Fire in Building - Assignment Example

ASSIGNMENTS by Student’s Name Code + Course Name Tutor’s Name Institution of Learning City, State Date Part B (LO 1) Heat transfer Given and Where and We are given: A = 8m, b = 10m, z = 1.20m, TH = 4750C, TC = 480C Therefore; X = a/z =8/1.2 = 6.667 and Y = b/z = 10/1.2 = 8.333 Calculating all the components in the F12 Formula: Arctan Y = 830 Therefore, Y arctan Y = 8.33*83 = 691.4 Arctan X = 810 Therefore; X arctan X = 81*6.667 = 540 Starting with the last component of the equation; Y/(1+X2)1/2 = 8.33/(1+6.6672)1/2 = 1.19 Arctan Y/(1+X2)1/2 = 49.950 Y(1+X2)1/2 = 83*6.74 = 559 X/(1+Y2)1/2 = 6.667/(1+8.332)1/2= 0.79 Arctan X/(1+Y2)1/2 = 38.30 X(1+Y2)1/2= 6.667*8.39 = 56 = = ln 5.2= 1.66 2/πXY = 2/(3.14*6.667*8.33) = 174.4 F12 = 174.4 [1.66 + 56*38 + 559*49.95 – 540 – 691] = 5.0 x 106 w/m2 Part C (LO 1) Fluid flows The ventilation parameter (AH)1/2 This segment critically describe the ventilation parameter (AH)1/2. The parameter is resultant from the examination of mass flow into a fire partition. The mass flow is indicated by If there is a variation of velocity over height h of the compartment, then Comparing the potential and kinetic energy at a specific height above the ground we get From which Hence Integrating this gives The ventilation parameter is then attained from The ventilation parameter is said to be proportional to the mass flow at the door way of the fire compartment and possess no relationship with the neutral plane and the thermal discontinuity plane. This section outlines the physical sense of each term in the governing equations of a fluid dynamics model. The momentum equation: This is obtained from the Newton’s first law F=Ma, Where F is the force, M is the Mass, and a is the acceleration. In the momentum equation below; The quantity Represents the force F of the fluid while mass M of the fluid is given by m = pdx dy dz And the acceleration along x direction is given by The Energy equation It is based on the first law of thermodynamics: the rate of energy change of a fluid particle is equal to the rate of heat addition plus the rate of work done (Tu, Yeoh, & Liu, 2013). = + The left-hand quantity is the rate of change of the energy inside a fluid element. The first quantity on the right-hand side (-div(pu)) is the net flux of the heat into the element while the last quantity is the rate of work done on the element due to body and surface forces (Zikanov, 2010). The continuity equation The first term indicates the change in density of the fluid while the second term is the convective term for the Net flow of mass across boundaries. The mass balance equation The equation symbolizes the rate of accumulation of mass that is equal to the difference between the mass inflow rate into the system and mass outflow rate from the system. Part D (LO 4) Fire suppression The three conditions essential for combustion and fire (the fire triangle) are oxygen, fuel, and heat. These elements must combine in a precise way for the fire start and keep burning. The concept of the fire triangle was developed by scientists to help understand the causes of fire and how to fight it. Heat is the source of ignition and includes anything that can emit a spark or a flame. Fuel refers to any combustible material such as paper, wood, flammable liquids and flammable gasses, etc. The source of Oxygen is the air in the atmosphere (Corbett, 2009). There are three associated methods of fire-fighting related with the action of water, foam, and neutral gas. These include starvation, smothering and cooling. Starvation involves cutting out the supply of fuel, smothering separates fuel from the oxidant, while cooling lowers the temperature and is usually done with water. Because foam is a stable mass consisting of bubbles and has a lower density than oil and gasoline, it is suitable in blanketing the surface of the fuel and hindering the burning process. Using an inert gas is a way of starvation, or simply cutting the oxygen supply. Carbon dioxide is usually used for starvation. This section reviews the different mechanisms of fire extinguishment (cooling of flame, reduction of fuel and oxygen, and interference with combustion reactions). The cooling is achieved using water. The water is directed towards the seat of the burning fire at a high pressure. Buildings are equipped with automatic sprinklers that run automatically when the pressure is above a set value. Others are hose reels that are connected to a water source ready for the fire emergency. There are also portable water extinguishers. Water can only be used to extinguish class A fires (Fires caused by solid fuels). Organic fuels have a lower density than water and will float and continue burning (Corbett, 2009). It cannot be used to fight electrical fires because it can case a short circuit, making everything electrically live. Reduction of oxygen or supply of fuel is achieved by use of foam or carbon dioxide. Foam is made by dissolving surfactants that give water surface properties. When pressure is applied on the mixture, a foam is formed. Several surfactants based on hydrocarbons, fluorocarbons, and hydrolyzed protein are used. The formulation varies with the class of the fire to be extinguished. Foam based fire extinguishers are used in extinguishing class A and class B fires. Carbon dioxide is also used in the reduction of oxygen supply to the fire. It has a triple point (the temperature and pressure where the three states of matter coexist) of 5 atm and -570C, compared to 6x10-3 atm and 00C of water. As a result, the carbon dioxide is an idea firefighting material