Project Overview & System Type
This project entailed a comprehensive CFD performance assessment of a mechanical dual-shaft smoke ventilation system designed for a multi-storey residential apartment development. The building incorporates a dual-core layout (North and South Cores), with each core utilizing a coordinated mechanical extract shaft and a mechanical air supply shaft to protect common residential circulation corridors and escape stairs.
The Engineering Challenge & Regulatory Framework
Compliance with BS EN 12101-13 and Smoke Control Association (SCA) guidelines requires smoke control systems to maintain a delicate pressure balance. While high extract rates (up to 7 m³/s) are necessary to remove smoke from common corridors, excessive negative pressure can compromise occupant safety. For a standard 1-meter-wide door fitted with a 45 N door closer, the maximum allowable corridor under-pressure is strictly limited to 50–60 Pa; exceeding this would make doors too heavy to open, trapping occupants. The engineering challenge was to design and validate a dynamic pressure control loop that maximizes smoke extraction upon detection while ensuring that corridor under-pressure never exceeds safe statutory thresholds during door-closed periods.
CFD Modelling & Analysis Methodology
Advanced transient CFD simulations were executed via Fire Dynamics Simulator (FDS). To model the system's automated control logic, a custom under-pressure control sub-model was integrated. The extract fan was programmed to run at full speed immediately upon smoke detection, while the supply fan was governed by a virtual pressure sensor located at the stair door, modulating its volumetric supply rate dynamically to cap corridor under-pressure. High-resolution mesh cell sizes were determined using characteristic fire diameter formulations to capture precise plume and soot dynamics. Soot density and visibility fields were analyzed across multiple horizontal slices from 0.0 m to 2.4 m above floor level.
Simulation Scenarios & Operational Timelines
Four rigorous fire scenarios were simulated across the North and South cores, focusing on typical residential apartment fires. The simulated timeline modeled ignition and rapid t-squared fire growth up to a peak of 1000 kW. At 289 seconds, the apartment door opens to simulate occupant evacuation, triggering smoke detection in the common corridor. The smoke shaft dampers automatically open, and the mechanical extraction system executes a 30-second ramp-up to full capacity (7 m³/s extract / 7 m³/s supply). This is followed by a transient means-of-escape phase and an active firefighting phase where the apartment and lobby doors are held fully open up to a total simulation time of 600 seconds.
Results & Performance Outcomes
The CFD results confirmed that the pressure-sensing modulation logic worked flawlessly; during door-closed sequences, the supply fan adjusted rapidly, successfully capping the maximum corridor under-pressure below the 50 Pa regulatory limit. Soot density plots demonstrated zero smoke contamination within the protected staircase across all scenarios. Tenability analysis during the means-of-escape phase showed that temperatures and visibility along the corridor escape routes remained well within tenable limits, and temperatures within the firefighting access paths were maintained within the "Routine" (<100°C) operational band for emergency responders.
Value Delivered & Compliance Impact
The CFD study provided robust, quantified evidence that successfully resolved comments raised during the third-party Qualitative Design Review (QDR) review. By proving that a dynamic, sensor-driven dual-shaft arrangement can safely balance high-volume smoke extraction with precise pressure control, the client avoided prescriptive design constraints. This optimized system ensured maximum life safety and full regulatory compliance while providing cost savings by eliminating the need for independent pressure-relief dampers.