Heat Transfer, Fluid Flows, and Fire Suppression - Coursework Example

Name: Title: Date: 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? The rate of heat transfer, 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. The ventilation parameter is defined by the height and the area of the opening in a compartment. The natural ventilation in a compartment is governed by the size of fire, its position within the enclosure, and the shape of the compartment (Ingason & Lönnermark, 2014). Fires that are influenced by ventilation are those whose rate of energy released is limited by the availability of oxygen in the compartment. The air inflow rate is related to the energy released by fire. The rates of air mass flows in and out of the building are related are shown in the diagram below. Fire in a compartment (Hurley, 2016) The flow of fluid through an opening that is influenced by the differences in densities is said to be having an Orifice-like flow. An Orifice flow is modelled as Bernoulli flow having a flow coefficient, Cd, which is dependent on the Reynolds number and the ratio of contraction. The total mass flow rate entering the compartment is integrated as shown below. Where h1 is the height of the neutral layer from the floor, Cd is the flow coefficient, u(z) is the velocity in form of z, the width, w, of the opening with an area, A0 which can be a window. A two zone model provides the solution in terms of the height of the opening, Ho, and the height of the neutral plane ZN. The ambient temperature ratio is. The equation for the flow into the building through an opening is Where mg = the rate of mass flow of hot gas though an opening in kg/s W0 is the width of the opening in m Cd is the coefficient of orifice constriction is the ambient air density in kg/m3 Xd is the interface height in m g is acceleration due to gravity is the ambient temperature in K Tg is the temperature of the hot gas in the upper layer in K XN is height of the neutral plane in m The rate of mass flow through an opening is described in the equation below. H0 is the height of the opening in m (Hurley, 2016). The interface between the lower and the upper layers in fires near or post flashover is near the flor level. Experiment has shown that the density flow through an opening is influenced by a ventilation factor. The widely used correlation is Where Ao is the area of the opening H0 is the height of the opening Ma is mass burning rate k is a constant which equal to 0.09 The out flow of hot gases through the opening and the inflow of cold gases are influenced by fire. External factors that may influence the results such as the effects of air conditioning system and mechanical ventilation are ignored. Thermal discontinuity occurs when the cold air outside the room flows in a contra-flow to the out flowing hot air close to the floor and below the level of the window (Feasey & University of Canterbury, 1999). b) The governing equations of a fluid dynamics model are shown below. Explain the physical sense of each term. Continuity equation The continuity equation shown below expresses conservation of mass It also expresses a continuous velocity as it is a partial differential equation. To obtain this equation, it is assumed that there is no change of phase. For two phases like water and steam, it is assumed that the rate of increase of mass of the first fluid is equal to the rate at which the mass of the second fluid is decreasing. The first and the second terms in the equation are the change in density with time and the change in velocity respectively (Currie, 2012). Momentum An equation that expresses conservation momentum of a fluid flow is The term represents the inertia force per unit volume of fluid. The first term represents a local change with time and the second term expresses the momentum convection. It covers the acceleration around an obstacle even at a steady flow. The term is nonlinear as it velocity appears quadratic. The other side of the equation represents the forces that causing the acceleration. P is a symbol for thermodynamic pressure; δi is Kronecker delta and represents the shear stress, and the symbol µ dynamic viscosity. The term expresses body forces like gravity that act on the fluid. The term accounts for the force on the surface. The term,, is diffusion which is the momentum exchange of the molecules. A good knowledge of the physical meaning of each term is important when approximating the full governing equation (Currie, 2012). Mass The equation for mass conservation is and The first two terms shows the change in mass of a given fluid with time. This is caused by the change in the properties of the fluid and change in position at any given time in a fluid flow. The third term represents diffusion and the fourth term shows a change due to heat being produced. Energy Energy equation is the principle of conservation of energy, which is derived from the 1st law of thermodynamics of a fluid mass flow. The equation is ; ; ; It based on the fact that a dynamic system is at rest at a given time, and after some event it is at rest again. Thus the change in internal energy is equal to the total work done on the system and any heat that was added. The equation signifies the conservation of fluid thermal energy. The rate of change of internal energy is represented by the left hand side of the equation which is a result of temporal and convective changes as a result of fluid flow. The term represents local change with time. The second term is a convective term. The term that contains P represents the influence of pressure on energy. The term is the rate of change of heat due to an external source. The remaining term expresses the action of surface stresses that result in the conversion of mechanical energy to a rise in temperature (Currie, 2012). 