CFD modelling of the effect of fire source geometry and location on smoke flow multiplicity

Understanding solution multiplicity of smoke flow at the same building configuration and ambient conditions is important for managing smoke flows and human evacuation in buildings. One of the known examples with solution multiplicity is in a simple single-compartment building on fire under an opposing wind. The occurrence of multiple solutions of smoke flow is induced by competing wind and thermal buoyancy forces. Under a given and moderate wind, the critical buoyancy flux ratio for the existence of smoke flow multiplicity, which is a ratio between defined parameters representing buoyancy force and wind pressure, is related to building height and opening area, as shown using a zone model. Computational fluid dynamics (CFD) simulations were used here to evaluate whether the behaviour of smoke flow multiplicity was affected by the geometry and location of the fire source(s). Our simulation results were in good agreement with previous macroscopic analysis results. A floor fire source can produce the largest smoke flow rate in the buoyancy-dominated flow regime among the tested cases while two corner sources can produce the smallest smoke flow rate. A floor source had a relatively large smoke flow rate in the wind-dominated flow regime while a point source had relatively small smoke flow rate. Moreover, a larger critical buoyancy flux ratio and a larger range of fire power in which smoke flow multiplicity existed were found for a floor fire source than for other sources. Switching of smoke flow solutions in building fires was found to depend on the initial conditions and the magnitude of flow perturbations.     

A Study of the Critical Velocity of Smoke Bifurcation Flow in Tunnel with Longitudinal Ventilation

Small longitudinal velocity cannot prevent backlayering in tunnel fire, while excessive longitudinal velocity will destroy stratification of smoke layer and lead to bifurcation flow. As smoke bifurcation flow proceeds, the longitudinal flow is divided into two streams and flow along both sidewalls of the tunnel ceiling. The critical velocity of bifurcation flow is the minimum value at which bifurcation flow starts to occur. To investigate the critical velocity of bifurcation flow, experiments and CFD simulations were conducted. Experiment was carried out in a reduced-scale tunnel, which is 8 m long, 1 m wide and 0.5 m high. The numerical research was performed using FDS. In simulation, the computational region of a tunnel is 200 m long, 10 m wide. The heat release rate (1 MW to 6 MW) and the height (4 m to 8 m) is changed in the 30 simulation scenarios. Theoretical analysis showed that the dimensionless critical velocity of bifurcation flow only depends on the dimensionless heat release rates, and a mathematical equation is proposed. The reduced-scale experiments indicated that the critical velocity of bifurcation flow is 1.48 times that of critical velocity for preventing backlayering, and the coefficient is in agreement with CFD simulation.     

Wind effects on smoke motion and temperature of ventilation-controlled fire in a two-vent compartment

High-rise building fire is often influenced by the ambient wind. Study concerning fire behavior in the compartment of high-rise buildings in wind environment is needed for exploring some effective methods used for evaluation of compartment fire smoke movement and control. In this paper, smoke flowing direction and temperature of ventilation-controlled fire in a two-vent compartment are studied when ambient wind blows to the vent at higher altitude. It is found that there is a critical wind speed, above which the direction of smoke movement is dominated by wind rather than by buoyancy. It is also found that ambient wind has a complex influence on smoke temperature in the compartment. When wind speed exceeds another critical value, only one steady state appears in the smoke temperature rising curve. Otherwise three steady states appear. Heat transfer through the compartment walls has great influence on the second critical wind speed.     

