【作者】 李萍；

【导师】 阳东；

【作者基本信息】 重庆大学 ， 供热、供燃气、通风及空调工程， 2021， 博士

【摘要】 近些年,由于地形限制以及地下空间的大量开发,倾斜隧道频繁出现。烟囱效应对倾斜隧道内烟气流动具有推动和阻碍作用,因此,与水平隧道相比,倾斜隧道内一旦发生火灾,烟气流动更为复杂,控制更为困难。纵向通风凭借其设备简单、模式转换方便等特点,被广泛用于隧道烟气控制。隧道发生火灾时,为保证隧道内滞留人员安全,纵向排烟方向应与行车方向一致。然而,当下行倾斜隧道采用纵向通风时,机械风压与热压作用方向相反,两者可形成对抗作用,从而可能导致隧道内出现多种烟气流动模式,甚至可能出现与预期排烟方向相反的流动模式。本课题主要研究纵向通风作用下倾斜隧道内火灾烟气行为的多样性以及其相应的通风控制方法。对于水平隧道,在火灾发生初期,为了确保下游区域滞留人员的疏散救援,隧道内上游横断面处的纵向风速应小于临界风速,但应达到限制风速。然而,由于倾角的存在,倾斜隧道内采用限制风速时,其烟气流动特性与水平隧道差别较大。本文通过盐水实验发现,在倾斜隧道中,烟囱效应使得上游横断面处的纵向风速小于临界风速,但达到限制风速时,上游逆流层将不能被控制在一定长度范围内。此外,当隧道内上游入口处的纵向机械风压由限制风速所需风压增大至临界风速所需风压且流动达到稳定状态时,烟气仍将会灌满整个上游区域并且从上游开口处排出。这是由于在倾斜隧道中,当上游横断面处的纵向风速为限制风速时,烟气发展早期在上游区域形成的烟囱效应会抵消一部分机械风压,导致风压增大至临界风速所需机械风压后仍不能诱导上游的纵向风速达到临界风速,最终烟气威胁到整个火源上游区域。因此,在倾斜隧道内,若要实现预期的排烟模式,在火灾发生初期上游横断面处的纵向风速应达到临界风速。采用限制风速实现对上游逆流长度以及下游烟气分层稳定性的有效控制仅适用于水平隧道发生火灾的初期,在下行倾斜隧道中失效。倾斜隧道垂直高度方向上累积的热压与机械风压之间的对抗导致其发展稳定后出现多种流动模式。本文通过盐水实验以及Fire Dynamics Simulator(FDS)数值模拟揭示了倾斜隧道内最终形成的稳态流动模式与风机启动时间相关。本文指出,在下行倾斜隧道中,在相同的边界条件作用下,隧道内可能存在四种流动模式,分别为:风压主导无逆流,风压主导有逆流,热压主导无逆流,热压主导有逆流,其中风压主导有逆流为不稳定流动模式。对于下行倾斜隧道,风压主导无逆流模式为设计预期的排烟流动模式。本文通过突变理论以及势函数分析法,提出任何风机启动时间下隧道内仍可形成预期排烟模式的临界风压理论模型,同时也推导出了任何风机启动时间下隧道内均会形成热压主导流动模式的临界风压理论模型。并以理论模型得到的关键参数作为输入参数进行了实验以及FDS数值模拟研究。理论计算、实验以及数值模拟结果均表明,当机械风压处于仅存在风压主导流动模式的临界风压与仅存在热压主导流动模式的临界风压之间时,在相同的边界条件下,隧道内将会出现多种稳态流动模式,此时隧道内的稳态流动模式与风机启动时间相关。因此,在倾斜隧道发生火灾时,为了确保烟气流向不受风机启动时间的影响且达到预期排烟模式,机械风压应不小于仅存在风压主导流动模式的临界风压。此外,研究结果表明,仅存在风压主导流动模式的临界风压不随火源位置发生变化,仅存在热压主导流动模式的临界风压以及临界风速所需临界风压随火源距离下游出口距离的增加而增大。仅存在风压主导流动模式的临界风压、仅存在热压主导流动模式的临界风压以及临界风速所需临界风压均随火源功率的增加而增大。在隧道火灾中,烟气层头部的蔓延速度影响滞留人员的疏散安全。本文采用盐水实验研究发现,风机启动时间对倾斜隧道烟气动态行为会产生影响。当风机未启动时,烟气层向火源两侧蔓延。当风机启动后,上游烟气层头部蔓延减缓,逆流层逐渐达到其最大长度,随后上游逆流层向火源位置处后退,最终逆流层消失。在整个动态蔓延过程中,下游区域的烟气层头部蔓延速度主要分为两个阶段。当风机未启动时,下游烟气层头部的蔓延速度与火源热释放速率的三分之一次幂成正比;当风机启动后,下游烟气层头部的蔓延速度也与火源热释放速率有关,与风机诱导的上游入口处的纵向风速无关。对于上游区域,当风机未启动时,上游头部的蔓延速度与火源热释放速率的三分之一次幂成正比;当风机启动且上游烟气层头部向火源位置处后退时,烟气层头部后退速度仅与风机诱导的上游入口处的纵向风速成正比,与火源热释放速率无关。为了连接地面交通网络,城市多分支隧道通常具有多个匝道和一定的纵坡。因此,由于多分支隧道中部分区段存在纵坡,当其采用纵向通风时,热压与机械风压的竞争也会导致隧道内出现多种稳态流动模式。单个倾斜隧道烟气行为分析为探究具有纵坡的多分支隧道烟气蔓延行为奠定了基础。本文以火源位置为中心,将多分支隧道划分为三个特征区域,分别为火源所在的分支,防烟区域,排烟区域。通过水力计算方法,将超过三分支的多分支隧道特征化为三分支结构。此外,本文基于势函数分析法,对带有入口匝道的三分支结构的三种典型火灾场景与带有出口匝道的三分支结构的三种典型火灾场景分别提出一种可抑制烟气流动多样性的机械风压分配方案。同时,由理论模型可知,通过调节多分支隧道分岔点处的全压可实现对各分支机械风压的分配。最后,利用FDS数值模拟方法展示了具有纵坡的多分支隧道内存在多种稳态流动模式,并且证实了提出的多分支隧道机械风压分配方案能够消除火源位于不同位置时烟气流动的多样性,为具有纵坡的多分支隧道的纵向排烟提供了设计方法。更多还原

【Abstract】 In recent years,due to the limitation of terrain and the development of underground space,inclined tunnels appear frequently.In inclined tunnels,stack effect can drive and impede the movement of smoke.Therefore,compared with the horizontal tunnel,once a fire occurs in the inclined tunnel,the smoke flow is more complex and difficult to control.Longitudinal ventilation has been widely used in tunnel smoke control system on account of its simple equipment and convenient conversion.In order to ensure the safety of stranded personnel in the tunnel fire,the longitudinal smoke exhaust direction shall be consistent with the driving direction.However,when the longitudinal ventilation is applied to inclined tunnel,the direction of fan-induced pressure rise and stack effect is opposite,which can form an antagonistic effect and lead to multiple smoke flow patterns.This paper mainly studies the multiplicity of smoke movement in inclined tunnel fire under longitudinal ventilation and its corresponding ventilation control methods.In the initial stage of fire,in order to ensure the evacuation and rescue of stranded individuals in the downstream area,the longitudinal ventilation velocity should be smaller than the critical velocity in horizontal tunnels,but should achieve the confinement velocity.However,due to the existence of inclination,the smoke flow characteristics of inclined tunnel are quite different from those of horizontal tunnel when the confinement velocity is adopted.Through brine water experiment,it is found that in the inclined tunnel,the stack effect makes the longitudinal ventilation velocity at the upstream less than critical