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2009 Issues Archive
19 August 2009
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Smoke signals
Software programs can give pointers to engineers about the way that smoke and flames are likely to move through a building, and so aid the adoption of safer designs.
Lee Hibbert
reports
The ferocious blaze that ripped through a 1960s tower block in south London last month, killing six people, came as a tragic reminder of just how quickly fire and smoke can spread, to deadly effect.
That’s why, on modern developments, engineers now use a wide range of sophisticated analysis tools to help design safer buildings. In particular, computational fluid dynamics (CFD) is being extensively applied to accurately model flame and smoke movement.
“Computer models have become much more widely used in recent years,” says Will Marshall, a principal engineer at Jeremy Gardner Associates, one of the largest fire engineering consultancies in the UK.
“There is a much greater range of programs available now as modelling techniques have improved and fire engineering has become more developed as a discipline. As well as developing more applications that perform complex analysis, more user-friendly interfaces and controls are being introduced,” he says.
One of the main CFD packages used by fire engineers is Fire Dynamics Simulator (FDS) developed by the National Institute of Standards and Technology in the US. The software solves numerically a form of the Navier-Stokes equations appropriate for low-speed, thermally-driven flow, with an emphasis on smoke and heat transport from fires.
Essentially, the software defines the space to be modelled as a number of cells: typically hundreds of thousands to a few million are used in modelling problems such as extended smoke reservoirs in shopping centres. For each of these cells, a simultaneous equation is generated for each of mass, momentum, energy and species, and then solved along with all the other cells for each time step to conserve each of the properties. Once the first time step is resolved it will move onto the next one (typically in the order of 0.05sec) before solving the next set of simultaneous equations, and repeating until the simulation is complete, which can often be 600 seconds or more.
Until recently, generating a text input file for FDS was done using a basic text editor. But, by forcing the user to type in each of the input parameters, there was a real possibility of typographical or syntax mistakes. Even if the inputs were well considered, where no input for a particular parameter was set, the FDS program would automatically use a default value. And if this default value was not appropriate, the calculation would probably not have given a reliable answer.
More recently, a commercial graphical user interface (GUI) called PyroSIM has been developed for FDS by a company called Thunderhead Engineering. This has greatly improved the speed of data entry and accuracy of syntax. It also provides users with graphical realtime views of the model, enabling errors to be quickly and easily identified, with menu structure set out in a number of windows to make the interface more manageable.
“The big benefit is the speed of entry,” says Marshall. “Using the text input approach, when you put a full stop instead of a comma it wouldn’t run and it wouldn’t tell you why. The GUI makes it all much clearer and quicker. The flipside is that it doesn’t offer quite so much transparency. Where with the input file you were forced to think about everything that you typed in, it’s now easier with the GUI to leave a box ticked or write something into the code that you maybe haven’t quite thought about.”
Once an accurate model has been run, the results can be analysed. FDS generates a 3D smoke output file as default for all simulations. This results in a video of the calculated smoke movement throughout the model. Marshall says there are a number of fire and smoke parameters that can be set, and these need to be checked before reliance can be placed on the visualisation, which can otherwise give a misleading impression of the results.
A fuller understanding of the results can be gained by considering a range of parameters, for example temperature, fire gases and visibility plots, to determine the presence and influence of the smoke layer. By using these different plots, greater understanding of the properties of the smoke can be achieved. Very cool, dilute smoke is unlikely to cause many concerns for life safety and may be acceptable at a lower level than hotter, denser smoke. “This detailed understanding of the results will allow this to be taken into account,” adds Marshall. “Using any one plot is not necessarily wrong but, to gain a full understanding of the simulation results, the more information that can be analysed the more informed your opinion can be, which can be achieved by viewing several output parameters.”
Marshall warns that, while computer programs are being increasingly used for fire engineering analyses, the programs need to be treated with an appropriate level of caution based on the level of complexity they contain.
“The complexity of the program often correlates with the number of assumptions and approximations which are required to be made,” he says. “The more complex the program, the greater the competency required and care that should be taken in checking the suitability of the model and results.”
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© PE Publishing, 19 August 2009
