A comparison of the use of fire zone and field models for simulating atrium smoke-filling processes

The smoke filling process for the three types of atrium spaces commonly built in Hong Kong are simulated using the two types of deterministic fire model: zone models and field models. The zone models used are the FIRST, CFAST, and CCFM.VENTS models developed at the Building and Fire Research Laboratories, NIST, USA and the NBTC one-room model of FIRECALC developed at CSIRO, Australia. The field model is a self-developed fire field model based on computational fluid dynamics theories. The results predicted by the two approaches are very similar. Simulation using a field model requires much more computing time compared with the use of a zone model, but it can give more detailed information on the fire-induced flow and temperature fields.     

中庭的形状系数与烟羽方程

分析了我国中庭空间几何形状的实地终统计调查结果，提出了形状系数的概念，并根据形状系数大小对中庭进行了分类。针对国内较普遍的瘦高型中庭(形状系数小于0．4)，按1／8比例建造了中庭火灾相似模型实验台，开展了烟气填充与机械排烟实验研究，得到了适用于这类中庭的稳态火灾烟气填充方程与反映受限烟羽特点的烟羽质量流量方程。     

Simulation on indoor aerodynamics induced by an atrium fire

Indoor aerodynamics induced by a fire in an atrium was simulated with the fire field model CC-EXACT. The package was developed from the computational fluid dynamics (CFD) model EXACT designed for studying indoor air flow induced by heating, ventilation and air-conditioning system. The results are compared with those predicted by the two-layer fire zone model FIRST. Both EXACT and FIRST are available from the Building and Fire Research Laboratory, National Institute of Standards and Technology (NIST), USA.Three different types of atria commonly found in Hong Kong of the same space volume of 2000 m³ were studied. The results of the interface height and average temperature of the hot-air layer predicted by the field model CC-EXACT were compared with those predicted by the zone model FIRST. Recommendations on selecting a suitable plume model for zone modeling are made.     

Experimental studies on natural smoke filling in atrium due to a shop fire

How smoke spilling out of a shop fire would fill up an atrium has been studied experimentally in this paper. Full-scale burning tests were carried out in the PolyU/USTC Atrium constructed in Hefei, China. Since balcony spill plume expressions appeared in the literature might not be applicable for a shop fire with finite width, two plume models for smoke spilling out of a shop fire inside an atrium were proposed and assessed. An equation on studying the smoke layer interface height with a two-layer approach was derived.     

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.     

Numerical study on smoke movement driven by pure helium in atria

The hot smoke test is often used for commissioning fire smoke management system in atrium buildings, in which liquid fuel is burnt to generate a buoyant plume mixed with artificial tracer smoke to model a fire smoke. The method is usually costly and often causes safety concerns. This paper studied an alternative method of using a cold smoke test, in which pure helium is used to create the buoyant plume. A method was developed to determine the supply rate of pure helium necessary to achieve the same buoyancy effect as that of the corresponding hot smoke test. Computational fluid dynamics (CFD) simulations of the helium smoke tests were conducted and compared to the measured hot smoke tests in a full-scale naturally ventilated atrium and a sub-scale atrium with mechanical ventilation. A new method was added in the CFD model to track the smoke layer height for the simulations of helium smoke based on the concentrations of smoke and helium. It is found that the predicted smoke layer heights based on the mass fractions of the tracer smoke are generally close to the measured ones in the hot smoke tests of different heat release rates. A non-dimensional temperature in the hot smoke test is also found closely related to the dimensionless helium concentrations in the helium smoke test for the atria modeled. Although the consumption of pure helium for a full-scale helium smoke test can be very high, it is promising to use the pure helium smoke test in the lab-scale experiments as the preliminary tests of full-scale and/or lab-scale testing of real fires.     

