Experimental measurements,integral modeling and smoke detection of early fire in thermally stratified environments

Building fires go through a series of stages. They start with a fire plume period during which buoyant fire smoke rises to the ceiling. A second stage is the following enclosure smoke-filling period. In this paper, the first stage is the subject, especially for the fire plume behavior in thermally stratified environments in large volume spaces. In NFPA 92B, Morton's integral equation was introduced for calculating the maximum plume rise, and beam smoke detectors were recommended for smoke detection design. In this work, experiments and CFD simulations were conducted in a small-scale enclosure and a large space to investigate early fire movements in temperature-stratified ambients. The results show that in a thermally stratified environment, the axial temperature and velocity of a fire plume decrease more quickly along the vertical axis than in uniform environment, and in some cases the fire plume ceases to rise. The previous integral equation was shown to underestimate the actual maximum height of a fire smoke plume, and also was unable to explain the differences of the maximum heights of low-density and high-density smoke plumes with the same stratification and outlet conditions. The integral equation was improved by introducing two correction factors, and extended for non-linear temperature stratified environments. A light section smoke detection method with three space-protected area was suggested and discussed.     

Steady State Ceiling Jet Behavior under an Unconfined Ceiling with Beams

Smooth horizontal and sloped ceiling jets have been studied extensively. The characteristics of these jets, i.e., temperature and velocity profiles, have been defined previously, particularly for horizontal ceilings. The presence of beams and obstructions pose a real challenge for both modeling and detector placement procedures. The present paper reports temperature profiles of ceiling jets for horizontal beamed ceilings, within beamed spaces that do not contain the plume impingement point. Furthermore, these results apply to an unconfined ceiling corresponding to large spaces or long enclosures where the end walls are far away. The research results can be highly useful for prediction of temperature of ceiling jet flows for spaces with continuous beams or joists, but may also be useful for other types of obstructions such as long light fixtures as well as smoke curtains in high bay areas. The correlation developed for temperature distribution within the bays of the beamed ceiling is based on the beam depth to ceiling height and is referenced to the smooth ceiling correlation developed by Motevalli. All data has been obtained from scaled experiments for non-dimensional fire sizes ranging from 0.0024 to 0.035. This range corresponds to fires from 38 kW to 560 kW for a 3 m ceiling height.     

Enclosure smoke filling revisited

Building fires go through a series of stages. They start with a fire plume/ceiling jet period during which buoyant fire gases rise to the ceiling and spread radially beneath the ceiling. A second stage, the enclosure smoke-filling period, follows; this second stage is the subject of this paper. It has been more than 20 yr since Zukoski first addressed the smoke filling stage of enclosure fires in terms of thermodynamic control volume concepts and fire plume entrainment, yet his analysis remains pertinent. This paper reviews and extends fire modeling concepts related to enclosure smoke filling developed by Zukoski. The mass-based analysis of Zukoski is recast in terms of the volumetric flow rates typically used for ventilation system design; it is extended to consider global average temperature rise and the effects of oxygen consumption on the maximum global average temperature rise that can be achieved in a closed-room fire. A spreadsheet template is developed to compare hand calculations based on a global analysis with numerical smoke filling calculations. Results of this comparison suggest that there is little difference in conditions predicted with the global hand calculations and the numerical smoke filling calculations; consequently, the hand calculations are suitable for preliminary fire hazard analyses.     

A Simplified Mathematical Model for Predicting the Vertical Temperature Profiles in Enclosure Fires Without Vertical Opening

This paper presents a mathematical model to predict the instantaneous temperature profiles in sealed or ceiling vented compartment fires. It has been observed in the existing research that in compartments without vertical opening, smoke fills the volume very soon indicating that the so-called one-zone type distribution forms quickly, and the gas temperature inclines linearly with height above the fire source. These characteristics are different from the smoke filling properties in enclosure fires with vertical openings. An assumption of linear distribution for temperature was introduced and a modified one-zone model was subsequently proposed in order to predict the transient smoke temperature profiles after the smoke fills the enclosure. With the knowledge of the heat release rate, the prediction model was established based on unsteady energy conservation by changing the heat loss factor using the semi-empirical models for fire plume and ceiling jet. Experiments including sealed and ceiling vented conditions were conducted to validate the model and the comparisons between measurements and predictions suggested the model can give fairly satisfactory estimations for the transient temperature profiles for both tests.     

