竖井截面尺寸对烟囱效应极限高度的影响研究

为研究隧道发生火灾时竖井截面尺寸的变化对隧道烟囱效应极限高度的影响，运用FDS对竖井截面形状为正方形时的6种不同截面尺寸的竖井进行了数值模拟，分析不同位置烟气运动速度随时间的变化。研究得出：竖井烟囱效应存在一个极限高度，且其随着竖井截面尺寸的增大而增大；根据模拟数据拟合得到了函数表达式并进行了模拟验证，可以用于估算0.4 m≤竖井边长≤3.0 m条件下竖井烟囱效应的极限高度。     

火源与排烟竖井的相对位置对重点排烟效果的影响研究

借助理论分析及数值模拟相结合的手段对某盾构城市地下道路火灾工况下，火源与排烟竖井的相对位置对重点排烟效果的影响进行模拟研究。研究结果表明单向排烟模式在一定的排烟量条件下，火源位置距离排烟竖井越近，排烟效果越好。当火源位于两个排烟竖井之间且偏向其中某一个竖井位置时，在排烟量允许的前提下，可以仅开启离火源位置较近的竖井风机排烟；当火源位于两竖井正中间位置时，可采用双向排烟模式；当火源恰好位于某个竖井正下方时，可以仅开启该竖井风机排烟，但采取该工况应综合考虑隧道内烟道板的耐火极限及耐火时间。     

高速地铁隧道压力波研究

根据地铁隧道结构,基于一维特征线理论,建立了含地铁区间隧道及竖井的隧道压力波计算模型。采用数值计算方法对地铁隧道压力波进行了研究,分析了列车运行速度、隧道横截面积、竖井横截面积及密封指数对车内压力变化的影响,根据压力变化的舒适度标准,给出了不同列车运行速度条件下高速地铁隧道设计参数的建议。     

外界风下竖井—走廊空间烟气蔓延特征研究

本文采用FDS火灾模拟软件针对外界风作用下竖井走廊空间的火灾烟气蔓延扩散特征进行研究,发现外界风、火源位置以及竖井窗口开启高度等对走廊内烟气能否进入竖井形成烟囱效应具有重要影响.外界风对走廊烟气进入竖井形成烟囱效应具有抑制作用,火源位于走廊中间时,利于积聚更多烟气,流入竖井形成烟囱效应.随着竖井窗口开启高度升高,烟囱效...     

秦岭终南山公路隧道火灾试验竖井通风的研究

为了弄清设有竖井的长大隧道发生火灾时隧道内温度及烟流的分布规律,在长大隧道发生火灾时能制定合理的隧道火灾救援方案,特进行秦岭终南山公路隧道火灾试验坚井通风的研究,此次试验按照流变规律模拟隧道火灾情况,进行三种火灾规模在三处位置的火灾实验,通过实验得出火灾时隧道内温度与火灾规模成正比,火灾火源点的最高温度与竖井的高度成正比关系,火灾的持续时间与竖井的高度成反比关系,找出了竖井通风情况下隧道火灾时温度纵向分布的规律。为隧道火灾时救援方案的制定提供依据。     

纵向通风与坡度对隧道竖井排烟影响数值模拟

以某公路隧道为研究对象,采用开启6个竖井的双向均衡排烟模式。利用FDS对纵向通风与坡度影响下的竖井排烟效果进行数值模拟,通过分析不同工况下竖井内的烟气扩散特性、温度场分布及烟气质量浓度变化,获得隧道内竖井排烟速率的变化规律。结果表明:火源位于隧道中间时,在无纵向通风和纵向风速较小时,竖井下方均会出现烟气层吸穿现象,排烟速率较低;风速增加,火源下游的竖井排烟速率较大;风速大于2.0 m/s时,火源下游的竖井出现边界层分离现象,排烟速率降低;改变隧道坡度并不影响竖井下方的自然排烟效果。     