中庭排烟系统设计优化分析

依照现行国家相关标准的规定并参考国外相关标准,对建筑中庭内两类典型设定火灾场景条件下烟羽流的质量流量和温度进行计算和比较分析。建议设计者充分理解和运用中庭烟羽流的发展变化规律,结合中庭自身的建筑特征,有针对性地采取适宜的技术措施,改进和优化中庭排烟系统设计,提高系统的安全性和有效性。     

Numerical analysis of tunnel thermal plume control using longitudinal ventilation

A CFD model of the 4th Beijing subway line was used to study the effect of longitudinal ventilation on heat and smoke plume movement in the tunnel. The critical ventilation velocity is correlated with the heat release rate for both a simplified heat fire source model and a complete combustion fire source model with methane gas as fuel. The influences of the heat source length and the fuel gas inlet geometry on the critical velocity are investigated for both fire source models. The results show that the influences of the combustion process and fire source area variation are not included in models based on Froude number preservation theory. Thus, Ri is no longer suitable as a dimensionless number for the critical ventilation velocity when the fire geometry or combustion conditions influence the results. The back-layering air temperature above the front of the fire source can be used to explain the different critical velocity variation regimes for all the simulation conditions.     

某室内商业步行街火灾烟气控制研究

为了研究室内商业步行街的火灾烟气控制效果,以某城市综合体为例,模拟室内商业步行街火灾,在中庭位置设置1.5 MW酒精火源,点火后分别联动开启排烟设施和手动开启自然排烟设施,观察两种工况下室内步行街内烟气蔓延情况,并测量试验区域温度等参数.试验发现,相较于顶部自然排烟,采用联动机械排烟的烟气沉降速率低17％,烟气层高度高...     

A Study of the Effect of Make-Up Air Velocity on the Smoke Layer Height with Symmetric Openings in Atrium Fires

This paper presents full-scale test results and CFD modeling of smoke conditions in atrium fires in the case of symmetric make-up air opening arrangements. The atrium used for the experiments was equipped with a smoke management system capable of exhausting 132 m³/s. Thermocouple trees were installed to measure the temperature along the height of the atrium. The N-percentage method was used to determine the smoke interface height from the measured temperature profiles. Make-up air velocities of 1 m/s, 1.5 m/s and 2 m/s were selected to investigate the effect of make-up air velocity on the smoke interface height in the case of 4-side and 2-opposite side openings. The fire size varied from 1 MW to 5 MW to cover the effect of small, medium and large fires. FDS was also used to simulate the atrium fires in order to compare the predictions with the full-scale test results and evaluate the accuracy of the developed correlations. Results show that the limit of 1 m/s is too restrictive in the case of symmetric opening arrangements. A correlation is proposed based on the full-scale test results that can be used to modify the smoke layer height obtained using plume equations so that the effect of make-up air velocity on the smoke layer height is considered.     

Scale Tests of Smoke Filling in Large Atria

The large Atriums of airports and railway stations facilitate the access to transport vehicles including shopping malls, cultural spaces, etc. For this reason, they are used by an elevated number of passengers and visitors. Numerous malls contain a large atrium too, as a principal access or as a food court, and they usually have high occupant loads. In case of fire, the smoke can affect human health seriously, and people may be unable to reach a safe place before being overcome by the conditions created by the fire. The traditional approach to fire protection by compartmentation is not applicable to these large volume spaces and the ability of sprinklers to suppress fire in spaces with high ceilings is limited. This work evaluated—using scale tests, fire computer modeling and analytical methods—a comparative analysis of the different results obtained for the smoke control in large atria when the smoke filling approach is applied. Smoke layer and plume temperatures have been registered during the scale test—based on the Froude Modeling—and they have been compared opposite to the FDS scale simulation and the FDS large scale simulation. Smoke layer descend has been studied and compared for the scale test, the computer simulations developed and the empirical equations used. The results demonstrated that the evacuation time calculation is conservative when the zone computer model CFAST, the field computer model FDS or the empirical equations are used, although it turns out to be difficult to define the interface height based on the temperatures registered during the scale tests. The zone computer models generate results faster than field computer models or smoke tests, so it would be necessary to develop better calculation algorithms to define the smoke layer interface.

