公路隧道火灾的数值模拟

应用火灾动力学模拟软件FDS对某公路隧道内的火灾过程进行了数值模拟，定量预测了不同工况条件下隧道内的烟气流动状况及温度分布，并在此基础上对隧道通风方式、耐火性能，以及火灾探测报警中的温度点漂移问题进行了讨论。     

一起隧道火灾模拟及灭火行动中排风机的干预

隧道火灾是消防部队在灭火救援行动中所遇上的一种特殊火灾，呈现以下特点：（1）隧道气流速度大，加之燃烧释放的热量不易散发，起火后热量迅速积累，隧道内温度骤升，导致火势发展迅猛；（2）通道狭长，大量烟气难以排出，人员疏散困难；（3）由于特殊火灾行为的影响，易加速火焰传播速度，隧道纵向坡度较大，一旦起火很容易形成烟囱效应。温度和烟会迅速传播，它的大部分能量被用以加热、流动通风的空气；（4）火灾扑救困难；（5）火灾损失大。     

含渐缩段卜形分岔合流隧道火灾临界风速研究

为探究“卜”形分岔隧道这一特殊隧道结构对隧道火灾临界风速的影响,运用FDS构建了主路渐缩分岔隧道、主路等宽分岔隧道与直线隧道3种结构的缩尺寸隧道模型,通过数值模拟分析隧道渐缩结构与分岔角度对火灾临界风速的影响。研究表明,对于主路渐缩的分岔隧道,当火源所在位置的局部隧道宽度减小时,所需的临界风量变小。而火源位置确定时,隧道的渐缩结构、分岔角度和分岔结构对临界风速的影响不明显,并提出一种适用于隧道工程渐缩段任意火源位置临界风量的计算公式。对于主路位置的火灾,提出无量纲临界风速与无量纲热释放速率的关系式,与前人直线隧道的变化规律相似,而较高的隧道高度导致临界风速的转折点较大。     

Flow around a cube in a turbulent boundary layer: LES and experiment

We present a numerical simulation of flow around a surface mounted cube placed in a turbulent boundary layer which, although representing a typical wind environment, has been specifically tailored to match a series of wind tunnel observations. The simulations were carried out at a Reynolds number, based on the velocity U at the cube height h, of 20,000—large enough that many aspects of the flow are effectively Reynolds number independent. The turbulence intensity was about 18% at the cube height, and the integral length scale was about 0.8 times the cube height h. The Jenson number Je=h/z₀, based on the approach flow roughness length z₀, was 600, to match the wind tunnel situation. The computational mesh was uniform with a spacing of h/32, aiding rapid convergence of the multigrid solver, and the governing equations were discretised using second-order finite differences within a parallel multiblock environment. The results presented include detailed comparison between measurements and LES computations of both the inflow boundary layer and the flow field around the cube including mean and fluctuating surface pressures. It is concluded that provided properly formulated inflow and surface boundary conditions are used, LES is now a viable tool for use in wind engineering problems concerning flow over isolated bodies. In particular, both mean and fluctuating surface pressures can be obtained with a similar degree of uncertainty as usually associated with wind tunnel modelling.     

Large eddy simulation of flow past rectangular-section cylinders: Side ratio effects

In this paper, the flow characteristics around rectangular-section cylinders with different side ratios (R=width/height) of 0.4-4 are studied for Re=10⁵. The effect of Reynolds number is also examined for some cases. Unsteady and three-dimensional computations are performed using large eddy simulation (LES) with two subgrid-scale models, the Smagorinsky (S-Model) and a one-equation model (OE-Model). An incompressible three-dimensional (3D) finite volume code with a collocated grid arrangement is used for solving the filtered Navier-Stokes equations. These equations are solved with an implicit fractional two-step method. A series of instantaneous and time- and spanwise-averaged resolved velocity, pressure, turbulent stresses, and streamlines are provided. Time-averaged global quantities such as the Strouhal number, the mean and the RMS values of drag force, the base suction pressure, the lift force, and the pressure coefficient are also calculated and compared with available experimental results.     

