Heat Transfer, Fluid Flow, Fire Suppression, and Heat Transfer - Assignment Example

COVER PAGE Table of Contents Part B. (LO 1) Heat transfer 3 Part C. (LO 1) fluid flows 6 Part D. (LO 4) Fire supression 12 Part E. (LO 1) Heat transfer 13 Part F. (LO 1) Electronics 16 References 1 Part B. (LO 1) Heat transfer (1) (2) Where and Figure 1. Parallel plates The formulae to be used (1) Where  is the view factor between the two surfaces where heat is being transferred?  Stefan Boltzmann Constant, 5.67x10-8 W/m2/K4  is the temperature of the hot surface in this case 475C  is the temperature of the cold surface in the current case 48C A is the area of the radiating surface in the case 10x8 Now to calculate the view factor the formula to be used is (2) Where = 8/1.2= 6.67and = 10/1.2=8.33 Now applying the formula by first calculating the components in the brackets independently the results will be 1- = 1.66 2- 6.67 3- 8.33 4- 6.67*tan-1 (6.67) = 543.43 5- 8.33*tan-1 (8.33)= 692.68 Now substituting for the components in the bracket F12=  Then applying Q= (7.47)(0.28)(0.56x10-8)(8x10)((475+273)4-(48+273)4) Q= (7.47)(0.28)(0.56x10-8)(8x10)((748)4-(321)4) Q= 288.129 kW 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. (10 marks) The term “”is what commonly is referred to as ventilation parameter or ventilation factor. This was brought forward by Kawagoe in 1958 as a means of evaluating post-flashover fire test data by flow analysis of an opening. The equation originated from an experiment whose implementation involved use of wood as fuel rooms with small windows (Babrauskas,1995). Figure 2 There is creation of a pressure difference between the outside and inside compartment when flow cross the vent opening and the variation of pressure comes as a result of temperature difference. The natural plane is a point within the vent opening where the pressure difference is zero. With the knowledge of pressure differences across the vent opening and Bernoulli’s being applied the distribution of velocity is obtained from. With the pressure difference and velocity being varied with height, the inner and outer mass can be contained through integration of velocity in the vent opening height starting from the neutral plane point, multiplying by the relevant density for the air, coefficient of discharge and the breadth of vent. Estimation of flow is achieved by application of Bernoulli’s Eq. (4) in the upper opening between point 1 and point 2, as can be seen in figure, (4) With the assumption that the velocity of the inner flow (air),=0 and = = Through substitution of the previous equation for both and gives (5) Equation (5) represents the air (inflow) velocity () at the bottom vent. Similarly, by application of the previous steps on the point 3 and 4 the pressure at point 4 is same as the ambient pressure, with negligible the hot gas velocity Substitution of the previous equation for both and gives (6) Because of the velocity variation in of the inflow and outflow at vent opening with the height from the neutral plane, the mass flow rate ()of the inflow and outflow is obtainable by use of the integration as follows. (7) With= the opening area m2, = the flow density kg/m3 By substitution in the previous equation for , and A=Width (w). Height (Z) In a similar fashion the mass outflow rate, (8) With two unknown heights hu and hl, they can be expressed as H=H0=hu+hl. Now by the equating the mass flow for the air and gas, we write: (9) Place Eq. (7) and (8) into Eq. (9), we get (10) But hu= H0-hl, we find (11) Now the mass flow of the air will be written as: When the vent area A=W.H0, the above equation can be modified as (12) With assumption that the gases are ideal, where the density is inversely proportion to the temperature, The term is the density factor and it has relation with the temperature ratio (Tg/Ta), with the being taken as ambient temperature is 293K, the ratio (Tg/Ta) =2.72 and the density factor will be 0.214, now Eq. (12) can be re-written as With use of the standard values of = 0.7, = 9.8 m/s2 and = 1.9 kg/m3, for a typical steady post-flashover H=0.5H.Now we can get the equation which expressed the relationship between the mass flow enter to an opening and the ventilation parameter at the natural plane:  (13) b) The governing equations of a fluid dynamics model are shown below. Explain the physical sense of each term. (10 marks) Considering a volume V that is bound by a surface S where the volume is fixed in space.the mass inside the surface is given by  Such that the rate at which mass decreases in  (1) The rate of mass flux out of (2) Where green’s formula is used in conversion to volume integral The integrand  may be expressed in Cartesian coordinates X=(x,y,z), V=(u,v,w) So that we have  (3) In order to have conservation of mass equation 1 and 2 are to be equal And this gives the continuity equation  Figure 3 There is introduction of surface stresses on a fluid element through the constitutive relation. The physical interpretation of this is There is generation of viscous stresses through velocity gradients which result to opposition of relative motion in the fluid elements. With reference to figure 2 across any of the plane AB, there is the action of a tangential viscous stress  where there is dragging of the fluid that is below AB by the faster fluid above AB while the fluid below drags the above fluid back (Bird, 2004). Even when there is no existence of velocity gradients, isotropic pressure will be felt by each of the elements. When in rest at the equilibrium, his would be equal to the thermodynamic pressure with regards to equation of state. Is the rate of change of mass of moving fluid element per unit time. With mass being conserved it can be stated that the time rate of change of mass for the fluid element will come to zero as the fluid is moving alongside the flow (Jones, 1999). Is the rate of change of energy of moving fluid element per unit time in the direction of motion of the fluid under consideration. This involves the application of the first law of thermodynamics to moving fluid elements. Here we have dissipation rate being dependant on the boundary conditions. The temperature at the boundary and pressure will determine the density of the fluid which will in turn is used in determination of dissipation rate at point A. 