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Journal of Zhejiang University SCIENCE A
ISSN 1673-565X(Print), 1862-1775(Online), Monthly
2025 Vol.26 No.7 P.666-678
Flow loss characteristics in parallel confluence sections of tunnels
Abstract: The construction of bifurcated tunnels is essential to advancing urban infrastructure systems, as they conserve land, reduce carbon emissions, and optimize traffic. However, the bifurcation structure of the parallel confluence section of such tunnels poses significant challenges in the design and operation of the tunnel ventilation system, in terms of both the internal and external environment. In this work, the flow and loss characteristics of parallel confluence sections are studied with numerical simulations and model experiments. The influences of the confluence ratio q and the confluence angle θ on the flow characteristics and loss mechanisms of the parallel confluence section are revealed theoretically. The results indicate that when q is small, the high-velocity airflow from the mainline entrains the low-speed airflow from the ramp, leading to flow separation at the upper connection between the parallel section and the gradual transition section; when q is large, the high-velocity airflow from the ramp entrains the low-speed airflow from the mainline, resulting in flow separation on the side of the confluence section adjacent to the mainline. Additionally, the mismatch between the airflow ratio Q and cross-sectional area ratio φ of the mainline tunnel and the ramp prior to confluence enhances the jet entrainment effect, increases the curvature of the streamline, expands the range of the flow separation area, and generates higher confluence loss coefficients |K13| and |K23| of the mainline and the ramp. For small q, K13 and K23 remain relatively constant with respect to θ, whereas for large q, both |K13| and |K23| decrease as θ increases. Finally, a semi-empirical formula is proposed to predict the loss coefficients for parallel bifurcated tunnels with confluence angles ranging from 5° to 15°. This study provides insights into the aerodynamic behaviour and loss mechanisms in bifurcated tunnels, offering guidelines for enhancing the efficiency of tunnel ventilation systems in tunnel-like underground infrastructure.
Key words: Parallel bifurcated tunnel; Confluence ratio; Flow characteristics; Loss coefficients
机构：1西安石油大学，电子工程学院，中国西安,710065；2浙江大学，平衡建筑研究中心，中国杭州，310028；3浙江大学，城市火灾安全工程研究中心，中国杭州,310058;4中国铁路隧道股份有限公司，中国郑州，450007;5中国计量大学，能源环境与安全工程学院，中国杭州,310018;6香港理工大学，智慧消防与城市韧性研究中心，中国香港，999077
目的：分岔隧道的平行式加速车道段中会产生空气和污染物的汇流现象，这给隧道通风排烟系统的设计及运营带来了难点和挑战。本文旨在探讨公路隧道平行式汇流段的流动特征及损失特性，并在此基础上，构建可供设计使用的隧道汇流损失系数的预测公式，以期为分岔隧道的节能型通风设计和高效控烟提供重要的理论参数依据。
创新点：1.通过解耦汇流比q和汇流角θ对平行式汇流段中流场特征的影响，阐明平行式分岔隧道的汇流损失机制；2.通过引入流场"突扩-渐缩"局部损失和流线夹角修正，提出能准确预测5°~15°平行式分岔公路隧道汇流损失系数的半经验理论公式，以弥补分岔公路隧道通风设计计算理论的不足。
方法：1.采用计算流体动力学方法，构建平行式汇流公路隧道气流运动的三维数学模型，并结合比尺模型，分析5°~15°的平行式公路隧道汇流段流动特征及损失系数随汇流比与夹角的变化规律（图8~12）；2.理论构建可供设计使用的平行式公路隧道汇流损失系数的半经验理论公式（公式(8)和(9)）。
结论：1.空气流经平行式汇流段时，两股速度不等的气流在平行加速段进行碰撞，形成速度不连续的断面，产生卷吸效应；在汇流比q较小时，主线高速气流卷吸匝道低速气流，使得平行加速段与渐变段连接处上部出现流动分离；而在q较大时，匝道高速气流卷吸主线低速气流，使得在汇流段靠近主线一侧出现流动分离。2.平行式汇流段损失主要是由结构突变及射流卷吸效应衍生的流速梯度变化、流线弯曲及流动分离引起的；伴随着q的变化，汇流前主线隧道与匝道间风量比Q与横截面积比φ的不匹配，将加大汇流段横断面的速度梯度，增强射流卷吸效应，强化流线弯曲及流动分离，从而产生更大汇流局部损失系数|K13|和|K23|。3.在q较小时，卷吸效应造成的流动分离范围及流线弯曲程度几乎不受θ影响，此时K13和K23基本不随θ变化；在q较大时，随着θ增大，主线一侧流动分离范围越小，匝道流线弯曲程度越弱，此时，主线和匝道汇流损失系数|K13|及|K23|均随之减小。4.基于Bassett公式，通过引入流场"突扩-渐缩"局部损失和修正流线夹角，提出了能够准确预测5°~15°平行式分岔公路隧道汇流损失系数的半经验公式。
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DOI:
10.1631/jzus.A2400405
CLC number:
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On-line Access:
2025-07-29
Received:
2024-08-19
Revision Accepted:
2024-10-20
Crosschecked:
2025-07-29