侧部重点排烟与水喷淋协同作用下隧道烟气运动研究

合理的水喷淋设计参数及排烟策略，可保证隧道有效排烟和烟气层的稳定性，为人员安全疏散提供有利环境。为研究侧部排烟模式下烟气失稳临界状态时最佳喷水流量和排烟口设计参数，采用FDS 对15 MW 火灾规模下，不同喷水流量、排烟量、排烟口间距及排烟口高度下19 组工况进行模拟计算。结果表明：喷淋流量越大，烟气层高度越高，隧道整体温度降低，改变喷水流量对控制烟气层的稳定性效益不大，隧道空间内有烟气滞留；排烟量为70 m3/s、排烟口间距为50 m、排烟口高度为3.2 m 或4.0 m 为烟气层稳定临界状态时的排烟口最佳参数，此时侧部抽吸力向上的分力与烟气的热浮力大于水喷淋拽曳力，烟气层较稳定，隧道空间内无旋涡烟气滞留，有利于排烟和人员疏散。     

室内烟气运动影响因素模拟研究

运用FDS模拟室内火灾烟气的运动规律,分析烟气层稳定性,以及门的尺寸、火源位置和火源面积对烟气温度及高度的影响。结果表明,具有稳定热释放速率的火源,燃烧一段时间后烟气层高度不会随时间发生变化;烟气层高度随门的高度和宽度增加而升高;火源处于房间中心时,烟气层高度随着门宽度增加迅速升高,与门高度的关系较小;随着火源面积增加,烟气层高度下降,温度升高。     

A computational study on effects of fire location on smoke movement in a road tunnel

In this work, a numerical model of tunnel fire is developed and aimed to investigate the influence of cross-sectional fire locations on critical velocity and smoke flow characteristic. It is shown that the critical velocity for a fire next to the wall is obviously higher than that for a fire in the middle or on the left/right lane. The ratio is estimated to be 1.12. The predictions of critical velocity from ‘small-fire’ models show a good agreement with that for a fire in the middle or on the left/right lane from CFD. The tunnel height at the fire location is proposed to be instead of the hydraulic tunnel height in the ‘big-fire’ model of Wu and Bakar for a fire next to the wall. The smoke moves backward in a tongue like form as the ventilation velocity is lower than the critical velocity. The back-layering length of a fire in the middle is shown to be approximate twice than that on the left/right lane under the same ventilation velocity, although they share the same critical velocity. Whereas a relatively short back-layering length for a fire next to the wall under the velocity of 2.6 and 2.7 m/s. In addition, a snaky high-temperature profile on the top wall at the initial downstream is observed for a fire on the left lane and next to the wall, and finally a steady and layered smoke flow. The likely cause of this phenomenon is subsequently explained in this study.     

空气幕对列车车厢火灾烟气的影响规律研究

为研究防烟空气幕对列车车厢火灾烟气的影响规律,建立了CRH2A 动车组一节车厢的内部模型,利用FDS 模拟软件对列车车厢火灾时期的烟气流动规律进行数值模拟。依据给定火灾场景下烟气水平运动速度设置空气幕水平切向速度,通过改变空气幕安装角度,研究车厢内部火灾时,在空气幕作用下车厢空间各区域烟气层高度的变化,温度及CO 浓度的分布情况。结果表明：火源功率为3 MW 时,空气幕安装角度θ=45̊ ,水平与垂直方向速度均为4 m/s 时,防烟效果较好。     

A two-dimensional numerical investigation of the oscillatory flow behaviour in rectangular fire compartments with a single horizontal ceiling vent

This paper presents a comparison of fire field model predictions with experiment for the case of a fire within a compartment which is vented (buoyancydriven) to the outside by a single horizontal ceiling vent. Unlike previous work, the mathematical model does not employ a mixing ratio to represent vent temperatures but allows the model to predict vent temperatures a priori. The experiment suggests that the flow through the vent produces oscillatory behaviour in vent temperatures with puffs of smoke emerging from the fire compartment. This type of flow is also predicted by the fire field model. While the numerical predictions are in good qualitative agreement with observations, they overpredict the amplitudes of the temperature oscillations within the vent and also the compartment temperatures. The discrepancies are thought to be due to three-dimensional effects not accounted for in this model as well as using standard ‘practices’ normally used by the community with regards to discretization and turbulence models. Furthermore, it is important to note that the use of the turbulence model in a transient mode, as is used here, may have a significant effect on the results. The numerical results also suggest that a linear relationship exists between the frequency of vent temperature oscillation (n) and the heat release rate ( ) of the type , similar to that observed for compartments with two horizontal vents. This relationship is predicted to occur only for heat release rates below a critical value. Furthermore, the vent discharge coefficient is found to vary in an oscillatory fashion with a mean value of 0.58. Below the critical heat release rate the mean discharge coefficient is found to be insensitive to fire size.     

