasized, as it facilitates the derivation of straight predictions on the level of damage that can be done on structural elements. Takede and Akita have discussed deeply the topic of regions with limited ventilations in their research work. The findings of the research was that increasing the opening area of a compartment brings about change of regimes that include; stable laminar burning, extinction, unstable oscillation and stable burning including a possibility of oscillation (Yang, D & Hu, L, 2010). There has been revelation from field models that it poses a great difficult to make an accurate prediction of thermal conditions as well as chemical species when ventilated compartments are being dealt with. It has also been found that for the case of formal ventilation progress where there is a well ventilated compartment, the performance of field models have been found to be good predictors of temperatures and species if there is a good account of experimental uncertainties (Merci , B & Vandevelde, 2007; Epstein, 1988; Bishop. et al. 1992). Compartment fire development stages Fire development in compartment can be divided in several stages with several environmental variables being put into consideration in the process of subdivision. Temperature can be used as a variable when the fire is being subdivided into the various stages. For cases where there is any form of control over the fire time and temperature relationship will be as shown in figure1. There are four distinct stages involved in fire development: ignition, growth, flashover, full development and the decay stage (Cooper, 1995). The ignition stage, also known as incipient stage, is where exothermic reaction takes place which is characterized by increased temperatures as well temperatures above ambient temperatures being recorded (Friedman, 1991). Ignition can be piloted ignition through flaming match, may also be trough sparks or it may be through any other pilot source or the other ignition source may be spontaneous ignition which could be brought about by accumulation of heat in the fuel. after the ignition process taking place flaming combustion or smoldering combustion may occur. The cause of fire ignition sparks which have minimal energy content, a heated surface or it may be due to a large pilot flame. There are three classifications of the sources of energy namely, chemical, mechanical or electrical (Beard, 1997). The rapidity of the spread of fire after its ignition is dependant on the size of the energy source. A glowing cigarette or a small spark may set in smoldering combustion which may take a considerably long time before a flame can come to life. Smoldering fires are associated by production of substantial toxic gases but low levels of heat. A pilot flame leads to production of flaming which then results rapit spread and growth of the fire. The manner in which the flame spreads id dependants on the place at which the fire starts. Figure 1: Stages of compartment fire development Growth stage The growth stage sets in after ignition and this may be slow or rapid depending on the fuel type, type of combustion, the level of oxygen available and the type of interaction with the surrounding. The growth stage is best described by the rate at which energy is released and also the rate at which the combustion products are being produced. A smoldering fire is usually regarded as being at the growth stage at which there is production of substantial level of toxic gases but energy production is at a low level. The growth process of smoldering fire may be very slow and there may also be a chance of the fire dying out before passing through all the growth stages. The growth stage may rapidly advance more so for the flaming combustion case, where there is a high flammability of fuel allowing the flames to spread rapidly. The flame manages to produce sufficient heat flux from the first burning fuel so as to ignite the adjacent fuel and the available fuel supply is sufficient and also the oxygen which provide favorable conditions for rapid fire growth (Fleischmann C.M.,& Parkes A.R.,1997). Where there is sufficient oxygen for combustion the fires are described as being fuel controlled. The transition from fire growth to the stage where the fire is fully developed is termed flashover. In fire safety engineering flashover is used to describe the point between pre-flashover and post-flashover. There are various definitions for flashover which are found in literature and this makes it not to be a precise term. In some of the criteria use in demarcation flashover point recognize a temperature of between 500-6000C, or the radiation towards the floor of the compartment being 15 to 20kW/m (Walton, W.D. & Thomas, P.H., 1995). Another criterion is that there should be appearance of flames from the openings in the compartment. The occurrences associated with flashover are as a result of various mechanisms resulting from fuel orientation, fuel properties, fuel position, and conditions in the upper layer and the geometry of the enclosure. Dembsey NA, et al. (1995) points out that flashover is not a mechanism but is a phenomenon that is associated with a thermal instability. After the growth stage through flashover the fire will be at the fully developed stage. In this stage there is maximum energy release of energy in the compartment and often this energy will be found to be limited the oxygen which is available. This is reference to as ventilation-controlled burning as the oxygen needed in the combustion process is assumed to gain entry into the compartment through openings. The un-burnt gases collect at the ceiling in ventilation controlled fires and as the gases make their way through openings there is burning that causes flames to stick out through openings. The average temperatures associated with this stage are very high ranging from 700 to 12000C (Norwegian Fire Research Laboratory, 1996). The final stage after the fully developed stage is the decay stage where the fuel is consumed; there is diminished energy release which results in a decline in the compartment temperatures. In this stage there is a