Fluid Dynamics of Fire Assessment - Assignment Example

Fluid dynamics of fire assessment Student Name: Student ID: Module Code Module Name: Coursework Title: Lecturer: Word Count: Date of submission: Q1: Classical mechanics of fluids 1.1 Navier-Stokes equations together with the equations of energy conservation Continuity: X – Momentum: Y –Momentum Z – Momentum Energy Equation of conservation: Where: t- Temperature, p-Pressure, q- Heat Flux, Re- Reynolds Number - Total Energy, - Density, - stress, - Prandtl Number () – Coordinates and,) - Velocity Components Physical meanings of each term in the momentum conservation equations The x, y, and z are the spatial coordinates, which are independent variables of some domain, and the time t. The pressure p, temperature T, and density, are contained in the energy equation via the total energy Et). On the other hand, the u component acts on the x direction, the v component acts on the y direction, and lastly the w component acts on the z direction, these three velocity vector components. These six components are dependent variables and functions of the independent variables (Niyogi, 2006, 102). are the terms that need turbulence modelling because that are stress tensor units. The pressure p, temperature T, and density r are the source term in the energy conservation equation 1.2 pressure drop in a venture meter Applying Bernoulli equation, Incompressible fluid Differential pressure Since velocity is related to flow rate, The circular cross-section Thus, Q2: Dimensional analysis 2.1 dimensions Dimensions are = The non-dimensional combination is equation C since the Reynolds number equation is true 2.2 dimensional analysis equations Q3: Heat Transfer, Thermochemistry and Fluid Dynamics of Combustion 3.1 PMMA steadily burns under low glow heat flux. The generation of carbon monoxide and carbon dioxide is constant in the entire process. When high heating is used, the PMMA has three stages. In the first stage, external sources are heated and PMMA decomposes generating monomer MMA and only few of other products. Secondly, MMA monomer decomposes produces smaller molecule products. Lastly, the produced molecules are oxidized through burning and heat is released from the process (Baker, 2002, 57). In the process, the elements under combustion are carbon dioxide and carbon monoxide. When the MMA monomer is heated at high temperature, it reacts with oxygen producing formaldehyde, methyl pyruvate, and acetone (Schaschke, 2005, 72). Amount of heat released by burning 2.5kg of PMMA Mass of PMMA= 2.5kg=2500g Molar enthalpy (Gyftopoulos, & Beretta, 2012, 62) 3.2 Fire reaction rate: this is the speed at which fire can spread after it starts. Factors affecting fire reaction rate a. Temperature: In any given fire reaction process, when temperature goes high or low it leads to commendable change that changes the curve used to analyse fire spread. This means that there are more or less particles in the burning material, which have gained activation energy from the temperature effect (Qing-Ming, 2011, 12). Comparing the curve in the graph below, when the number of molecules is constant there is no difference under the two curves, which becomes the same. However, for the molecules in the curve the curve T2 have more average energy, which means there is a higher number of particles that has more necessary energy to react. This has two factors that can be considered: Particle energy increment due to rise in average energy in the particles. Increase in velocity of particles increasing the particle-to-particle successful collisions having the necessitated activation energy. As a result of these two factors, the rate of fire reaction increases. b. Concentration and pressure Considering an increase in concentration or pressure of the fuel or chemical, more particles are supplied in the given space, which in turn increases molecule collision.  Particles will collide more often As this increases, concentration or pressure, the reaction rate goes higher c. Physical state When particles are maintained in the same physical state phase like (gas/gas) or (liquid/liquid) it makes is easier for the mixing of the two particles. As a result, there is high rate of collision for the particles. Nevertheless, given that one of the fuels used in solid, then the reaction may take place of the solid surface. The smaller solid particles sizes, the higher the reaction surface area that is taken So exquisitely divided powder responds more rapidly than the similar stuff in a great big chunk! A position that can come up with a two gases or two liquids it is only if they are cannot mix; this will imply that the reaction has a possibility to take place and the point of contact of the two fluids. d. Catalyst A catalyst exploits the process of altering the pathway of energy for a chemical reaction. It renders an optional means (mechanism) that reduces the Activation Energy entailing