The Effect of Aspect Ratios on Critical Velocity in Tunnel Fires
The influence of aspect ratio, A s (height/width for a tunnel of rectangular cross section), on the critical velocity in tunnel fires was studied by computational fluid dynamics (CFD) simulation. According to previous researches, the influence of aspect r
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Changcheng Liu, Song Lu, Ruifang Zhang, Hui Yang, Xudong Cheng, and Heping Zhang
Abstract
The influence of aspect ratio, As (height/width for a tunnel of rectangular cross section), on the critical velocity in tunnel fires was studied by computational fluid dynamics (CFD) simulation. According to previous researches, the influence of aspect ratio on critical velocity presents different tendencies. Someone believes the critical velocity increases with As; others think that the critical velocity decreases when As is larger than unity. According to previous experimental results and CFD results, in this paper, it is inappropriate to use aspect ratio as the only factor which affects the critical velocity. Both As and the tunnel width should be considered. By introducing the characteristic fire diameter, the difference of relationship between critical velocity and aspect ratio is unified, and a new critical velocity model is developed. The critical velocity varies with the one-fifth power of the heat release rate for under-medium fires. Keywords
Tunnel fire Aspect ratio Critical velocity Smoke control
Nomenclature H ΔT
V A As Cp D D* Fr g H hc L
Hydraulic diameter of the tunnel (m) Temperature difference from ambient (K) Volumetric flow rate (m3/s) Tunnel cross-sectional area (m2) Tunnel aspect ratio Specific heat (J/kg K) Diameter of fire source (m) Characteristic fire diameter (m) Froude number Gravitational force (m/s2) Tunnel height (m) Heat transfer coefficient (W/m2 K) Characteristic length in Eq. 95.5 (m)
l Pr Q Re T Uc V Vp W
Length (m) Prandtl number Heat release rate (kW) Reynolds number Smoke temperature (K) Critical ventilation velocity (m/s) Ventilation velocity (m/s) Velocity of fire plume (m/s) Tunnel width (m)
Greek Symbol ρ0
Ambient air density (kg/m3)
Subscript C. Liu S. Lu R. Zhang H. Yang X. Cheng H. Zhang (*) University of Science and Technology of China, Jinzhai 96, Hefei 230026, China e-mail: [email protected]
F M
Values used in full-scaled models Values used in scaled models
# Springer Science+Business Media Singapore 2017 K. Harada et al. (eds.), Fire Science and Technology 2015, DOI 10.1007/978-981-10-0376-9_95
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C. Liu et al.
Introduction
Toxic smoke with high temperature produced in fire is the most important cause of casualties [1]. Tunnel can be considered as a long narrow confined space, and when fire occurs, there are no vents but only two exits. This makes the combustion inadequate and smoke production increased, which leads a serious threat to personnel safety. Therefore, the control of smoke is a very important endeavor during a tunnel fire. Especially, it is significant to stop the backflow of hyperthermal toxic gases to guarantee the evacuation of people from the tunnel. However, the ventilation may strengthen the fire and lead severe results [2, 3]. The critical ventilation velocity Uc which defined as the minimum airflow velocity to prevent smoke back layering has been widely used for smoke control in tunnel fires. It already b
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