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41 CHAPTER SEVEN ANALYTICAL FIRE MODELING--LITERATURE REVIEW Information from numerous test reports has shown that road to solve the large scales of motion and model the small scales tunnel fire tests are expensive, require precise measurements, that are assumed to be universal. The Large Eddy Simulation and are difficult to reproduce. Numerical modeling can be results in a transient solution to the actual Navier­Stockes repeated and allows for easy change of control parameters. equations valid for a low-speed (low Mach number) buoyancy- Numerical modeling helps researchers to understand the driven flow. physical processes and influences from design parameters. Validation of numerical models against the fire tests helps to A specific CFD model called SOLVENT was developed expand the models to new projects. However, validation of as part of the Memorial Tunnel Fire Ventilation Test Program numerical fire models is complicated and in most cases not for simulating road tunnel fluid flow (ventilation), heat transfer, successful, although some parts of the numerical model can and smoke transport. SOLVENT can be applied to all venti- be verified against tests and field measurements, considering lation systems used in road tunnels, including those based on that appropriate measurements have been made during the natural airflow. SOLVENT has not been used for modeling fire tests. fixed fire suppression systems and has some other limitations. Theoretical models, especially computer-based models, Other CFD programs, both commercially available and in can be very valuable in assisting tunnel fire safety decision the public domain, have been used to model fire scenarios making. However, such models are also capable of being in road tunnels, the list of which is too numerous to include misleading. Nowadays, computer-based simulation models here. The most common and powerful for tunnel applications are widely used to calculate propagation of smoke and hot are ANSYS (Fluent CFD) and CFX verified against testing gases to assess the means to improve tunnel safety. They enable results. the simulation of the interaction of various fire parameters. Initially, the strengths and weaknesses of each program CFD software can model emergency fire operating con- are investigated. Validation of the results against experimental ditions in tunnels and predict the resulting contaminant con- data or another equivalent program is encouraged. Some pro- centration levels. In areas of geometrical complexity, CFD is grams have limitations and are unable to model the required the appropriate tool for predicting 3D patterns of airflow, processes, including water-based fire protection, moving traf- temperature, and other flow variables, including concentration fic (sliding mesh), wall roughness, and so forth. Validation is of species, which may vary with time and space. CFD was challenging for tunnel fire modeling because the experimental developed as a scientific tool for the investigation of aero- data are far from absolute, given the complexity of the phys- dynamic and thermodynamic processes. Nowadays, CFD ical process. Good experimental data are required. software is considered as the design tool of choice for obtain- ing an optimum design, because experimental methods are It becomes difficult to check the CFD model and results. costly, complex, and yield limited information. However, In many cases it is up to the artistic, inventive ability of the it requires in-depth knowledge of physical processes and engineer who created the model. The knowledge and experi- numerical models and, preferably, testing experience from ence of the user becomes crucial. Users may employ different the numerical modeler. inputs in applying the same models or use different deter- ministic models to the same case, both of which produce dif- Many commercial CFD packages have been developed in ferent results. Some studies showed significant differences recent years. A Fire Dynamics Simulator (FDS) is a CFD when the same user applied two different CFD-based models model of buoyancy-driven fluid flow from a fire. A separate to the same case. The user must be knowledgeable about tunnel code called Smokeview (OpenGL graphics program) is used fire science, as well as the model's limitations and applicable to visualize data output from an FDS. These applications conditions. can also be configured to model pollutant levels outside the portals and around the exhaust stacks of tunnels. Both of Assessment of models and their results are important and these public domain programs are under active development must be conducted by experienced people. It is important to and can be obtained from the National Institute of Standards establish a procedure for producing comprehensive, iterative and Technology (NIST). FDS uses the Large Eddy Simulation assessment of fire models.