Manual to Compartment Oxidation Method the Outcomes of Airing Switch Mode - Assignment Example

Introduction to Compartment Fires Chemical and physical phenomena are the two forms of fire divisions that are highly recognized even though the both have high level of interaction in nature. The interaction is also found to be non-linear in terms of fire fuel, the surrounding environment and its flame and this makes it a complex affair to clarify the quantitative estimate process. Compartment fire is an option that gives a better understanding of the behaviour of fire. A compartment fire is categorized as being any fire that is in a closed space such as a building, a storage unit or theatre where the limit of heat transfer and oxygen supply to and from the fire is controlled (Quintiere, 2006). The description of the relationship between compartment fires and fire dynamics can be a good start to this section. Finding a fundamental approach capable of replicating the stages of fire as an alternative to reliance on traditional methods is the best alternative (Drysdale, 1998). The main purpose in such an approach is to have to an understanding of the engineering perspective. This therefore makes studying fire dynamics by a fire protection engineer to be analogous to a chemical engineer studying chemistry. Computational fluid dynamics and fire zone model There has been a lot effort made by many researchers in the field of fire protection where there emphasis has been directed towards the development of software and modelling techniques. In solving of fundamental fire equations computational fluid dynamics (CFD) and fire zone model usually come into application (Kim, 1993; Quintiere, 1980; Wakatsuki, 2001). The frequent use of the zone model is attributed to its lower cost in addition to it being less demanding for computing. Large Eddy Simulations (LES) is utilized the FDS simulator which is a development initiative of Institute of Standard and Technology (NIST). The FDS utilizes the basic techniques of LES with large quantity of data being tabulated in which a huge fraction of the energy system transferred with there being a need to have direct resolution so as to represent a flow process having the desired accuracy. When small scale eddies are used there is additional benefit of reduction in computational demands this being beneficial in terms of having an improved speed in the simulation process (Bullen, 1978; Emmons, 1997). It is not difficult to obtain transient solution because when using LES modelling the need for averaged parameters is eliminated (Wakatsuki, 2001). A close look at the outcomes of models reveal a very close match and this is as strong evidence of reliability of models when used in the prediction of temperature and velocity of situations which are well defined provided the grid resolution is appropriate in addition to the boundary conditions being properly specified. In the process of conduction a demonstration a sketcher case can be used because of its benefits in giving of important dimensions under study in modelling validating studies. In CFD modelling k-episilon is used and as a consequence LES technique which is linked to FDS model results into production of a temporal resolution which is important in the evaluation of entrainment. From this it clear that the use of time averaged technique will impact seriously on total amount of air being entrained in the fire. A mathematical approach is taken when it comes to the development of FDS based model. The mathematical approach is associated with CFD models where a lot of emphasis is placed on reducing of heat flow transfer brought about by the fire. In the turbulence sub model emphasis is directed towards flow experienced in fires with its importance being evident in practical engineering where turbulence is being experienced. The desired degree of accuracy is very important when it comes to the choice of sub modelm (Karlsson and Quintiere 2000). In many situations the Large Eddy Simulation has been found to be the method that is most applicable in FDS (Yang, D & Hu, L, 2010). Direct Numerical Simulation (DNS) is a model where there is use of fine grid resolution making it possible to model turbulent flow without the need of having a sub-grid approximation. The DNS finds its application in compartment fires it addition to research application (Merci , B & Vandevelde, P. 2007). By use of eddy simulation it is practical to have a simulation of all fluctuations which are larger in size than the size of the model mesh size. When estimations for small eddies are being made uncertainity is low owing to the uniform character of eddies (Novozhilov, 2001). The sub models which are used in combustion are able to handle the processes involved in the conversion of fuel and oxygen into products and generation of heat and thus enable FDS to have the ability of simulating fire and the effect the fire has on the immediate environment in addition to showing the flow of the fluid. The finite rate reaction model and the mixture fraction combustion model are the two models that are put into use (Bullen &Thomas , 1978). When it comes to checking for resolutions of DNS calculations in the resolving of the diffusion gas species the finite rate reaction has been found to be preferable. 