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) Combustion is a chemical reaction whereby substances combine with oxygen and produce heat. The three conditions necessary for combustion include the fuel, heat source and oxygen. They are normally represented in form of a triangle whose sides represent the conditions as shown below. (Mannan, 2014) Fire does not occur if one of the conditions is not present. The heat needed to start fire is usually supplied by an external source and then start with combustion process. The three conditions provide a clue of how fire can be extinguished. Fire goes off if one condition is removed. This is the objective of fire fighting. The methods used to extinguish fire are: Cutting off the fuel supply Removal of heat Stopping the supply of oxygen. (Mannan, 2014) (b) Critically review different mechanisms of fire extinguishment (cooling of flame, reduction of fuel and/or oxygen, and interference with combustion reactions). (10 marks) Interference with combustion reactions The aim of this technique is to suppress the chemical reaction of the flame. This is done by use of agents that disrupt the reaction by use of agents that interferes with the combustion chain reaction that is critical to sustain combustion process. When the chain carriers near or in the reaction area is removed, the chain reaction is disrupted and the fire will not sustain itself (Mannan, 2014). Cooling of flame Fire is suppressed by removing the source of heat. Therefore the techniques that can cool the flame are useful in distinguishing fire. For example since water has a high heat capacity, it can cool the flame by absorbing heat. Water also changes to vapour and in the process absorb heat in form of latent heat of vaporisation, thus producing a cooling effect (Iafc, 2012). Reduction of fuel and/or oxygen Fire can be extinguished by cutting off the supply of oxygen by creating a blanket over the fire, thus separating oxygen from the fuel. In the absence of oxygen, the fire goes off. This is applied in putting of petroleum fire by use of foam. Fire can also be distinguished by cutting off the fuel supply. This is normally applied in a situation where there is a fuel leakage such as oil leakage. In this case the valve in a pipe that supply oil is closed (Iafc, 2012). (c) Review fire protection using water. Analyse the reason for Halon phase-out. (5 marks) [20% of the assignment] Water is commonly used to extinguish fire in various fire incidents, except in petroleum fire and fires associated with electricity. Fire extinguishers spray water on fire at high pressure. This not only produce a cooling effect on fire, but it also cause turbulence that may destabilize the burning process. As water change to gases, it produces a cooling effect through latent heat of vaporization. Halon which is the combination of halogen and hydrocarbon can be used to extinguish fire. However, its use has been phased out due to its damaging effects on the ozone layer. The use of Halon and other chlorofluorocarbons (CFCs) results in depletion of the ozone layer and thus leads to global warming. It was banned in Montreal (Speegle, 2013). 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: How would the “thermal penetration depth” of skin vary with time for someone in a developing room-fire environment? An individual inside a developing room fire will be affected significantly by ‘thermal penetration depth’ of the skin and the temperature continues to rise. As the temperature rise to 440C the individual body begins pain. The table below shows the response by human skin when it is exposed to fire. 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: The time that an individual take to feel pain at a steady heat flux of 100kJm-2s-1 is calculated as follows. Where T is 440C (The temperature human skin begins to feel pain as shown in the table above) T0 = 370C (The normal body temperature) β = 1.7 kW s1/2m-2K-1 (mean thermal inertia) = 100kJwm-2s-1 which is constant t =? The skin will begin to burn at 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? Using the formula V = 230V, I = 13A, t = 60 x 60 s, m =, Temperature change, If the initial temperature 230C (296K) Then the temperature after an hour will be 5,865 K References Feasey, R., & University of Canterbury. (1999). Post-flashover design fires. Christchurch, N.Z: School of Engineering, University of Canterbury. Hurley, M. J. (2016). SFPE handbook of fire protection engineering. . Ingason, H., Li, Y. Z., & Lönnermark, A. (2014). Tunnel fire dynamics. . Iafc, (2012). Fundamentals of Fire Fighter Skills, Jones & Bartlett Publishers Mannan, S. (2014). Lees' process safety essentials: Hazard identification, assessment and control. Currie, I. G. (2012). Fundamental Mechanics of Fluids. Hoboken: CRC Press. Speegle, M. (2013). Safety, health, and environmental concepts for the process industry. Clifton Park, NY: Delmar, Cengage Learning. Read More