Computational Fluid Dynamics and Fire Zone Model - Essay Example

Computational fluid dynamics and fire zone model In many research that have been done emphasis has been directed in coming up with softwares and modelling techniques. The computational fluid dynamics (CFD) and the fire model have been cited by Wakatsuki (2001) and Beard (1997) as being the most commonly used in finding solution to fire equations including compartments such as aircrafts. Zone model are most popular because of being considered to be cheaper as well as less complicated in their application. The FDS simulator was pioneered by National Institute of Standards and Technology (NIST) where there is utilization of Large Eddy Simulations (LES), with primary LES technique is used. In the primary LES technique large data is tabulated where a substantial percentage of the energy system is transferred calling for direct resolution so that the process can be presented with the desired accuracy. Use of eddies at small scale presents the advantage of reduced need of calculation and thus translating to overall improvement. Obtaining transit solution does not present any difficult for LES modelling as the need of averaged parameters is eliminated (Bullen, 1978; Wakatsuki, 2001). A close look of results from models have shown existence of very close match, an indication that the models are reliable when used in finding the velocity and temperature for well defined situations where there is appropriate grid resolutions as well as well defined boundary conditions. When investigation fire a sketcher case may be used so as to demonstrate so that important features can be applied in modelling validation studies. CFD models involve use of k-episilon technique and as a result LES technique which is used in FDS model is used in coming up with a temporal resolution which is important in evaluating entrainment. This can be taken as a pointer that in time average technique there maybe substantial impact on the total air entrainment in the air. In developing of FDS model, the mathematical approach which is taken is commonly applied in CDF models where emphasis is usually placed reducing the flow of heat that is being generated by the fire. Developing of FDS basic models involve use of mathematics as an important instrument, this is the approach that is commonly used for CFD models where emphasis is put on reducing the heat being produced by the fire. A sub-model in turbulence region places emphasis being placed in the flow of heat within the fire; the importance of this is observed in practical engineering where turbulence is common. Bullen (1978) points out that the level of accuracy desired is an important consideration when choosing a sub model and more often large Eddy simulation is most applied method in FDS (Merci & Vandevelde, 2007). Using a fine grid resolution it is possible for turbulent modelling to be done with sub-grid approximation being unnecessary and this is called Direct Numerical Simulation (DNS). DNS can be applied in compartment fires such as that which is likely to be experienced in aircrafts in addition it is also important for research application. It is possible to simulate all the fluctuations above the mesh size by use of eddy simulation. The uncertainty that comes with estimation of small eddies is minimal owing to the uniformity of eddies (Novozhilov, 2001). The combustion models have the ability too treat conversion processes of fuels and oxygen into combustion products and heat and thus it is thus possible for FDS to be used in simulating fire as well as its associated effects on the immediate environment is addition of the fluid flow being simplified. In this case both the mixture fraction combustion and the finite model are applicable (Emmon, 1997). The finite model is considered to be the most appropriate for use when the resolutions of DNS calculations are being checked in the resolution of the diffusion of the gas species. The fig 1: shows the relationship between limiting oxygen volume fraction and gas temperature used in the extinction model in FDS to determine whether burning can take place. In the fire extinction model, there will be spontaneous reaction when the process of mixing oxygen and fuel takes in the Burn zone while there is no reaction for the case where the mixing of fuels takes place in the No burn zone as seen in figure 1 (Bishop. et al. 1992). In this instantaneous model the major end products of combustion are CO2 H2O, CO and soot with the proportion of the products being dependant how fast the reaction is taking place. Thus the amount of product production is highly dependant on rate at which fuel is being consumed (Epstein, 1988). Many experiments have performed with the purpose of assessing the variation of minor and major carbonaceous species and the soot present with time in the upper layer in the ISO 9705 room (Drystlale et al, 1985). Fire researchers, regulatory bodies and professional like engineers and lawyers make use of fire models such as the NIST fire dynamics simulators for the purpose of design (for the case of engineers) and for analysis for safety measures where fire instances are concerned such as cases of post fire forensic and establishing the amount fuel consumed in a fire incident. It is important for compartment fire behaviour to be thoroughly understood, as this makes it possible for straight predictions on the extent of damage that is likely to be done