possibility of the fire going from ventilation controlled to fuel controlled. Burning Rate There is a great need of having accuracy in the prediction of burning rate characteristics as this is crucial in the estimation of specific fire effects within a compartment, for instance, when it comes to the analysis of the effect of post-flashover fires on structures have basis on how severe the burning is and the time taken for burning. The expected duration of burning can be described by the concept of fuel loading, provided there enough air. Burning rates have been made use of by some authors in the description of mass loss rate of fuel which completely differs in meaning though the two rates may follow similar trends. Mass loss rate for fuel involve at which the condensed fuel will decompose into hot gases due to re-radiation from surrounding hot boundaries. If it is assumed that there are no inverts present in the fuel, the relationship existing between mass loss rate be described as Fuel mass loss rate = Burning rate + Rate of unburned fuel gases and soot. (Utiskul, 2006): The alternative way of writing this is  the global equivalent ratio is another parameter which is used in the escription of relationship between burning and rate of loss of mass. This is usually the case where where no provision is given for unburned fuel gases can be written as follows (Quintierre, 1997; Drysdale1998):  Where, r represents the stoichiometric mass of fuel to air ratio. Suppose the global equivalent ratio has a value greater than one (> 1) – this will be an indication a distinct burning regime named under-ventilated (fuel rich) compartment fires, meaning there no sufficiently burn the oxygen to ensure complete oxidation of the fuel. But when < 1 – it is an indication that a compartment fire burning regime which is over supplied with oxygen (over-ventilated) where no unburned fuel exists. As can be seen from equation above, the induced mass inflow of air into the compartment fire is dependant on ventilation factor and can be expressed as (Karlsson and Quintiere 2000);  Hence, the compartment equivalency ratio can then be given as;  From the evaluation, equivalence ratio can be defined ratio of mass loss rate of fuel and the induced mass inflow of air in the room vent that is normalized by stoichiometric ratio. It follows then that the mass loss rate of a fuel need to be accurately known so that there can be correct prediction rate of burning as seen with under-ventilated regimes where burning depends on the amount of air supplied and the rate of fuel consumption (stoichiometric mass fuel to air ratio). Past research work by Parkes and Fleischmann (1996, 1997) found the global equivalent ratio for a range of opening factors, for vent with  was found to be approximately 1.7 and that for  tends be closer to a global equivalence ratio of 3. Factors Affecting Compartment Fires Having an understanding of the factors affecting compartment fires is integral in the success of a research of this nature because this are identified as the main parameter which can essentially be utilized in the analysis of investigation. It has been reported that physical factors such as the characteristics of the compartment, its configuration, the characteristics and availability as well as ventilation profile are the determining factor on how fire develop after ignition. authors such as Grimwall et al (2010), Babrauskas (1982), Thomas et al (1980), and Quintiere and McCaffery (1980) have done investigation on these factors and have tried explaining the individual effects on the fire development. Studies which were carried out by these authors’ indicated correlation that, size and geometry contribute highest in the determination of the growth pattern of any fire on ignition. They are in agreement that, once a fire is ignition occurs in a room, there will be production of hot gases, which will then rise due to buoyancy until when they hit the ceiling of the room and then form a hot layer underneath the ceiling. If heat radiated from fire is increased it will result into an increase in temperature of the hot layer, which in turn increases temperature levels of the surrounding boundaries. Building aerodynamics and applications to fire engineering Building aerodynamics find application in a variety of problems in fire engineering including smoke control and design of buoyancy driven roof vent. The studies have found new applications in areas determination of wind effects on the spread on fire spread which is close to the current study. There has been vast experimental data that has been collected from wind tunnel studies with full scale measurements being collected and utilized in designing of building structures in the last two decades. However, in the recent past there is much attention on effect of wind on internal environment of building due to the necessity of taking account of the effect in the design of various smoke control system so as to ensure the existence of large pressure difference do not affect the occupants who may be attempting to escape by confining the smoke to the point of fire. The mechanics of flow dynamics together with building dynamics play a vital role in containment of fire. In this field there are laws of aerodynamics which are used to indicate how buildings are made susceptible to negative effects of fire. This science also finds application in tall buildings so as to reduce the effect of wind on the building structure (Robertson et al. 1980, p.201). In recent studies there is a shift to investigation of the impact aerodynamics on spread of fire in buildings. The cross-section area of the building, design sign details of ventilation shafts and the height of the building are some of the factors that are that has effect on aerodynamics attributes of a building. These factors are the centre of focus in research with the aim of assessing and modeling the role of structural aerodynamics with regards to fire spreads and safety. (Whalen et al. 1998; Gutierrez-Montes et al. 2008; Chow 1996). Wind characteristics is also found to be of equal importance just as the shape and building