The Development of Compartment Fires Understanding - Assignment Example

FIRE IN BUILDING By Student’s name Course code and name Professor’s name University name City, State Date of submission Table of Contents Table of Contents 2 Part A. (LO 1-3) Experiment Write-up 3 Part B. (LO 1) Heat transfer 12 Part C. (LO 1) fluid flows 15 Part D. (LO 4) Fire Supression 21 Part E. (LO 1) Heat transfer 24 Part F. (LO 1) Electronics 26 References List 28 Part A. (LO 1-3) Experiment Write-up Aims The aims of this experiment are derived from the fact that compartment fires experience various stages or phases. These include the ignition stage, growth stage, flashover stage, fully developed stage and finally the decay stage in that order. It has been noted in various experimental setups by various scholars in the field of fire control that during the early stages of development the compartment parameters usually have no effect on the growth. Some of the observable characteristics; smoke grows producing hot gases, forming a layer is at the ceiling level and exits through the existing compartment openings, generation of smoke which progresses steadily and in cases where there are no openings the generated smoky descends to the floor, fire is likely to grow to a full blown stage leading to the flashover stage in event of fuel availability or exhaustion automatically sets in leading to the decay stage. While this experiment allows the development of compartment fires’ understanding, it aims at assessing the development from the ignition stage through to the decay stage. This understanding shall be brought forth by setting up experiments to observe the effects of controlling fuel and limiting ventilation in two separate instances. Apparatus A firebox with the nominal internal dimensions of 0.65mx0.34mx0.38m was deployed for this small scale modeling experiment. The firebox was constructed to prototypical of apartment fundamentals i.e. with a roof, walls and a floor. The external layer was modeled from a 0.025m thick of a monolux 500 layer while one of the walls was constructed from a fire resistant glass. The firebox consisted of an adjustable 225mm high door designed to open up to 0.15m wide The firebox floor was also designed in such a way that the axle supporting fuel was passed through a 0.45m opening. Further to this, the firebox was fixed to a steel frame mounted with wheels for ease of handling. The firebox contained three preinstalled columns with four thermocouples spread through the rear wall to the floor hole. Figure 1: Side elevation with the indicative dimensions. Figure 2: Front elevation showing nominal equipment dimensions. Figure 3: Front elevation of firebox with the indicative dimensions. Thirteen thermocouples were also deployed for carrying out temperature measurements in this experiment. These thermocouples were classified type ‘K’ as they were made of nickel - aluminum and nickel – chromium covered with stainless steel sheet. While one thermocouple was installed on the protruding area on the floor hole, the remaining ones were deployed on the rear wall of the experimental compartment setup. The thermocouples setup were projected into the firebox at approximately 200mm and positioned on the three columns located 0.15m, 0.315m and 0.48m from the compartment opening with connection to a squirrel data loggers meant for recording the temperatures. Methodology This experiment utilized polymethyl methacrylate (PMMA) square slabs as small scale compartment fire fuel as they are widely used in this illustration. The fuel samples to be used were of 0.012m (12mm) or 0.25m (25mm) thickness and approximately 100mm X 100mm, or 150mm x 150mm or 200mm x 200mm. After the measurement of all the internal dimensions of the firebox for verification purposes, the sample location was verified and recorded to ensure the integrity of the experiment. All thermocouples inside the firebox were cleaned using a paper towel so as to remove the excess sooth from previous experiments. These were then connected to the squirrel logger and recordings done for the data logger and thermocouple channel relation. The balance was switched on and an appropriate sample tray measured for tare weight after which the ‘tare’ button was pressed. A single PMMA sample at a time was measured for mass and recorded accordingly. All thermocouples were straightened and the so as to protrude into the firebox and the squirrel data logger started in readiness for the experiment. A PMMA sample whose weight had been verified was then ignited using a lit taper while ensuring personal safety. The vent opening was immediately adjusted to a desired size and a small piece of paper placed in front of the box to observe any signs of flashover during the experiment. The height of the smoke layer, mass of PMMA sample and any other observations were recorded concurrently at a span of 1 minute. The back draught was recorded at 15-20 minutes from ignition for all experiments carried out. The data logger was stopped once the experiment was over. What was measured and accuracy The data obtained was tabulated as shown in the sample table 1 below for weight loss against time recorded at a span of 30 seconds for