CFD simulation tests were carried out to study the asymmetric flow phenomenon caused by the combination effect of tunnel slope and longitudinal fire location in a naturally ventilated tunnel. The result shows that the tunnel slope and the longitudinal fire location can both create the induced longitudinal flow solely. The longitudinal flow within tunnel induced by the stack effect caused by the tunnel slope is normally unidirectional, flowing uphill and the speed increases gradually with the tunnel slope. While the longitudinal flow induced by the thermal pressure difference caused by the uneven distribution of upstream and downstream smoke transportation can be bidirectional, which depends on the longitudinal fire location (also known as the downstream and upstream tunnel length difference, ΔL = L − L). Therefore, the induced longitudinal flow under the combination effect of tunnel slope and longitudinal fire location is very complex. For tunnels going uphill from left (upstream) to the right (downstream) portals, the two effects are positively added when the fire is located at the upstream tunnel (ΔL > 0), while the two effects are counteracted when the fire is located at the downstream (ΔL < 0). To quantify the strength of asymmetric flow caused by the two effects, an empirical equation of mass flow rate of induced longitudinal flow is proposed. Meanwhile, a model to predict the smoke back-layering length under the two effects is also proposed and validated by former experimental data. A prediction model on the upper critical fire position where the two effects are completely canceled out is proposed. Another critical position (the lower critical fire position) where the smoke flow can achieve unidirectional transportation in an inclined tunnel under natural ventilation conditions is deduced as well.

中文翻译：

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自然通风隧道坡度与纵向火灾位置对不对称流场的联合影响研究

开展CFD模拟试验，研究自然通风隧道中隧道坡度和纵向火灾位置共同作用引起的不对称流动现象。结果表明，隧道坡度和纵向火灾位置均可单独产生诱导纵向流。隧道坡度引起的烟囱效应引起的隧道内纵向流一般是单向的，向上坡流动，速度随隧道坡度逐渐增大。而上下游烟气输送分布不均匀引起的热压差引起的纵向流动可以是双向的，这取决于纵向火灾位置（也称为上下游隧道长度差，ΔL = L − L） 。因此，隧道坡度和纵向火灾位置共同作用下诱发的纵向流非常复杂。对于从左（上游）到右（下游）入口上坡的隧道，当火灾位于上游隧道（ΔL > 0）时，两种效应正相加，而当火灾位于下游隧道时，两种效应被抵消。下游 (ΔL < 0)。为了量化这两种效应引起的不对称流的强度，提出了诱导纵向流质量流量的经验方程。同时，还提出了两种效应下烟气回层长度的预测模型，并通过以往的实验数据进行了验证。提出了两种效应完全抵消的上临界火位预测模型。还推导了在自然通风条件下烟流在倾斜隧道内实现单向输送的另一个临界位置（下临界火灾位置）。