Study on the confinement velocity in centralized smoke exhaust mode with top double vents

The confinement velocity in centralized smoke exhaust mode with top double vents is investigated through reduced-scale experiments and numerical simulations. An equation for calculating the dimensionless confinement velocity and smoke back-layering length in this mode is derived from theoretical analysis, model experiments, and numerical simulations. The findings indicate distinct confinement velocity transition points for both normal-type and slit-type smoke vents, and that both sensitive and insensitive areas increase with the increase of fire source heat release rate. For the normal-type smoke vent, the confinement velocity to total confinement velocity ratio is approximately 0.6, with the smoke back-layering length to tunnel hydraulic diameter ratio around 3. For the slit-type smoke vent, these ratios are approximately 0.85 and 2, respectively. The ratio can be calculated to get the corresponding confinement velocities and smoke back-layering lengths. The confinement velocity and total confinement velocity for the slit-type smoke vent are lower than those for the normal-type, demonstrating a more significant smoke control effect and superior exhaust performance.