The Role of Radiation in Fire Spread Between Neighbouring Buildings - Assignment Example

Assignment Details Theoretical questions (total 30 marks) T1. Review the main features of radiation and analyse the radiating gases produced in combustion. Explain the role of radiation in fire spread between neighbouring buildings and discuss the requirements for space separation (6 marks). Answer: Radiation energy can travel in vacuum which means that it does not need any medium to transfer its energy from one location to another. In the case of fire radiation, the radiant heat emitted by radiating gasses produced in combustion can easily be transmitted to the closest combustible material, resulting in uncontrollable fire propagation. The radiant properties of combustible materials imply that proximities between buildings, walls, and neighboring areas must be far enough so as not to allow radiant energy to consume remote combustible materials. This also implies that exposed materials must be heat-resistant in order to localize (and hence control the spread) of radiant heat. T2. Critically analyse the effect of enclosure ventilation on combustion and the composition of smoke (6 marks). Answer: The production of smoke during fire incident is dependent on two factors – the type of combustible materials burnt and the amount of oxygen present during combustion. When there is not enough oxygen in a given enclosure, complete combustion does not occur which results to an increase in the amount of particle in the smoke produced. In other words, enclosure with poor ventilation is more likely to produce soot and smoke in any event of fire. T3. Analyse the effect of varying compartment/building geometry and fire location on the production of smoke. Compare various plume types including: axisymmetric plumes, adhered plumes, spill plumes etc (6 marks). Answer: Compartment geometry and the location of fire affect the production of smoke in two ways: first, the geometry of the compartment dictates the shape of the smoke plume that will be emitted and second is the location of the fire in the compartment will determine the shape of the smoke plume emitted during combustion. Smoke plumes are columns of smoke produced during combustion. If the compartment (or the opening of the compartment) has a circular shape, then it is most likely that the smoke plume produced will take a cylindrical shape. Consequently, an enclosure with a square opening will have smoke columns (during combustion) that take the shape of the square. Generally speaking, there are three types of smoke plumes – axisymmetric, adhered, and spill plumes. Axisymmetric smoke plumes are smoke plumes that are symmetrical to its axis of propagation. Axisymmetric smoke plumes are typically defined columns of smoke emanated from the source. Adhered plumes are smoke columns that, because of the geometry of the compartment, have to adhere to the walls during propagation. Lastly, spill plumes are smoke plumes that have spread horizontally (due to either compartment geometry or fire location). T4. What is the function of smoke control? When and where is it required? Critically analyse methods of smoke control (6 marks). Answer: Smoke control systems are installed to protect lives and properties as well as to prevent the spread of smoke and fire in the building in the event of fire. Smoke control systems are typically installed in the ventilation systems in buildings and in areas where the HVAC is installed. The standard for use of smoke alarm systems vary by region although the mechanisms involved in their operation are very similar to each other. Generally speaking, smoke alarm systems offer an efficient way to control the spread of smoke and fire in a given floor area. Smoke detectors are typically installed along with wall dampers which inhibit the spread of fire and smoke in an enclosure, allowing the occupants to reach safety with ample time. In large areas, two or more smoke control systems are typically installed. T5. Critically analyse the use of standard fire curves for determining fire resistance. How and why does the approach