velocity,but can reach the confinement velocity,the upstream backlayer will not be controlled within a certain length range.In addition,when the fan-induced pressure rise at the upstream portal in the tunnel increases from the fan-induced pressure rise required for confinement velocity to the fan-induced pressure rise required for critical velocity,and the flow reaches a stable state,the smoke will fill the whole upstream area and be discharged from the upstream portal.This is because in the inclined tunnel,when the longitudinal ventilation velocity at the upstream is equal to confinement velocity,the stack effect formed in the upstream area in the early stage will offset part of the mechanical fan-induced pressure rise.This phenomenon will lead the longitudinal ventilation velocity at the upstream to fail to reach the critical velocity after the fan-induced pressure rise increases to the fan-induced pressure rise required for critical velocity,and finally the smoke threatens the whole upstream area of the fire source.Therefore,in the inclined tunnel,in order to achieve the expected smoke exhaust mode,the longitudinal ventilation velocity at the upstream at the initial stage of fire occurrence should reach the critical velocity.The effective control of upstream backlayering length and downstream smoke stratification stability by confinement velocity is only applicable to the initial stage of fire occurrence in horizontal tunnel and hardly apply to initial stage of fire occurrence in downward inclined tunnel.In inclined tunnel,the competition between fan-induced pressure rise and stack effect accumulated in the vertical height direction leads to multiple steady-state flow patterns.Through brine experiment and fire dynamics simulator(FDS)numerical simulation,this paper reveals that the final flow state in the tunnel is related to the fan activation time.This paper points out that in the downward inclined tunnel,under the same boundary conditions,there may be four flow modes in the tunnel,namely:fan-dominated flow pattern without backlayering,fan-dominated flow pattern with backlayering,buoyancy-dominated flow pattern with backlayering,buoyancy-dominated flow pattern without backlayering,in which fan-dominated flow pattern with backlayering is an unstable flow mode.For the downward inclined tunnel,the fan-dominated flow pattern without backlayering is the expected smoke exhaust flow mode.Through catastrophe theory and potential function analysis method,this paper puts forward the theoretical model of critical fan-induced pressure rise that can still reaches the expected smoke exhaust mode in the tunnel under any fan activation time,and deduces the critical fan-induced pressure rise that any fan activation time will lead to the buoyancy-dominated flow pattern.The key parameters obtained from the theoretical model are used as input parameters for experiments and FDS numerical simulation.The results of theoretical analysis,experiment and numerical simulation show that when the mechanical fan-induced pressure rise is between the critical fan-induced pressure rise for the only existence of fan-dominated flow pattern and the critical fan-induced pressure rise for the only existence of buoyancy-dominated flow pattern,there will be multiple flow patterns in the tunnel under the same boundary conditions.At this time,the steady-state flow mode in the tunnel is related to the activation time of the fan.Therefore,in order to ensure that the smoke flow direction is not affected by the fan activation time in inclined tunnel fire and reaches the expected flow mode,the mechanical fan-induced pressure rise shall not be less than the critical fan-induced pressure rise for the only existence of fan-dominanted flow pattern.In addition,the results show that the critical fan-induced pressure rise for the only existence of fan-dominated flow pattern does not change with the fire source location,and the critical fan-induced pressure rise for the only