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.
中文概要
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。
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Project supported by the Natural Science Foundation of Zhejiang Province (No. LY19E080028), the Key Research and Development Project of Zhejiang Province (No. 2018C03029), and the Fundamental Research Funds for the Central Universities (No. 2017QNA4023), China
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Zhang, X., Zhang, Th., Hou, Yg. et al. Local loss model of dividing flow in a bifurcate tunnel with a small angle. J. Zhejiang Univ. - Sci. A 20, 21–35 (2019). https://doi.org/10.1631/jzus.A1800298
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DOI: https://doi.org/10.1631/jzus.A1800298
Key words
- Bifurcate tunnel
- Dividing flow
- Local loss mechanism
- Flow separation characteristics
- Computational fluid dynamics (CFD)
- Theoretical formula