compared to water. It is kept is pressurized cylinders in liquid form, and as the valve is opened, there is rapid evaporation that gives a cloud of very cold carbon dioxide. Because the gas is denser than air, it forms a blanket that covers the burning material. The carbon dioxide extinguishers cannot be used on Class A fire because the pressure can disperse the burning pieces spreading the fire. Interference with the combustion reaction is achieved using BCF (Halon 1211) and dry powders. The BCF itself does not burn and has a boiling point of -4 degrees Celsius, making it easily liquefied by atmospheric pressure at room temperature. It has good smothering properties and comes out with low pressure than that of carbon dioxide. The BCF consumes the free radicals in the flame, terminating the propagating chains of reaction (AICE, 2005). BCF extinguishers are suitable for all classes of fires. BCF would be phased out in the light of Montrial protocol because they have a depleting effect on the ozone layer. Dry powders include dry chemicals for class B and C, which have sodium bicarbonate as the major constituent. Others are multipurpose dry chemicals for classes A, B, and C with mono-ammonium phosphate as the major constituent. The powders are contained in the containers and are driven out by either carbon dioxide cartridges or pressurized nitrogen. Sodium bicarbonate undergoes an endothermic equilibrium reaction, where the equilibrium constant increases with temperature. The endothermic reaction takes up the fire heat, producing carbon dioxide and water. The sodium bicarbonate, therefore, extinguishes by absorbing the fire heat and giving out byproduct that cause a smothering effect (AICE, 2005). Fire suppression by water is due to the heat of vaporization. The molar heat capacity of water is 75J/K/Mol while its specific heat capacity is 4.18J/K/g. One liter of water will, therefore, absorb about 313kJ of heat in changing temperature from 250C to 1000C. The molar heat of vaporization of water is 42kJ/mol. Therefore, one liter of water at 100 degrees Celsius will absorb 2300kJ on complete vaporization. Part E (LO 1) Heat transfer The surface temperature over the skin can be estimated from(Herman, 2007): T0 We are given: Ts = 317K (the temperature taken as 44 degrees since we are looking for pain and burn) T0 = 310K (normal body temperature) Q = 100w/m-2/s (assumed to be constant over time) β = 1.7 kWs1/2m-2K-1 t = ? Therefore, the time the person will feel the pain and burn under constant heat flux of 100w/m2/s is found by (Herman, 2007): )2 Where Ts = 317K (the temperature taken as 44 degrees since we are looking for pain and burn) T0 = 310K (normal body temperature) Q = 100w/m-2/s (assumed to be constant over time) β = 1.7 kWs1/2m-2K-1 Therefore; )2 = 11122 seconds or 11122/3600 = 3 hours This means that the person will feel the pain burn after 3 hours if he continues to stay at 100w/m2/s Temperature (0C) Human skin response Time in Seconds Time in hours 44 The skin begins to feel pain 11,122 3 48 The skin receives a first-degree injury 27,453 7 55 The skin receives a second-degree burn injury 73,513 20 (Source: NIST) The calculations show clearly that the thermal penetration depth of the skin varies with time. For instance, in a room with low heat, the skin begins to sweat almost at 37 degree Celsius. This increases fast as the skin temperature increases and the time duration of the fire continuously in a developing room fire environment. In this case, the human skin performs the thermal thick for the heat, and an adult can extinguish the fire as soon as there is a feeling of pain at 43 to 44 degrees Celsius within seven hours. On the case of a big fire, the human skin performs thermal thin and receives first-degree burn injury at 48 degrees Celsius within seven hours. The thermal penetration depth varies from child, adult, and older people skin. Part F (LO 1) Electronics We are given; V = 230V, I = 13A, Density of copper, p = 8960 kg/m3, specific heat capacity of copper = 390 J/Kg/K, Volume of the copper wire = 2*2.5*10-6 = 5*10-6 M3 The mass of the copper wire is therefore given by; Density * Volume = 8960 * 5*10-6 = 0.0448Kg Energy dissipated E = VIt (time is one hour)(Beaver& Powers, 2010) E = 230 x 13 x 3600 = 10,764,000J 0.1% of this energy is lost in heating the wire. This is equivalent to 0.1% *(10,764,000J) = 10,764J It is assumed that the wire was initially at room temperature. Change in temp is given by VIt = mcθ, where θ is the change in temperature (Beaver & Powers, 2010) Therefore; 10764 = 0.0448*390 θ 10,764/17.472 = θ Which gives θ = 616K The final temperature is therefore given by 616-298 = 318K or 450C Note: Initial temperature was assumed to be the room temperature because copper is a good conductor (298K) References American Institute of Chemical Engineers (AICE). (2005). Guidelines for fire protection in chemical, petrochemical, and hydrocarbon processing facilities. Hoboken: John Wiley & Sons. Beaver, J. B., & Powers, D. (2010). Electricity and magnetism: Static electricity, current electricity, and magnets. United States: Mark Twain Media. Corbett, G. P. (2009). Fire engineering's handbook for firefighter I & II. Tulsa (Okla.: PennWell. Herman, I. P. (2007). Physics of the human body. Berlin: Springer. National institute of science and technology (NIST) (January 2016), Fire dynamics. Retrieved http://www.nist.gov/fire/fire_behavior.cfm Tu, J., Yeoh, G. H., & Liu, C. (2013). Computational fluid dynamics: A practical approach Zikanov, O. (2010). Essential computational fluid dynamics. Hoboken, N.J: Wiley. Read More