that more particles now have the necessitated energy required to undergo a productive collision. An example of the reactions is the combustion of methane gas in presence of oxygen it releases carbon dioxide and water. Q4: Characteristics of Flames & Fire Plumes 4.1 Fire plume. Axisymmetric plume model Radiant heat transfer of the plume fire is dependent on the source as well as well as the destination of the fire in a model. Considering the horizontal fire, it gives an estimate of the source location. On the other hand, ventilations in a compartment control fire flow in a compartment, thus making more prone to spreading in cases where the ventilations are sufficient to provide the required amount air to support burning. Where there are fewer ventilations, fire spread is limited and it is controllable, which makes such a scenario easy compartment to control fire spread in a compartment. Lastly, fire spread in a compartment from a central point is characterised by increase in temperature and reduced oxygen, this means that, when the temperatures go high, then there is more support for fire to spread. From the models 4.1 a and b, axisymmetric fires estimates how important can flame L in the event of determining associated hazards when considering the burning fuel. A correlation of how flame heights have been developed as indicated in the equation below: Where: Flame height (m) Non-dimensional parameter Fire source diameter (m) 4.2 factors that affect the spread of the flame on the solid fuel surface a. Fuel: fuel undergoes combustion when exposed to heat. Most of the fuels have combinations of carbons, oxygen and hydrogen in different ratios. For combustion to take place in liquid or solid fuels, it occurs at the top of the fuel surface where when the fuel is heated vapour is created in that region (Gibbings, 2011, 70). This heat can either be from the ambient, exposing the fuel to existing fire, or exposure to an ignition source. When this heat is exposed to the fuel, it causes pyrolysis releasing the combustible products, which burn in the presence of air and an ignition source. b. oxidizing agent: oxygen is the oxidizing agent on earth. Mixing chemical oxidizers can allow fire to burn in the absence of the atmospheric oxygen. Depending on the type of fuel used, combustion can still be started in the atmosphere with low oxygen. When the environmental temperature rises, the need to use more oxygen is minimized with the usage reduced to up to 14 -16% of oxygen. In some conditions when the flame is beyond flashover point fuel can burn in the absence of oxygen. Thus, if the environment is hot, the amount of oxygen required is reduced. c. Heat: The heat constituent of the tetrahedron constitutes the heat energy higher than the minimal level required discharging vapours of fuel and starting ignition. Therefore, heat can be defined in terms of heat rate or intensity (kilowatts or Btu/sec). When considering heat in fire it generates heat that leads to ignition, which in turn results promotion of fire growth as well as spreading of fire on solid fuel. This maintains a cycle producing the fuel and ignition (Simeon, 2010, 82). d. Chemical Chain Reaction that is Uninhibited. Burning is a complex group of reactions leading to rapid fuel oxidation releasing light, heat, and other by-products. When the oxidation process is slow, such as rusting newspaper yellowing, the amount of heat released is less, hence there is no combustion. Self-sustained burning takes place when enough surplus heat from a reaction that is exothermic radiates back to the fuel producing vapors that start ignition even when there is no source of ignition (Simeon, 2010, 83). Solid combustion can occur through two main mechanisms: smoldering and flaming. Combustion of solids can occur by two mechanisms: flaming and smoldering. Smoldering is when the solid burns on the surface at a lower rate where less heat is released. On the other hand, Flaming burning occurs in the fuel gas or vapor phase. When considering liquids and solids, flaming takes place on the surface. Smoldering may undergo transition when the solid fuel has received enough total energy or oxygen is enough to help in speeding up the combustion process. Bibliography Niyogi, P. (2006) Introduction to Computational Fluid Dynamics. New Delhi: Pearson Education. Baker, C.R. (2002) Introductory Guide to Flow Measurement. New York: John Wiley & Sons. Qing-Ming, T. (2011) Dimensional Analysis: With Case Studies in Mechanics. New York: Springer Science & Business Media. Schaschke, C. (2005) Fluid Mechanics: Worked Examples for Engineers. Warwickshire: IChemE. 62 Gibbings, J.C. (2011) Dimensional Analysis. New York: Springer Science & Business Media. 70 Gyftopoulos, E. & Beretta, G.P. (2012) Thermodynamics: Foundations and Applications. New York: MacMillan Courier Corporation. 62 Simeon, O. (2010) Fluidized Bed Combustion. New York: CRC Press. 82 Read More