Numerical Investigation on the length of the near-field region of smoke flow in tunnel fires

In a tunnel fire, smoke flow is often divided into two regions: the near-field and the far-field. The former is a three-dimensional radial flow region close to the fire source and the latter is a onedimensional steady flow region generally formed a certain distance away from the fire. Although the efficiency of the smoke exhaust system is influenced by the region, limited research has been done to determine the demarcation point between the two regions. In this study, a total of 27 tunnel fire computer simulations were conducted to examine the effect of heat release rate (HRR) and the tunnel's cross-sectional dimensions on the location of the demarcation point. Rather than relying on the tunnel's height, which has been commonly used in previous tunnel fire studies to determine smoke flow characteristics, a hydraulic diameter of the tunnel's cross-section was employed in this study. An improved correlation for the location of the demarcation point is proposed by comparing a theoretical dimensionless model to numerical simulation results.