Numerical simulation of the spread of smoke in an atrium under fire scenario

The Fire Dynamics Simulator code is used to investigate the smoke movement in an atrium under fire scenario. At first, by comparing with experimental data of the atrium fire under low and high heat release rates, reasonable model constants of C_s and Pr_t and appropriate grid system are determined for simulating smoke movement in the atrium, the simulation results are in good agreement with those experimental data. Then, the performance of different smoke exhaust methods in the atrium is studied. Smoke filling processes are investigated under different natural and enhanced smoke exhaust methods. Simulated results show that natural smoke exhaust method is preferred when the smoke exhaust vents are located at the ceiling of the atrium. On the other hand, when the smoke exhaust vents are located on the walls of the atrium, the higher positions of the smoke exhaust vents are preferred. In addition, the influence of the fire source locations on the smoke spreading process is presented in this paper, three kinds of fire source locations are studied, they are central fire, side wall fire and corner fire. Results indicate that the descending process of the smoke layer is the slowest when the fire source is at the corner of the atrium.     

Natural smoke filling in atrium with liquid pool fires up to 1.6 MW

Experimental studies on natural smoke filling in an atrium induced by a liquid pool fire up to 1.6 MW were carried out. The new full-scale burning facility, the PolyU/USTC Atrium constructed at Hefei in China, was used. Five sets of hot smoke tests with diesel pool fires of 2×2 m placed on the floor were carried out. All openings were closed, except leaving a small vertical vent of 0.2 m high for supplying fresh air. Transient variations on the mass of the burning fuel, the vertical temperature distributions and the smoke layer interface heights were measured. Results compiled from the tests were compared with those predicted by a smoke filling model developed from plume equations; the NFPA smoke filling equation; and a model developed by Tanaka and co-workers.     

A simple two-layer zone model on mechanical exhaust in an atrium

Provision of make-up air is essential in designing mechanical exhaust system in a compartment. There are always problems in determining the inlet positions for supplying make-up air. In this paper, a zone model for studying the effect of different positions of make-up air supply on the performance of a mechanical exhaust system in an atrium will be developed. Traditional two-layer approach with an upper smoke layer and a lower air layer will be assumed.Three scenarios of extraction with different relative positions of the air inlet are studied. These are scenarios with the smoke layer interface lying above, within, and below the air inlet. Conservation of mass and energy are considered for each scenario to study the smoke filling process. Transient variations of smoke layer temperature and interface height will be predicted under different fire sizes, exhaust rates and make-up air conditions.Full-scale burning tests in an atrium were conducted to justify the predicted results. In addition, results predicted by this zone model will also be compared with those predicted by Computational Fluid Dynamics (CFD) with the software Fire Dynamics Simulator FDS version 3.1; and another zone model CFAST version 5.0.1. It is observed that the predicted results from this new zone model agreed well with experiments and CFD results. However, results predicted by CFAST deviated from experiments for the scenario with the smoke layer interface lying below the air inlet.     

Numerical studies on atrium smoke movement and control with validation by field tests

Many computer fire models were developed in the literature with the rapid advancement of information technology. With the possibility of implementing engineering performance-based fire codes, fire models are used frequently in hazard assessment. Among the different approaches, fire field models using the technique of computational fluid dynamics (CFD) are widely used. The approach takes the advantage of predicting the fire environment in a ‘microscopic’ picture. Air flow pattern, pressure and temperature contours can be predicted. However, it is not easy to validate the CFD predicted results. Most of the field models are only validated by some experiments not specially designed for such purpose. There are very few studies on comparison with field measurements in actual sites. Whether those models are suitable for use are queried, leading to challenges. In this paper, the CFD tool fire dynamics simulator developed at the National Institute of Standards and Technology in USA will be applied to study atrium fires. Smoke layer interface height and air temperatures inside the atrium are simulated. The experimental data on atrium hot smoke tests carried out recently was used. CFD results predicted can be validated by comparing with the experimental results.     