火源—竖井间距对自然通风隧道火灾烟气流动特征的影响

通过数值计算,研究顶部开口自然通风隧道火灾火源–竖井间距对烟气流动特征与竖井排烟效率的影响。考虑因素有火源–竖井间距、竖井断面尺寸。结果表明：随着火源–竖井间距的增大,竖井前方来流烟气的质量流量增大,且竖井的排烟效率逐渐降低,竖井内空气卷吸量减少;当火源–竖井间距较小时,竖井更有利于排出更多的热量,竖井后方的温度降低幅度更大,烟气可以被控制在更小的范围内。此外,随着竖井截面尺寸的增大,竖井的排烟效率增加,且增大竖井的宽度更有利于增加竖井的排烟量。因此建议当相邻竖井的间距较大时,可适当增加竖井的截面尺寸和竖井高度。     

高层建筑火灾中电梯竖井烟气流动特性的模拟研究

高层建筑中诸多竖井,比如楼梯间、电梯竖井,是建筑火灾中烟气竖向蔓延的主要通道。在烟囱效应的作用下,若高层建筑竖井设计合理,利用电梯竖井进行自然排烟具有一定的可行性。本文利用正交实验的方法对"典型建筑"在"标准火源"条件下的烟气流动特性进行场-网模拟,电梯竖井作为场区域,其它空间作为网络区域,采用CONTAM软件实现了两区域之间的耦合计算。模拟计算得到了竖井顶部开口面积、各楼层送风量、电梯门密封性以及不同着火楼层对电梯竖井内烟气流动的影响。计算结果表明,竖井顶部开口面积对自然排烟量与中和面位置影响程度均最大。     

竖井内羽流上升驱动力实验分析

为得到竖井内烟囱效应与浮力对羽流上升造成的影响,搭建了小尺寸竖井火灾实验平台,测量了不同燃料、不同油池尺寸条件下两端开放及底部封闭竖井内温度分布,对各驱动力进行了理论分析.结果表明:两端开放竖井内火灾热烟气上升受到烟囱效应与自身浮力的共同作用,且烟囱效应驱动力占主导;底部封闭竖井内热烟气上升仅靠自身浮力作用;两端开放竖井内烟气受到向上的驱动力更大.在相同火源及竖井尺寸条件下,两端开放竖井内向上驱动力与底部封闭竖井内向上驱动力的比值随油池直径的变大而增大,平均斜率为0.32.     

火灾工况下公路隧道竖井通风模式试验研究

为了建立火灾工况下有效的竖井通风模式,通过大比例火灾模型试验,对不同通风模式下,主隧道、风道及竖井内温度场的传播分布、烟流蔓延扩散规律进行了研究。试验模型隧道长100 m,内径1.8 m,设有直径1 m的送风竖井、排风竖井各一座。火源采用燃烧床盛放油料模拟,试验中设定了A、B、C三个火灾规模用以模拟实际隧道火灾场景,考虑了三个火灾位置:火灾位置I、II和III。试验结果表明随着通风风速、火灾规模、火灾位置的不同,隧道、通风道及竖井内温度场分布及烟流流动差异很大,而且随着时间的推移,其分布发生显著变化。这表明当隧道中发生火灾时,应根据火灾点与竖井的相对位置分阶段,实施不同的通风模式。基于试验结果,建议了秦岭隧道火灾时的有效通风模式。     

Modeling for predicting the temperature distribution of smoke during a fire in an underground road tunnel with vertical shafts

In this study, fire experiments using a 1:20 model-scale tunnel were conducted to investigate the performance of natural ventilation in an underground road tunnel with six vertical shafts. The experimental parameters were the heat release rate of a fire source and the height of the shafts, and nine experiments were conducted in total. Furthermore, simple models were developed for predicting the temperature distribution of the smoke flowing under the tunnel ceiling. The following results were obtained: (1) In the experiments, the form of the smoke exhausted from the shaft became plug-holing when the shaft height was 1.0H_t, and became boundary layer separation when the height was 0.24H_t. (2) The average efficiency of heat exhaust was 0.16 when the form was plug-holing, and was 0.12 when the form was boundary layer separation. (3) When the form was plug-holing, the ratio of entrainment of fresh air became almost constant regardless of Ri. On the other hand, when the form was boundary layer separation, the ratio of entrainment of fresh air was smaller than that under the condition of plug-holing. (4) The temperature distribution under the tunnel ceiling predicted by the models agreed with that measured by the fire experiments in all cases.     

