A Probabilistic Inferential Algorithm to Determine Fire Source Location Based on Inversion of Multidimensional Fire Parameters

A probabilistic inferential framework is proposed to utilize transient temperature data measured at the ceiling of the compartment to determine location of fire source, as well as size of fire, based on Bayesian inferential theory. This approach treats the problem as one of parameter estimation, expressed as a function of posterior probability distribution based on the qualitative of agreement between predicted temperatures and observed temperatures at the sensor locations. A comparison of the measured temperature from full-scale burn tests with predicted temperatures from in verse problem solution algorithm indicate the error to be less than 5% when fires are small, but the error increases to more than 10% for large size fires. The accuracy of the inverse problem solution algorithm can be improved by utilizing data from sensitivity studies carried out on fire source location errors and heat release rate errors.     

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.     

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.     

Turbulence statistics in a fire room model by large eddy simulation

Fire and smoke movement in a room is influenced by the turbulence characteristics (such as Reynolds stress, turbulent heat flux, etc.) of the flow and temperature fields. In order to accurately predict fire and smoke movement by computational fluid dynamics (CFD), it is necessary to verify these turbulence quantities. The purpose of this study is to predict the turbulence structure of the flow and temperature fields due to a fire in the compartment by large eddy simulation (LES) using detailed experimental data to verify the simulation results. The results show reasonably good agreement with experimental data for both the mean flow properties and the turbulence quantities with the exception of the region near ceiling. This study provides useful information for verifying LES technique when applied to compartment fires.     

Energy balance in a large compartment fire

The National Institute of Standards and Technology (NIST) and the Nuclear Regulatory Commission (NRC) are collaborating to assess and validate fire computer codes for nuclear power plant applications. This evaluation is being conducted through a series of benchmarking and validation exercises. The goal of the present study was to provide data from a large-scale fire test of a simulated nuclear power plant cable room. The experiments consisted of a hydrocarbon spray fire with a 1 MW heat release rate, burning in a single compartment 7 m wide, 22 m long, and 4 m high. Measurements included the vertical temperature profiles, heat flux to the compartment surfaces, the velocity and temperature at the compartment doorway, and the total heat release rate. From these measurements, an energy balance was considered, in which it was determined that nearly 74% of the fire's energy went to heat compartment surfaces, 22% escaped through the doorway, and 4% heated gases in the compartment.     

Temperature and velocity correlations in room fires for estimating sprinkler actuation times

Automatic sprinklers are increasingly used in residential occupancies to provide active fire protection. These sprinklers, known as quick response and residential sprinklers, may be located either at the ceiling (pendent-style) or on a wall (sidewall-style). Though several fire models are available for estimating actuation times for sprinklers located under unobstructed ceilings, these use engineering correlations that do not apply to residential-sized rooms. Thus, data are needed for estimating sprinkler actuation times for residential occupancies.This paper reports on fire tests that were conducted in various sized rooms to obtain temperature and velocity data for 73 kW, 100 kW, and 147 kW fires. The data were then used to develop nondimensional correlations for temperature and velocity at the sprinkler locations. The temperature data revealed a significant temperature transient in the hot gas layer, and thus a nondimensional correlation describing the transient phenomenon was developed. These correlations compared reasonably well with experimental data, and they were used to estimate the sprinkler actuation times. The estimates were in reasonable agreement for the pendent sprinkler, except for the smallest fire in a 4.27 m by 4.27 m occupancy. The estimates for sidewall sprinkler acuation were significantly lower than experimental values. This may have been due to the sprinklers' heat losses, which were not accounted for in the calculation.     

Measuring modified glass bulb sprinkler thermal response in plunge and compartment fire experiments

Automatic fire sprinklers use a heat sensitive element such as a glass bulb or fusible link to respond to the heat from a fire. The response of commercial fire sprinkler glass bulbs has been extensively characterised in convection-dominated dry gas flows but in real fires there may be more factors that influence the heat transfer to the bulbs such as radiation from the fire or cooling from adjacent sprinkler sprays. The time of activation is the only indication of the thermal response of typical commercial fire sprinklers using glass bulbs to a fire, but direct temperature measurement using a modified proxy may provide a better understanding of how sprinklers respond in a complex environment. Modified glass bulbs have been created that allow a thermocouple to be inserted in the bulb for direct temperature measurement. In this paper, the thermal response of sprinklers with these modified bulbs has been observed in hot-air wind tunnel plunge experiments and full scale room fire experiments. At the time of activation the measured temperature of the modified sprinklers was found to be higher than the nominal activation temperature specification for the unmodified sprinklers. For the compartment fires, a thermal response model generally predicted longer sprinkler activation times based on ceiling jet temperature and velocity measurements than was observed experimentally.     

