Inclined tunnels serve as a vital role in modern urban transportation networks. Nevertheless, the height-induced stack effect caused by the tunnel inclination during tunnel fires, results in smoke movement with multi-directional flow patterns, thus making the smoke flow in a very complicated manner. This study investigates the inlet ventilation velocity with flow field characteristics analysis for inclined tunnel fires under natural ventilation. Three flow patterns (i.e. “bidirectional flow”, “transitional flow”, and “unidirectional flow”) are clearly identified according to different airflow directions and smoke stratification. Moreover, theoretical analysis reveals that the flow patterns are principally governed by the interactive effects of thermal buoyancy (or fire HRR) and inertia forces (or induced velocity) concurrently. Herein, a modified Richardson number Ri', which essentially reflects the ratio of buoyant effect to inertial effect, has been proposed to determine the flow patterns in the inclined tunnel fires. Specifically, when Ri' < 1.91, the airflow inertia force dominates the flow field structure, which causes the fire smoke to be a unidirectional flow with the longitudinal ventilation flow, and thus forms well-mixed gas. As Ri' increases, the buoyant effect becomes more prominent, which triggers the intermittent mixing regime occurred with fire smoke in induces a transitional flow state and partial stratification. When Ri' increases to 17.57, the buoyancy is predominant and leads the fire smoke to be complete stratification. In this case, the fire smoke and entrained air flow in opposite directions, resulting in a bidirectional flow within the tunnel. In addition, it is found that the inlet ventilation velocity increases with the increase of slope or tunnel length, but remains relatively unchanged by their combined influence under a fixed absolute tunnel height difference. Finally, considering the stratification characteristic in three flow patterns, a semi-empirical correlation to estimate the stack effect-induced velocity has been proposed. The proposed framework is validated by comparing with multi-scale experimental and numerical results from previous major studies. The research findings reveal the formation mechanism of the multi-directional flow patterns in inclined tunnel fires, which resolve the smoke transportation characteristics and illustrate the smoke flow dynamics intrinsically.

中文翻译：

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自然通风下倾斜隧道火灾中的诱导流型和入口速度研究


倾斜隧道在现代城市交通网络中起着至关重要的作用。然而，隧道火灾期间隧道倾斜引起的高度诱导烟囱效应，导致烟雾以多方向流动模式移动，从而使烟雾流动非常复杂。本研究研究了自然通风下倾斜隧道火灾的入口通风速度和流场特性分析。根据不同的气流方向和烟雾分层，明确识别三种流动模式（即“双向流”、“过渡流”和“单向流”）。此外，理论分析表明，流型主要受热浮力（或火 HRR）和惯性力（或诱导速度）的交互效应同时控制。在此，提出了一个修正的 Richardson 数 Ri'，它基本上反映了浮力效应与惯性效应的比率，以确定倾斜隧道火灾中的流动模式。具体来说，当 Ri' < 1.91 时，气流惯性力主导流场结构，导致火烟与纵向通风流呈单向流动，从而形成混合良好的气体。随着 Ri' 的增加，浮力效应变得更加突出，从而触发了与火烟一起发生的间歇性混合状态，从而诱导过渡流态和部分分层。当 Ri' 增加到 17.57 时，浮力占主导地位，并导致火烟完全分层。在这种情况下，火势烟雾和夹带的空气沿相反方向流动，导致隧道内双向流动。 此外，研究发现，入口通风速度随坡度或隧道长度的增加而增加，但在固定的绝对隧道高度差下，受其综合影响保持相对不变。最后，考虑到三种流型的分层特征，提出了一种半经验相关性来估计堆栈效应诱导的速度。通过与以往主要研究的多尺度实验和数值结果进行比较，验证了所提出的框架。研究结果揭示了倾斜隧道火灾中多向流型的形成机制，解析了烟雾输送特性，并从本质上说明了烟雾流动动力学。