Experimental study of natural roof ventilation in full-scale enclosure fire tests in a small compartment

An analysis of full-scale fire test experimental data is presented for a small compartment (3×3.6×2.3 m). A square steady fire source is placed in the center of the compartment. There is an open door and a horizontal opening in the roof, so that natural ventilation is established for the well-ventilated fire. A parameter study is performed, covering a range of total fire heat release rates (330, 440 and 550 kW), fire source areas (0.3×0.3 m and 0.6×0.6 m) and roof ventilation opening areas (1.45×1 m, 0.75×1 m and 0.5×1 m). The impact of the different parameters is examined on the smoke layer depth and the temperature variations in vertical direction in the compartment. Both mean temperatures and temperature fluctuations are reported. The total fire heat release rate value has the strongest influence on the hot smoke layer average temperature rise, while the influence of the fire source area and the roof opening is smaller. The hot smoke layer depth, determined from the measured temperature profiles, is primarily influenced by the fire source area, while the total fire heat release rate and the roof opening only have a small impact. Correlations are given for the hot smoke layer average temperature rise, the buoyancy reference velocity and the total smoke mass flow rate out of the compartment, as a function of the different parameters mentioned. Based on the experimental findings, it is discussed that different manual calculation methods, widely used for natural ventilation design of compartments in the case of fire, under-predict the hot layer thickness and total smoke mass flow rate, while the hot layer average temperature is over-estimated.     

Influence of tunnel width on longitudinal smoke control

The objective is to carry out experiments on scale models and CFD calculations in order to study the influence of tunnel width W on critical velocity (for a given tunnel height H). By definition, the critical velocity is the minimum longitudinal velocity needed to prevent smoke back flow when a fire occurs in a tunnel. Two different experimental reduced scale models are used: the first one is a thermal model using a propane gas flame to simulate the fire and the second one is a densimetrical model in which the fire-induced- smoke is represented by a continuous release of an isothermal buoyant mixing. In both approaches, for aspect ratios W/H greater than unity, it is noticed that the critical velocity decreases when the width increases, as predicted by theory, but for low values of the aspect ratio (i.e. when W<H) and for high enough fire heat release rates, the critical velocity significantly increases with tunnel width. This can be associated to a change in the transverse flow pattern close to the buoyant source. Complementary CFD calculations are also presented in order to describe the influence of the lateral confinement on smoke plume spreading and then, on critical velocity.     

The Use of Positive Pressure Ventilation Fans During Firefighting Operations in Underground Stations: An Experimental Study

Positive pressure ventilation (PPV) fans are widely used by the fire service during firefighting operations in buildings. Fans are positioned to create a flow through the enclosure. This flow can remove the smoke after the fire or affect the direction of the smoke to support firefighting operations. In subway stations, it is less common to use PPV fans. Here, 106 full-scale tests with up to four fans have been performed in a training building that represents a subway station. The fans were used as extraction fans. The generated flow through the subway station has been measured. The critical velocity for a hypothetical tunnel (W × H: 3.17 × 4.15 m) attached to the subway station has been calculated as 2.37 m/s. Reaching the critical velocity has been used as criterion for ‘success’. All combinations with four fans exceed this velocity, supporting the idea that the fans could be used to facilitate a firefighting operation. The location of the fans was varied. Combinations with three fans on the platform and one at the top of the staircase performed better than combinations with two fans on the platform, one on the landing and one at the top of the staircase. There is an optimum value for the distance between the fans on the platform and the first step of the staircase. This value depends on the angle of inclination of the fans. The fans were not capable of creating a flow that exceeded the critical velocity in the station itself (L × W × H: 60 × 7.15 × 4.53 m). However, a velocity of 2.40 m/s corresponds to a flow rate that will limit the backlayering distance in the station to 15 m. This was only achieved by tests with four fans (three on the platform and one at the top of the staircase).     