算子识别反问题模型优化与数据优化的联合反演技术

针对算子识别反问题,分析了解的不适定性与模型误差、数据误差的关系,建立了基于模型优化和数据优化的联合反演技术,提出了适合同时处理数字式数据与非数字式数据的量化单调消噪方法。建立了数值反演可靠性概念,包括正演算子可靠性、正演计算可靠性、测量设计可靠性、反演算法可靠性、反演计算可靠性、测量数据可靠性,并建立了相应的可靠性定量评估方法。通过一个岩土工程的算子识别反问题的工程应用与数值试验说明:其一,这一联合反演技术实质是一门系统性的优化技术,能够显著提高数值反演的可靠性和准确度;其二,应用可靠性定量评估方法,能够客观地、定量地获得反问题解估计的质量评定。     

The critical ventilation velocity in tunnel fires—a computer simulation

In ventilated tunnel fires, smoke and hot combustion products may form a layer near the ceiling and flow in the direction opposite to the ventilation stream. The existence of this reverse stratified flow has an important bearing on fire fighting and evacuation of underground mine roadways, tunnels and building corridors. In the present study, conducted by the National Institute for Occupational Safety and Health, a CFD program (fire dynamics simulator) based on large eddy simulations (LES) is used to model floor-level fires in a ventilated tunnel. Specifically, the critical ventilation velocity that is just sufficient to prevent the formation of a reverse stratified layer is simulated for two tunnels of different size. The computer code is verified by checking the computed velocity profile against experimental measurements. The CFD results show the leveling-off of the critical ventilation velocity as the heat release rate surpasses a certain value. At this critical ventilation, the ceiling temperature above the fire reaches a maximum for both tunnels. The velocity leveling-off can be explained from this observation. An extended correlation of Newman (Combust. Flame 57 (1984) 33) is applied to the temperature profiles obtained by CFD. At the critical ventilation, temperature stratification exists downstream from the fire. The computed critical ventilation velocity shows fair agreement with available experimental data taken from both horizontal and inclined fire tunnels. The CFD simulations indicate that the Froude modeling is an approximation for tunnel fires. The Froude-scaling law does not apply to two geometrically similar fire tunnels. The CFD results are compared with two simple theories of critical ventilation by Kennedy et al. (ASHRAE Trans. Res. 102(2) (1996) 40) and Kunsch (Fire safety J. 37 (2002) 67).     

Postflashover fires — an overview of the research at the national research council of Canada (NRCC), 1970–1985

The NRCC model of fully developed compartment fires is discussed. Although the mathematics involved is quite simple, it allows a rather comprehensive simulation of the fire process. The model offers an explanation for the findings that ventilation control is related to the pyrolysis mechanism and is not a result of scarcity of air in the fire compartment, and that thermal feedback is of secondary importance in the burning (pyrolysis) of cellulosic fuels. Another feature of the model is the introduction of the normalized heat load concept. The normalized heat load is a scalar quantity that depends on the total heat absorbed by the compartment boundaries during the fire incident, and is practically independent of the temperature history of the fire. A simple explicit formula has been proposed and proved experimentally to describe the normalized heat load for real-world fires with fair accuracy. The normalized heat load concept offers a simple means for converting fire severities into fire resistance requirements, and makes it possible to design buildings for prescribed levels of structural fire safety. The potential of fires to spread by convection and the expected characteristics of fires of noncharring plastics are also discussed. Reference: T. Z. Harmathy, Postflashover Fires—An Overview of the Research at the National Research Council of Canada (NRCC), 1970–1985,Fire Technology, Vol. 22, No. 3, August 1986, p. 210.     

Actuation of sprinklers at high ceiling clearance facilities

As buildings with a high ceiling clearance are becoming increasingly common, making proper assessments of whether or not the ceiling sprinklers would actuate becomes very critical to designing adequate fire protection systems for such buildings. Two sets of fire test data under high ceiling clearances pertinent to growing 3-dimensaional fires and steady plane pan fires were analyzed to estimate maximum ceiling heights from the given fire sources that would allow actuation of ceiling sprinklers. The threshold fire sizes that would actuate ceiling sprinklers at a given ceiling clearances were also computed for growing fires and steady pan fires. Comparisons of the estimated threshold fire sizes between the growing fires and the pan fires indicate that assessing sprinkler actuations based on pan fire tests, which is a commonly used practice, will be likely to lead to a wrong conclusion. The analysis shows that a much smaller fire size than what pan fire tests might indicate would be needed to actuate sprinklers on high ceilings.     