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 use of the extinction model where the combination of the gas and oxygen are in the burn zone as seen in figure 1, there will be an instantaneous reaction when mixing of oxygen and fuel occurs in the cells. In the case where the combining of gas and oxygen is in the “No Burn” zone the mixing of fuel and oxygen will not result into a reaction which is described as a null reaction. (Quintiere, 2004; Epstein, 1988). Some of the products that are likely to be produced in the instantaneous reaction model include CO2 H2O, CO and soot this being proportionate to the rate of consumption of fuel. It is therefore necessary that when giving the product yields the mass of fuel involved need to be put into consideration (Thomas, 1981; Bishop et al. 1992). There are numerous experiments which have been performed whose main aim has been to have an assessment of the major and minor carbonaceous species and the amount of soot present at specific time intervals in the upper layer of fire of the room (; Merci , B & Vandevelde, 2007 ). Law enforcement agents, fire researchers, fire engineers and regulatory bodies make use of fire models like NIST fire dynamics simulators in the design process and in the analysis of safety measures fire where instances like post fire forensic applications and in the determination the quantity of fuel consumed in the fire process (Drystlale et al, 1985). Understanding of compartment fire behaviour is important as this is a window to being able to make predictions on the impact fire is likely to have on structural elements. The topic of regions with limited ventilations has been of interest to many authors with Takede and Akita being pointed out as major contributors of research in the area. An increase in the opening area of the compartment is associated with change in regimes from stable laminar, extinction stage, unstable oscillation and stable burning stage which (Epstein, 1988; Bishop. et al. 1992). Findings from field models have revealed that having accurate predictions of thermal conditions and the chemical species involved is difficulty where ventilated compartment fires are concerned. In the case where there is access to a well ventilated compartment the performance of field models have been found to good performance in the prediction of temperature levels and species provided there is proper account of experiment uncertainties. (Merci , B & Vandevelde, 2007; Yang, D & Hu, L, 2010 ) . Stages in compartment fire development. Compartment fire development can be into different stages where several environmental variables are put into use. Temperature distribution is used when describing compartment fires with various stages being identified. In the cases where no attempt is made to have control over the fire the relationship between temperature and time together with the stages of growth will be as shown in figure 2. According to Cooper (1995) the stages include the ignition stage, growth stage, flashover stage, full development stage and decay. At the ignition stage, also known as the incipient stage involves the process of production of exothermic reactions whose characteristics are increased temperature that is well beyond the ambient temperature (Friedman, 1991). The process of ignition can be as a result of piloted ignition by flaming match, sparks, other pilot sources or may be brought about by spontaneous ignition that may result heat accumulating in the fuel. Smoldering and flaming are the two types of combustion that may occur after fire has been ignited. The fire ignition source can be spark having minimal energy content a large pilot flame or the source may be a heated surface. The broad sources of energy that may cause fire ignition are mechanical, chemical or electrical (Beard, 1997). Rapid growth of fire of the fuel source will be realized where the source of energy is large. In the case where glowing cigarettes or spark is involved there may be smoldering combustion which may take considerably a long time before flaming is realized. Low heat production and substantial toxic production level are the distinct characteristics of smoldering fires. In the case of pilot flame there is production of flaming combustion resulting into a quick spread and growth of fire. The point where the ignition starts will have effect on the spread of the fire. Figure 1: Stages of compartment fire development The growth stage which follow the ignition stage may be slow or rapid depending on the type of fuel, the type of combustion, the supply level of oxygen and the nature of interaction with the surrounding. During the stage of growth fire is described by the rate of release of energy and the type and quantity of combustion gases being produced. A smoldering fire will be regarded as fire which is still at the growth stage where there is low level of heat production but substantial amount of toxic gases production. The smoldering