on structural element to be made. Studies made on the topic of regions having limited ventilation have shown that when the area of the opening in a compartment is increased will result into the regime changing, where the regime change include stable laminar burning, extinction, unstable oscillation and stable burning with chances of oscillation (Yang, D & Hu, L, 2010). Field models have shown that a lot of difficult is experienced in attempt of making accurate thermal predictions and the chemical species when ventilated compartments are being dealt with. The other finding is that formal ventilation involving well ventilated compartments, performance of field models serve as very good predictors of temperatures as well as species as long as experimental uncertainties are well accounted for (Bishop. et al. 1992). Stages of compartment fire development The development of fire in compartment has several stages where a number of environmental variables are considered in coming up with the subdivisions. When demarcating the stages being undertaken by the fire temperature is usually used as the variable. Figure 1 gives a picture for a typical case where there is a form of control over fire time and temperature. Ignition, growth, flashover, full development and decay are the four stages which are part of fire development. The ignition stage (incipient stage) involves exothermic reaction where there is increased temperature which is well above the ambient temperature (Friedman, 1991). The type of ignition which may cause fire include sparks, piloted ignition by flaming match, spontaneous ignition as a result of heat accumulating in the fuel or through any other source. In an aircraft loose electrical connection may be one of the most likely causes of ignition. After fire has been ignited there may be flaming combustion or shouldering combustion depending on the environment in which the ignition takes place. The sources of energy which are likely to cause fire are electrical, chemical or mechanical (Beard, 1997). The size of the energy source plays a role in determining how fast the fire is able to spread. A fire ignition involving a glowing cigarette or a small spark is likely to result into a smoldering combustion that may take quite a long time before emerging into a flame. This is an indication that it is possible to avoid such fires developing into advanced stage in an aircraft if the personnel are alert such that they can detect the fire before it fully grows. When the fire is at the shouldering stage, there is likely to be production of toxic gases with the heat generated being low. Figure 1: Stages of compartment fire development growth stage The growth stage follow the ignition stage and the stage may either be slow or rapid this being dictated by the amount of air available, the combustion type being undergone, and the interaction with the environment. The growth stage described by putting into consideration the rate of heat release in addition to the rate of combustion products production. A smoldering fire, where production of toxic gases is witnessed with low rate of energy production, falls at the growth stage of fire development. In the smoldering fire growth may be very slow to the extent that the fire may even die out before undergoing all the growth stage. The advancement of the growth stage maybe be so abrupt especially for the case of the flaming combustion, which is characterized high level of flammability fuel making the spread of fire to be so rapid. The flame is able produce sufficient heat energy from the rapidly burning fuel and this is able to ignite the adjacent fuel, there is sufficient supply of fuel and also air supply is sufficient to sustain rapid fire growth (Fleischmann and Parkes, 1997). The fires are said to be fuel controlled if the air supply is sufficient. Flashover is the transition from fire growth to fully developed fire. From the perspective fire safety engineering flashover refers to point between pre-flashover and post-flashover. Many definition of flashover are given in literatures and this makes it not to be a precise term. Sometimes a temperature of between 500-6000C or a radiation emission towards the compartment of between 15 to 20Kw/m2 is used in identifying flashover point (Walton, W.D. & Thomas, P.H., 1995). The other which is used in identifying the flashover point is the point when flames will appear in the openings of the compartment. What happens during flashover emanate from a number of mechanisms linked to fuel orientation, fuel positioning, property of the fuel and the compartment upper layer conditions as well as the geometry of the enclosure. According to Dembsey et al. (1995) flashover is not to be taken as a mechanism but rather as a phenomenon that results from thermal instability. The fire enters the fully developed stage after flashover. In fully developed stage maximum heat flux is released from the fire in the compartment where the actual amount is controlled by the amount of air available as far as ventilation controlled burning is concerned. The gases which have not yet undergone combustion will move in the upper parts of the compartment and when the gases come out through windows they start undergoing combustion which results in flames sticking out though the available openings. The average temperatures in the compartment at this stage will range between 700 and 12000C (Norwegian Fire Research Laboratory, 1996). The