Analysis of Transitional Characteristics of Impulse Fan Smoke Control Performance With Increasing Heat Release Rate in an Underground Parking Garage

In this study, the variation in smoke control performance of impulse fans in an underground parking garage as the heat release rate (HRR) increased was investigated numerically. Fire Dynamics Simulator version 6.7.0 was used, with impulse fans modeled by imposing velocity vectors at the VENT boundary conditions. Six scenarios were considered for HRR values of 3, 7, and 21 MW, both with and without impulse fan operation. The mass flow rate difference (<i>Δ</i>m<sub>peak</sub>) across sections and the vertical distribution of airflow velocity were compared, and a jet–buoyancy dominance index (<i>Π</i><sub>jet/buoy</sub>) was introduced to quantitatively evaluate the flow-governing mechanism. The results showed that with impulse fan operation, the mass flow rate increased in the upstream and near-fire regions, whereas non-uniform flow structures such as backflow and recirculation occurred in the downstream regions, and became more pronounced as HRR increased. In terms of velocity distribution, the effectiveness of the impulse fans decreased as HRR increased, and buoyancy effects became dominant, particularly in the upper region. The <i>Π</i><sub>jet/buoy</sub> decreased with increasing HRR, indicating a transition from jet-dominated to buoyancy-dominated flow, consistent with the observed changes in mass flow rate and velocity distribution. These findings quantitatively demonstrate the variation in impulse fan smoke control performance and the transition of flow-governing mechanisms with fire size, providing fundamental data for understanding the underlying physical phenomena.