COMPBRN III — A fire hazard model for risk analysis

COMPBRN III is a deterministic fire hazard computer code designed to be used in a probabilistic analysis of fire growth in a compartment. Its primary application to date has been the assessment of fire risk in the nuclear power industry. COMPBRN III follows a quasi-static approach to simulate the process of fire growth during the pre-flashover period in an enclosure. Physical models which quantify the thermal hazard (including temperature and heat fluxes) during a compartment fire are developed. Simulations of experiments are performed to test the accuracy of the improved hot gas layer model used in this version of COMPBRN in predicting the behavior of large-scale fires.     

Determination of Separation Distances Inside Large Buildings

In this study, an analytical framework is developed to determine the hazards posed by an uncontrolled fire burning indoors. This scenario, unlike unconfined outdoor fires, has received little attention in the literature and associated engineering methods for risk evaluation are unavailable. The present analyses are limited to overventilated fires burning in large non-combustible buildings. Hazards are evaluated on the basis of thermal radiation and firebrand transport. Thermal radiation is assessed using a solid flame radiation model; transport of firebrands is evaluated taking into consideration the convective ceiling layer established by the fire plume. Given the considerably different geometry of the scenario of interest herein, as compared to unconfined fires, efforts are placed in developing a rigorous physical and mathematical approach so as to make the developed methodology sufficiently general. The model derived is validated against limited heat flux data obtained for free-burn fires (up to 50 MW) involving Class 2 commodity rack storage arrays. In addition, general trends are investigated using a hypothetical sample scenario. Results show that thermal radiation is the main phenomenon driving the hazards encountered in indoor fires; firebrand transport, due to ceiling confinement, presents a much lesser hazard.     

Thermal Analysis of Hollow Steel Columns Exposed to Localised Fires

In fires in large compartments like enclosed car parks, airport terminals and industrial halls, the uniform distribution of gas temperature of post-flashover stages are unlikely to occur; in these cases, the thermal actions of a localised fire must be taken into account. In order to design steel structures for a localised fire, very detailed data concerning the development of temperatures in steel is required. EN 1991-1-2 presents a simplified model for calculating the temperatures in ceiling slabs and in the beams that may support such slabs; however, no simplified calculation model for the heat transfer in vertical elements, such as columns, is yet available. There is a need for more experimental data on real scale structures exposed to localised fires. A research project on the evaluation of temperatures in steel columns exposed to localised fires was carried out at the University of Coimbra. Full-scale natural fire tests were used to test columns, instead of conducting the usual furnace tests. This paper presents and discusses the results of the experimental tests on unprotected hollow steel columns exposed to localised fires, each of them simulating a distinct fire scenario according to different fire loads, positions and ventilation conditions. During the fire tests, real measurements showed flame heights and burning times different to those preliminarily estimated: flame heights had been conservatively predicted; while, the duration of the burning had been significantly underestimated.     

燃烧训练室内部温度分布特性分析

基于集装箱式燃烧训练室，开展不同规模火源和开、关门工况的6组燃烧实验，研究其内部温度分布规律，为消防人员制定训练方案、开展实战训练提供参考。实验结果表明：关门会使火源产生瞬间燃烧现象，开门使火源燃烧升温速率增大，顶棚处温度最高；关门燃烧时，小、中、大火的最高温度分别为331、463、752 ℃，随着距火源水平距离的增加，温度先快速降低后小幅回升，1.2 m高度处平均温度为100~180 ℃；在关、开门的中火燃烧对比实验中发现，开门面积越大，高温持续时间越长，火源外各点温度下降明显，1.2 m高度处的最高温度平均在70 ℃左右。     

A combined overall and surface energy balance for fully-developed ventilation-controlled liquid fuel fires in compartments

As part of the research to extend the understanding of fully-developed wood fires to non-cellulosic fuels, the outline of a theoretical energy balance for a liquid fuel fire in a compartment is presented. A computer solution of the heat balance is described and the results of simulated fires are given to illustrate the uses of the model and the limitations of the assumptions made in the theory.The results show systematic departures from the well known assumption of the constancy of the ratio of burning rate to ventilation rate; this can account for some of the scatter commonly found in measurements of this ratio.     