Numerical analysis of different ventilation schemes during the construction process of inclined tunnel groups at the Changheba Hydropower Station,China

During the excavation process of underground caverns, the rational selection of the ventilation scheme is very important for the safety and health of construction workers. The flood discharge tunnel groups at the Changheba Hydropower Station are selected as a case to study the design of ventilation schemes in inclined tunnel groups; these groups are characterized by a gradient of approximately 10% and a complex intersecting relationship among the tunnels. The Computational Fluid Dynamics (CFD) method is used to simulate the fluid dynamics in tunnel groups when different ventilation schemes are employed. Four ventilation schemes with the same duct at different positions along the transverse section are formulated, and the scheme approaching the right side with most of the construction adits is adopted in engineering after a comparative analysis, as it offers a well-distributed velocity field and sufficient security distance. The study reveals that flow vortices appear in the tunnels with a long axis length ranging from 5 m to 20 m; the observation that the flow velocity on the transverse sections is away from the heading face indicates that a low-velocity area is always present in the vicinity of an air duct, and the security distance on the upstream side is 60% shorter than on the downstream side with the same air-blower when the tunnels have a 10% gradient. In addition, when the excavation distance rises 200 m, the ventilation condition in the tunnels, especially in the areas around tunnel intersections, is greatly improved by the completion of pilot tunnels and shafts in advance.     

基于计算流体动力学方法的风荷载对高层钢建筑影响的数值评价

进行了风对联邦航空咨询委员会(CAARC)标准高层建筑影响的综合数值研究。采用计算流体动力学(CFD)技术,如大涡流模型(LES)、RANS方程模型等,用于预测风荷载的作用以及风在建筑物周围流动的规律。本研究的主要目的是探索有效可靠的方法来利用CFD技术评估风对高层建筑的影响。计算结果与大量的试验数据进行了比较(这些试验数据来自7个风洞试验),并对引起数值预测与试验数据误差的原因进行了识别和讨论。通过比较发现,采用LES和SGS模型可以得到满意的作用在高层建筑上的平均或脉动风载预测值;修正的RANS模型在大多数情况下可以得到较好的结果而且具有快速解决问题的优势。此外,表面有起伏的建筑物的空气边界层气流场的典型特征可以用数字表达。分析发现,入射风的速度分布主要影响建筑物的平均压力系数,而入射风的紊流强度分布对风力的变化有显著影响。因此,需要正确模拟入射风风速分布及紊流强度分布,采用CFD方法精确预测风对高层建筑的影响。CFD技术和相关的数字处理方法为设计者提供了一个有效的方法来估计风对高层建筑的影响,并满足详细的风洞试验的要求。     

Influence of floor motions in wind tunnels on the aerodynamics of road vehicles

The effect of a moving floor on the flow around a simplified car with a typical fastback geometry is investigated. Two large-eddy simulations of the flows with stationary and moving floors are made and both instantaneous and time-averaged results are compared. It is found that the floor motion reduces drag by 8% and lift by 16%. Changes in the flow are found to be global but are largest close to the floor and on the rear slanted surface of the vehicle. The wake flow is found to be relatively insensitive to the floor movement, in agreement with previous experimental observations. The periodicity of the flow events is found to be dependent on whether the floor is moving. Power spectral density of both the lift and the drag contain only one dominant frequency peak when the moving floor is adopted as compared to scattered spectra in the stationary floor case. Changes in the qualitative picture of the flow are limited to the flow near the floor and on the slanted surface of the body. However, changes in the surface pressure on the body and the history of the flow show the need of a moving floor in experimental and numerical simulations.     

Using large eddy simulation to study particle motions in a room

As people spend most of their time in an indoor environment, it is important to predict indoor pollutant level in order to assess health risks. As particles are an important pollutant indoors, it is of great interest to study the airflow pattern and particle dispersion in buildings. This study uses large eddy simulation (LES) to predict three-dimensional and transient turbulent flows and a Lagrangian model to compute particle trajectories in a room. The motion of three different types of solid particles in a decaying homogeneous isotropic turbulent airflow is calculated. By comparing the computed results with the experimental data from the literature, the computational method used in this investigation is found to be successful in predicting the airflow and particle trajectories in terms of the second-order statistics, such as the mean-square displacement and turbulent intensity. This Lagrangian model is then applied to the study of particles' dispersion in a ventilated cavity with a simplified geometry for two ventilation scenarios. It is shown that light particles follow the airflow in the room and many particles are exhausted, while heavier particles deposit to the floor or/and are exhausted. PRACTICAL IMPLICATIONS: The results of this paper can be used to study dispersion of infectious diseases in enclosed spaces in which virus or bacteria are often attached to particles and transported to different rooms in a building through ventilation systems. In most of studies, the virus or bacteria have been considered to be gaseous phase so there is no slip between virus/bacteria and air. The results in this paper show that heavier particles are submitted to gravity and are sensitive to the ventilation strategy.     