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. (5 marks) In order for fire to start we need to have to components: oxygen, fuel and source of heat. If any of the three is missing then fire will not be started and if there is fire, then if any of these three is taken away then the fire will go out (IFSTA ,2008). Figure 4: Source:https://www.google.com/search?q=fire+triangle&biw (b) Critically review different mechanisms of fire extinguishment (cooling of flame, reduction of fuel and/or oxygen, and interference with combustion reactions). (10 marks) The three method of fire fighting are through cooling, smothering or by removal of the fuel. When water is used to extinguish fire most of the available heat is removed when water is converted to vapour. This is because evaporation process involves high level of energy absorption (Tewarson, 1995). Water also extinguishes fire through smothering where the vapour generated is able to substantially dilute the oxygen that is required for combustion and thus resulting to the fire being extinguished. Foam extinguishes fire by creating a layer between the fuel and oxygen supply which means oxygen is eliminated and thus fire being extinguished. (c) Review fire protection using water. Analyse the reason for Halon phase-out. (5 marks) As it has been seen by use of water extinguishing of fire is both by cutting off the supply of oxygen (asphyxia) and cooling effect. Cooling is found to be the most essential element is mastering a fire when it comes to the case of closed area. The water to be used in fire fighting may be accessed through pressurized fire hydrant, where pumping will be from sources such as rivers or lakes, delivery by water tanker trucks, dropped from water bombers. In 1960s Halon 1211 was seen to be best solution in fire fighting. This view however changed after realization that it had a problem of causing ozone layer depletion. The halon phase-out was declared under Montreal Protocol and it was replaced by what is referred to as zero ozone depletion potential (ODP) (Safelincs limited, 2015). Part E. (LO 1) Heat transfer The mean thermal inertia of skin has been estimated as 1.7 kW s1/2m-2K-1. Formula (3) estimates the surface temperature of skin exposed to a constant heat flux: (3) How would the “thermal penetration depth” of skin vary with time for someone in a developing room-fire environment? There are a number of effects rendered on a person in terms of burn injuries from fires. The burns injuries happen as a result of some conditions being present; the skin being exposed exposure to a hot surface, gas, or as a result of thermal radiation. The injuries inflicted by fire can be classified roughly depending on the extent of the damage inflicted. In the case where the epidermis is affected we talk of first degree injury, while where dermis and subcutaneous tissue are affected then we have Second degree and Third degree injuries respectively. Figure 5: the different burning degrees.(http://www.home-remedies-for-you.com/remedy/Burns.html) It is thus of great importance to have measurement of the impact of fire in human skin or body. According to (Cox, 2007) penetration depth is a rough indicator of how extremely the heat has managed to penetrate into the tissue with time in seconds as a result of a sudden heating pulse. The thermal penetration depth is defined as: (17) Where: = the thermal penetration depth (m), = thermal diffusivity (m2/s), = the exposing time (s). From Eq. (17) it is clear that thermal penetration depth and time are not related linearly. The depth or thickness of the skin that exposed to a fire is divided into different types as shown in figure (5) Figure 6 the burn depth. 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: As from the table of human skin response it can be seen that the human skin will begin feeling pain feeling pain when the exposure temperature is 44 The time to elapse before the skin reaches a temperature of 44 is given by Where is the mean thermal inertia= 1.7 kW s1/2m-2K-1  = is the skin surface temperature =44 (317K) = The normal body temperature (310K) = Heat flux = 100 kJ m-2 s-1 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: Heat energy generated = VIt= 230x13x3600= 10764000j Cross section of wire = 2.5 mm2= 2.5/(1000000)=0.0000025m2 Volume of wire = 0.0000025m2x2= 0.000005m3 Mass of wire = Volume of wire x density of wire Density of wire =  mass of wire m = 0.000005x8960 = 0.0448kg Energy lost through heating = 0.001x10764000= 10764j From Where VIt=10764j Assuming general environment temperature T0 of 310K Th- T0 10764j = 0.0448x390x(Th- 310) References Babrauskas, V. & Greyson, S. (1992). Heat Release in Fires, E. & F. N. Spon, London. Babrauskas, V. (1995). “Burning Rates,” SFPE Handbook of Fire Protection Engineering, 2nd ed., National Fire Protection Association, Quincy, MA,. Cox, B., 2007. Introduction to Laser-Tissue Interactions, s.l.: s.n. Fine R. A. (2011). Observations of CFCs and SF6 as ocean tracers. Ann Rev Mar Sci.; 3:173-95. Fire Tests (1993). ISRN LUTVDG/TVBB--3070--SE, Department of Fire Safety Engineering, Lund University, Lund, Sweden. IFSTA (2008). "Essentials of Fire Fighting and Fire Department Operations 5th Edition" Rodney C. (2004). Scientific American Inventions and Discoveries, p.351. John Wiley & Songs, Inc., New Jersey. ISBN 0-471-24410-4 Bird, R.B., et al. (2004). Transport Phenomena, 2nd edition,Wiley. Safelincs limited (2015). Phase out of Halon in Portable extinguishers. Farlesthorpe Road Industrial Estate, Alford, LN13 9PS Särdqvist, S., “Initial Fires: RHR, Smoke Production and CO Generation from Single Items and Room Tewarson, A., “Generation of Heat and Chemical Compounds in Fires,” SFPE Handbook of Fire Protection Engineering, 2nd ed., National Fire Protection Association, Quincy, MA, 1995. Tewarson, A. (1996). “Flammability,” Physical Properties of Polymers Handbook, Ed. Mark, J.A., NFPA, (1985). Guide for Smoke and Heat Venting, NFPA 204M, National Fire Protection Association, Quincy,MA,. Jones, W.P. (1999) “turbulence modelling” Imperial College Appendix 1 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 Read More

