Project Overview & System Type
This case study assesses a proposed mechanical ventilation and smoke clearance system serving a basement car park. The arrangement combines mechanical extract ventilation, make-up air introduced through the entrance ramp geometry, and five induction fans used to drive airflow into low-movement regions and improve overall circulation throughout the car park.
The Engineering Challenge & Regulatory Framework
The brief was to confirm that the same installed system could provide reliable day-to-day pollution control and robust post-fire smoke clearance despite an updated car park layout and revised internal volume. The assessment was completed in line with the project Qualitative Design Review, relevant British Standards, and established smoke ventilation guidance, with the CFD study acting as the primary performance-verification tool.
CFD Modelling & Analysis Methodology
NIST Fire Dynamics Simulator was used to model two operating conditions: normal pollution-control mode at a target 6 air changes per hour and smoke-clearance mode at a target 10 air changes per hour after a vehicle fire. Revised extract rates of 5.8 m³/s and 9.6 m³/s were applied to reflect the latest geometry. The model used 0.1 metre cells in the fire region with larger cells elsewhere, while induction fans were represented using momentum-equivalent modelling so that thrust and bulk airflow patterns matched the intended hardware performance without excessive computational cost.
Design Fire & Verification Strategy
The design fire was a conservative 4 MW fast-growth t-squared vehicle fire using polyurethane fuel properties and a soot yield of 0.1. The fire was allowed to grow to peak output, remain active until assumed suppression at 900 seconds, and then extinguish so that the smoke-clearance stage could be evaluated directly. System ventilation performance was verified using a tracer-gas decay methodology aligned with ASTM E741, providing a practical measure of how effectively the scheme removes contaminants and eliminates stagnant regions.
Results & Performance Outcomes
In the 6 ACH pollution-control case, the system achieved a sustained global ventilation rate of roughly 6 to 7 ACH within around 100 seconds. Airflow distribution became progressively more uniform over time, and only minor isolated low-flow pockets were identified around the perimeter. These were not persistent enough to be considered significant stagnation zones.
In the 10 ACH smoke-clearance case, the system exceeded its nominal target and delivered effective ventilation rates of approximately 13 to 14 ACH. Fire-driven buoyancy improved mixing and promoted smoke transport towards the extract system, and by around 400 seconds all assessed zones exceeded the required local ventilation criteria. Separate induction-fan verification also showed close agreement between modelled thrust and manufacturer values in both low- and high-speed modes, supporting confidence in the simulated airflow behaviour.
Value Delivered & Compliance Impact
The study demonstrated that the ventilation strategy is fit for purpose in both everyday and fire conditions. It confirmed that the revised arrangement achieves the required air change rates, avoids persistent dead zones, distributes airflow effectively across the full footprint, and clears smoke efficiently after a design fire. That evidence provides a strong technical basis for the project fire engineering strategy and supports the suitability of the installed ventilation concept.