Ellipsoidal Solid Flame Model for Structures Under Localized Fire

This paper presents a model to evaluate the thermal energy transfer between a localized fire and the surfaces exposed to it, without the flame impinging the ceiling of the semi-open compartment. Although this type of fire may not have significant consequences for the structure as a whole, it is capable of triggering other disasters such as explosions and larger fires, which is why its study becomes increasingly important. Currently, this accident is analyzed using either sophisticated or semi-empirical numerical models available in the literature. The former uses computational fluid dynamics (CFD), which acceptably reproduces the fire, although with high computational cost. In turn, the semi-empirical models generally present conservative results. The proposed model presents variants in classic simple models available in the literature with the aim of being a tool that allows designers to estimate the thermal fields resulting from this type of fires at the preliminary structure design stage. In this model, the thermal analysis is performed using a finite element program, considering relevant parameters that characterize the fire such as: heat release rate, location and equivalent diameter of the fire source, among others. Through subroutines, the finite element model considers (a) a modification of hot gases temperature field based in a classic simple model and (b) proposition of a new geometry of the flame. The estimated radiative heat flux employs a solid ellipsoidal flame whose height changes according to the heat release rate. The convective heat flux is evaluated using a model for localized fire. Efficiency and accuracy of the methodology are checked by comparing the simulation results with those obtained by sophisticated models developed in fire dynamic simulator (FDS). The cases studied consider: (a) the replication of the experimental test conducted at Luleå University and (b) an offshore platform deck under localized fire action. The results of the first case confirm that the FDS replicates the experimental measurements with high accuracy. Finally, the results show that the proposed model allows to realistically represent the temperature fields generated by the fire, with relatively low computational cost compared to the CFD models for cases (a) and (b), therefore it is possible to use it to develop preliminary analyses in other fire scenarios.     

Characterizing Heat Release Rates Using an Inverse Fire Modeling Technique

A ubiquitous source of uncertainty in fire modeling is specifying the proper heat release rate (HRR) for the fuel packages of interest. An inverse HRR calculation method is presented to determine an inverse HRR solution that satisfies measured temperature data. The methodology uses a predictor-corrected method and the Consolidated Model of Fire and Smoke Transport (CFAST) zone model to calculate hot gas layer (HGL) temperatures in single compartment configurations. The inverse method runs at super-real-time speeds while calculating an inverse HRR solution that reasonably matches the original HRR curve. Examples of the inverse method are demonstrated by using a multiple step HRR case, complex HRR curves, experimental temperature data with a constant HRR, and a case with an experimentally measured HRR. In principle, the methodology can be applied using any reasonably accurate fire model to invert for the HRR.     

The Response Time of Different Sprinkler Glass Bulbs in a Residential Room Fire Scenario

The response time of fire sprinklers is essential for their performance, especially in applications where life safety protection is desired. The earlier the sprinkler activates, the smaller the size of the fire. Most commercial residential sprinklers are fitted with 3 mm, 68°C glass bulbs. However, thinner sprinkler glass bulbs with lower operating temperatures are available. The aim of this study was to determine the response time—and the corresponding heat release rate—of different glass bulbs in a residential room fire scenario. A series of tests were conducted inside a compartment measuring 3.66 m by 3.66 m having a ceiling height of 2.5 m. The compartment was either enclosed or had two walls removed to provide a more ventilated scenario. A propane gas burner was positioned at one of the corners. The mass flow rate of the gas was controlled such that either ‘slow’, ‘medium’ or ‘fast’ fire growth rate scenarios were simulated. In each test, nine Response Time Index (RTI) and operating temperature combinations were tested. Each test was replicated three times. In addition, two commercial fire detectors were tested. The results show that the fire is considerably smaller upon activation with a combination of a low RTI and a low operating temperature, as compared to the 3 mm, 68°C glass bulb typically used for residential sprinklers. The operating temperature proved to have a larger impact on the results than the RTI. The heat from the fire was typically detected by the fire detectors prior to the activation of the sprinkler glass bulbs, especially for the ‘slow’ and ‘medium’ fire growth rate scenarios.     

Assessing the Sprinkler Activation Predictive Capability of the BRANZFIRE Fire Model

This paper describes an investigation into the sprinkler response time predictive capability of the BRANZFIRE fire model. A set of 22 fire/sprinkler experiments are simulated where the sprinkler activation time and the heat release rate (HRR) for each individual experiment had been determined. The experiments provided data for use in validating the sprinkler activation prediction algorithms in the BRANZFIRE zone model. A set of base case values were chosen and input files constructed for the simulations. The experiments were then simulated by the fire model using both the NIST/JET ceiling jet and Alpert’s ceiling jet options (which are the two ceiling jet correlations available in the BRANZFIRE zone model). The fire model included a heat transfer calculation for the temperature of the heat sensitive sprinkler element. Different sprinkler operational parameters such as the conduction factor, response time index (RTI) and the sprinkler depth below ceiling were also varied to assess the sensitivity of their effect on the activation time. Results showed that using the NIST/JET ceiling jet algorithm gave a closer prediction of the sprinkler response time in a small room than Alpert’s correlation. This was expected, since the former includes the effect of a hot upper layer while the latter applies to unconfined ceilings. The experiments available for comparison had been conducted inside an enclosure with a developing hot upper layer. The findings also signified that changing the sprinkler operational parameters can change the predicted sprinkler activation time significantly.