fire may take considerably a long time to grow to the next stage and there may be a likelihood of the fire dying out without passing through all the other stages. In the case of flaming combustion the growth stage can occur very rapidly the favorable conditions being high flammability of the fuel, a conducive environment allowing rapid spread of the flame, availability of sufficient heat flux from the initial burning fuel making it possible for adjacent fuel to be ignited and where there is enough supply of fuel and oxygen that is favorable to rapid growth of the fire (Fleischmann C.M.,& Parkes A.R.,1997). In the case where the supply of oxygen is sufficient combustion fires are said to be fuel controlled. Flashover is the transitional stage between the fire growth to the point the where the fire is fully grown. In fire safety engineering the flashover stage is used in the description of the region lying between pre-flashover and post-flashover. A range of definitions are given for flashover in different literatures as a result of which making it not being a precise term. In terms of temperatures some of the criteria used recognize 500-6000C or the radiation to the floor of a range of 15 to 29 kWm2 being the demarcation points (Walton, W.D. & Thomas, P.H., 1995). The point at which there is occurrence of flames at the openings in the compartment is another criterion which I used in describing flashover. The events associated with flashover are due to mechanisms that are associated with the properties of fuel involved in the fire, the location of the fuel,the conditions of the upper layer and also the geometry of the enclosure. According to Dembsey et al (1995) flashover is not a mechanism but rather a phenomenon which is associated with instability. The fully developed stage is the stage reached after the fire passes the growth stage through the flashover. At the fully developed stage the rate of release of energy is at the maximum in the compartment where the energy amount is dependant on the level of oxygen supplied. This applies to the case of ventilation-controlled burning where the amount of oxygen needed for the combustion process is assumed to enter the compartment through openings. The burning process is such that any un-burnt gases will tend to collect at the ceiling of the compartment in the case of ventilation controlled fires and as the gases make their way out of the compartment via the openings they will catch fire resulting in flames sticking out through openings. The average temperatures at this stage are very high in the range of 700 to 12000C (Norwegian Fire Research Laboratory, 1996). The decay stage is the final stage in combustion process where the fuel is fully consumed, energy release is diminished resulting into a reduced temperature levels in the compartment. This stage also be the point where the fire changes from ventilation controlled to fuel controlled. Extinction behaviour and heat release rate The point where there is extinction is determined by the level of oxygen concentration, the level of local temperature and the heat flux available at the surface of the fuel. This is clearly demonstrated at a point where the concentration of oxygen close to the fuel at the floor is plotted against the gas temperature close to the floor. Stable oscillation occurrence is a form of extinction marking the point where the flame is almost suppressed as a result of reduced supply of oxygen. This is a result an outward flow occurring via the bottom vent followed by regeneration of the flame with an inward induction of air flow into the compartment (Merci , B & Vandevelde, 2007 ). From the results which have been obtained from experiments it has been clearly shown that there will always be a drop in the level of oxygen at the extinction stage whenever an increase in ambient temperature is recorded (this will result from heating the compartment) and a decrease in the limiting oxygen at extinction being recorded. An investigation of a theory showing agreement with results obtained from experiments have has been investigated. In this theory the stagnant 1-D burning rate model used for safety liquid pool fire burning is used as illustrated in equation 1 and also the corresponding governing equation representing the flame temperature. Theoretically there is an assumption that the temperature level is 13001 C at the extinction stage with the flame radiation effect being negligible (Quintiere, 2004; Bishop. et al. 1992). For small B-number the burning rate per unit area is given by:  Equation 1 Where  representing convective heat transfer coefficient, r representing stoichiometric oxygen and fuel mass ratio,  stands for the boiling point of the liquid fuel (surface temperature), and  representing the external radiation. In the equation for the flame temperature ( ) is given by  Equation 2 On substituting for burning rate in equation 1 and equation 2 the relation between oxygen, temperature, and external heat flux ( for a given flame can be given as =  In the equation  is the local ambient temperature while  is oxygen mass next to the flame.  