the 100x100mm and weighed approximately 140g prior to commencement. Temperature increment was also recorded against time at an interval of 10 seconds each for all the thermocouples in both experiments and this data logged accordingly. The data obtained was accurate since the parameters of this experiment were handled in the most accurate manner to ensure the integrity. This can also be concluded right from the manner in which the analysis graphs came out in a desirable manner in comparison to the existing studies. Table 1: Excerpt of data obtained from the 100mm x 100mm whose PMMA Sample weighed 140 grams prior to ignition. Time (seconds) Weight (grams) Time (seconds) Weight (grams) 30 139.4 1230 82.6 60 139.2 1260 79.4 90 139.2 1290 76.2 120 139.0 1320 72.6 150 139.0 1350 69.2 180 139.0 1380 66.2 210 138.6 1410 62.8 240 138.6 1440 59.6 270 138.6 1470 56.6 300 138.4 1500 53.6 330 138.2 1530 50.6 360 138.0 1560 47.6 390 137.8 1590 44.6 420 137.4 1620 42.0 450 137.0 1650 39.0 Data obtained The data obtained and tabulated as shown in the sample above was then converted into graphs for analysis and comparison purposes as shown in the analysis section below. Timeline of the fire As expected, mass loss of the sample in the 200x200mmm experiment was faster than that of 100x100mm opening owing to the availability of the paramount ingredient of fire which in this case is oxygen. The time consumed in exhausting 140g of PMMA fuel sample in 100x100mm opening was approximately 2500s as per our graph while that of 200x200mmm compartment opening only consumed 1500s which makes a major difference between the two scenarios. Personal observations Smoke in 100x100mm opening was more than in 200x200mm ventilation and fire went off severally before re-lighting up on its own. Again the development of fire was slower in this case as compared to 200x200mm whose fire developed quicker due to the availability of oxygen. The temperatures in 200x200mm were also seemingly higher than those of 100x100mm due thereby consuming a higher weight of sample quicker as is portrayed in the graphs were drafted from the data achieved. Explanation & analysis of data This experiment was typical of the fire development phases as observed from the temperature increase against time graphs shown below. The incipient stage in both cases was experienced once the PMMA fuel samples had combined with oxygen accelerated by heat in a chemical reaction that resulted to fire in what is referred to as ignition. This was then followed by the fire growth stage in which the two experiments saw a steady increase in temperature thus leading to thermal layering and the deadly flashover. The fully developed stage was experienced with observations such as rapid fuel consumption and peak temperatures. The decay stage then experienced with a significant reduction in the three component of fire i.e. oxygen, fuel and heat thus leading to a smoldering effect on any existing combustibles (National Fire Protection Association, 2005). Figure 4: Graph indicating combustion temperature increment with time of for 100x100mm compartment opening. Figure 5: Graph indicating combustion temperature increment with time of for 200x200mm compartment opening. Figure 6: A graph of fuel mass loss against time for 100x100mm ventilation. Figure 7: A graph of fuel mass loss against time for 200x200mm ventilation. Evaluation of the experiment This experiment achieved its objectives in enhancing compartment fires knowledge in relation to the size of the opening. On the other side, the four phases of fire were also encountered as part of this study, with the flashover phenomenon being observed. While it is evident from common knowledge that a larger opening i.e. as seen in the case of 200x200mm opening is likely to lead to quicker consumption of fuel due to availability of oxygen it may be though that temperatures in such fires are low. However it has been ascertained beyond no reasonable doubts that fire in larger openings or ventilations are more dangerous than smaller openings due to sudden flashover and higher temperatures that are experienced therein. Mass loss in these two experiments was also observed to be inversely proportional to time in both experiments which is the reason behind the direct proportionality of temperature to time up to the fully developed stage. After that it becomes inversely proportional due to diminished fuel and exhausted heat release rates (HRR). Part B. (LO 1) Heat transfer The underside of a smoky layer 10m x 8m is radiating like a flat, isotropic plate at 475C to the floor of a compartment 1.20m below. The mean emissivity is 0.35 and the floor is homogenous/flat plate at 48C. What is the rate of heat transfer from the smoke to the floor? Useful figures & formulae: (1) (2) Where and Figure 8: Parallel plates. Solution Basic dimensions required to solve the rate of heat transfer; , and For heat transfer, total area of the compartment has to be considered since heat is being emitted in all directions. Therefore; While direction to which heat transfer is being evaluated; Solving for and ; Solving equation 2 by substituting values of and ; To carry out the heat transfer we deploy formulae (1) below; Stefan-Boltzmann constant Therefore; (Schauer, et al., 2005) Part C. (LO 1) fluid flows a) Critically discuss the “ventilation parameter” and how it relates (if at all) to the mass flow at the doorway of a fire-compartment, to the neutral plane, and to the thermal discontinuity plane. b) The governing equations of a fluid dynamics model are shown below. Explain the physical sense of each term. Reference: W.P.Jones, Imperial College February 1999 “turbulence modelling” “Ventilation Parameter” The ventilation parameter on the burning area has been known to affect the mass burning rate from experimental set ups for this study as simple as the reduce scale models. The basic parameters widely known to affect the burning behavior of compartments have been documented over time as the opening conditions, fuel properties, the fuel bed area, fire position and size of the compartment. The effect of ventilation size and shape can only be represented by height (H) and area (A) which are the basic dimensions expected for any opening. Studies carried out in the have in often cases indicated that fuel and ventilation controlled fires are quite different in that gravitational forces, ambient air density and the fuel surface area set in (Fenghui, 1988). The ventilation parameter has an outstanding effect on compartment fire in the way that burning fuel is transformed through various phases of fire thus the need to establish its relationship with mass flow at the doorway of a compartment, neutral plane and the thermal discontinuity plane. The ventilation parameter is associated with mass flow at the doorway of a compartment in that the two vary in accordance with other in a direct proportionate manner. The mass loss is thus reduced when the ventilation is reduced and vice versa as caused by vitiation in oxygen. The burning rate is thus controlled by the mass flow which also emits less heat flux thus reducing mass loss rate as shown in the graph below obtained in studies by Mizukami et al. (2008). ` Figure: A graph indicating compartment mass flow rate in relation to the ventilation parameter . Thermal discontinuity and the neutral plane vary with direct proportionality with the increasing size and height and positioning of the ventilation. This means that having a larger ventilation parameter would result to a more transient behavior in compartment fire with flashover results in compartment fires. The case of thermal discontinuity and the neutral plan with respect to the ventilation parameter is such that they both coincide during the flashover stage thus reducing the efficiency of subsequent combustion in case of continued fire (Barham, 2006). The Governing Equations of Fluid Dynamics Model The law of conservation of mass when applied to a fluid flows past an infinitesimal fixed control volume yield the equation of continuity; In the equation above, ρ is used to denote the density of the fluid while U represents the fluid velocity at any given time. Increase in density in the control volume over time is also represented by the first term of our equation above. The second term of the equation represents the mass flux rate per unit volume due to the surface that surrounds the control volume. The figure below shows a typical Eulerian approach that is used to arrive at this vital fluid dynamics equation; Figure 9: Eulerian approach. In utilizing this approach, the volume is fixed and the flow recorded respectively. In the Lagrangian approach, changes in fluid element properties are recorded while applying the Cartesian coordinate system to arrive at the velocity vector which is in turn utilized to arrive at the continuity equation, while keeping in mind the laws of conservation in play. The divergence conservation law is also applied as an alternative when the fluid element is incompressible. Thus the equation below is used for incompressible fluids; The second Newtonian Law of momentum has been used to obtain one of the most important equations of fluid dynamics modeling. Increase of momentum per unit volume is represented by the first term of this equation while the rate at which momentum loss occurs is symbolized by the second term and both of these are derived under a strictly controlled volume. Generally, this equation can be applied for both non-continuum and continuum flow where shear-stress tensor loses are not put into consideration to a great extent. The energy equation is derived from the first law of thermodynamics for a fluid passing through an infinitesimal volume. Both sides of the equation represent the rate at which energy increases or decreases while keeping volume controlled. In the right hand side equation it can be observed that energy losses are catered for due to decreased velocity and is represented in terms of total energy lost by convention through the control surface indicated in the figure above for Eulerian approach of deriving these continuity equations. The significant heat loss or gain through conduction to or from the control surface is also catered for in the same equation as a different form of energy which is also important in this study (Pletcher, Tannehill, & Anderson, 2013). Mass conservation can be conservative of