排烟口附加组合挡板对隧道火灾排烟效果的模拟研究

建立单孔隧道模型,采用FDS 数值模拟分析不同高度的组合挡板对隧道机械排烟的影响。结果表明,相较于普通排烟口,安装组合挡板可以有效提高隧道机械排烟效率,改善排烟口处气体压力场的分布状态,减小垂直方向的排烟风速,加大水平方向的排烟风速,从而降低吸穿现象发生概率。经模拟验证,发现组合挡板高度是影响吸穿现象发生的重要因素,组合挡板会使排烟口前方烟层厚度变小,当烟层厚度小于挡板高度时就会发生吸穿现象。通过比较挡板高度分别为50、65、80、95、110 cm 几种情况的模拟效果,发现H=50 cm 工况排烟口处的烟气浓度最高,单位时间排烟量最大,排烟效果最好。     

滨海软黏土加速蠕变特性试验研究

为了研究滨海软黏土加速蠕变特性,采用典型的滨海相软黏土进行室内蠕变和加速蠕变试验,研究不同围压、加荷比、振动频率、动应力比对软黏土蠕变特性的影响,并通过对比得出了不同试验条件下滨海软黏土的加速蠕变的变化规律。试验结果表明:滨海软黏土具有非线性应力–应变关系的流变特性,其蠕变试验曲线呈衰减型,蠕变变形大部分应变量发生在蠕变前期,变形随时间增长趋于一个定值;静载作用下,围压越大初始蠕变速率越大。初始加荷压力一致,加荷比越大破坏应力越小。循环动荷载作用下,应变在动荷载阶段变化缓慢,而动荷载结束后进入稳定阶段后应变发生突变,急剧变大最后趋于稳定;此外在不同振动频率和动应力比下试样承受荷载等级不同,均存在一个临界安全荷载,超过这个临界值试样会发生蠕变破坏现象。     

室外风对高层建筑竖井内烟气运动影响的数值模拟

采用火灾模拟专业软件FDS对不同火源位置、不同风向条件下火灾烟气的运动进行模拟,测定典型位置处温度、速度、CO及CO2体积分数变化情况。实验结果表明:在近地风场中,风向对竖井内烟气蔓延的影响大小顺序为迎风>背风>侧风,竖井开口位于迎风面时,外界风对竖井内烟气运动影响最大:火源位于中性面以上时,烟气通过竖井与前室的开口向竖井内蔓延,并向下运动;而火源位于中性面以下时,前室内烟气向外部运动,竖井内无烟气流入。     

Continuous monitoring of the dynamic behavior of a high‐rise telecommunications tower

This article describes the experimental evaluation of the dynamic effects induced by wind on a high‐rise telecommunications tower based on a permanent monitoring system. Monte da Virgem telecommunications tower is located near the city of Porto (Portugal), and its structure consists in a reinforced concrete shaft and a steel mast, with a total height of 177 m. The monitoring system includes accelerometers, anemometers, and a meteorological station, allowing the characterization of the maximum accelerations of the structure and wind regimes during a period of 6 months. The analysis of the results enabled identifying specific events, denominated as critical events, for which the dynamic response of the tower under wind actions appears significantly amplified due to wind aeroelastic instability phenomena in the steel mast. The automatic identification of the critical events was based on the application to the acceleration's records of an autoregressive model and estimation of its optimal order number based on a singular value decomposition. The results proved the robustness and efficiency of the proposed technique in identifying the number, duration, and maximum amplitude of accelerations associated to the critical events, envisaging its potential integration in structural health monitoring systems.     

Control of smoke flow in tunnel fires using longitudinal ventilation systems – a study of the critical velocity

