Project Overview & System Type
This project involved a detailed fire safety and environmental assessment of a mechanical impulse ventilation system serving a single-storey enclosed basement car park spanning approximately 474 m². The engineered system utilizes a dual-purpose strategy incorporating low-profile, high-velocity induction (jet) fans operating in tandem with a main mechanical extract fan shaft.
The Engineering Challenge & Regulatory Framework
Enclosed car parks require robust mechanical systems to handle two distinct operational demands: day-to-day pollution control to clear hazardous carbon monoxide (CO) accumulations (complying with Approved Document F) and emergency smoke clearance during a vehicle fire to assist firefighting operations (complying with Approved Document B). The engineering challenge lay in optimizing the positioning, orientation, and thrust of the induction fans within a low-ceiling basement environment. Improper placement could cause airflow short-circuiting, smoke logging in parking bays, or excessive turbulence that disrupts the natural stratification of smoke, thereby hindering safe firefighting access.
CFD Modelling & Analysis Methodology
Computational Fluid Dynamics (CFD) simulations were executed via Fire Dynamics Simulator (FDS) to assess the aerodynamic performance of the jet fans and main extract plant. To rigorously evaluate the volumetric efficiency and air exchange patterns throughout the irregular car park geometry, a tracer gas concentration decay method was implemented. The simulation tracked velocity vectors, local air change rates, and concentration gradients across a dense computational mesh. Localized stagnancy was rigorously assessed by identifying any zones where the local air change rate fell below 50% of the design target or where air velocity stalled below 0.1 m/s.
Simulation Scenarios & Operational Timelines
The impulse system was evaluated under two primary operational configurations:
- Pollution Control Mode: Configured to achieve a continuous ventilation rate of 6 Air Changes per Hour (ACH) across the entire car park volume.
- Emergency Smoke Clearance Mode: Configured to ramp up to full capacity to deliver a minimum of 10 Air Changes per Hour (ACH).
The simulation evaluated the performance of 2 high-velocity induction fans strategically distributed along the primary drive lanes to mechanically direct the air mass towards the main extraction core.
Results & Performance Outcomes
The CFD results validated the mechanical layout, showing that the 2 induction fans successfully established a continuous, high-momentum air stream that swept across the parking bays towards the extract points. Grille flow rates and fan thrust profiles matched the engineering specifications. The stagnancy verification confirmed that the primary drive lanes and parking bays remained free of hazardous dead zones, with the local exchange rates safely exceeding the 50% threshold. Velocity vector fields confirmed that the induction jets did not induce adverse recirculation loops, ensuring effective clearance in both modes.
Value Delivered & Compliance Impact
The CFD assessment provided the client with verifiable proof of compliance with both Approved Document B and Approved Document F, satisfying the rigorous requirements of the local Authority Having Jurisdiction. By optimizing fan placement and thrust through simulation, the design achieved full smoke and pollution clearance without requiring excessive numbers of jet fans, directly reducing the building’s capital plant costs and ongoing operational energy consumption.