Tenability conditions and filling times for fires in large spaces

Motivated by recent demands on regulatory reform, closed form solutions are developed for the filling times and upper layer temperatures for fires in large spaces including the volume expansion term that was neglected in previous similar efforts. The solutions evolve from (a) utilizing the air entrainment to a buoyant plume from a point source having the same convective heat release as the fire and (b) applying an energy balance for the hot layer. Heat losses to the surfaces of the enclosure and provisions for smoke control by natural ventilation are also considered in an approximate way. Although analytic solutions for the filling times exist in the literature if the volume expansion term is neglected, this work is the first to (a) present analytic solutions for the upper layer temperature including the volume expansion term and (b) incorporate heat losses and smoke control by natural ventilation. The present predictions agree with recent numerical results (Fire Sci. Technol. 19(1) (1999) 27), which agree with experimental data and consequently, the present results in turn agree well with experimental data (Fire Sci. Technol. 19(1) (1999) 27). They are also corroborated by asymptotic analysis worked out in Appendix A. For certain large spaces, the results show that critical times for evacuation or rescue operations from fire brigade depend on the upper layer temperature reaching high enough values to cause harm by radiation to occupants or fire fighting rescuers. Thus, critical times in large spaces do not result from the smoke layer descending below a critical height (e.g. 2.1 m from the floor), as they do for small spaces. The present results for large spaces having high ceiling clearance do not agree with CFAST calculations because the entrainment equation for the fire plume in CFAST is different from the one in this work.     

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.     

Influence of fire ignition locations on the actuation of smoke detectors and wet-type sprinklers in a furnished office

Experiments were conducted in a full-scale model office equipped with movable and fixed fire loads to explore the influence of ignition source (movable fire load(s)) conditions on smoke detector and sprinkler actuation. The interior plan dimension is 5.7 m × 4.7 m and the net ceiling height is 3.3 m. Both northeast and southeast wings have a 2.1 m × 0.9 m single door to be opened. Seven fire scenarios (seven different ignited fire load configurations) under natural ventilation were investigated experimentally. The results show that the amount of fire load at the initial stage in a room fire does not markedly affect smoke generation and does not significantly impact the actuation time of the smoke detectors. When the fire source is located near a corner, the plume corner effect greatly increases; smoke detectors and sprinklers can activate quickly and effectively actuate the fire suppression. When the fire source is located in the room's center, given the uncertainty regarding smoke detector and sprinkler actuation, it may not be possible to control the fire spread.     

A numerical study of atrium fires using deterministic models

The smoke filling process for the three types of atrium space containing a fire source are simulated using the two types of deterministic fire model; zone model and field model. The zone model used in this simulation is CFAST (Version 3.1) developed at the Building and Fire Research Laboratories, NIST in the USA. The field model is a self-developed CFD model based on full consideration of the compressibility and k–ε modeling for the turbulence. This article is focused on finding out the smoke movement and temperature distribution in atrium spaces. A computational procedure for predicting velocity and temperature distribution in fire-induced flow is based on the solution of three-dimensional Navier–Stokes conservation equations for mass, momentum, energy, species etc. using a finite volume method and non-staggered grid system. Since air is entrained from the bottom of the plume, total mass flow in the plume continuously increases. Also, the ceiling jet continuously decreases in temperature, smoke concentration and velocity; and increase in thickness with increasing radius. The fire models, i.e. zone models and field models, predicted similar results for the smoke layer temperature and the smoke layer interface heights. This is important in fire safety, and it can be considered that the required safe egress time in three types of atrium used, in this paper is about 5 min.     

A simplified calculation method on maximum smoke temperature under the ceiling in subway station fires

To assess the impact of smoke on the ceiling in subway stations, the maximum smoke temperature under the ceiling was studied theoretically and experimentally with two sets of small-scale experiments conducted. The results show that the maximum smoke temperature under the ceiling complies with the Alpert equation in which fire keeps distant from the walls in subway stations whereas fire adjacent to the end wall leads to the maximum smoke temperature under the ceiling decaying exponentially against the increased distance between the fire and the wall. In addition to the Alpert equation, a correlation determining the maximum smoke temperature is developed by taking the end wall effect into account. Consequently, a simplified calculation method involving the Alpert equation and the correlation is established. The method is applicable to practical fire engineering designs for subway stations.     

Calculating fire plume characteristics in a two-layer environment

Methods are developed to determine axial gas flow conditions within a weakly buoyant plume that passes from an ambient quiescent environment, in which the plume originates, to an upper layer at elevated temperatures. The methods are appropriate for inclusion in two-layer analysis of enclosure fire. In particular, they are first steps in developing a prediction of actuation time for thermally activated automatic sprinklers exposed to an enclosure fire. Results obtained with various methods are compared with measurements in a 1.22 m diameter cylindrical enclosure. National Bureau of Standards Reference: David D. Evans, “Calculating Fire Plume Characteristics in a Two Layer Environment”, Fire Technology, Vol. 20, No. 3, August 1984, p. 39. Note: This paper is a contribution of the National Bureau of Standards and is not subject to copyright.     

