Abstract: Centralized dedicated smoke extraction is commonly adopted at the tunnel bifurcation nodes of underground interchange projects. Multiple tunnel sections share one exhaust fan, forming a unilateral centralized smoke-exhaust mode. Constrained by terrain conditions, the tunnels of underground interchanges form longitudinal gradients. Under fire conditions, the density difference between the hot flue gas and ambient air induces the chimney effect inside tunnels, substantially changing the longitudinal flow characteristics of smoke and increasing the difficulty of smoke control for unilateral centralized exhaust. This study investigates a unilateral centralized smoke-exhaust mode with symmetric openings of smoke-exhaust valves. The influences of longitudinal gradient, exhaust flow rate, and heat release rate (HRR) on the smoke-spread distance are systematically analyzed through full-scale numerical simulations, and corresponding predictive models are established. Three results emerged from the study are as follows: (1) The upstream smoke-spread distance decreases nonlinearly with increasing exhaust flow rate or decreasing longitudinal gradient, whereas the downstream smoke-spread distance decreases with increasing exhaust flow rate. At the same longitudinal gradient, a higher HRR requires a larger exhaust flow rate to control the upstream smoke spread. (2) A dimensionless prediction model of smoke-spread distance, derived through a dimensional analysis, is established. The smoke-spread distance depends on the HRR, longitudinal gradient, and exhaust flow rate. (3) The additional buoyancy-driven force caused by the increased HRR superimposes and reciprocally reinforces the driving force induced by the chimney effect in the upstream region, and counteracts and weakens the driving force in the downstream region. Therefore, the exponent of HRR is much smaller in the downstream dimensionless prediction model of smoke-spread distance than in the counterpart upstream model.

Key words: highway tunnel, unilateral centralized exhaust, chimney effect, smoke spread