Coherent structures of the flow around a surface-mounted cubic obstacle in turbulent channel flow

The coherent structures of the flow developing around a surface-mounted cubic obstacle placed in turbulent channel flow (Re=40.000 based on bulk velocity and channel half-width) are investigated. The technique of the proper orthogonal decomposition is applied to extract the coherent structures of the flow from a database obtained with the use of a finite volume computational code for the numerical integration of the three-dimensional time-dependent incompressible Navier-Stokes equations; for turbulence modeling the LES approach is followed and the RNG subgrid-scale closure is used. The three-dimensional time-dependent velocity field is computed and 100 non-dimensional time steps of the turbulent statistically steady state are considered. The decomposition is performed on two appropriate subdomains around the cubic obstacle and a “reduced” velocity field, reflecting the contribution of the most energetic eigenfunctions of the decomposition, containing up to 95% of the turbulent kinetic energy of the original flow field, is reconstructed; results are presented in terms of vorticity showing the temporal dynamics of the coherent structures in the selected subdomains.     

自动喷水灭火系统在地铁隧道工程应用的可行性

分析了地铁隧道火灾的特点和隧道消防设施设置的现状，提出隧道内设置自动喷水灭火系统的设想，运用场模拟FDS程序，通过一假定的隧道火灾算例，分析比较了设置自动喷水灭火系统和不设置时隧道火灾的烟气、温度发展情况，得到了两种情况下隧道内温度和烟气对人构成的威胁和对隧道结构造成的破坏情况。结果表明，在地铁隧道内设置自动喷水灭火系统有利有弊，但综合考虑设置自动喷水灭火系统是可行的。     

基于FDS的高速公路隧道火灾人员疏散研究

由于高速公路隧道狭长且封闭的管状空间结构形式,一旦发生由交通事故引发的火灾将会对隧道内人员的生命安全构成严重威胁.文章基于FDS分析不同火灾工况下隧道内的温度、CO浓度以及能见度变化,利用Pathfinder软件模拟不同车辆堵塞密度和疏散方向以及疏散人数下人员的疏散逃生过程,分析影响疏散时间的因素.通过研究得到不同工况...     

One-dimensional analysis of the global chimney effect in the case of fire in an inclined tunnel

A one-dimensional computational analysis is presented of the global chimney effect due to a fire source in an inclined tunnel. Fire-induced flows are not accounted for in the present study: the computed velocity, due to global heating, is to be superposed as an initial and/or boundary condition in detailed computational fluid dynamics (CFD) simulations in a region of interest. The results must not be used as a replacement for such detailed simulations in e.g. network problems. Two calculation options are explained for the global chimney effect. The first method is based on the global heating of the entire tunnel part between the fire source and the higher positioned tunnel outlet. In the other method, the initial fire-influenced region is restricted to a certain distance and, as soon as a prescribed temperature rise is met, the fire-influenced region length increases until the tunnel outlet is reached. The treatment of the enthalpy flow rate through the fire-influenced region boundaries is discussed. In order to identify the most important parameters and to quantify their influence on the global chimney effect, a sensitivity study is presented with respect to the different model ingredients. The following quantities are examined: fire source heat release rate, tunnel inclination angle, tunnel length, tunnel wall roughness, ambient temperature, ambient pressure conditions, tunnel cross-section area, and fire source position. Attention is focused onto the fire-influenced region mean temperature and the air velocity at the tunnel inlet. The inlet air velocity is quantified by means of a correlation, taking into account the parameters mentioned.     

