21 February 2022
The Relevance of CFD for Shopping Mall Design
Shopping malls alternate between being mundane, functional or a venue for amusement, based on customer needs. Since many customers visit shopping malls at any given time for any given reason, mall design must carefully consider the inhabitants’ safety. One of the main aspects of safety in an enclosed space concerns fire. Advances in computational fluid dynamics (CFD) software have made it possible to analyse and foresee potentially dangerous circumstances, especially in malls. Though the study of CFD contributes significantly to the comfort and safety of buildings, it’s worth understanding why fire and smoke analysis, in particular, is critical in malls.
First, let’s recap what we mean by CFD.
Computational fluid dynamics, or CFD, is a methodology that uses numerical analysis and data to analyse, predict and solve problems that involve fluid flows. Calculations are performed to simulate the free flow of fluids and their interaction (liquids and gases) with different surfaces.
Computational fluid dynamics is also known as heat transfer. Data from CFD calculations can help analyse several conditions, such as:
- impact of exhausts on the environment
- smoke and fire risks
- indoor environment quality
- natural ventilation systems
- wind loading
- heating/cooling load calculations.
Of these conditions, smoke and fire risks have the ability to quickly spiral out of control and inflict heavy damages, both to life and finances. The causes for a fire in a shopping mall may be varied, but along with preventing potential fire causes, studies have been conducted on how to contain a developing fire. So, what are the factors that affect the development of a fire? Important factors include:
- Properties of the fuel
- Quantity of fuel
- Natural or mechanical ventilation in the space
- Volume and ceiling height of the space
- Location of the fire
- Temperature and wind conditions
The fire then develops and spreads by transferring heat energy through conduction, convection or radiation to the closest objects or life forms in its vicinity. The calculations involved in CFD help develop systems to prevent, contain and extinguish any potential fires. The main calculations that CFD helps compute to deal with fire are:
- Fire analysis
- Smoke analysis
- Temperature analysis
- Evacuation analysis
- Smoke images
- Temperature plots
- Visibility plots
- Fan velocity plots
Typically, the initial part of air and then fire simulation involves using CFD to predict indoor air flow speeds on different building levels. Then, a model is created of coupled thermal and air flow simulation to check several conditions, including potential overheating in summer.
Software, such as Autodesk CFD, helps model and analyse fire dynamics. It also helps assess life safety in large, complex buildings, such as shopping malls. Smoke hazards can be minimised, and by analysing various kinds of fire and sources of fire, smoke exhaust system designs can be optimised.
Other than Autodesk CFD, PyroSim and Pathfinder are software that can be useful to:
- Create models and be used for simulation calculations
- Develop fire and smoke analysis
- Provide evacuation analysis
- Use GUI (graphical user interface)
One drawback to Pathfinder, though, is that it does not have fire dynamic simulators. PyroSim has features that allow it to actually simulate a fire, starting from any point in a shopping mall.
Another key aspect of fire protection that CFD can assist with is evacuation. Shopping mall evacuation simulation can be conducted using CFD methods for reliable results. The effect of fire on the emergency exit process of people is an important aspect to consider. A fire in the central area of a mall can lead to a fast TET (total evacuation time). This is estimated for a fixed population density and is based on the change of heat release rate, soot yield, soot density and the layout of the mall. Though some amount of human movement can be estimated, accurately predicting evacuation behaviour is difficult, because human nature and crowds are fundamentally unpredictable, especially during times of panic.
Using mall CFD simulations and atrium simulations, smoke clearance and ventilation system weaknesses can also be identified and prevented. Aspects studied include immersive smoke dispersal, temperature distributions, velocity planes and visibilities. These factors are studied according to time lapse and video formats. Carbon monoxide and smoke detectors are modelled, and a potential fire scene is run to test the system. The movement of smoke can be predicted with significant accuracy, including around irregular shapes and encompassing a range of air movements. Depending on the results, smoke layer height, humanly tolerable temperatures and visibility within smoke can be maintained at prescribed levels in smoke exhaust systems.
