Critical fan-induced pressure rise for preventing smoke flow multiplicity in longitudinally ventilated sloping tunnel fires: Potential function analysis and numerical simulations

Abstract

Tunnel fires remain a major concern around the world, especially in sloping tunnels. We investigated the critical conditions for preventing smoke flow multiplicity in longitudinally ventilated sloping tunnel fires and identified multiple flow patterns for smoke movement (i.e., different steady states could be achieved under identical boundary conditions). This results in significant challenges for the design and efficient operation of longitudinal ventilation systems in tunnels. We conducted a potential function analysis and numerical simulations to investigate the critical fan-induced pressure rise for preventing smoke flow multiplicity in a downhill sloping tunnel. Calculation formulas regarding the critical fan-induced pressure rise for preventing smoke flow multiplicity, Pj,max′, were obtained. We demonstrated that a fan-dominated pattern with back-layering flow is physically unstable, and that the fire source location and source strength have an evident impact on smoke flow multiplicity, and on critical values (i.e., Pj,max′andPj,min′) of fan-induced pressure rise. The magnitude of pressure rise derived from critical velocity Pj,cr′ is not sufficient to prevent smoke from spreading upstream in sloping tunnels. Multiple steady states can be achieved if the magnitude of fan-induced pressure rise is between Pj,cr′and Pj,max′. Moreover, Pj,max′ increases more rapidly than Pj,cr′ as the strength of the source increases. The Pj,cr′ is approximately 10–30% less than Pj,max′ when the strength of the source surpasses 20 MW. Therefore, in applications, Pj,max′, rather than Pj,cr′ is essential to ensure the effectiveness of longitudinal ventilation design for smoke control.