Computer Modelling in Fire Investigation - Term Paper Example

Introduction The use of mathematics and science in fire dynamics can be traced back to 1940s (Nelson, 2002). Because of application of math in this field of fire dynamics not having lasted for a long time, the field is seen as being young with no advanced development. But putting mathematical and scientific application aside fire can qualify as the oldest and most widely studied phenomenon. Types of models The one type of fire modeling that can be traced back to the history of man is the actual burning process of fuels and putting the results under examination. The studies are still in use up to date and they form the basis of the profession of fire protection. Currently standardized test such as ASTM D1230, D2859 and E603 are used in the illustration of hazards that can be attributed to different types of fuels. Physical fire modeling can be considered as the first major class of fire dynamics modeling, which involves testing and demonstrating fire phenomenon using various types of fuels under different scenarios. The type of tests and demonstration may be categorized as either being small-scale or full scale. In full-scale tests fire situation is replicated through creation of a structure or an item which has similar geometric dimensions and there is an attempt for fire phenomena to be reproduced. For the case of small scale test replication of a scenario of fire is done through the creation of a structure or an item whose geometric dimensions and other relevant variables will be scaled down in the attempt of reproducing the fire phenomena. The physical model can be seen as paving way to mathematical models. In mathematical modeling mathematical equations are used in the description of the physical models (Beyler, et al 2002). Simply stated, physical models are observed by scientist with aim of coming up with equations on the basis of thermal science principle so as to have a match with the physical behavior that is observed. The mathematical equations put into use may range from algebraic equations which are simple and which are used in the prediction of basic fire phenomenon like the case in calculation of fire height calculation to much more complicated partial differential equations applied in the prediction of fire phenomenon in enclosures. In the basic hand calculations use of algebraic equations which are developed through experimental correlations which are applied in the estimation of fire phenomenon which have simple configurations. This is used in obtaining quick estimation of a given scenario. It is also important to note that in the higher level mathematical equations which find application in advanced computer fire models, both zone and field models also have their basis on the hand calculations and experimental correlations. The hand calculations implementation is done by use of computer spreadsheets such as Microsoft excel as a chain of calculations which make the application to be easy to handle. Fire Dynamics Tools (FDTs) is considered to be the most popular; this being a creation of US Nuclear Regulatory Commission and the model is still under the support of the commission. The change from basic hand calculation to engaging the use of more advanced computer software in fire modeling is traced to 1975 (Nelson, 2002). Zone model is seen to be the most common computer modeling that is used in the prediction of fire behavior on the basis of the movement made by the gases into two large zones. The two zones in zone modeling development is based on physics and dynamics in the fire enclosure, which includes fire plumes, entrainment of air and the combustion products of fire. ASET-BX (Available Safe Egress Time Basic), FPETOOL and CFAST (Consolidated Fire and Smoke Transport) and FASTlite are the zone models which are in common use. In this model the temperature and height of the smoke layer in a chamber is calculated and this is of help in the assessment of fire and assessment of the smoke spread more so in a situation where escape of occupants is considered to be a major factor. The predicted temperature of the hot smoke layer is utilized in prediction where the room is untenable and where there is high likelihood of progressing to flashover stage. happen to have contact with the upper zone by the process of radiation or conduction. Computational fluid dynamics (CFD) which is also known as field model is the other type of mathematical computer. In field models a computer is divided into many tiny cubes or calculation units ranging in hundreds to thousands on the basis of the user inputs. Field models are associated with more calculation when compared to the zone models. In these models each cell is calculated by use of higher level mathematics in relating energy transfer and the flow of fluids. In CFD there is application of basic laws of momentum and conservation of energy at cell level balancing with all the adjacent cells. There are three computer fire models which are: Building Research Association of New Zealand fire (BRANZfire); Fire Dynamics Simulator (FDS) and Consolidated Model of Fire Growth and Smoke Transport (CFAST). CFAST creation and release was in the early 1980’s by NIST which continues to offer support to the model; the support that has seen the release of version 6.0.10. NIST was also behind the release of FDS in 2000 and it also supports the model leading to the fifth version being released. The Building Research Association of New Zealand (2003) was behind the creation and subsequent release of BRANZfire in 1997. There are other models which are for specialized use including: Post-flashover models such as COMPF,SFIRE-4 which give time-temperature history for energy, species and mass and is able to evaluate the integrity of structure in fire exposure in post-flashover rooms. Thermal and structural response models such as FIRES-T3, TASEF and HEATING7 for finite element calculations that tests the structural endurance of a building or a specific component of the building by use of failure conditions such as tensile strength with respect to temperature Fire protection DETACT-QS, DETACT-T2, LAVENT which gives sprinkler and dector response times for specified fires Smoke movement model such as CONTAM96, Airnet, MFIRE that illustrates how smoke and gas species are dispersed. Egress models including Isafe, buildingEXODUS, ELVAC, EVACS, EXITT which are probabilistic models for escape of people by use of smoke conditions and occupant and egress variable Computational fluid dynamics (CFD) and the fire model the most commonly applied when it comes to finding solutions fire equations (Beard,1997). Zone model have popularity as they are associated with low cost and because of their simplicity in the application. In