How can CFD help ensure the safety of people in a shopping mall if a a fire has started?
Since one of the factors that determine the containment of fire is the ventilation system in the building, choosing the right ventilation system is critical. Factors, such as the building’s position, wind exposure and entrance and exit locations, must be considered. Sufficient fresh air must circulate at all times.
The use of CFD can help plan for the inclusion of Natural Smoke and Heat Ventilation Exhaust systems (NSHEVs) in systems design. These NSHEVs are simple, economical and efficiently remove smoke from a building. They are sensitive to interior wind flows and help improve fire safety by:
- increasing evacuation time available, by reducing the threat of hot, toxic gases produced in the fire
- slowing fire growth by limiting smoke temperature and reducing heat radiation to flammable materials
- reducing the amount and temperature of smoke
The dynamics of a fire in a shopping mall throws up certain unique situations and factors.
Shopping mall fire safety has its own considerations.
The large floating populations in shopping malls, combined with expensive property, large merchandise stocks, kitchens, restaurants and regular employees, require a foolproof fire protection solution. Various retail outlets and people of diverse backgrounds in shopping malls mean that evacuation could get chaotic. With a sound, well-researched and effective fire protection system design assisted by CFD calculations, damages can be minimised.
In shopping malls, voice alarm systems, emergency lighting and smoke control systems all play a vital role in fire protection. A fire alarm system in shopping malls must connect zoned sprinklers, smoke control provision, secondary power supplies, emergency lighting and manned control centres. These systems can be integrated using CFD calculations.
Typically, a mall hosts addressable point-type optical smoke sensors, with dual heat/smoke sensors in electrical rooms and heat sensors in the basement car park areas to help provide smoke-free escape routes for rapid evacuation and minimum smoke inhalation. Once elevators and ventilation systems are automatically shut down, people can evacuate through smoke-protected areas and exits. Additional precautions include the installation of smoke curtains and smoke extracts in the open mall areas.
Precautions are taken by considering the possible causes of fire. Causes include arson, smoking inside the mall and unsafe storage of flammable materials or products. In addition, fires can be started by electrical malfunctions, open flames, sparks, hot surfaces in restaurants and paradoxically – water pipe freezing. Each of these situations can be simulated with the right CFD software.
In colder regions of the world, a curious problem in shopping malls is the freezing of fire protection sprinkler pipes. How does this happen?
Some malls install wet-pipe sprinklers. The problem starts when temperatures dip to 40 degrees Fahrenheit or below. The sprinklers may freeze, which is why experienced fire protection service providers install dry-pipe, pre-action, dry-pendent or dry-sidewall sprinklers in unheated areas, such as exterior zones, attics and storage rooms. Still, if the sprinklers are activated and not properly drained, the water remaining in the pipes can freeze. Since frozen water increases in size by 10%, this could increase pressure on the pipes so that they burst. Fittings and sprinklers could also burst, resulting in the release of large volumes of water and time-consuming, expensive repair, leading to possible financial losses. Also, if the sprinkler has an ice block, it will inhibit the flow of water during a fire, and the sprinkler may not even activate.
Fire protection services, using the precise calculations of CFD, must be designed by fire design services providers who are aware of the risks of extremely cold temperatures on sprinkler piping, considering potential burst pipes and the installation of dry-pipe, pre-action, dry-pendant or dry-wall sprinklers.
Efficient fire protection systems are crucial for the safety of customers, employees and merchandise in shopping malls. In conjunction with sound hydraulics and plumbing design services, fire prevention and fire protection systems design services for shopping malls need the expertise and experience of technically well qualified fire design services providers who are proficient in using CFD for critical calculations and simulations. In Western nations, these professionals can be expensive, and so, fire design services are increasingly being sourced overseas, where large numbers of technically qualified personnel have long-term experience with international codes and regulations for fire services design and are significantly more cost effective.
Ultimately, shopping malls must ensure fire prevention and fire protection preparedness and using the latest software for computational fluid dynamics makes the process as effective as possible.