Fire Dynamics for Firefighters - Assignment Example

T1 Basing on physics definition, radiation is any process in which energy travels through a medium or through space, at last is absorbed by another body.Other fields often associate the word with ionizing radiation (e.g., as occurring in nuclear weapons, nuclear reactors, and radioactive substances), but it can also refer to electromagnetic radiation (i.e., radio waves, infrared light, visible light, ultraviolet light, and X-rays) which can also be ionizing radiation, to acoustic radiation, or to other more obscure processes. What makes it radiation is that the energy radiates (i.e., it travels outward in straight lines in all directions) from the source. This geometry naturally leads to a system of measurements and physical units that are equally applicable to all types of radiation. Fires which reach flashover produce acutely lethal products of combustion, it includes smoke, carbon monoxide, acrolein, hydrogen chloride, hydrogen cyanide, nitrogen oxide(s), and so on, in addition to heat and insufficient oxygen. These gases are often produced in large quantities and moved to remote areas to cause life-threatening situations. inhalation of these toxic products of combustion is a major cause of death in fires.For these reasons, any combination of finishes, combustible building materials, or contents and furnishings that could result in flashover (full-room involvement) in a few minutes represents a severe fire hazard in many types of occupancies The spread of fire between neighbouring buildings is as a result of radiative transferring the presence of local high temperature igniting sources, i.e., spark s or flying brands. According to a report on example of the spread of fire by radiation by J.H. MoGuire and G.W. Shorter Report No. 145 of the Division of Building Research Ottawa August 1958: It was interesting to note that the secondary fires on the side walls of t h e two neighbouring dwellings had only been initiated on the vertical faces directly exposed to radiation from the burning dwelling. The undersides of t h e eaves were only discoloured where flames from the secondary fire had played upon them; t h e under edges of certain exposed boards were not discoloured at all. It was concluded, therefore, that the spread of fire to houses A and C resulted from radiative heat transfer. Its of importance in the field of fire technology to be able to predict the space separations between buildings which will prevent a fire in one building from spreading to another, Despite this, there is little reliable information on which such predictions can be based. According to the report above; On the assumption that the radiation incident on the piece of timber which did not ignite originated largely from the burning areas of house B, the black body temperature of these areas has been calculated to be 890°C f 60°C. If this assumption were not valid the true black body temperature would be lower than this. The pilot ignition of the wood on the one side of both the neighbouring houses would have been prevented had the separations between the houses been greater than 24 feet, or had the concrete-brick side walls of house B been carried up to the roof. T2 If the compartment is unventilated but there is no danger of a back draft occurring, opening the door does affect the fire conditions momentarily, but the situation soon relaxes. Nothing dramatic is seen in the latter case either. Enclosure ventilation clearly improves the survival probability in the compartment: the visibility dramatically improves, and the temperatures are low everywhere else except for the fire room. T3 Where the configuration of the space warrants it (e.g., at a high vaulted ceiling) adjoining smoke control zones may overlap along the common border, to include the special geometric feature of the space on either smoke zone and use it to prevent smoke migration between the zones . Optimal Positive pressure ventilation rate flow rate depends on the room geometry, the location of the fire etc. The flow rate has to be high enough to push the flames back to the fire room but low enough not to mix the gases so vigorously that the fire room becomes too difficult to enter. In the present case, the optimal positive pressure ventilation flow rate was 4 - 6 m3/min corresponding to 128 - 192 m3/min full scale for a compartment of about 80 m3 total volumes. T4 Extracting smoke from the fire compartment as a part of operational fire fighting would be most beneficial, but it always poses some risks when initiated during the fire. In some cases the smoke ventilation is advantageous: the hot gases are removed from the fire enclosure, the visibility improves and the enclosure cools down. The opposite happens in some cases: When there is accelerated burning rate, more smoke is spread around, and the temperatures rise. The unique consequence is the initiation of a backdraft, where the pyrolyzed gases ignite instantaneously