FDS there is use of Large Eddy Simulation (LES) which involves tabulation of large data with a high percentage of energy being transferred and this makes it necessary to direct resolution to ensure that the process presentation is done with the desired accuracy. Using eddies at small scale level comes with the advantage of cutting down on calculation requirements and these brings about overall improvement. Also in LES modeling no difficulty emerges in coming up with transit solution with the need of having averaged parameters being done away with. (Bullen, 1978). A close observation of the model results has revealed a close match in the actual experiment results and model results, this being a pointer or the reliability of the models when they are used in obtaining the velocity and temperature involving situations that can be described as being well defined with appropriate grid resolutions with boundaries which are well defined. In fire investigation, there may be use of a fire sketcher in the demonstration with important features being applied in modeling of the validation studies. In CFD models k-episilon techniques is applied and thus necessitation the use of LES technique is used to come up with temporal resolutions that is important in entrainment evaluation. This is a clear indication that in the application of time average technique, there is likely to be a high impact on total air entrainment. When coming up with FDS, the same mathematical approach taken will also find application in CDF modeling with emphasis being placed minimization of heat flow generation from the fire. 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. A sub-model located in the turbulence region will have emphasis in heat flow within the fire; the relevance of this is witnessed in engineering situations where turbulence flow is common. The accuracy level desired is a very important factor that needs to be considered when it comes to making choice of a sub model and it is more often found that large Eddy simulation will be the most applied FDS method (Bullen, 1978) By use of a fine grid resolution method turbulent modeling can be done where it would involve sub-grid approximation and this is known as Direct Numerical Simulation (DNS). Computer modeling use for investigations NFPA921 is a guide used for Fire and Explosion Investigation and it is required that those involved in fire investigation should take a systematic approach in analyzing the origin, the cause of the fire and the person responsible. The scientific method is the option chosen to be the systematic approach. Testing of a hypothesis is one of the important steps undertaken in the scientific method with computer fire modeling being a scientific method that is generally accepted in the test of a hypothesis. Its primary application involves testing the hypothesis by engaging a scientific method with regards: understanding the fire in terms; analysis of timelines; survival chances of occupants; analysis of fuels and analysis of post-fire indicators. Understanding the fire: Through computer modeling a fire investigator may be able to understand how fire evolved. Computer fire modeling may be of assistance through assessment of the relationship between rate of heat release of fuel involved in the fire and other variables like the amount of CO produced and the radiant ignition. For scenarios which may be complex it calls for undertaking of multiple runs with a range of ventilation variables being involved, and this gives the user multiple outcomes whose effects need to be analyzed. The model also has the ability to compute the minimum energy requirement that will enable a compartment to pass through flashover in addition to addressing timing at flashover stage and at full room involvement stage. Through modeling the investigator may be able to evaluate the sufficiency of fuel for flashover and the damage that could be registered after the fire incident as a result of heat flux emanating from burning objects. Timeline analysis Through the use of the model several issues relating to time may be established and this could be of great assistance in making sense of a witness accounts, the possibility of away out, comparing of the injuries incurred in relation to the level of fire development, to find more on the activation and the interaction of the elements of fire protection and in ignition evaluation and issues relating to time to ignition. In general through the use of computer modeling the fire investigator is able to get an object analysis that will have progressed in a fire incident. Survivability analysis The by-products of fire that include toxic gases, the flames and reduced visibility may have adverse effects on the occupants of an affected building. The different by-products are associated with tenability limits whose analysis may be accomplished through use of computer fire models. Computer modeling may then be used in analyzing the egress and addressing escape issues. Analyzing post-fire indicators The investigator may make use of the fire model in comparing the post-fire damage or the physical evidence at the site of fire to there results generated by the model. The model have the capability of giving insight in terms of heat transfer and the accompanying effects the transfer will have on materials. Visualization of fire Phenomena The other important aspect of fire modeling is conversion of the mathematical outputs into computer graphics in three dimensions. Both FDS and CFAST have associated animation software that gives animation of fire and thus helping in visualization of fire. A god example is where two animation scenarios are created at station nightclub one showing the fire spread with sprinklers in working condition and the other scenario where sprinklers were not functional. Multiple Hypotheses Computer fire modeling is at the centre stage when it comes to scientific method. The models give an objective alternative of testing the hypothesis that will have been put forward. The investigator’s hypothesis or any other person’s hypothesis is tested so that it may be considered valid or may be done away with. For example it may be claimed at a certain point of fire incident a room could have been filled with some. The model would then be used to approve or disapprove the statement. References Bayler, C., Carpenter, D. , Dinenno, P. (2008). Introduction to fire modelling. Fire Protection Handbook 20th Edition. National Fire Protection Association: Quincy,MA. Beard AN. (1997).Fire models and design. Fire safety J;28:117,38. Bullen M. L. &Thomas P.H. (1978). Compartment fires with non-cellulosic fuels. Proc Combust Inst. Nelson, H. (2002). From Phlogiston to Computational Fluid Dynamics. Society of Fire Protection Engineers (SFPE). Read More