non-conservative; these equations are obtained through application of the underlying physical principles such as density and volume and their respective variations due to the control surface. The non-conservative bits of the equation for mass conservation are therefore obtained by considering the fluid elements in the field of flow. The equation below represents this discussion; Lastly, for flows in which chemical reactions are likely to take place, it is assumed that the temperatures at any given time are relatively low to permit explosions and for a gas, it is supposed to be calorifically perfect. Increase in temperature renders the gas calorifically imperfect even when vibrations exist on the surface of flow. Gas dissociation at sea level is estimated at approximately 4000K and it is possible for fluid dynamics to be estimated no matter the condition at any given time hence the auxiliary equations are required. In cases where chemicals react with each other, equilibrium or non-equilibrium conditions may be observed. Where reaction rates are zero, the rate of production of byproducts that may affect the fluid dynamics is set at zero thus the mixture is said to be frozen (Pletcher, Tannehill, & Anderson, 2013). Part D. (LO 4) Fire Supression a) Analyse three conditions essential for combustion and fire (the fire triangle). Specify three associated methods of fire-fighting and relate these methods with the action of water, foam, or neutral gas. b) Critically review different mechanisms of fire extinguishment (cooling of flame, reduction of fuel and/or oxygen, and interference with combustion reactions). c) Review fire protection using water. Analyse the reason for Halon phase-out. The Fire Triangle Fire is a chemical reaction where oxygen combines with a combustible substance in the presence of ignition to release heat. On the contrary, while solids and liquids are expected to produce dioxides and monoxides of their respective compositions due to the oxidation process facilitated by the presence of oxygen, there are other materials such as vapor which may be produced depending on the material’s moisture content. From the above statements, it is true to state that fire can only be developed under certain circumstances which are also seen to be a combination of forces. This hence leads to fire being referred as an adiabatic planar combustion wave in which three conditions are capitalized. These conditions are limited to fuel, oxygen and heat also known as fire triangle and if one of them is missing at any given point then fire shall not occur (Mannan, 2014). Figure 1: Fire triangle (Mannan, 2014). The above schematic diagram of a fire triangle clearly shows the requirements for fire to occur. In order to start a fire there must be fuel which in this case should be readily combustible such as plastic, oil, gas, wood etc. The combustibility in this case is affected by characteristics such as the shape, moisture content and its quantity. A heat source according to our triangle is necessary since it represents ignition on the combustible medium. Heat aids in the spread of fire as the temperature of the medium rises giving off flammable vapors. As fire spreads, temperature rises leading to the preheating of the surrounding areas enhancing its ease of spread and the respective travelling path. Apart from the two essential conditions needed to cause fire, oxygen is required for it to stay alight. While atmospheric air consists of 21% of oxygen, it is necessary to note that oxidation during combustion only requires 16% in terms of composition to keep it going (Elite Fire, 2013). Fire-Fighting in Relation to Action of Water, Foam, or Neutral Gas Fire-fighting mechanisms basically work in line with the fire triangle in that one basic element has to be eliminated. It is also important to note the fire elimination mechanisms such as smouldering of fire after it runs out of fuel, suffocation on eliminating oxygen and finally loss of heat due to cooling down of fire. It is therefore clear that any trials directed towards elimination of fire should be based on the essentials. In relation to water, it will be noted that heat is lost on cooling the temperatures of the fuel hence stoppage. On the other hand, introducing foam shields or insulates the fuel bringing forth the extinguishing action. Lastly, the neutral gas further dilutes oxygen below the 16% threshold thereby leading to extinguishing of fire (Elite Fire, 2013). Critically Review Different Mechanisms of Fire Extinguishment The main approaches towards fire extinguishment are well represented by cooling of flame, reduction of fuel and/or oxygen, and interference with combustion reactions ( also known as chain reactions). In removing heat, an absorption medium must be applied to act as a heat exchanger. While water is the most commonly utilized medium, it may not be utilized on all fuels depending on the possible resultant chain reaction. While fuel removal may not be practical in most scenarios which mostly involve gases and liquids, shutoffs are carried out in to cut down the flammability. Removal of oxygen basically involves dilution or displacing it with inert gases such as nitrogen or carbon dioxide