Performance-Based Fire Safety Design
The objectives of performing CFD modelling and simulation of performance-based fire safety design were to characterise the behaviour of fire, smoke and ultimately determined the robustness of the proposed fire safety design. Our CFD simulation results addressed the following issues:
Enclosure: defined the enclosure being studied in the CFD modelling. The enclosure details included the identity of the enclosures that belonged in the fire model analysis, the physical dimensions of the enclosures included in the fire model, and the boundary conditions of each enclosure.
Fire Locations: the fire locations used in the CFD modelling followed the proposed fire locations established in the Singapore Fire Safety Engineering Guidelines (SFSEG) 2015, published by SCDF.
Fire Characteristics: the source fire was the critical input for the fire scenario and was often described as the ignition source. The source fire was typically characterised by a heat release rate although other important aspects included the physical dimensions of the burning object, its composition, and its behaviour when burning. The heat release rate could have been specified as a continuous function of time or it might be an array of heat release rate and time data. The fire characteristics used in the CFD modelling followed the fire hazards as specified in Table 3.1 of the SFSEG 2015.
The variables employed to describe the fire were:
a) Heat release rate (HRR)
b) HRR per unit area
c) Fire perimeter
d) Elevation from finished floor level
e) Soot yield
f) Heat of combustion
g) Toxic substances (for fractional effective dose, FED)
h) Radiative heat flux (for FED analysis)
CFD Software: SCDF only recognised 3 simulation software for this purpose. The software we employed, ANSYS Fluent, was one of these recognised software.
Computational Domain: the computational domain was as close as reasonably practicable to the actual enclosure. Where inlet and/or exhaust vents were located at the domain boundaries, we included an additional 5m buffer outwards to account for the aerodynamics of the vents. Where wind effects were being modelled, the domain was extended correspondingly to take this into consideration.
Temperature: in general, ambient temperature in air-conditioned spaces was taken to be 24 deg Celcius while that of non air-conditioned spaces, 32 deg Celcius.
Smoke Management Systems: relevant smoke management system i.e. engineered smoke control system, smoke purging system or smoke vents was included in the CFD modelling.
Report: the report included output quantities for visibility, temperature and velocity. Additional output quantities e.g. carbon monoxide, heat flux, FED could also be included in the report but this depends on the scope of the performance-based design. For slice parameter, slices in all three planes (x, y and z-planes) were included. A cut along the centreline of the fire origin and slices at critical areas in the model were included. More slices could be included if a spill plume forms part of the analysis. At least 2 slices of Z-plane at 1.7m (estimated human height level) and 2.5m above the finished floor level were included. Additional slices at critical areas in the model could be included if required.
Within the building & environment industry, we provide simulation services in the following areas:
- Building acoustics analysis
- Building performance
- Cooling tower exhaust ventilation
- Data centre temperature control adequacy
- Generator exhaust extent of recirculation
- HVAC ducts optimisation
- Optimisation of solar chimney design
- Performance-based fire safety design
- Pollutant (plume or particles) emission
- Wind driven rain
Buildings consume much energy and contribute to greenhouse gas emissions. The quests are thus to reduce the carbon footprints of buildings and their energy usage. Since buildings are relatively expensive to build, their performance must be known and ensured prior to construction.
CFD simulations analyse impacts of changes, reduce time and expense, answers 'what if' scenarios, provides comprehensive data which are difficult to obtain from experimental tests and allow alternatives to be studies before optimum designs are confirmed.
Our CFD services have improved building performance in the areas of thermal comfort analysis (velocity, temperature, RH), design of natural ventilation, solar and daylighting analysis,embodied energy, energy usage, architectural design, room layout, HVAC sizing for occupants comfort and optimising the locations of diffusers and returns.