here represent the local ambient temperature while  represents oxygen mass fraction near the flame, respectively. The external radiation is given as  =  where  refers to the temperature measurement experienced in upper compartment. Heat release and dimensionless variables Through the use of the quasi-steady rough theory the vital variables which are dimensionless can be pointed out and this is the basis of global presentation of data. The energy conservation in a compartment with uniform properties within the compartment can be written for the case where μ=0 (this being a representation of a small supply of fuel) as cp(-) = -hAs(-) In this equation  stands for the rate of outflow at the top vent, T is the temperature and  being ambient temperature outside the compartment this being assumed to be constant. In the equation cp represent specific heat,  is the power of the fire or the chemical energy dissipated in the compartment, h is total heat transfer coefficient while the compartment surface area is given by As.  can be made dimensionless by substituting for  and the dividing by  thus yielding the equation  = 0.5Cd 1/2+. The equation gives the dependence of temperature in this case is the dependant variable. This can be expressed as T = function  Using conservation species  = , In the equation  is the oxygen mass fraction in the compartment, and  is the heat of combustion per unit mass of air and on substituting  in order to make  dimensionless using  the resulting equation is  =  1/2+ This can also be expressed as  = function  Considering the effect of oxygen and radiation resulting from smoke with a view factor F the expression of the compartment burning rate will be = + This can be expressed in the form =function (,) It is also possible for the fire power to be expressed as one of the following depending which has the lower value  or  =  or , In the above expression s represents stoichiometric air fuel mass ratio and the symbol  is the heat of combustion for every unit mass of fuel consumed. In general the dependant variables can be represented as =function With the fuel terms comprising of L,, and s. Types of combustion models Laminar flamelet model. This is the model allows the incorporation of finite rate chemistry effects involving turbulent combustion. In this model there is an assumption that occurrence of combustion is local in microscopic elements in the turbulent flames. The concentration of the major species and the temperatures are given by mixture fractions. It is possible to compute the probability density function (PDF) in the standard form by flow field prediction of Favre-avarage mixture fraction mean, variance and gravitational constant. In circumstances where we have two parts of PDF one which will be a Beta function will go towards the turbulent jet and the other being a Delta function. Detailed chemical-kinetic computations can be done by Flamelet model incorporation with turbulence modeling (Norwegian Fire Research Laboratory, 1996). . Constrained equilibrium method. This involves temperature calculation, species concentration through correlation of variables and mixture fractions and through rigorous calculations of laminar diffusion flame coming up with the required evaluations. The strong energy losses experienced through radiation in sooting diffusion flames makes in impractical to have the radiation model incorporated in the model. The elimination of this problem is the application of constrained equilibrium which involves equilibrium calculations being taken and an enthalpy value imposed on the problem. This approach is believed to have equivalence with having the system subjected to some form of cooling. When performing the equilibrium computations the thermodynamics knowledge of the systems is of vital importance with detailed information about the reactions being irrelevant, thus making it possible to relax to equilibrium assuming we have the reaction (Kerrison, 1998). The implication of this is that a reaction mixture at equilibrium composition is independent of reaction mechanism. The energy is given by the equation () = - Where h is the static enthalpy,  is the velocity component,  represents density of the mixture,  represents transport coefficient,  stands for jth coordinate,  describes the mass fraction of species I, the Lewis number of a species i is represented by  while  is the heat flux. After enthalpy distribution has been obtained calculation of heat loss factor can be done using the equation  = , Where h is enthalpy which is calculated from equation,  is the enthalpy for isothermal mixing and  being the enthalpy value in fully burning conditions. Through the equation it two dimensional look-up table can be generated that can give the flow properties as functions of mixture fraction thus producing heat loss factor. Eddy break-up model. In the eddy break-up model the solution of an explicit equation is given for fuel mass fraction The equation involved being +) -  = -. In the eddy break-up combustion model the assumption is that there is a fast chemistry implying turbulent mixing is rate controlling. In the turbulence