thereby reducing its concentration in air. Lastly, interrupting the chain reaction is the most recent technology applied in the modern extinguishing agents. This involves introduction of halogenated agents and other dry chemicals which are meant for suspending or bonding with free radicals hence cutting off the chain reaction (Voelkert, 2009). Fire Protection Using Water Protection of premises using water is one of the cheapest option in the market as compared to gaseous substances and modern revolutionary methods. Further, water is a safe method that does not have any toxic effects that may lead to asphyxiation, low documented damages, no environmental impacts and highly efficient in fire suppression. Early studies championing the use of water as a fire suppressant optimized the droplet parameters to be used in the water mist and jet applications. Water mist has seen the usage of tiny droplets as very efficient in the suppression of mist explosions for example. It is however unfortunate that some of the chemicals introduced for usage with water suppressants such as Halon 1211 and Halon 1301 had to be phased out during the development stage due to the negative environmental effects that they posed to the environment (Hurley, et al., 2015). Part E. (LO 1) Heat transfer The mean thermal inertia of skin has been estimated as. Formula (3) estimates the surface temperature of skin exposed to a constant heat flux: (3) Reference: 1995 Physiol. Meas. 16 213 (http://iopscience.iop.org/0967-3334/16/4/002) How would the “thermal penetration depth” of skin vary with time for someone in a developing room-fire environment? If the heat flux to the skin was a steady 100 kJ m-2 s-1 when would a person with normal pain threshold and skin texture experience pain and a burn: Data from NIST: http://www.nist.gov/fire/fire_behavior.cfm º C Response 37 Normal human oral/body temperature 44 Human skin begins to feel pain 48 Human skin receives a first degree burn injury 55 Human skin receives a second degree burn injury 62 A phase where burned human tissue becomes numb 72 Human skin is instantly destroyed Solution The mean thermal inertia is estimated as; While; Normal body temperature; Temperature at which human skin begins to feel pain; It will take normal human skin 0.0111s to start feeling pain. Human skin experiences a burn at 48º C i.e. Normal body temperature; Human skin receives a first degree burn injury; A human skin would therefore experience a burn at 0.027s when exposed to a heat flux of . Part F. (LO 1) Electronics An extension lead is 2m long and carries current down a 2.5 mm2 copper conducting wire, driven by a 230V potential difference. If the lead is drawing 13 Amps current and 0.1% of the energy is lost to heating the wire, how hot would it get after an hour? Justify any assumptions you make. Hint: Solution (1) Where; V is the voltage passing through the conductor, I is the current passing through the conductor, t is time, m is mass also a product of volume and density of material, c is the specific heat capacity and is the change in temperature. Rearranging equation (1) above we get; (2) Calculating mass; Imposing energy loss coefficient on the solution above; References List Barham, R. (2006). Fire Engineering and Emergency Planning; Research and Applications. New York: Taylor and Francis. Elite Fire. (2013, April 25). Back to Basics with the Fire Triangle. Retrieved from Elite Fire: http://www.elitefire.co.uk/news/basics-fire-triangle/ Fenghui, J. (1988). A simple method for prediction of mass burning rates in compartment fires. International Association for Fire Safety Science, pp 404-409. Hurley, M. J., Gottuk, D. T., Hall, J. R., Harada, K., Kuligowski, E. D., Puchovsky, M., et al. (2015). SFPE Handbook of Fire Protection Engineering. In M. Hurley, A review of water mist fire suppression systems - fundamental studies (pp. pp. 32-50). New York: Springer. Mannan, S. (2014). Lees' Process Safety Essentials: Hazard Identification, Assessment and Control. Oxford: Butterworth-Heinemann. Mizukami, T., Utiskul, Y., Naruse, T., & Quintiere, J. G. (2008). A Compartment Burning Rate Model for Various Scales. Fire Safety Science-Proceedings of the Ninth International Symposium (pp. pp. 839-848). International Association for Fire Safety Science. National Fire Protection Association. (2005). User's Manual for Nfpa 921: Guide for Fire and Explosion Investigations. London: Jones & Bartlett Learning. Pletcher, R. H., Tannehill, J. C., & Anderson, D. (2013). Computational Fluid Mechanics and Heat Transfer, Third Edition. London: CRC Press. Schauer, C., Mazzolani, F., Huber, G., Matteis, G., Trumpf, H., Koukkari, H., et al. (2005). Improvement of Buildings' Structural Quality by New Technologies: Proceedings of the Final Conference of COST Action C12, 20-22 January 2005, Innsbruck, Austria. Estimating temperatures in compartment fires (pp. 171-187). Innsbruck, Austria: Taylor & Francis. Voelkert, J. C. (2009). Fire and Fire Extinguishment: A brief guide to fire Chemistry and Extinguishment theory for fire equipment service technicians. Retrieved from Amerex Fire: http://amerex-fire.com/wp-content/uploads/2011/12/Fire_and_Fire_Extinguishment.pdf Read More