size, when it comes to the determination of wind induced internal as well as external pressure distribution for a building (B. S. Kandola 2008). Wind characteristics Wind velocity always varies with height above the ground and the details on the variation and pressure coefficient have been revealed by Sachs, 1972; Macdonald, 2975; Scanlan, 1978. The region close to earth’s surface experiences more profound variation of wind velocity with height and this region is referred to as terrestrial boundary layer. The terrestrial boundary layer is represented graphically as seen in figure 3 where the various curves is a representation various surface roughness. In the figure category 1 refers to a plain area such as a dessert and open sea while areas falling under curve category 4 have high roughness such as places with several buildings. Wind is the main agent of dispersion and its impact on containment of fire or its spread is dependant on its speed, direction, persistence and level of turbulence (Mannan 2005, p.16/283). For there to be accurate assessment of fire spread in internal scenario and external scenario, it is important to use wind speed and direction data which coincides with environmental location of the fire. For the case of open environment things like surrounding vegetation and hills are put into consideration while in built environment there changes are expected as wind passes through crevices, windows and ventilators, resulting into creation of an envelope which has similarity to forest fires experienced in high altitudes canyon basis (Sharples et al. 2010). The presence of strong winds has two fold impacts on the spread of fire. For compartment fires when presence of high wind speeds which gains entry into the compartments will result into super-heated gases and smoke plumes spreading fire rapidly in the compartment. This impact plays a vital role in intra –compartment flow dynamics of fire (Yung 2008, p.119). a good instance is where the extend of smoke and heated gas spread is dependant on forces such as buoyancy force and restriction on flow of wind. According to Yung (2008, p.120), the heat release is dissipated through compartment wall conduction, wind convection through openings and wind-based radiation through openings. Figure 3 After having some understanding on how wind velocity relates with height it important to have a look at wind flow over buildings and especially for the case of tall buildings. It have been revealed from research done on tall buildings as well as wind tunnels that the windward surface of a building will have a positive pressure while all the remaining surfaces will experience a negative pressure as illustrated in the figure. Figure 4 As can be seen from the figure 4 the air flow resulting from the wind that heads in the windward surface of the building will be slowed down until that point it comes to rest at s, a point referred to as stagnation point. There will be an increase in total pressure with the increasing height up the stagnation point which marks the start of a decrease in pressure towards the roof. Below the point s the flow of air is towards the ground level and at the point the air hits the ground level, there will be turning and the flow is reversed towards the building again as a result of the oncoming wind. This movement will then result into a vortex formation in front of the building as can be seen from the diagram. The pressure experienced is usually given in terms of pressure coefficient defined by The pressure will be highest at the stagnation point on the windward side of the building and this point is located at a height of approximately four-fifths of the building height. B. S. Kandola (2008). The leeward side will have a negative pressure and the pressure is likely to be constant which will be as a result of flow separation experienced at the corners of the building. A good understanding of the pattern created by the wind in built environment plays an important role when it comes to the design of robust building envelopes, the developing of building codes and also in determining of the effect of wind variables for various buildings. There is a lot effort which is being directed towards the development of physical modeling techniques which can be used in development models to be used in investigating on the impact of wind based on boundary layer (Cermak et al. 1998). When wind flows over a building a global impact will be exhibited on the structures. Generally wind load are compost mean and time varying segments where the study of the impact the flow of wind over a building is performed in two different modes one with aero-elastic and the other without effects on the building. Building with aero-elastic effects of winds will need a complex modeling of wind flow. The aero-elasticity phenomenon involves studying of inertial , elastic and aerodynamic forces that have effect on the building structure (Hodges et al. 2002, p.xiv). For the case where the structure does not involve aero-elastic effects, static modeling will be applicable by use of geometrically scaled external features (Cermak et al. 1998, p.25). For fire spread related effects for urban built environments, the external thermal environment and structural load related to cooling will be determined as a function of convective heat fluxes emerging from the structures and also the streets (Defraeye and Carmeliet 2010). Wind tunnel studies that was done on built structures which had rectangular cross-section obey the power law velocity profile with the wind facing direction will have high atmospheric pressure in comparison to other surfaces which may not be in contact with the wind. Wind induced internal flows The effect of wind on the movement of air within an ideal building assumed to be tightly constructed or fully sealed and having all the windows and doors being closed will be negligible but the wind of the same strength will have a significant impact on a building which is loosely constructed or for buildings whose windows and/or doors are open resulting in an increased and complex airflows. Wind induced internal flows is the flow of