Multiple patterns of heat and mass flow induced by the competition of forced longitudinal ventilation and stack effect in sloping tunnels

The understanding of heat and mass transfer phenomena and underlying flow and thermal mechanisms in a tunnel fire is critical for smoke control and human evacuation. Critical velocity has been acknowledged as the most critical parameter to guarantee the effectiveness of smoke control using longitudinal ventilation in tunnels. However, in slopping tunnels, even under the conditions where the expected smoke flow route and the corresponding critical velocity can be achieved, another flow pattern which has an opposite smoke flow direction and higher gas temperatures is also likely to be generated. This phenomenon has not attracted enough attention. In this paper, multiple patterns of heat and mass flow in a slopping tunnel induced by the competition of buoyancy and forced ventilation were examined using laboratory experiments, Computational Fluid Dynamics simulations and theoretical analysis. Both dynamic and steady state analyses were performed to explain the transient evolution behaviors and final steady solutions for these flow patterns. The constraints and the prerequisite conditions for the existence of multiple flow patterns or only a certain pattern were expressed in terms of three main influencing parameters including fire heat release rate, pressure rise induced by mechanical ventilation system and the fire source location. For the regime where both fan-dominated and buoyancy-dominated patterns can occur, the initial flow state and the flow evolution process determines the steady state solutions, indicating the activation time of the mechanical ventilation plays an important role in the final flow pattern. This study has particular relevance to understanding the complex heat and mass flow behaviors in a sloping tunnel fire and it provides new implications for smoke control.