岩石类材料的分叉分析及其工程应用

根据Ottosen用特征值方法得到的非关联弹塑性材料发生不连续分叉的临界硬化模量和局部化方向的解析解 ,给出了岩石类材料常用的Mohr -Coulomb和Drucker -Prager准则的临界硬化模量和局部化方向 ,并对二者单轴受压时的局部化方向进行了比较。对平面应变混凝土压板进行分叉分析 ,得到其不同位移荷载步下的局部化区的形成过程和最终破坏模式。并对某拱坝进行弹塑性分叉分析 ,计算结果表明坝踵处最先发生应变局部化 ,是拱坝的薄弱部位。     

The impact of atrium shape on natural smoke ventilation

The performance efficiency of natural smoke ventilation in atria spaces are influenced greatly by several design decisions such as atrium shape, height, size and openings location. This paper investigates the impact of atrium shape (horizontal profile) on smoke ventilation performance in naturally ventilated atria. Three different configurations (square, rectangular and triangular prism) with the same area, height, and hence, volume were tested. The smoke ventilation performance is being assessed in terms of smoke filling time using a computational fire dynamic simulator (FDS). FDS is used to simulate the natural smoke filling resulting from atrium fire in the three configurations. The smoke layer interface height as a function of time and soot mass fraction and temperature as a function of height have been registered during the simulation. The predicted transport lag time for initial formation of the smoke layer beneath the ceiling (ceiling jet) was compared for the three tests. In order to test sensitivity of the shapes, all other parameters were designed to be similar in the three tests, and the same fire scenario was applied including inlet and outlet area, and fire size and location. The results showed that the rectangular configuration contributes better to smoke ventilation, and that the triangular configuration is the most critical in terms of smoke filling time, followed by the square configuration.     

The Impact of a Change on the Size of the Smoke Compartment in the Evacuation of Health Care Facilities

Evacuation in health-care facilities is complex due to the physical impairment of the patients. This kind of evacuation usually requires the assistance of the workforce members. A proposed change of NFPA 101, Life Safety Code, would increase the maximum allowable size of a smoke compartment (a space within the building enclosed by smoke barriers on all sides that restricts the movement of smoke) in health-care occupancies from 2090 m² to 3700 m², almost double the size. This study aims to analyse the impact of this change in the required time for evacuating patients during a fire in order to understand the consequences of that potential change. This paper is focused on the area where the patient’s rooms are located. The evacuation scenario is a floor plan comprised of four smoke compartments. To analyse the proposed change, the smoke barriers between two adjacent compartments were removed in a floor plan and three ratios of number of patients per one staff member were considered (4:1, 3:1 and 2:1). A computational methodology was conducted to calibrate the model STEPS for simulating assisted evacuation processes. In addition, Fire Dynamic Simulator (FDS) was used to simulate the fire and smoke spread in a table and a PC to compare fire and evacuation results The evacuation results show that the change of the smoke compartment size increases the mean evacuation time by 23%; however, the fire results show that the available safe egress time is 16 min for both smaller and large smoke compartment. The ratio of the number of patients per staff member is also a strong factor that increases the evacuation up to 82% when comparing the ratios of 2 patients per staff member and 4 patients per staff member.     