decay stage succeed the fully developed stage and at this stage fuel has been fully consumed, release of energy is very low and thus resulting into lower temperatures in comparison to the development stage. The fire at this stage would change to being fuel controlled from ventilation controlled. Aircrafts materials and fire regulations The use of polymer composite materials which is highly being adopted in aircrafts is a great fire hazard due to their high flammability. Exposure of the composites to a temperature range of between 300 and 4000C will result into the decomposition of the material resulting into the release of heat energy, toxic volatile gases, smoke and soot (Smith, et al. 2005). The organic fibres used in the reinforcement of the composites are susceptible to decomposition and they are likely to play important role in the heat generation, smoke and fume incase of fire incident in aircrafts. The composite have a tendency of softening, creeping and being distorted when subjected to a moderate heat of between 100 to 2000C with the possibility of the damage from the heat and flame making the load bearing structures to be distorted, buckle and eventually failing (Mouritz, 2006). . The heat, gases and smoke emanating from burning composite materials in addition to the effect on the structural integrity may quickly bring the safety of an aircraft to jeopardy. Because of conventional composite materials being highly susceptible to fire their use in aircraft applications is highly curtailed. Aircraft fires are classified as great hazard because of the limited time available for combating and extinguishing the fire so that passengers and the aircraft crew are put out of danger. Without use of materials of appropriate fire resistance the fire hazard is of high severity suppose there is cabin flashover, this being likely to be experienced in few minutes after the start of the fire incident. A fire incident in a cargo-hold will need to be extinguished by pilots in not more than 2 minutes otherwise any delay will lead to the growth of the fire being beyond the limit of being extinguished by the fire-suppression systems on-board the aircraft. When extinguishable fire starts in the aircraft a pilot will have up to 14 minutes to land and evacuate, so as to avoid the possibility of being incapacitated by smoke or fumes. The FAA has been given the mandate of determining the fire safety regulations in materials manufacturing of civil aircrafts in the US. The regulation of FAA is used globally in the aviation industry. Aircraft fires can be ramp, in-flight or post-crash where ramp fires occurs when the aircraft has been parked at the terminal ramp, the occurrence of such cases case is rare (Smith, et al. 2005). The dominant fire occurrences in aircraft are experienced during the flight are post flight and as a result FAA regulations emphasis is on smoke, fire and toxicity of cabin materials in post-crash fire scenario with the passengers being expected to make there way out of the aircraft in not more than 5 minutes after crash-landing without incapacitation, injuries or hindrance by heat, smoke or toxic fumes generated by cabin materials under combustion. All materials that are vulnerable to fire (mostly all non metallic) which are used in the aircrafts are to adhere to FAA regulations on flammability. FAA mandates several fire tests to be undertaken to ascertain fire performance of the materials where the key fire properties that are being put into consideration being generation of smoke, the total heat energy emitted and heat flux generated. The performance limits for heat and smoke on materials that will ensure delayed flashover and thereby increasing the time for passengers to escape is set by FAA. Flashover in the aircraft compartment is characterized the hot smoky layer below the ceiling being ignited. The smoky layer has combustion products which are partially burned emerging from smoldering materials. At flashover stage the chance of survival in a burning aircraft is non-existent with the temperatures rising rapidly and the flames spreading at alarmingly fast rate (Diehl, 2013). The safety regulations stipulates that test materials are not to release a total energy beyond 65kW/m2 for a time period beyond 2 minutes while the highest rate of heat release not go beyond 65 kW/m2 in the fire minutes when the test is being undertaken (Mouritz, 2006). The regulations serve as a measure of ensuring that materials to be used in aircraft compartment are not contributing to growth and spreading of fire in the first minutes of crash landing. References Beard AN. (1997). Fire models and design. Fire safety J;28:117}38. Bishop , S & Drysdale, D (1995) Experimental Comparison With A Compartment Fire Model. International Communications in Heat and Mass Transfer. Bullen ML, Thomas PH (1978). Compartment fires with non-cellulosic fuels. Proc Combust Inst. Dembsey NA, et al.( 1995) Compartment fire near-field entrainment measurements. Fire Safety Journal 24:383}419. Diehl, Alan (2013) "Air Safety Investigators: Using Science to Save Lives-One Crash at a Time." Xlibris Corporation. Drystlale. Et al (1985). Smoke prcjducticm in tires, Small scale experiments. Fit-oSufity: Scicww utzd Ett~itwritt~. ASTM STP 8X?. T.Z. Harmathy. Ed. (American Society forTesting and Materials. 1985) pp. 285 300. Emmons HW (1997). A universal flow orifice formula. 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