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.     

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.     

Fire detection, location and heat release rate through inverse problem solution. Part I: Theory

A proposed method of detecting, locating and sizing accidental fires, based on the solution of an inverse heat transfer problem, is described. The inverse heat transfer problem to be solved is that of the convective heating of a compartment ceiling by the hot plume of combustion gases rising from an accidental fire. The inverse problem solution algorithm employs transient temperature data gathered at the ceiling of the compartment to determine the location and heat release rate of the fire. An evaluation of the proposed fire detection system, demonstrating the limits on the accuracy of the inverse problem solution algorithm, is presented. The evaluation involves operating the inverse problem solution algorithm on transient temperature data from computer simulated compartment fires. The simulated fire data are generated assuming fires with quadratic growth rates, burning in a 20 m wide by 20 m deep by 3 m high enclosure with a smooth, adiabatic ceiling. The accuracy of the inverse problem solution algorithm in determining the location of a fire is shown to be insensitive to the errors in the fire model used in the forward problem solution, but sensitive to errors in the measured temperature data. The accuracy of the heat release rate of the fire is sensitive to both errors in the fire model and errors in the temperature data. The validity of the use of computer simulated data in the evaluation is verified with a second evaluation using fire data interpolated from published measurements taken in large-scale compartment fire burns.     

Lag times associated with fire detection and suppression

The effectiveness of fire detection systems and fire mitigation strategies can be related to three distinct time lags associated with building fires: a transport time lag, a detection time lag, and a suppression time lag. The impacts of these lag periods on fire detection and suppression are developed. Transport lag periods are considered in terms of available correlations of fire plume and ceiling jet data, detection lag periods in terms of available heat detector response models that use these data correlations. Suppression lags are developed in terms of expected response times for automatic and manual suppression. Example calculations are presented.     

Wind effects on atria fires

A series of unsteady atria fire calculations are performed using a finite-volume CFD program on two and three dimensional generic buildings immersed in simulated atmospheric boundary layers. The model results reveal that external winds can modify the infiltration and exfiltration of air through external doors and windows, distort thermal and smoke columns rising above test fires in the atria, cause the plumes to impact directly against atria walls, and modify the resultant filling of elevated atria spaces. In some cases aggressive fire “whirls” form, which can enhance fire strength, enclosure mixing, and exposure. Results are compared qualitatively with similar physical model experiments.     

Theoretical Analysis and Experimental Study on Whirling Flames in Enclosure Fires

Whirling flames are not commonly observed in enclosure fires, but some special hazards will be induced for their characteristics once they occur. Employing the theories of vorticity transport and fire dynamics, the formation mechanism of whirling flames in enclosure fires has been analyzed, and the factor expression governing the formation of whirling flames occurring in single rooms with ceiling and lower openings, which are familiar to us in industrial buildings, has been deduced. Using a small-scale firebox with a lower door and a ceiling opening, the experiments of whirling flames in enclosure fires were carried out. Based on the experimental results, the characteristics of whirling flames in enclosure fires were studied, and the basic parameters of fires with and without whirling flame were compared. Combining the results of the theoretical analysis and experimental study, the formation criterion of swirling flames in enclosure fires were derived. Furthermore the reasons that whirling flames cause increases in some parameters, such as hot gas layer temperatures and floor radiant heat flux, were analyzed.     

Experimental study on transverse smoke temperature distribution in road tunnel fires

To assess the impact of smoke on the ceiling in tunnel fires, the smoke temperature under the ceiling was studied experimentally with small-scale experiments. This study focused on the transverse smoke temperature distribution in road tunnel fires as the longitudinal one has been widely researched. Comparison for the transverse and longitudinal smoke temperature distributions near the fire was conducted and the difference was researched. A correlation determining the transverse smoke temperature distribution under the ceiling was developed by taking the fire location into account.     

Transport Time Lag Effect on Smoke Flow Characteristics in Long-Narrow Spaces

This paper focuses on the smoke transport lag time at the early stage of fires in long-narrow spaces, which is defined as the time from fire onset to the time when smoke reaches a given position on the ceiling. For a heat detector at a specific location on the ceiling, the smoke transport lag time is a part of the response time of the heat detector. Especially when the heat release rate is relatively small at the early stage of fires, the smoke transport lag time will be very long, which will hence lead to the increase of heat detector response time. It is clear that the prediction of smoke transport lag time is critical to the activation time of the heat detector. However, previous studies have much focused on fire characteristics in long-narrow spaces, leaving very few on the transport time lag. Therefore, in this study, a theoretical model regarding smoke transport time lag was developed for both steady and time-dependent fires based on the weak-plume theory. This model was validated by a series of reduced-scale experiments. It can be concluded from comparison that the predictions of this model agree reasonably well with the corresponding experimental results. Using the proposed method, the dimensionless equations of smoke transport time lag, velocity and temperature considering the smoke lag effect in a long-narrow space for time-squared fires were also theoretically deduced. Additionally, to further determine the applicability of ‘Quasi-steady’ state assumption for time-squared fires, a calculation method regarding the critical time was also developed. The outcomes from this study will be beneficial to the development of fire detection model in long-narrow spaces.     