timescale ,  can be given as  = min, Where  gives time-averaged mass fractions of the fuel  is the oxidant and  being the product. The stoichiometric oxygen-to-fuel mass ratio is given as “s”,  is density of the fluid,  is the laminar viscosity while  representing turbulent viscosity. Similarly  stands for laminar Prandtl number, stands for turbulent Prundtl number with t being time transient cases. CR and CA represents constants whose determination is empirical dependant on the mixing model and the reaction rate chemistry. Radiation models Discrete transfer radiation method: This is a ray tracing technique where the scattering coefficient in fine soot particles is in the range of 0.1m this being considered as negligible when compared to the corresponding absorption coefficient. Calculating the absorbent coefficient ka in the gaseous and soot mixture may require the use of Hubbard and Tien concept where Rayleigh absorption limit and Elsasser narrow-band model are applied for soot and the absorption of gases respectively. On evaluating the CO2 contribution to Planck mean absorption coefficient, the vibration-rotation bands that are considered include 15, 10.4, 9.4, 4.3, 2.7, and 2.0 m. In the case where H2O involved there is inclusion 6.3, 2.7, 1.9, 1.4 m vibration-rotation band and 20 m pure rotation. Incorporation of radiation in the flamelet model. Because in flamelet model there is a unique relationship between species concentration and temperature with mixture fraction accounting for global irradiative heat exchange the approach becomes complicated. After solving the equation () = - The equation  = , is applied in the calculation of heat loss factor. Because of heat loss factor having a direct relationship with radioactive heat loss, it is assumed that the sensible heat is proportional to the heat loss factor. This makes it possible to calculate temperature by making an assumption that sensible enthalpy loss is equal to the difference between the enthalpy from solution of energy equation and adiabatic enthalpy. References Bishop , S & Drysdale, D (1995) Experimental Comparison With A Compartment Fire Model. International Communications in Heat and Mass Transfer. Bullen ML, Thomas PH (1978). Compartment fires with non-cellulosic fuels. Proc Combust Inst. Drystlale. Et al (1985). Smoke prcjducticm in tires, Small scale experiments. Fit-oSufity: Scicww utzd Ett~itwritt~. ASTM STP 8X?. T.Z. Harmathy. Ed. (American Society forTesting and Materials. 1985) pp. 285 300. Emmons HW (1997). A universal flow orifice formula. In: NISTIR 6030, vol. 1. Gaithersburg, MD:National Institute of Technology and Standards. Epstein M. (1988). Buoyancy-driven exchange flow through small openings in horizontal partitions. J Heat Transfer. Friedman, R.,(1991). “Status of Mathematical Modeling of Fires,” FMRC Technical Report RC81-BT-5,Factory Mutual Research Corp., Boston, 1981. Walton, W.D. and Thomas, P.H. (1995). “Estimating Temperatures in Compartment Fires,” in The SFPE Handbook of Fire Protection Engineering, 2nd ed., National Fire Protection Association, Quincy, MA, Large scale compartment fires(1995). experimental details and data obtained in test comp-27. Report-OTO94024, Health and Safety Executive. Norwegian Fire Research Laboratory, (1996). Blast and fire engineering for topside structures, test programme F3, confined jet and pool fires. Dembsey NA, et al.( 1995) Compartment fire near-field entrainment measurements. Fire Safety Journal 24:383}419. Fleischmann CM, Parkes AR. (1997) Effects of ventilation on the compartment enhanced mass loss rate. Fire Safety Science-Proceedings of the Fifth International Symposium. p. 415}26. Peatross M.J.& Beyler C.L.(1997). Ventilation effects on compartment fire characterization. Fire Safety Science-Proceedings of the Fith International Symposium. p. 403}14. Beard AN. (1997). Fire models and design. Fire safety J;28:117}38. Fan W.C. & Wang Z.W.( 1997) A new numerical calculation method for zone modelling to predict smoke movement in building fires. Fire Safety Science-Proceedings of the fifth International Symposium,. p. 487}98. Hasofer A.M. & Beck V.R.(1997). A stochastic model for compartment fires. Fire Safety Journal ;28:207}25. Peacock R.D. et al.(1998). Issues in evaluation of complex fire model. Fire Safety Journal ;30:103}36. Luo M.(1997). Application of field model and two-zone model to flashover fires in a full-scale multi-room single level building. Fire Safety Journal 29:1}25. Friedman R.(1992) An international survey of computer models for fire and smoke. Fire Proting Engng ;4:81}92. Jia F. (1997). The prediction of fire propagation in enclosure fires. Fire Safety Science-Proceedings of the fifth International Symposium,. p. 439}50. Kerrison L. (1998). A two-dimensional numerical investigation of the oscillatory behaviour in rectangular "re compartments with a single horizontal ceiling vent. Fire Safety Journal;30:357}82. Kim IK, & Ohtani H, 1993. Experimental study on oscillating behaviour in a small-scale compartment fire (short communication). Fire Saf J Merci , B & Vandevelde, P (2007) Experimental study of natural roof ventilation in full-scale enclosure fire tests in a small compartment . Fire Safety Journal. 