JZUS - Journal of Zhejiang University SCIENCE

Journal of Zhejiang University SCIENCE A 2019 Vol.20 No.1 P.21-35

Local loss model of dividing flow in a bifurcate tunnel with a small angle

Author(s): Xin Zhang, Tian-hang Zhang, Yun-ge Hou, Kai Zhu, Zhi-yi Huang, Ke Wu
Affiliation(s): College of Civil Engineering and Architecture, Zhejiang University, Hangzhou 310058, China; more
Corresponding email(s): wuke@zju.edu.cn
Key Words: Bifurcate tunnel, Dividing flow, Local loss mechanism, Flow separation characteristics, Computational fluid dynamics (CFD), Theoretical formula

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Abstract: To provide a theoretical basis for the flow diversion control of a bifurcate tunnel, the flow separation characteristics and local loss model at the tunnel bifurcation were analyzed by combining numerical simulation and theoretical derivation. The results showed that the sudden change of boundaries interrupts uniform flow when air flows through a tunnel bifurcation, causing changes in flow velocity and direction. When the diversion ratio β is small, the flow is separated on the downstream mainline tunnel sidewall close to the bifurcation point and the ramp sidewall away from bifurcation point; when β is large, the flow is separated on the downstream mainline sidewall away from bifurcation point and the ramp sidewall close to bifurcation point. The local loss on flow division is caused mainly by velocity gradient changes and flow deflection and separation. When the air flux ratio q of the downstream mainline tunnel to that of the ramp is equal to their cross-sectional area ratio ϕ, local loss coefficients are at their minimum; when q>ϕ, the loss coefficients decrease with the increase of β, but the loss coefficient for the ramp increases as the bifurcation angle rises. When q<ϕ, the loss coefficients increase with the increase of β, but the loss coefficient for the ramp declines as the bifurcation angle rises. Finally, a theoretical formula to predict the dividing flow local loss coefficient of a bifurcate tunnel is established based on the airflow deflection angle assumption. The proposed model has a higher precision in prediction than other formulas.

小夹角分叉隧道分流局部损失模型

目的:掌握分叉隧道的空气流动特征与阻力损失特性是进行分叉隧道通风设计和控制的关键. 本文旨在探讨小角度分叉结构中的流动特征及局部损失机制,并基于流动分离机制构建可供设计使用的分叉隧道分流局部损失系数的理论公式.
创新点:1. 揭示气流在小角度分叉结构中的流动分离特征及损失机制; 2. 提出流向偏转角假设,建立可供设计使用的分叉隧道分流局部损失预测模型.
方法:1. 通过数值模拟,获得隧道分叉处的流动特征(图5、6a和6d),以及分流局部损失系数随分流比及夹角的变化规律(图6b和6c); 2. 通过理论推导,构建小夹角分叉结构的分流局部损失系数预测公式(公式(18)和(21)); 3. 通过现场实测,验证预测公式的可靠性(图15).
结论:1. 空气在隧道分叉处的分流将导致流速和流向的变化; 当分流比β较小时,流动分离出现在靠近分叉点一侧的主线边壁和远离分叉点一侧的匝道边壁; 当β较大时,流动分离出现在远离分叉点一侧的主线边壁和靠近分叉点一侧的匝道边壁. 2. 当分流后主线与匝道的流量比q等于两者的面积比ϕ时,主线及匝道的分流局部损失系数ξ₁₂和ξ₁₃最小; 当q>ϕ时,ξ₁₂和ξ₁₃均随β的增大而减小,且ξ₁₃随着θ的增大而增大; 当q<ϕ时,ξ₁₂和ξ₁₃均随β的增大而增大,且ξ₁₃随着θ的增大而减小. 3. 基于隧道分叉处的流动分离机制,提出了空气流向偏转角假设,构建了可用于预测分叉隧道分流局部损失系数的理论公式,与已有文献公式相比,具有更好的预测精度.
关键词：分叉隧道; 分流; 局部损失机制; 流动分离特征; 计算流体动力学; 理论公式

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