By Wojciech Węgrzyński
Introduction
Smoke control solutions in car parks are not consistent in various regions of the world and depend heavily on local regulations. In some countries, these systems are obligatory — in Poland, for instance, smoke control is required in all enclosed car parks larger than 1500 m². However, many countries do not have rules for car park ventilation in fire mode, and some outright forbid the use of solutions such as jet-fan systems.
This worldwide discrepancy has its roots in common misunderstanding of the role of smoke control systems in fire safety. It is often said that “the jet-fan systems do not meet their expected performance” — but two factors in this statement can be wrong: the system design or the expectations.
This paper presents the variety of jet-fan–based solutions, with an explanation of what can be achieved with such systems and the features that influence its performance the most. The cases are illustrated with CFD simulations, although the technical aspects of the simulations are not discussed here. (The simulations are described in detail in our book1, and some of the information may be shared in English upon request.)
Benefits of Smoke Control
The most important benefit of a smoke control system is the ability to change the (a) flow paths and (b) energy distribution in a car park. Without any smoke control system, the fire may grow to a large size that cannot be controlled through firefighter operations, leading to extreme damage (Liverpool, 31.12.2017) or even a collapse of the building above the car park (Moscow, 8.10.2017). Not every fire will grow to such size; such events can be considered rare. However, from the risk analysis point of view, the risk of the destruction of the building due to a fire in a car park may be unacceptable.
A smoke control system employing jet fans may completely change the conditions in which firefighters will operate, by lowering the temperature of the smoke or providing a smoke-free access path. A properly designed system enables firefighting operations that otherwise would be impossible. Figures 1 and 2 illustrate such a case.
Figure 1 shows smoke concentration and temperatures at the height of 2 m in a car park fire of 20 MW (approx. five vehicles), after reaching a steady-state flow field. The top and bottom façades of the car park are open, and the distance between them is 100 m, so the building may be considered naturally ventilated. Despite this, the car park is completely filled with smoke, and there is no possibility of entering the building or locating the fire.
Figure 2 shows the same fire in the same building, but with three rows of 400 mm 50 N jet fans. This straightforward and reasonably inexpensive solution allowed clearing most of the building from smoke and substantially limited the temperatures at the path of entry for firefighters. It is unlikely that the system itself would significantly affect the growth of the fire or the local consequences to the structure. However, the difference in the flow field, temperature distribution and — in many cases — creation of safe passage for firefighters is can be expected from an effectively performing jet-fan solution.
Figure 1: Conditions in a naturally ventilated car park during a 5-vehicle fire, steady state. Plots at 2 m above the ground.
Figure 2: Conditions in a mechanically (jet-fan) ventilated car park during a 5-vehicle fire, steady state. Plots at 2 m above the ground.
Sources of knowledge related to the design of jet-fan systems are limited. A comprehensive overview of the design, modelling and hot-smoke testing of jet fan systems is available1, 2. Essential data related to jet-fan system design are available in NEN3 and NBN4. Some relevant information for CFD modelling of jet-fans is also available5-7.
What Will the Jet-Fan System Provide?
Jet-fan systems can work either as a solution to limit the temperatures in a car park or provide a safe entry path for firefighters. These are two main modes of operations, often referred to as smoke clearance and smoke control. The differences between these modes are illustrated in Figure 3. What differentiates these solutions from each other are the design choices — simply put, the smoke control requires larger exhaust rates and stronger jet-fans, and thus is the more-expensive solution.
Figure 3: The different types of jet-fan systems provide different conditions in the car park.
A familiar controversy in the design of jet-fan systems is about their operation in the evacuation phase. As shown in these examples, the inherent feature of the horizontal ventilation is that the smoke will fill parts of the car park, due to the fan operation. To avoid causing issues with the evacuation of people from these areas, the easiest solution is not to activate the jet-fans until the evacuation is over (or RSET has passed).
In most cases, the smoke will form a buoyant layer underneath the car-park ceiling and spread through the car park. The undisturbed layer may maintain its buoyancy for extended periods of time (up to 7–8 minutes). However, the designer must be aware that this is possible only in car parks with sufficient height (not less than 2.9 m)1. Illustration of such behaviour is presented in Figure 4 and was also visible in the media coverage of the Liverpool Echo fire (31.12.2017).
Figure 4: Buoyant layer underneath car park ceiling maintained for a prolonged time (6 minutes) during a 600 kW hot smoke test.