42 () p523-535 Novozhilov, V (2001) Computational fluid dynamics modeling of compartment fires. Progress in energy and combustion science. 27 p611-666 Wakatsuki K. (2001). Low ventilation small-scale compartment fire phenomena: ceiling vents. MS Thesis, Department of Fire Protection Engineering, University of Maryland, College Park, MD. Yang, D & Hu, L (2010) Comparison on FDS predictions by different combustion models with measured data for enclosure fires. Fire Safety journal. 45 Andronicus M. et al (1998) Non-accidental burns in children. Burns 24: 552–8 British Burn Association (2008) Emergency management of severe burns course manual, UK version. Wythenshawe Hospital. Manchester E. et al (2009). Emergency and early management of burns and scalds. Br Med J 338: 937–41 Hudspith J and Rayatt S (2004) First aid and treatment of minor burns. Br Med J 328:1487–9 La Hei et al(2006).Laser Doppler imaging of paediatric burns: burn wound outcome can be predicted independent of clinical examination. Burns 32: 550–3 Read More

In the process of conduction a demonstration a sketcher case can be used because of its benefits in giving of important dimensions under study in modelling validating studies. In CFD modelling k-episilon is used and as a consequence LES technique which is linked to FDS model results into production of a temporal resolution which is important in the evaluation of entrainment. From this it clear that the use of time averaged technique will impact seriously on total amount of air being entrained in the fire.

A mathematical approach is taken when it comes to the development of FDS based model. The mathematical approach is associated with CFD models where a lot of emphasis is placed on reducing of heat flow transfer brought about by the fire. In the turbulence sub model emphasis is directed towards flow experienced in fires with its importance being evident in practical engineering where turbulence is being experienced. The desired degree of accuracy is very important when it comes to the choice of sub modelm (Karlsson and Quintiere 2000).

In many situations the Large Eddy Simulation has been found to be the method that is most applicable in FDS (Yang, D & Hu, L, 2010). Direct Numerical Simulation (DNS) is a model where there is use of fine grid resolution making it possible to model turbulent flow without the need of having a sub-grid approximation. The DNS finds its application in compartment fires it addition to research application (Merci , B & Vandevelde, P. 2007). By use of eddy simulation it is practical to have a simulation of all fluctuations which are larger in size than the size of the model mesh size.

When estimations for small eddies are being made uncertainity is low owing to the uniform character of eddies (Novozhilov, 2001). The sub models which are used in combustion are able to handle the processes involved in the conversion of fuel and oxygen into products and generation of heat and thus enable FDS to have the ability of simulating fire and the effect the fire has on the immediate environment in addition to showing the flow of the fluid. The finite rate reaction model and the mixture fraction combustion model are the two models that are put into use (Bullen &Thomas , 1978).

When it comes to checking for resolutions of DNS calculations in the resolving of the diffusion gas species the finite rate reaction has been found to be preferable. 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 use of the extinction model where the combination of the gas and oxygen are in the burn zone as seen in figure 1, there will be an instantaneous reaction when mixing of oxygen and fuel occurs in the cells.

In the case where the combining of gas and oxygen is in the “No Burn” zone the mixing of fuel and oxygen will not result into a reaction which is described as a null reaction. (Quintiere, 2004; Epstein, 1988). Some of the products that are likely to be produced in the instantaneous reaction model include CO2 H2O, CO and soot this being proportionate to the rate of consumption of fuel. It is therefore necessary that when giving the product yields the mass of fuel involved need to be put into consideration (Thomas, 1981; Bishop et al. 1992). There are numerous experiments which have been performed whose main aim has been to have an assessment of the major and minor carbonaceous species and the amount of soot present at specific time intervals in the upper layer of fire of the room (; Merci , B & Vandevelde, 2007 ).

Law enforcement agents, fire researchers, fire engineers and regulatory bodies make use of fire models like NIST fire dynamics simulators in the design process and in the analysis of safety measures fire where instances like post fire forensic applications and in the determination the quantity of fuel consumed in the fire process (Drystlale et al, 1985). Understanding of compartment fire behaviour is important as this is a window to being able to make predictions on the impact fire is likely to have on structural elements.

Read More