The “critical velocity” is the minimum air velocity required to suppress the smoke spreading against the longitudinal ventilation flow during tunnel fire situations. The current techniques for prediction of the values of the critical velocity for various tunnels were mainly based on semi-empirical equations obtained from the Froude number preservation combining with some experimental data. There are a few uncertainties in the current methods of prediction of the critical ventilation velocity. The first is the influence of the fire power on the critical ventilation velocity. The second is the effect of the tunnel geometry on the critical velocity. Both problems lead to the issues of the scaling techniques in tunnel fires. This study addressed these problems by carrying out a series of experimental tests in five model tunnels having the same height but different cross-sectional geometry. Detailed temperature and velocity distributions in the tunnels have been carried out. The experimental results showed that the critical velocity did vary with the tunnel cross-sectional geometry. It was also shown clearly that there are two regimes of variation of critical velocity against fire heat release rate. At low rates of heat release the critical velocity varies as the one-third power of the heat release rate, however at higher rates of heat release, the critical velocity becomes independent of fire heat release rate. Analysis of the distribution of temperature within the fire plumes showed that there were two fire plume distributions at the critical ventilation conditions. The change of the fire plume distribution coincided with the change of the regime in the curves of the critical velocity against fire heat release rate. The study used dimensionless velocity and dimensionless heat release rate with the tunnel hydraulic height (tunnel mean hydraulic diameter) as the characteristic length in the experimental data analysis. It was shown that the experimental data for the five tunnels can be correlated into simple formulae which can be used for scaling. The new scaling techniques are examined by applying the scaling techniques to the present experimental results and three large-scale experimental results available in the public literature. A good agreement has been obtained. This suggests that the scaling techniques can be used with confidence to predict the critical ventilation velocity for larger-scale tunnels in any cross-sectional geometry. Comprehensive CFD simulations have been carried out to examine the flow behaviour inside the tunnels. Validation against the experimental results showed that the CFD gave slightly lower but satisfactory prediction of the flow velocity. However the temperature prediction in the fire region was too high. The findings from the CFD simulations supported the ones from experimental tests.     

地应力对深大竖井稳定性影响研究

长大公路隧道通风竖井通常建设在地应力作用的岩体区域,地应力状态是影响竖井工程稳定性最重要的因素之一。为探明高地应力状态对深大竖井围岩的应力演化和变形规律的影响,依托米仓山特长隧道通风竖井工程,采用数值模拟对不同应力系数的竖井施工过程进行分析,研究了不同应力条件下大断面竖井沿不同深度的围岩应力特征及径向位移规律。研究结果表明:相差较大地应力系数对围岩位移分布规律产生较大影响;围岩径向位移与围岩级别及竖井深度呈正相关性,围岩级别变差比竖井深度增大对围岩径向位移的影响更为明显;但当竖井深度超过200 m时,随地应力系数增大竖井径向位移速率急剧增大;不同围岩级别条件下随竖井深度和应力系数的增大,径向应力和切向应力都呈增大趋势,但增大趋势较平缓;研究成果可为类似竖井工程的科研、设计和施工提供参考。     

Analysis of lateral earth pressure on a vertical circular shaft considering the 3D arching effect

The lateral earth pressure on a vertical circular shaft is investigated using both experiments and numerical analyses. The study focused on quantifying the magnitude and distribution of the lateral earth pressure, which was measured by considering the three-dimensional arching effect. A framework for determining distribution of the earth pressure based on centrifuge model tests and 2D FE analysis is introduced. The FE modelling techniques and the constitutive relationships of the soil are presented in detail. Parametric analyses showed that the arching effect on the lateral earth pressure is highly dependent on the diameter and height of the shaft, the internal friction angle and the cohesion value of the soil, the end-bearing conditions and the flexural modulus of the shaft. The study found that when the arching effect is considered, the lateral earth pressure on a vertical circular shaft is approximately 80% less than that calculated using Rankine’s theory. The study also found that the arching effect of the soil is more significant for flexible vertical shafts than for rigid vertical shafts embedded in weathered soil.     

Smoke flow bifurcation due to opposing buoyancy in two horizontally connected compartments

We consider two horizontally connected compartments with four openings and one compartment on fire; the height of one compartment is fixed, while that of the other varies. A simple small-scale experimental study using air was performed to demonstrate the existence of two stable flow solutions at the same height ratio. Moreover, a macroscopic zonal model is used to theoretically analyze smoke flow direction and smoke mass flow rate. Under different ranges of height ratio, four patterns of smoke flow are identified and studied. One of the solutions is unstable, while the other three solutions are stable, as shown by nonlinear dynamic analysis. Therefore, two stable solutions of smoke flow exist at the same height ratio and ambient conditions when the height ratio is larger than a critical value, and the critical value is shown to be a transcritical bifurcation point. The implications of smoke flow bifurcation on smoke control in an atrium or building with a stairwell are also discussed.