Performance of Jet-Fan Systems
The performance of jet-fan systems will depend on multiple factors, including:
- Exhaust capacity (average airflow velocity in the car park)
- Size of the fire
- Shape and size of the car park
- Jet-fan thrust
Figure 5 shows how these factors can be illustrated with CFD simulations, performed on two different car park models. The first model was a narrow car park with a width of 24 m and length of 105 m. This shape is often considered as optimal for jet-fan ventilation. The second model shared all of the first characteristics, with the difference in width — 40 m. Both car parks shared the same height of 3 m.
Figure 5: Car park models.
The performance of jet-fan systems will start goes back to the concepts of smoke clearance and smoke control. Figure 6 shows the smoke concentration, temperature and flow-path plots in a 9,60 MW fire within a narrow car park, ventilated with a 45 m³/s and 65 m³/s exhaust rate systems. The former system provides smoke clearance and the latter smoke control. In the clearance system, the smoke travelled almost 60 m upstream, which means that the smoke source will be obscured, while the control system provides relatively clean of smoke path of entry (the plots are at the height of the 2 m; if this case is analysed in 3D, the conditions seem much better).
Despite the fact that smoke clearance system does not stop the smoke from lodging in the building, the temperature upstream is significantly reduced compared to the temperature downstream, so firefighter operations would be possible. Obviously, smoke control would make the identification of the seat of the fire, extinguishing and rescue operations considerably easier.
Figure 6: Comparison of performance of two different exhaust rates in the same car park (model 1) at 9,60 MW fire.
It is important to note that these exhaust rates create the “clearance” and “control” effect within this particular architecture, and this particular fire – in every scenario, the parameters of the system to obtain the same goal will be different. Figures 7 and 8 show the conditions for different exhaust rates, fires and geometry of the car park.
In the “narrow” car park, Figure 7, and 4 MW fire (often used as a single-vehicle design fire size), system with capacity of 55 m³/s can be considered as maintaining smoke control, while at the fire of 8 MW (multiple vehicles design fire), the system capacity to reach the same goal is much higher. If the same fire occurs in a larger car park, a significantly larger exhaust rate will be required to remove the smoke with the same efficiency, and the problem with smoke backlayering will be more severe. In the wide car park and the 8 MW fire case, the 77 m³/s system has a similar performance to the 55 m³/s system in the case of the narrow car park.
It is imperative that these three critical features of the system design — size of the design fire, exhaust capacity rate and architecture of the car park — are considered together. Despite some attempts, there is no suitable hand calculation method to do this, so CFD simulations seem necessary.
Figure 7: Performance of jet-fan system at various exhaust capacities in a narrow car park.
Figure 8: Performance of jet-fan system at various exhaust capacities in a wide car park.
The design fire can be considered as an artificial construct – simplification of the whole range of possible outcomes of a fire in a building to allow the dimensioning and design of the safety features. A choice of the design fire will drive the design, but it does not mean that the system will cease to operate if a real fire exceeds the prediction of the designer. What will change is the performance of the solution — a system that may be considered as smoke control solution at 4 MW design fire will still play an essential role in lowering smoke temperatures at higher heat release. To illustrate this, Figure 9 shows a comparison of the performance of two different exhaust rate systems (45 m³/s and 77 m³/s) for four different sizes of fires.
Figure 9: Comparison of the performance of two different exhaust rates, at different design fire sizes.
The exhaust rate is not the only parameter that will strongly influence the performance of a jet-fan system. An important factor to consider is the total thrust of jet fans used in the system. This parameter must be balanced with the exhaust rate, since insufficient thrust will lead to uneven velocity distribution in the car park cross-section and increased smoke backlayering. Thrust that is too high will cause unexpected flow patterns to form, often against the direction of the flow and along the walls of the building. The amount, location and size of the jet fans must be thus adjusted carefully, based on the results of the CFD analysis. Comparison of the effects of the different thrusts of jet fans is shown in Figure 10.
Figure 10: Comparison of the performance of a jet-fan system with different types of jet fans.
Conclusion
This short parametric study shows a “tip of the iceberg” overview of considerations in designing jet-fan systems. Multiple other important factors also can be at play: air inlets, combining natural and mechanical supply, height of the jet-fan installation, location of jet fans, use of deflectors, scenario of operation, etc. However, despite being difficult in the design, jet-fan systems often provide a cheap and efficient solution for smoke control in car parks. If well-designed (and understood by all stakeholders), these systems can be considered as a valid tool in fire safety.
Wojciech Węgrzyński is with Research Institute (ITB)
References
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