When in rest at the equilibrium, his would be equal to the thermodynamic pressure with regards to equation of state. Is the rate of change of mass of moving fluid element per unit time. With mass being conserved it can be stated that the time rate of change of mass for the fluid element will come to zero as the fluid is moving alongside the flow (Jones, 1999). Is the rate of change of energy of moving fluid element per unit time in the direction of motion of the fluid under consideration.

This involves the application of the first law of thermodynamics to moving fluid elements. Here we have dissipation rate being dependant on the boundary conditions. The temperature at the boundary and pressure will determine the density of the fluid which will in turn is used in determination of dissipation rate at point A. 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. (5 marks) In order for fire to start we need to have to components: oxygen, fuel and source of heat.

If any of the three is missing then fire will not be started and if there is fire, then if any of these three is taken away then the fire will go out (IFSTA ,2008). Figure 4: Source:https://www.google.com/search?q=fire+triangle&biw (b) Critically review different mechanisms of fire extinguishment (cooling of flame, reduction of fuel and/or oxygen, and interference with combustion reactions). (10 marks) The three method of fire fighting are through cooling, smothering or by removal of the fuel.

When water is used to extinguish fire most of the available heat is removed when water is converted to vapour. This is because evaporation process involves high level of energy absorption (Tewarson, 1995). Water also extinguishes fire through smothering where the vapour generated is able to substantially dilute the oxygen that is required for combustion and thus resulting to the fire being extinguished. Foam extinguishes fire by creating a layer between the fuel and oxygen supply which means oxygen is eliminated and thus fire being extinguished. (c) Review fire protection using water.

Analyse the reason for Halon phase-out. (5 marks) As it has been seen by use of water extinguishing of fire is both by cutting off the supply of oxygen (asphyxia) and cooling effect. Cooling is found to be the most essential element is mastering a fire when it comes to the case of closed area. The water to be used in fire fighting may be accessed through pressurized fire hydrant, where pumping will be from sources such as rivers or lakes, delivery by water tanker trucks, dropped from water bombers.

In 1960s Halon 1211 was seen to be best solution in fire fighting. This view however changed after realization that it had a problem of causing ozone layer depletion. The halon phase-out was declared under Montreal Protocol and it was replaced by what is referred to as zero ozone depletion potential (ODP) (Safelincs limited, 2015). Part E. (LO 1) Heat transfer The mean thermal inertia of skin has been estimated as 1.7 kW s1/2m-2K-1. Formula (3) estimates the surface temperature of skin exposed to a constant heat flux: (3) How would the “thermal penetration depth” of skin vary with time for someone in a developing room-fire environment?

There are a number of effects rendered on a person in terms of burn injuries from fires. The burns injuries happen as a result of some conditions being present; the skin being exposed exposure to a hot surface, gas, or as a result of thermal radiation. The injuries inflicted by fire can be classified roughly depending on the extent of the damage inflicted. In the case where the epidermis is affected we talk of first degree injury, while where dermis and subcutaneous tissue are affected then we have Second degree and Third degree injuries respectively.

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