Numerical study on plume bifurcation in longitudinally ventilated tunnel fires

Abstract

The buoyant plume theory is one of the essential fundamental theories of tunnel fire dynamics. In this work, a special phenomenon, “plume bifurcation,” was studied numerically. The multi-dimensional characteristics of the buoyant plume under different ventilation conditions and aspect ratios are analyzed. The plume bifurcation is formed by the density-gradient counter-rotating vortex (CVP). Results show that the plume bifurcation only occurs when the longitudinal ventilation velocity reaches a certain value. The bifurcation angle increases rapidly at first and decreases slowly as the ventilation velocity increases. Meanwhile, the bifurcation angle increases with tunnel width when the tunnel aspect ratio is small; and is insensitive to the tunnel width under a large tunnel aspect ratio. A modified Richardson number Ri’ is proposed to determine the plume morphology. When Ri’ > 3.8, the plume is dominated by buoyancy and keeps single; when Ri’ < 2.7, the entrainment intensity is large enough to tear the plume into two sub-streams and results in plume bifurcation; and when 2.7 ≤ Ri’ ≤ 3.8, there is a transition region, where the plume has the trend to extend along the transverse direction, but no obvious bifurcation can be found. Finally, the relationship between the “plume bifurcation” and “smoke layer bifurcation” is analyzed and discussed. This research reveals the multi-dimensional plume characteristics and improves the classical plume theory under tunnel fire scenarios.