wind within the atmospheric boundary layer with its induced pressure load on building surface and inside the building through the openings found in structural openings such as windows and vents (Stathopoulos and Baniotopoulos 2007, p.1). There is substantial variation in wind induced flows, on basis of the type of building structure. The internal pressure induced by wind may take a substantial proportion of design specific wind load of a building. Where the steady wind flow conditions exist, there is a rapid build up of internal pressure surrounding the window openings so as to neutralize the external pressure emanating from the windward direction (Holmes 2007, p.152). There is occurrence of wind induced ventilation is as a result of the build up pressures in addition to the momentum of air pressure where the opening is considerably wide. Mean pressure data is the most popular method used in the prediction of wind induced ventilation. However, the method is associated with some discrepancies when measurements are made (Choiniere et al, 1992). For the case of completely air sealed buildings, the outside wind will not have any effect of outside wind flow on fire. For the case where the building has some openings, the internal flow of air is affected to a great extend by the outside air pressure. There can be serious effect on the functioning smoke extracting vents. This causes the smoke or hot gases to flow to other aras of the building by the already existing airflows. The effect of smoke spread inside the building and having a positive air pressure as shown in figure Figure 5 Effect of wind on the compartment fire Even when it is considered that a building is constructed tightly or loosely or the construction being done to the best possible standards when an incident of fire occurs the windows are likely to shatter as a result of existence of gases at very high temperatures, or by the occupants or the fire fighters and this may bring about a significant increase in fire growth or spread of smoke throughout the floor where the fire originates (Council on Tall Buildings and Urban Habitat, 1992; B. S. Kandola, 2008). Effect of wind (forced air flow) on fire conditions and the spreading of smoke have often been taken as an issue in tall buildings. The outcomes of various studies have indicated that when wind blows into broken window of a room which is on fire it is likely to be turn a “routine room and contents fire” into full explosion fire. This phenomenon has been the cause death of many fire fighters, serious injuries especially where the buildings are high rise where it is necessary to fought from the inside of the building as height becomes a constraint. When a fire originates from a compartment it causes physical damage to the occupants in addition to causing damage to other compartments in the building. An enclosed fire causes the release of harmful substances which include heat, smoke particles and toxic gases. Fire attributes affected by wind Wind affects the Spreading of superheated gases and smoke. A blowing wind will generate a positive pressure from the windward direction while the leeward side where wind enters from the windward side, will have a negative pressure. Suppose fire is originating from the windward side, the wind will have on tribute to the impact of fire inside the building together with the smoke spread. In the case of the leeward side there is restriction of spread of fire by wind. Fire which is at the stage of full development will is associated with ejections coming out of windows and other openings. The Himoto et al (2009) investigation on a reduced-scale experiment made a conclusion that the role of window flame trajectory, the rise of temperature in the compartment, the velocity of wind experienced outside the compartment as the major contributing factors causing the fire to spread to other compartment. Practical tests carried out against real-life buildings Broken or open windows in a building affected by fire will tend to turn small room and material fire into a raging thunderstorm. The impact of the fires may turn out to be catastrophic on fire-fighters as well as the other people who may be stuck inside the building. There are numerous experiments which have been done by standard fire fighting bodies regarding fire fighting personnel and occupant safety. The observations which were made from the underlying tests were as follows. Impact of pressure ventilation fans (PVF): The observation made was that PVF do not have the ability to control the over effects of wind. However, when the devices are put into use together with door control and wind control devices (WCD), the PVFs had the ability to maintain a reasonable safety level to the fire fighters to enable them operate and evacuate occupants. Role of wind control devices (WCD): The WCDs substantially reduced the temperature in the building by 50% within minutes of deployment. WCDs were also seen to be successful in acting as complete mitigation of externally induced wind velocity and to a variety of thermal set ups with no failure being seen. Effect of applying water externally In the experiment there was also the observation of the various methods of applying water externally through the building. This method involved the use of fog stream into the fire compartment window, the streaming of fog from the floor below to the room window and also an actual stream of water flowing from the floor below through the window opening. In all the three uses there was the reduction of internal temperatures through the suppression of fires by at least 50%. The overall effect of wind on compartment was found to be able to sufficiently reduce through the use of the above mentioned steps. All in all the obvious effects of wind and the resultant pressure distribution around tall buildings have not been fully utilized in fire safety design of modern buildings in various countries and having a comprehensive analysis in this area will likely discover crucial factors that can be used in the determination of wind impacts in compartment fires. 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