Large eddy simulation and zonal modeling of human-induced contaminant transport

An immersed boundary method for particulate flow in an Eulerian framework is utilized to examine the effects of complex human motion on the transport of trace contaminants. The moving human object is rendered as a level set in the computational domain, and realistic human walking motion is implemented using a human kinematics model. A large eddy simulation (LES) technique is used to simulate the fluid and particle dynamics induced by human activity. Parametric studies are conducted within a Room-Room and a Room-Hall configuration, each separated by an open doorway. The effects of the average walking speed, initial proximity from the doorway, and the initial mass loading on room-to-room contaminant transport are examined. The rate of mass transport increases as the walking speed increases, but the total amount of material transported is more influenced by the initial proximity of the human from the doorway. The Room-Hall simulations show that the human wake transports material over a distance of about 8 m. Time-dependent data extracted from the simulations is used to develop a room-averaged zonal model for contaminant transport due to human walking motion. The model shows good agreement with the LES results. PRACTICAL IMPLICATIONS: The effect of human activity on contaminant transport may be important in applications such as clean or isolation room design for biochemical production lines, in airborne infection control, and in entry/exit into collective protection or decontamination systems. The large eddy simulations (LES) performed in this work allow precise capturing of the local wakes generated by time-dependent human motion and thus provide a means of quantifying contaminant transport due to wake effects. The LES database can be used to develop zonal models for the bulk effects of human-induced contaminant transport. These may be incorporated into multi-zone infiltration models for use in threat-response and exposure mitigation studies.     

Large eddy simulation of compartment fire with solid combustibles

For properly describing practical building fire processes with solid combustibles, the pyrolysis kinetics model of solid combustibles and the large eddy simulation (LES) approach are applied to the simulation of the thermal decomposition of the polyurethane foam (PUF) slab and the space fire spread in a compartment. The instantaneous variations of the heat release rate of the PUF slab, the smoke temperature, and the smoke interface height with time are obtained under different ventilation conditions. They are in agreement with the measured data. The ventilation conditions have distinct effects on the interactions between the pyrolysis of the PUF slab and the space fire spread. Influenced by the space fire spread, the heat flux on the top plane of the PUF slab exhibits a non-uniform distribution. The PUF slab is consumed in an asymmetric manner.     

Experimental data and numerical modelling of 1.3 and 2.3 MW fires in a 20 m cubic atrium

Atria and large spaces are common architectonical features in modern buildings such as high rises, auditoria, warehouses, airports and mass transport stations among others. There is currently an international trend towards the performance-based design for fire safety of these building elements. This design process relies heavily on fire modelling but the knowledge in fire dynamics and the movement of smoke in atria and large spaces still presents some gaps. This paper aims at contributing to close these gaps and reports the three Murcia Atrium Fire Tests conducted in a 20 m cubic enclosure using pools of 1.3 and 2.3 MW. Detailed transient measurements of gas and wall temperatures, as well as pressure drop through the exhaust fans and airflow at the inlets were recorded. The study also includes the effect of the mechanical exhaust ventilation. Results have been compared with those predicted by the computational fluid dynamics (CFD) model Fire Dynamics Simulator FDSv4. In general terms, the comparisons between experiments and simulations show good agreement, especially in the far field of the plume, but the accuracy is poor at the lower plume region and near the flame.     

Validation of FDS for the prediction of medium-scale pool fires

Fire dynamics simulator (FDS) has been applied to simulate a medium-scale methanol pool fire. The simulation used predominantly the existing features in FDS except that an additional sub-grid-scale combustion model based on the laminar flamelet approach of Cook AW and Riley JJ [Combust and Flame 1998;112:593–606] was used alongside the default mixture fraction combustion model for comparison. The predictions of the two different combustion models for temperature and axial velocity distributions were found to be in reasonably good agreement with each other and the experimental data. The pulsating nature of air entrainment was demonstrated by the air entrainment velocity fluctuations and the instantaneous velocity vectors, which revealed formation and shedding of vortices and the well-known “neck-in” at a distance of approximately one diameter from the pool surface. The predicted variations of air entrainment at different heights agreed well with some published data and correlation. Although the limitation of the code in predicting the puffing frequency was noticed as the spectra of temperature fluctuations failed to demonstrate any dominant frequency, the present study has demonstrated the capability of FDS to deliver reliable predictions on most important parameters of pool fires.