causing a severe explosion. The positive effects of forced smoke ventilation carried out by external fans were known in Finland already in the 40’s /1, 2/: “The objective is that the accumulation of heat and production of smoke gases is stopped altogether by starting the ventilation as early as possible. In a cool room, a bright flame is easy to extinguish.” The inventor of the so-called Fenno-Vent method, Mr. Leo Pesonen, wrote in 1958 that he planned to “initiate scientific research on Fenno- Vent that he had been vainly expecting for 22 years.” Only now in the 90’s, over 30 additional years later, scientific interest is finally being shown in the method. Methods of smoke control: The outside air intakes for make-up air and for shaft pressurization, and the points of discharge of any fans used for smoke exhaust, should be located with sufficient remoteness to preclude recirculating smoke into the building, taking into account prevailing winds and geometry of the building envelope. In order to enable activation of the smoke control systems that is an appropriate response to the detected fire condition, where the smoke control system incorporates dual-purpose HVAC equipment or portions of the HVAC air distribution system, the HVAC zones should coincide with or be nested in the smoke control zones. In addition, all heat, smoke and water flow detection devices that are incorporated in the smoke control design shoud are of the point addressable type, to afford the necessary flexibility of the design. Furthermore, the automatic sprinkler system zones should be coordinated with or be nested in the smoke control zones. T5 Fire resistance can be defined as the ability of structural elements to withstand fire or to give protection from it (2). This includes the ability to continue to perform a given structural function. A relatively new method for determining fire exposure used by fire protection engineers is to first calculate the fire load density in a compartment. Then, based on the ventilation conditions and an assumed source of combustion determine the compartment temperature at various times. Another factor considered in the analysis is the effect of active fire protection systems e.g. sprinklers or fire brigades on the growth of the fire. The size and timing of the fire growth determined by fire analysis is sensitive to changes in the fuel load over time and changing ventilation conditions during the fire. This method of fire analysis requires special software and extensive training and is used only in very large or unusual buildings. Once the temperature time relationship is determined using a standard curve or from the method described above, the effect of the rise in temperature on the structure can be determined. For low and moderate structural fire severity situations, determination of the adequacy of a steel member by comparing the temperature reached in a design fire with the limiting temperature based on the member heat sink characteristics, extent of insulation and utilization factor is becoming increasingly common fire engineering design practice. Basing on this, it’s relevant to have an accurate and widely applicable parametric fire model as is practicable. The standard time-temperature curve used in the examples is the ISO 834 curve. The two parametric time-temperature curves used in the paper are the Eurocode parametric curve and a recently developed one termed the BFD curve. The latter was found to fit the a wide range results of actual fire tests closely than existing parametric curves and is mathematically simpler .The BFD shape curve and the parameters used to define it has a strong relationship to both the pyrolysis coefficient (R/Avhv0.5) and the opening factor, F02. The curve also models fire development without the need for time shifts. This approach vary for onshore and offshore application as: It uses a single and relatively simple equation to generate the temperature of both decay phases and the growth of a fire in a building. Three factors are required to derive the curve. They include a shape constant for the curve, the time at which this maximum temperature occurs and the maximum gas temperature. If desired, the shape constant can be different on the growth and the decay sides to model a very wide range of natural fire conditions and test An overview of the background to the BFD curve and illustrations on its use in a simple fire engineering design application, where the adequacy of a steel beam is checked using the Eurocode parametric curve and the BFD curve to represent the fire is presented in Copyright © 2004 John Wiley & Sons, Ltd. References 1 McGuire, J.H., Heat transfer by radiation, Department of Scientific and Industrial Research (DSIR) and Fire Offices' Committee, Fire Research Special Report 2 Joint ACI-TMS Committee 216, “Standard Method for Determining Fire Resistance of Concrete and Masonry Construction Assemblies (ACI 216.1-07/TMS 0216.1-07),” American Read More