隧道长度对烟气分布影响数值模拟

采用数值模拟方法,研究隧道长度对火灾烟气质量流量及烟气层厚度的影响。隧道长度分别设定为100,200,300,400,500 m。结果发现：设定条件下,烟气蔓延距离大于200 m时,烟气沉降明显,烟气质量流量和厚度呈现先增大后变小的趋势;烟气层下方存在诱导气流,二者之间的剪切作用共同影响烟气质量流量和厚度的变化;200 m范围内,诱导气流的速度随着与火源距离的增加而增大,主要是由于烟气层温度较高,浮力效应较强,从而卷吸下层冷空气,导致烟气质量流量和厚度增大,下方诱导气流空间减小,导致速度增加;200 m范围外,烟气层温度大幅降低,浮力效应减弱,诱导气流惯性力在二者剪切作用下占据主要作用,从而卷吸上层烟气,导致烟气质量流量和厚度减小。     

Application of FDS to Adhered Spill Plumes in Atria

In a recently published article (Poreh et al., Fire Saf J 43(5):344–350, 2008), Poreh et al. carried out a number of experiments in a small-scale atrium. They investigated the mass flow of the spill plume in case of fire emerging from an adjacent room or corridor. Based on these experiments, the equation for the mass flow rates of adhered spill plumes in atria was adjusted. In our article, we repeat the experiments in a computational fluid dynamics (CFD) program. The results agree well, both with the experiments and the suggested formula. After this first validation, large-scale CFD-simulations are carried out. It appears that the equation suggested by Poreh et al. is only valid in the case of a uniform smoke layer depth. If the smoke layer has a more complex configuration, the formula is no longer reliable for the design of the smoke and heat exhaust ventilation system.     

Definition and experimental evaluation of the smoke “confinement velocity” in tunnel fires

An experimental study is carried out on a reduced scale tunnel model (scale reduction is 1:20). The main objective is to evaluate the longitudinal velocity induced into a tunnel when a fire plume continuously released is confined and extracted between two exhaust vents located on both sides of the fire source. For the experimental simulations, fire-induced smoke is simulated by an air and helium mix release. Smoke flow is symmetrical as regards the fire location and experiments are realized for an half tunnel with only one vent activated downstream the source. The vent extraction flow rate is step by step increased and the length of the stratified smoke layer downstream the vent as well as the longitudinal fresh air flow induced, are measured. A confinement velocity is then associated to the minimum value of the longitudinal air flow needed to prevent the smoke layer propagation downstream the vent. This velocity is evaluated for several values of the fire heat release rate and finally compared with the corresponding critical velocity obtained for a longitudinal ventilation system.     

Using FDS to Simulate Smoke Layer Interface Height in a Simple Atrium

This study examines the possible effects of various make-up air supply arrangements and velocities in an atrium smoke management system. Variations include velocities ranging from 0.5 to 3.0 m/s. The arrangement of make-up air supply injection points include symmetrically located vents placed low in the spaced, an array of vents distributed from the floor to the ceiling, and asymmetrically located vents. Fire Dynamic Simulator version 4.06 is applied to simulate ten scenarios in a 30.5 m cubical domain with a fire source simulating a stack of pallets with an approximate peak heat release rate of 5 MW. Results show that make-up air supply velocities should be diffused such that little to no velocity effects reach the fire. Make-up air should be supplied to the fire symmetrically for the best chance of not disturbing the fire plume. Disturbing the fire and smoke plume results in a significant increase in the smoke production rate, as evidenced by a deeper smoke layer.     

On the use of time constants for specifying the smoke filling process in atrium halls

The time constant proposed to specify the smoke filling time in an atrium hall for design purposes is further evaluated in this paper using the plume expression fitted by dimensional analysis. The zone model FIRST developed at the Building and Fire Research Laboratory, NIST, U.S.A. is used to simulate the smoke filling time. Predicted results of the zone modelling simulation supported the fact that the time required to fill 80% of the atrium space with smoke is related to its time constant. Full-scale experimental results on smoke filling processes in atria available in the literature are used to justify this parameter. Use of this quantity to specify the smoke filling time for an atrium space is recommended with a selected design fire.