vary for onshore and offshore applications (6 marks). Answer: The building materials used in onshore infrastructure and offshore infrastructures typically differ due to the difference in their applications. For this reason, the fire resistance curves used in the onshore building materials differs from that of the offshore building materials. Onshore building materials most likely use materials rich in cellulose like wood furniture, door panels, wood-based structural support, paper, and other combustible materials. For this reason, the standard fire curve used in onshore building regulations follow the celluloid curve whereas the offshore building regulations use the hydrocarbon fire curve in order to measure the resistance of the materials against fire. Numerical exercises (total 70 marks) N1. Using Arrhenius’ law compare the chemical reaction rates at two temperatures, 450 and 900K, where the activation energy is 180kJ/mole. Comment on the results (2 marks). Soln: k900 / k450= Ae(-180) / ((0.008314)(900)) / Ae(-180) / ((0.008314)(450))= 5.73 x 10-8/ 3.28 x 10-15 = 1.62 x 107. As can be noted, the chemical reaction rate increases by a billion times when the heat involved in the chemical reaction is doubled. N2. Calculate the maximum pre-explosion heating and induction period for polyvinyl nitrite for the storage temperatures of 20C, 100C and 200C. Comment on the results (2 marks). From: T(293K )= cpRT2/Ek0Qe-E/RT = (1090)(8.3144)(293)2/ (155)(2.13x1020)e-155/(8.3144)(293) = 2.51 x 10-14s T(373K)= cpRT 2/Ek0Qe-E/RT = (1090)(8.3144)(373)2/ (155)(2.13x1020)e-155/(8.3144)(373) = 3.63 x 10-14s T(473K)= cpRT2/Ek0Qe-E/RT473 = (1090)(8.3144)(473)2/ (155)(2.13x1020)e-155/(8.3144)(473) = 5.90 x 10-15s As the temperature increases, the induction period decrease proportionally which means that chemical reactions at a higher temperature combust easily. N3. Calculate the maximum diameter of a cylindrical tank, which provides secure storage of the explosive substance plumbum azide upon the temperatures 20C and 150C providing heat transfer coefficient at the tank surface of 25 W/(m2.K) (2 marks). From: d20C = 1/e * [4*25*8.3144*2932/EQk0e(-E/8.3144*293)] = 1/e*[7.1x107/ EQk0e(-E/2427.8)] d150C = 1/e * [4*25*8.3144*4232/EQk0e(-E/8.3144*423)] = 1/e*[1.48x108/ EQk0e(-E/3516.9)] (please plug in values for density (r), Activation energy (E) and Heat release rate (Qk0) of plumbum azide then compute for d) N4. Given the parameters of the powder, E=85 kJ/mole, k = 1.0 W/m.K, ρ=800 kg/m3, Qk0 = 1.95x1011 J/kg.s determine the critical plane layer thicknesses (above which presents an explosion hazard) of the dried powder in an oven at 200C and 500C (3 marks). From: crit = 0.878 for a plane layer from the Frank-Kamentskii parameter L200C = sqrt{(0.878)*[1*8.3144*473]/[85*800*1.95x1011*e(85/8.3144*473) = 50.4 microns L500C = sqrt{(0.878)*[1*8.3144*773]/[85*800*1.95x1011*e(85/8.3144*773) = 0.64 microns N5. The initial temperature of a flammable mixture is 293 K and the initial pressure is 1 atm. The mixture is burned in a closed vessel and the adiabatic flame temperature 2300K is achieved. Calculate the pressure in the burnt mixture. Calculate the laminar burning velocity assuming a propane-air mixture under this pressure (3 marks). From Pf = 1 (2,300/293) = 7.85 atm Vb = Vb0(7.85/1)0.25 to Vb0(7.85/1)0.33 (look for Vb0 value for propane-air mixture from the table provided in your class Vb = = 1.67Vb0 to 1.97Vb0 N6. A mixed fuel is composed of methane (volume fraction 0.3), propane (0.2), carbon monoxide (0.25) and hydrogen (0.25). Calculate the lower flammability limit for the mixture (oxidiser is air) (2 marks). From: LFL = 1 / (0.3/0.05 + 0.2/(0.021) + 0.25/0.125 + 0.25/0.04) = 1/(6+9.5+2+6.25) = 0.0425= 4.25% N7. Consider a 2.9m diameter pan fire of methyl alcohol with a heat release intensity of 500 kW/m2 of surface area. Calculate the mean flame height under normal atmospheric conditions (2 marks). Lf = 0.23Q2/5 – 1.02D = 0.23(500)0.4 – 1.02(2.9) = 0.25m N8. A stack of wood pallets (1.5m x 1.5m) burns with a total heat release rate of 2300kW under normal atmospheric conditions. Calculate