Cooling Tower Exhaust Ventilation
The objectives of performing CFD modelling and simulation of cooling tower exhaust were to determine the extent of exhaust recirculation into the cooling towers and to establish the temperature profile around the vicinity of cooling towers. Our CFD simulation results addressed the following issues:
Recirculation into air intake units: in building and construction industry, due to the space constraint on the building roof, it was common to locate the cooling towers in close proximity to one another. Moreover, trellis were installed above these cooling towers for aesthetic reason. Because of such a scenario, there was a possibility that the exhaust from a cooling tower recirculated into its own air inlet or inlets of other cooling towers. This might eventually affect the performance of the cooling towers. Thus, our CFD simulations predicted whether recirculation occurred based on the proposed arrangement and locations of the cooling towers.
Temperature profile: a temperature profile around the cooling towers was also established to determine whether elevated temperature affected the cooling towers performance or whether they were acceptable to the efficient operations of the cooling towers.
Impact by neighbouring buildings: due environmental wind conditions, the performance of a group of cooling towers might be affected by neighbouring buildings. This was especially so for shorter buildings being surrounded by taller neighbouring buildings which might act as obstacles to wind flow.
Simulation scenarios: multiple scenarios were considered to study the impact of different variables. These variables included wind direction, wind speed, variable performance level of discharge equipment, ambient temperature conditions and height of trellis.
The objective of performing wind-driven rain (WDR) simulations was to determine the effectiveness of a building’s weather protection features whilst still maintaining the effectiveness of its natural ventilation and smoke evacuation design capabilities. Our CFD simulation results answered 3 important questions:
Wind-Driven Rain (WDR): was the weather protection effective against wind driven rain? If rain droplets did penetrate the interior spaces, where was the exact location, what was the extent, distant and quantity? Which façade was affected the most? The objective was to ensure the safety of the building users and to minimise, if not prevent, rain water-induced damage to furniture and building structure.
Thermal Comfort: the weather protection might be effective against WDR but did it compromise the natural ventilation of the building? How effective were the louvres in allowing sufficient air to enter the interior space? Did the wind meet the recommended velocity of 1.15 m/s for 31 deg Celcius or 1.5m/s for 32 deg Celcius? The objective was to minimise the overdesign of weather protection feature which ultimately reduced natural ventilation.
Smoke Ventilation: in the event of a fire breakout in the station, what was the natural movement of smoke and the likely places of it getting accumulated? How effective were the louvres in allowing the smoke to escape to the atmosphere? How effective was the natural ventilation in evacuating smoke? The objective was to ensure the building design did compromise building fire safety even if the design was effective against WDR and promotes Thermal Comfort.
Bottom Line: In several projects, all of the above areas of study (WDR, Thermal Comfort and Smoke Ventilation) must now concurrently meet relevant government agency requirements.
Pollutant (plume) Emission
The objectives of performing CFD modelling and simulation of plume emission were to determine the effectiveness of the pollutant discharge equipment and to optimise the system so that the emission would not recirculate to fresh air intake units of the building and affect neighbouring buildings.
In addition, there were other exposure limits set out by OSHA PEL, NIOSH REL, ACGIH which defined the permissible exposure limits for a given exposure period, as well as other critical concentrations such as odour threshold limit for the case of ammonia smell nuisance. Thus, it was prudent to ensure that the designed pollutant discharge was able to meet the required emission & exposure limits.
Recirculation into air intake units: in building and construction industry, it was common to locate both fresh air intake units and plume discharge equipment in close proximity such as on a building roof. Hence, there was possibility that discharged pollutant was drawn backed into the building through the air intake units. This could be the result of insufficient plume height and locations of the equipment.
Impact on neighbouring building: due environmental wind conditions, pollutant dispersion could be carried over to neighbouring buildings, which might have air intake units on their roofs or occupants with rooms exposed to the plume. If the plume height was not high enough to overcome neighbouring buildings, these plumes would have negative consequences on these buildings. This was especially so for taller neighbouring buildings or neighbouring building on a higher topography.
Simulation scenarios: multiple scenarios were considered to study the impact of different variables. These variables included wind direction, wind speed, variable performance level of discharge equipment and ambient temperature conditions.