existence of buoyancy-dominanted flow pattern and the critical fan-induced pressure rise required by the critical velocity increase with the increase of the distance from the fire source to the downstream portal.The critical fan-induced pressure rise for the only existence of fan-dominanted flow pattern,the critical fan-induced pressure rise for the only existence of buoyancy-dominanted flow pattern and the critical fan-induced pressure rise required for critical velocity all increase with the increase of fire source power.In tunnel fire,the propagation speed of the head of smoke layer determines the effective time and safety for the evacuation of stranded individuals.It is found that the fan activation time will affect the dynamic behavior of smoke in inclined tunnel.When the fan is not activated,the smoke spreads to both sides of the fire source.When the fan is activated,the propagation speed of the upstream smoke layer slows down,and the backlayer gradually reaches its maximum length.Then,the upstream backlayer retreats to the fire source,and finally the backlayer disappears.In the whole dynamic spreading process,the propagation speed of the head of smoke layer in the downstream area is mainly divided into two stages.When the fan is not activated,the propagation speed of the head of the smoke layer is directly proportional to the one-third power of the heat release rate of the fire source;When the fan is activated,the propagation speed of the head of the smoke is related to the heat release rate of the fire source,and independent of the longitudinal ventilation velcity at the upstream portal induced by the fan.For the upstream area,when the fan is not activated,the propagation speed of the upstream smoke head is directly proportional to the one-third power of the heat release rate of the fire source;When the fan is activated and the head of the upstream smoke retreats to the fire source,the retreating speed of the head of the smoke is only related to the longitudinal ventilation velocity at the upstream portal induced by the fan,and independent of the heat release rate of the fire source.In order to connect the ground transportation network,urban multi-branch tunnels usually have multiple ramps and a certain slope.Therefore,due to the existence of slope in multi-branch tunnel,when longitudinal ventilation is adopted,the competition between stack effect and mechanical fan-induced pressure rise will also lead to a variety of steady-state flow modes in the tunnel.The analysis of smoke behavior characteristics of single inclined tunnel lays a foundation for exploring the smoke propagation behavior of multi-branch tunnel with longitudinal slope.Taking the fire source location as the center,this paper divides the multi-branch tunnel into three characteristic regions,the branch where the fire located,smoke prevention region and smoke exhaust region,respectively.Through the hydraulic calculation method,the multi-branch tunnel with more than three branches is characterized into a three branch structure.Furthermore,based on the potential function analysis method,a general optimization fan-induced pressure rise combination scheme which can inhibit the diversity of smoke in multi-branch tunnel is proposed for three typical fire source scenarios of three branch structure with on-ramp and three typical fire source scenarios of three branch structure with off-ramp.In addition,according to the theoretical model,the fan-induced pressure rise of each branch can be adjusted by changing the total pressure at the bifurcation point of multi-branch tunnel.Finally,FDS numerical simulation method is used to show that there are many steady-state flow modes in multi-branch tunnel,and proved that the ventilation scheme of multi-branch tunnel can eliminate the diversity of smoke flow when the fire source is located at different positions,and provides a design method for longitudinal smoke exhaust of multi-branch tunnel with longitudinal slope.更多还原