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。     

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.     

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.     

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.     

高海拔隧道全尺寸火灾烟气及温度场特征试验研究

通过对海拔为4100m的高海拔隧道进行全尺寸火灾试验,揭示高海拔隧道火灾烟气下沉及温度场变化特征。试验采用三种不同尺寸火源(0.8m2、1.0m2、2.0m2),对隧道火灾烟气蔓延特征、火区最高温度、隧道拱顶纵向温度分布进行研究。试验研究结果表明：隧道火灾试验初期及燃烧稳定阶段,火源附近隧道上层烟气与下层冷空气分界明显,火灾后期烟气下沉严重;较小风速有利于高海拔隧道小规模火灾烟气逆流层纵向和垂向蔓延的控制。隧道火灾温度场研究表明：隧道火灾温升速率随火源热释放率增大而增加;火源附近20m范围内温度衰减速率较快,远火源区域隧道拱顶纵向温度衰减较慢,趋于平缓;通过对火源上方拱顶烟气温度分析,发现隧道火灾探测采用差温报警模式较定温报警模式更加有效,并得出10℃/min的温升速率可基本满足高海拔隧道小规模火灾的初期报警;隧道拱顶纵向温度分布规律导致火源远场烟气下沉严重而近火源区域烟气层化较好的特征。高海拔隧道火灾温度分布特性试验研究,可为高海拔隧道火灾动力特性研究提供依据,为高海拔隧道人员疏散逃生提供指导及建议。     

水电站不同火源强度烟气运动模拟

应用FDS模拟水电站地下主厂房火灾烟气填充与流动动态过程,分析不同火源强度下烟气分层规律.重点分析火源强度为10 MW时的烟气填充过程、顶棚射流温度和横向烟气温度的变化特点.结果表明,烟气到达某一高度的时间随火源强度的变化服从一阶指数衰减;顶棚射流温度和烟气层横向温度变化具有很强的规律性;下层空气温度不高但浓度相对较高;同一水平面的烟气浓度分布很不均匀,距火源30 m以外处的浓度是距火源中心10 m以内处浓度的2～3倍.     

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.     

Enclosure Smoke Filling and Management with Mechanical Ventilation

Enclosure smoke filling and management are addressed from the standpoint of the volumetric flow rates commonly used for mechanical ventilation system design. In this context, fire-induced gas expansion is treated as a volumetric source term. A two-layer analysis developed previously for enclosure smoke filling without mechanical ventilation is extended to consider the impact of mechanical ventilation on smoke layer descent rates and conditions within the smoke layer. A spreadsheet-based model of enclosure smoke filling developed in conjunction with the previous unventilated analysis is also extended to consider both mechanical extraction and injection systems. Some implications of mechanical ventilation on the development and descent of a smoke layer in an enclosure fire are discussed.     

Analysis of experimental hydrocarbon localised fires with and without engulfed steel members

Localised fires can represent an important hazard to structural safety of buildings where a fully generalised fire cannot develop or when it is at its early stage. Plume correlations given in the codes are valid for undisturbed plume and it is not known whether the presence of a structural element engulfed into the localised fire can affect the validity of such correlations. In structural design, this may lead to highly conservative assumptions or, even, to possible misuses of the correlations. In order to provide insight into this issue, a comprehensive experimental programme aimed at providing data on hydrocarbon localised fires with and without engulfed vertical steel members was performed. In detail, a series of 22 tests of circular hydrocarbon pool fires in well-ventilated conditions of diameters ranging from 0.6 m to 2.2 m were performed with diesel and heptane. The particular aspect of these tests is that they were performed by means of a system that controlled the fuel flow and thus the rate of heat release (RHR) of the fire. The flame length and the temperatures of the fire plume measured experimentally were compared with existing plume correlations, data in the literature and the Eurocode correlations. The results show that: the presence of the column contributed to “straighten” the flame; although pool fires with same diameters were characterised by the same RHR, the flame length was different depending on the fuel type; experimental gas temperatures were lower than the temperature correlation given in the Eurocodes. In sum, the correlations included in the Eurocodes provided reasonable predictions in terms of flame length and of fire plume temperature rise around a steel vertical element located along the centreline of the localised fire.     

Fire simulation in road tunnels

The catastrophic tunnel fires since the year 1999 and a series of accidents in some tunnels in the summer of 2001 triggered extensive discussions and proposals relating to tunnel safety. When a fire occurs in a tunnel, and in absence of sufficient air supply, large quantities of smoke are generated, filling the vehicles and any space available around them. Unless a strong flow is created and maintained, hot gases and smoke migrate in all directions. With a weak airflow, smoke forms a layer along the tunnel ceiling and can flow against the direction of forced ventilation, interfering with personal evacuation. This paper shows the results of a computer fire simulation in a tunnel and the results of this simulation: air velocity, air temperature and wall temperature in the case of fire. The simulation started before the emergency ventilation system is activated and continued with the fans activated to control the smoke.