双平行圆形隧道稳定的塑性极限分析上限解

从塑性极限分析上限法的基本原理出发，通过分析单圆形隧道4种类型垮落机制所获得的稳定率上限解，构建黏土层中双平行圆形隧道的垮落机制，阐述双平行圆形隧道稳定与垮落间的临界稳定分析过程，导出浅土层中双平行圆形隧道稳定率的上限方程。该方程根据隧道间不同距离，综合两隧道重叠（单隧道）、两隧道相接触、相互影响的双隧道以及互不影响的两单隧道的稳定特征。通过该方程进一步讨论土重对双隧道稳定率上限解的影响。最后，由离心模型试验结果证实其上限解的正确性。     

室内火灾烟气流动随机性研究述评

评述了目前国内外室内火灾烟气流动随机性研究的现状、室内空间火灾烟气流动随机性的基本原因、影响烟气流动随机性的因素以及这些因素的影响机理等.     

Smoke production,radiation heat transfer and fire growth in a liquid-fuelled compartment fire

A detailed investigation is described of the interaction between fire development, smoke production and radiative exchange in a half-scale ASTM compartment in which the source is a heptane pool fire. Measurements of heat flux, fuel mass loss rate, ventilation flow rates, temperature and soot volume fraction are reported for the compartment for varying door widths. Data from the compartment are compared with open pool fire measurements using the same equipment. The confined geometry is shown to exert a strong influence on pool fire development and suggests that considerable caution is needed in employing open pool fire data as boundary conditions for CFD simulation. Numerical simulations based on the direct calculation of radiative exchange between the liquid fuel surface, the smoke-laden environment and bounding walls do reproduce the behaviour observed when combustion, soot production and radiation are modelled in detail and finely resolved spatially.     

基础隔震结构高宽比限值研究

本文研究目的是得到隔震结构在各种工况下的高宽比限值,这对隔震结构设计是必要的。橡胶隔震支座不能产生拉应力和隔震支座压应力不超过容许值是保证隔震结构在强震中不产生倾覆的充分条件。基于这两个条件,本文推导了隔震结构高宽比限值的显式并给出了针对不同建筑类别、不同设防烈度、不同场地条件和不同隔震层阻尼比的高宽比限值。在支座的轴力计算中,考虑了水平地震作用、竖向地震作用和重力荷载代表值的共同影响以及荷载的最不利组合。研究发现,当控制条件为支座不产生拉应力时,高宽比限值随隔震结构周期的增加而增加;当控制条件为支座压应力不超过容许值时,高宽比限值随隔震结构周期的增加而减小。因此,存在一个临界周期使高宽比限值取得极大值。研究还发现,存在一个最大的隔震结构周期使高宽比限值等于零或隔震层位移超过容许值。将隔震结构的周期与临界周期和最大隔震周期比较,就可以得到相应高宽比限值的表达式。     

酒店中庭自然排烟方式的数值模拟

分析青海某庭院式酒店中庭区域的烟气蔓延,通过模拟得到排烟口高度处烟气层内热流、质量流、体积流随时间变化的情况,分析建筑自然排烟系统的有效性,并对比排烟口布置位置对排烟效果的影响。通过计算得出排烟窗面积为内庭院面积的10%时能够保障建筑的消防安全。在4.0 MW的火源功率下,火源稳定之后150s左右烟气层稳定在30～32m高度处;自然排烟口位于庭院中心处的排烟效果优于排烟口位于四周。     

热压与风机动力共同作用下多分支隧道内排烟气流的多解性

多分支隧道的排烟与补风路径较多,热压与风机动力的竞争可能造成其通风排烟模式具有多解性。针对某一多分支隧道的防排烟工况,利用理论分析建立了各种气流模式的控制方程,通过数学方法获得了理论解。结果证明,在按照预期设计选定通风排烟模式与风机以后,多分支隧道内的排烟气流仍然可能存在多种状态,风机的运行工况点也会随之漂移,导致排烟方向可能与设计预期完全相反。研究还发现,通过改变风机选型能起到抑制排烟气流出现多解的作用。