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No smoking policy

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technical & practice

Computational fluid dynamics can provide an illustration of how smoke and fire strategy will perform in reality

Within a very small space of time a fire has the potential to ignite, develop and cause enormous damage to buildings and structures, presenting life-threatening danger to the occupants. The ease with which this can occur is seen on a daily basis. While the capital cost of damage to buildings is significant, the impact on life is even greater, with around 650 fatalities and 18,000 injuries per year in the UK alone. Around 70 per cent of these fatalities are caused by smoke inhalation. Although these figures are reducing, there remains scope for improving safety further by using modern technology. Fire protection is an important issue in any building design, but particularly in high-risk and high-occupancy buildings. Engineers responsible for this are required to prove that a building will afford occupants a safe passage to an exit under specific fire scenarios.

When designing for fire and smoke management, the fire protection engineer is typically looking at providing a system that will allow occupants in a building a means of escape in a suitable timeframe after detecting the fire. They cannot be exposed to untenable temperatures or levels of smoke that may overcome them or hamper visibility through escape routes. The structural integrity of the building must also be maintained for a minimum period of time, at least until the emergency services can arrive.

Several options are available when designing a fire protection and smoke management system:

lsprinkler systems, triggered by high temperatures, are used to suppress a fire by extinguishing it or limiting its spread;

lventilation and extraction systems are tuned to limit the exposure of occupants to smoke - the primary cause of fatality;

lbarriers and baffles can be added to limit the spread of smoke into particular areas.

A good design must be effective and practical in the strategic location of these facilities, requiring a detailed understanding of an unpredictable process - the transport of heat and smoke from the fire zone. Computational fluid dynamic (CFD) methods simulate this complex flow and heat transfer process, providing valuable data. This offers several key benefits:

lcost-effective investigation of a design prior to construction;

lcomprehensive data such as temperatures, smoke levels and resulting visibility throughout the entire area modelled;

ldetailed visualisation of the air and smoke flow allowing any problems to be diagnosed.

The use of CFD tends to be focused on the design of high-occupancy buildings and structures. Several factors have contributed to the increase in the application of CFD for fire and smoke modelling. Modern architecture can provide many opportunities for smoke to permeate areas of a building that are not local to the fire source and potentially increase the spread of smoke or fire. Atria, for instance, have become popular features for promoting natural ventilation but provide a direct route for smoke from a fire on a lower level to rise to upper floors. Also, the trend towards constructing taller buildings, both in the UK and abroad, has resulted in inevitably longer escape routes. This has in turn put greater pressure on evacuation strategies, and required an increasingly in-depth analysis to help ensure efficacy.

Satisfying the inspectors In many cases, unconventional layouts or ventilation schemes mean that a conventional approach to fire protection and smoke management would impose a scheme that is impractical or even prohibitively expensive to implement. Nowadays, the results of CFD analysis are taken by the regulatory authorities as evidence that a proposed scheme satisfies the necessary requirements.

CFD as a technology has advanced rapidly and is now much easier to use, so that engineers without previous CFD experience can create and analyse models quicker than ever before. At the same time, high-powered computing resources are becoming more affordable and so models can be built and solved in shorter timescales. These factors have led to CFD becoming a feasible option for building design teams.

There are several examples of where CFD has been implemented to aid building design and give confidence in the safety of evacuation procedures. In Germany, a serious fire at D³sseldorf Airport in 1997 led to a review of the fire protection strategies in place at other airports around the country. Following this, the plan for renovation of Terminal 1 at Frankfurt Airport was reappraised to discover if the fire protection scenarios in place as part of the redesign could be optimised. CFD was used as part of this process. One of the problems identified was the thickness of the smoke layer in regions with low ceilings. Fans and smoke outlets were carefully positioned to ensure that a nearly smoke-free layer - around 2m high - was provided at floor level to allow safe evacuation.

The Budapest Sports Arena in Hungary was a prominent city landmark, completely destroyed by fire in December 1999. Unsurprisingly, the design of the new stadium to replace this put a lot of emphasis on fire safety.

The layout of the smoke management equipment was tested, using the simulation capabilities of CFD to investigate three different fire scenarios. In each case, results showed that occupants at the highest grandstand level would not be subjected to harmful rising smoke within a 10-minute window, allowing adequate time to vacate the building. Also, temperatures within the smoke cloud were sufficiently low to ensure the structure would not be compromised.

Ultimately, if CFD results show that preventative technology and smoke clearance ventilation will eliminate the problem, then these mechanical interventions can be factored into the building's approved fire strategy.

The successful use of CFD in the previous two examples begs the question, why don't all consultants use this technology? The answer is that it must be remembered that it is not a 'black box' tool; it is not a rule of thumb; and it is not, as with Building Regulations Approved Documents, a universal guide. CFD requires, inevitably, a fire protection engineer to specify the behaviour of the fire and interpret the results. This is an added cost, but could pay dividends.

Commonly, the engineer will specify a 'design' fire heat release and smoke generation rate for the fire - specific to what is being burned and how long it is expected to burn in a given environment. The results will allow them to track what happens in terms of heat and smoke movement and gauge the conditions for tenability of the occupants. This typically provides a reliable solution approach, given sensible input quantities. More advanced techniques are looking to predict fire spread and suppression as well. However, these are still much less validated approaches, often introducing unknown or low reliability parameters into the simulation that can impose varying levels of uncertainty in the final results.

Where possible, the approach used should be validated against sensible metrics from physical tests or published data at the early design stage, particularly with novel designs. To a lesser extent, some final comparisons can be performed with minor tests during the final commissioning.

CFD is a powerful tool enabling fire protection engineers to design modern buildings. Previously designs were often based solely on experience and judgement. That's fine, but the considered application of this tool in competent hands will improve fire engineering and hopefully continue the reduction in capital cost, injuries and fatalities.

Steve McCormick is senior engineer at Fluent Europe. Contact steve@ fluent. co. uk or visit www. fluent. com

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