Abstract In a longitudinally ventilated tunnel, the backlayering flow of buoyant contaminants appears when the ventilation velocity is lower than the critical velocity. In this study, we use brine-water experiments to investigate the backlayering length and the flow structure of the backlayering flow. It is found that the “seemingly stagnant” backlayering flow actually consists of two sub-layers moving towards opposite directions. The distribution of the reduced gravity in the entire model tunnel is measured using a light attenuation technique. The contaminant concentration in the backlayering flow is significantly larger than that in the downstream current. A dimensionless parameter is proposed to normalize the density distribution in different backlayering flows. In the scenario without boundary heat transfer, the mass exchange between the buoyant fluid and ambient fluid also can reduce the buoyancy force of the backlayering flow. A model is proposed to estimate the backlayering length without boundary heat loss. Because both boundary heat loss and mass exchange contribute to the buoyancy attenuation of smoke backlayering flow, the effect of the boundary heat transfer on the backlayering length is quantitatively analyzed by comparing the present model with previous models of smoke backlayering. The results are helpful for understanding the buoyant backlayering flow in longitudinally ventilated tunnels.

中文翻译：

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强制纵向通风隧道中浮力污染物的回层流

摘要 在纵向通风隧道中，当通风速度低于临界速度时，会出现浮力污染物的背层流动。在这项研究中，我们使用盐水-水实验来研究背层长度和背层流的流动结构。发现“看似停滞”的背层流实际上由两个朝着相反方向移动的子层组成。使用光衰减技术测量整个模型隧道中减少的重力的分布。后层流中的污染物浓度明显大于下游流中的污染物浓度。提出了一个无量纲参数来归一化不同背层流中的密度分布。在无边界传热的情况下，浮力流体与环境流体之间的质量交换也可以降低背层流的浮力。提出了一个模型来估计没有边界热损失的背层长度。由于边界热损失和质量交换都对烟气背层流的浮力衰减有贡献，因此通过将本模型与以前的烟气背层模型进行比较，定量分析了边界传热对背层长度的影响。结果有助于理解纵向通风隧道中的浮力背层流动。由于边界热损失和质量交换都对烟气背层流的浮力衰减有贡献，因此通过将本模型与以前的烟气背层模型进行比较，定量分析了边界传热对背层长度的影响。结果有助于理解纵向通风隧道中的浮力背层流动。由于边界热损失和质量交换都对烟气背层流的浮力衰减有贡献，因此通过将本模型与以前的烟气背层模型进行比较，定量分析了边界传热对背层长度的影响。结果有助于理解纵向通风隧道中的浮力背层流动。