the mean flame height above the top of the pallet stack (3 marks). : Lf = 0.23Q2/5 – 1.02D = 0.23(2,300)0.4 – 1.02(1.5) = 10.75m N9. Estimate the heat release rate above a free-standing steady pool fire with 2.1m diameter. Assume a net heat of combustion of 33.4 MJ/kg fuel, a mass burning rate of 40 g/(m2.s) and a combustion efficiency of 0.75 (2 marks). From: where A = d2/4 Q = (2.12/4)(40)(0.75)(33.4x106) = 3,470,000W or 347kW N10. Estimate the average heat flux received by the surface of a free standing Heptane pool fire (3 marks). For Heptane, Lf = 318kJ/kg and m”f = 0.05kgm2s From: q”f = (318)(0.05) = 15.9kW/m2 N11. Calculate the diameter of the kerosene pool fire, which provides a 200kW sustained fire. Assume the combustion efficiency is 0.75 (3 marks). From: where H=46.2MJ/kg and mf = 0.039 kg/m2s d = 4Q/mfH = (4*200,000)/(3.1416)(0.039)(0.75)(46.2x106) = 0.188 m. N12. Estimate the wavelength corresponding to maximum emissive power of a hot body of dull red light. Calculate the corresponding frequency (3 marks). ≈ 0.7m f = c/= (3.0x109)/(0.7x10-6) = 4.2x1014Hz N13. Calculate the increase in emitted power if the temperature is increased by a factor of 2 (3 marks). From: E(T) = T4, when T0 =2T, we get E(T0) =(2T)4 = 16T4 which means that the emitted power increases by 16 times. N14. Two parallel surfaces of square shape are located opposite each other at the distance equal to the side of the square. Estimate the view factor (3 marks). Because all the energy that would leave square surface A will reach square surface B and vice versa, it is highly likely that the view factor for both surfaces ≈ 1 N15. A 5x3.5x2.8m (height) room is fully involved in fire. The hot gas-soot media inside the room is homogenous and contains combustible products of hydrocarbon fuel. The estimated mole fraction for carbon dioxide 0.14 and for water vapour is (0.10. Soot volume fraction is 0.22x10-6. The flame inside the room is of cherry red colour. Calculate the total emissivity of the media and the rate of energy emission by thermal radiation from the broken window 2.0m x 1.5m (6 marks). Total Emissivity of the Media: egas= eCO2 + eH2O – eCO2*eH2O = 0.226 emixt = egas + esoot – egas*esoot = 0.226 Thermal Radiation Emission E = emixt*T4A = (0.226)*(5.6x10-8)*(T4)(3) = 3.9x10-8T4 N16. Estimate the rate of C0 production in the free burning of polystyrene C8H8 with an area of 4.0m2 (3 marks). N17. Calculate the mass concentration of combustion products, generated by burning flexible polyurethane, that corresponds to a 50% lethal probability after 8 and 30 minutes exposure (3 marks). N18. Estimate the extinction coefficient of the smoke produced by the flaming combustion of 0.2kg of polystyrene-foam C8H8 in a 5m x 5m square room of 2.4m height (3 marks). N19. Consider a 2.0 m diameter pan fire of methyl alcohol. Calculate the flame height under normal atmospheric conditions, the maximum plume velocity, and the temperature at a height of 5m. Assume the radiative heat fraction is 25% (3 marks). N20. Calculate the plume mass flow rate as a function of height for the fire in example N19. Give the numerical values of the plume mass flow rate at the heights which area equal to the flame height and double the flame height (3 marks). N21. A fire with a constant heat release of 100 kW is developed in a compartment with overall dimensions 8.0 x 5.0m and 3.0m height. Calculate the time required for the smoke layer interface to descend to a height of 1.7m (3 marks). N22. A fire in a vented shopping mall of rectangular cross-section 10m width generates smoke with the production rate of 33 kg/sec and a temperature rise of 167K above ambient. Calculate the depth of screens required (3 marks). N23. Consider a 1.8m diameter pan fire of methyl alcohol. Calculate the maximum temperature rise under the ceiling (height = 12m) directly above the fire and 3m away (3 marks). N24. Assuming the following room geometry, 3.5m x 3.5m floor area, 2.3m height with a door opening of 1.8m height and 0.65m width, calculate the heat release rate necessary to cause flashover using different approaches (4 marks). Read More