Effect of Variable Jet Fan Speeds on Characteristics of Smoke Produced from Fires Inside Tunnels

Jet fans are commonly used in tunnels to provide ventilation and support evacuation during fire scenarios. These systems typically operate at constant speeds, which vary based on fan capacity, location, and spacing. The novelty of this study lies in evaluating the performance of variable-speed jet fans in comparison to constant speed jet fans and free ventilation in a full-scale tunnel, by investigating their influence on smoke velocity, temperature distribution, and visibility at various tunnel locations under different heat release rates (HRRs). All simulations were conducted using PyroSim as a Fire Dynamics Simulator (FDS), based on the real dimensions of the Ismailia Tunnel in Ismailia, Egypt. Variable-speed jet fans were configured to operate at incremental linear speeds starting at 4.8 m/s to reach 24 m/s using a series of five pairs of jet fans. An additional case was included, starting at 8 m/s and increasing to overcome the reverse smoke flow generated at the highest HRR, which ranged from 6 to 200 MW. The results presented that at a fixed HRR of 16 MW, the smoke movement at a height of 6.8 m decreased by 36–44% with variable-speed jet fans, and an 83% reduction under free ventilation compared to the constant-speed jet fans case. The maximum velocity difference between upper and lower tunnel sections reached 14.8 m/s at the highest HRR, decreasing by 19%, 43%, and 44% as HRR values decreased, particularly in the ventilation direction. For temperature distribution, the upper tunnel temperature decreased by 68% with constant-speed jet fans and by 35–42% with variable-speed jet fans. Regarding visibility, at an HRR of 16 MW, variable-speed jet fans reduced visibility by 7–13% at 300 m from the fire source relative to constant-speed fans, but improved visibility by 65% compared with free ventilation due to enhanced smoke extraction near the ceiling.