Study on induced flow patterns and inlet velocity in inclined tunnel fire with natural ventilation

Abstract

Inclined tunnels serve as a vital role in modern urban transportation networks. Nevertheless, the height-induced stack effect caused by the tunnel inclination during tunnel fires, results in smoke movement with multi-directional flow patterns, thus making the smoke flow in a very complicated manner. This study investigates the inlet ventilation velocity with flow field characteristics analysis for inclined tunnel fires under natural ventilation. Three flow patterns (i.e. “bidirectional flow”, “transitional flow”, and “unidirectional flow”) are clearly identified according to different airflow directions and smoke stratification. Moreover, theoretical analysis reveals that the flow patterns are principally governed by the interactive effects of thermal buoyancy (or fire HRR) and inertia forces (or induced velocity) concurrently. Herein, a modified Richardson number Ri', which essentially reflects the ratio of buoyant effect to inertial effect, has been proposed to determine the flow patterns in the inclined tunnel fires. Specifically, when Ri' < 1.91, the airflow inertia force dominates the flow field structure, which causes the fire smoke to be a unidirectional flow with the longitudinal ventilation flow, and thus forms well-mixed gas. As Ri' increases, the buoyant effect becomes more prominent, which triggers the intermittent mixing regime occurred with fire smoke in induces a transitional flow state and partial stratification. When Ri' increases to 17.57, the buoyancy is predominant and leads the fire smoke to be complete stratification. In this case, the fire smoke and entrained air flow in opposite directions, resulting in a bidirectional flow within the tunnel. In addition, it is found that the inlet ventilation velocity increases with the increase of slope or tunnel length, but remains relatively unchanged by their combined influence under a fixed absolute tunnel height difference. Finally, considering the stratification characteristic in three flow patterns, a semi-empirical correlation to estimate the stack effect-induced velocity has been proposed. The proposed framework is validated by comparing with multi-scale experimental and numerical results from previous major studies. The research findings reveal the formation mechanism of the multi-directional flow patterns in inclined tunnel fires, which resolve the smoke transportation characteristics and illustrate the smoke flow dynamics intrinsically.