Early fire smoke movements and detection in high large volume spaces

The maximum rise equation was obtained by differential solution of the integral equations of a plume in linearly thermally stratified environment and its applicability in piecewise linearly stratified environment was discussed. To better understand early fire smoke movements in thermally stratified environments in large volume spaces, a detailed study of smoldering cotton wick and flaming diesel oil smoke plumes in thermally stratified environments in a small scale enclosure was investigated by experimental measurements and CFD simulations. The reasonably good agreements of the experimental results and the simulated results indicate that the thermally stratified environment intensifies the decreases of the axial temperature and velocity of a fire smoke plume until makes it stop at a maximum height. Comparisons of the maximum plume heights between the experimental measurements and the integral equation results show that the available equation underestimates the actual maximum heights of fire smoke plumes and is unable to predict the influence of smoke density upon the maximum heights. A new smoke detection method of light section image detection was suggested to overcome the shortcomings of conventional beam-type smoke sensors and detect the early fire effectively.     

火源位置的不同对自然排烟的影响

在双区模型的基础上,给出墙边羽流和轴对称羽流的排烟稳态方程,最终求得烟气层密度以及高度。通过FDS模拟,得出不同工况下的排烟效果,验证了理论计算结果。得出结论认为,同种火源条件下,墙边羽流模型由于烟气产生速率小于轴对称羽流模型,烟气层密度较大,高度较高。     

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.     

Numerical study of water spray interaction with fire plume in dual chambers connected to tall shaft

The interactions between water droplets and fire plume in a two compartmental enclosure connected to tall shaft are numerically investigated. The cooling and drag effects of water particles on thermal plume characteristics are analyzed under natural and forced ventilation conditions. Numerical study is performed by Large Eddy Simulation (LES) using Fire Dynamic Simulator (FDS) code. The water droplets oppose the smoke buoyancy force and reduces the ceiling vent discharge rate. For higher sprinkler operating pressure the drag force dominates buoyancy force and stops the plume propagation through horizontal passage. The critical sprinkler operating pressure that leads to smoke logging is identified. The horizontal vent mass flow rate decreases linearly with water spray discharge rate. The forced air stream supplied at low velocity assists buoyancy force and eliminates smoke logging. However, higher ventilation velocities intensify cooling effect by increasing the interactions between water droplets and thermal plume. The model employed has been validated with the existing experimental results available in the literature.     

大空间烟气控制性能化设计影响因素模拟

利用FDS数值模拟大空间烟气流动情况,探讨大空间火灾烟气控制系统性能化设计中的影响因素.针对设计火灾荷栽、火源位僵、大空间的形状系数和大空间体积对烟气层高度和烟气温度的影响进行模拟分析,对大空间火灾烟气控制系统的性能化设计具有重要意义.     

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。     

某地铁车站热烟实验研究

在某地铁车站站厅和站台开展热烟实验,研究烟气流动情况,测量火灾时羽流及距离地面2 m高度处顶棚温度变化情况,在此基础上评价其排烟系统性能。结果表明,该车站内烟控系统能达到排烟目标;采用人员疏散出入口作为自然补风口,可避免烟气对疏散出入口的侵袭;在已建成建筑内采用受控的火源和烟源,用热烟实验方法可模拟真实火灾场景并测试排烟系统性能,也可以作为验证和评估大型复杂建筑内防排烟设计的手段。     

防火分隔及排烟措施对综合管廊电缆火灾烟气蔓延的影响

以某综合管廊为研究背景,利用PyroSim模拟软件建立的仿真计算模型,研究了设立挡烟垂壁、改变防火门开启程度、增设排烟设施等情况下的烟气蔓延规律.研究发现:在烟气未充满管廊时,挡烟垂壁会使烟气蔓延速度降低,烟气蔓延速度与挡烟垂壁高度成反比;防火门打开会使火灾烟气蔓延至相邻防火分区,烟气蔓延速率与防火门开启程度成正相关;...     

Fire detection, location and heat release rate through inverse problem solution. Part II: Experiment

The design and fabrication of a prototype video fire detection system, which can locate a fire and determine its heat release rate, is described. The operation of the prototype system is demonstrated in a series of small-scale tests. The system utilizes a video camera to monitor an array of passive sensors distributed around the compartment to be protected. Each of the sensors is made up of a temperature-sensitive sheet that changes color at a prescribed temperature. In the event of an accidental fire, the plume of hot combustion gases rising from the fire will cause the temperature-sensitive sensors to be activated and change color. The times and locations of sensors changing color are used as data for an inverse problem solution algorithm, which determines the location and the heat release rate of the fire. A small-scale evaluation of the prototype video system is presented in which the prototype system is used to detect, locate and determine the heat release rate of a 2·4 kW burner placed in a 2·75 m wide by 2·75 m deep by 1·5 m high test enclosure. The accuracy of the prototype system in locating and determining the heat release rate of the small flame source placed in the reduced-scale enclosure is reported. In addition, the ability of the prototype system to make approximate measurements of the optical thickness of smoke in the enclosure, along camera-sensor lines-of-sight and then to use these measurements to locate and track the growth of a smoke plume is demonstrated.