The Use of CFD Calculations to Evaluate Fire-Fighting Tactics in a Possible Backdraft Situation

This paper is an attempt to integrate theoretical Computational Fluid Dynamics (CFD) calculations with practical fire-fighting tactics commonly used when arriving at the scene of an underventilated fire. The paper shows that CFD has a great potential in improving understanding and creating better effectiveness in the estimation of fire-fighting tactics. If burning has occurred in a lack of oxygen for a long time, excessive pyrolysis products may have accumulated in the fire compartment. If air is suddenly introduced in the compartment a backdraft may occur. The CFD code used for the simulations is fire dynamics simulator (FDS). In this paper, we focus on the conditions that can lead to backdraft, and not the deflagration or rapid combustion in itself. Therefore, the simulations focus on the gravity current and the mixing process between cold fresh air and hot smoke gases by considering a uniform temperature inside the building as initial condition. The different tactics studied include natural ventilation, positive pressure ventilation (PPV) and dilution by water mist. Their effectiveness is observed comparing them with a reference scenario, where no action is taken. The main objective of natural ventilation is to find the fire source, and the venting is more effective with several openings. Tactics involving PPV are very effective in evacuating the unburnt gases, but increases the mixing, and consequently the probability of backdraft during the early stage of operation. On the other hand, the addition of water mist can reduce the danger of backdraft by reducing the concentration of unreacted combustible gases below the critical fuel volume fraction (CFVF), where ignition cannot occur. If the dilution level is insufficient the danger of backdraft is increased, mainly because the process of gases evacuation is longer due to cooling, which reduces the density difference between hot and cold gases. During a fire-fighting operation, the choice of tactic depends mainly on whether there are people left in the building or not, but also on the fire-fighters’ knowledge of the building’s geometry and the fire conditions. If the situation shows signs of strongly underventilated conditions, the danger of backdraft has to be considered and the most appropriate mitigation tactics must be applied.     

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.     

Influence of Obstacles on the Development of Gravity Current Prior to Backdraft

The phenomenon of backdraft is closely linked to the formation of a flammable region due to the mixing process between the unburned gases accumulated in the compartment and the fresh air entering the compartment through a recently created opening. The flow of incoming fresh air is called the gravity current. Gravity current prior to backdraft has already been studied, Fleischmann (1993, Backdraft phenomena, NIST-GCR-94-646. University of California, Berkeley) and Fleischmann (1999, Numerical and experimental gravity currents related to backdrafts, Fire Safety Journal); Weng et al. (2002, Exp Fluids 33:398–404), but all simulations and experiments found in the current literature are systematically based on a perfectly regular volume, usually parallelipedic in shape, without any piece of furniture or equipment in the compartment. Yet, various obstacles are normally found in real compartments and the question is whether they affect the gravity current velocity and the level of mixing between fresh and vitiated gases. In the work reported here, gravity current prior to backdraft in compartment with obstacles is investigated by means of three-dimensional CFD numerical simulations. These simulations use as a reference case the backdraft experiment test carried out by Gojkovic (2000, Initial Backdraft. Department of Fire Safety Engineering, Lunds Tekniska Högskola Universitet, Report 3121). The Froude number, the transit time and the ignition time are obtained from the computations and compared to the tests in order to validate the model.     

Subgrid scale laminar flamelet model for partially premixed combustion and its application to backdraft simulation

Flame spread after air is suddenly introduced to a vitiated compartment in backdraft is between non-premixed and premixed flame spread under a ventilation-controlled condition. And it is necessary, but difficult to numerically simulate it. In this paper, an attempt of backdraft simulation is introduced. Numerical models including a subgrid scale laminar flamelet model and a partially premixed model are imbedded in FDS3.0 source code for backdraft simulation. Some significant fire characteristics reported in previous backdraft experiments can be seen in the numerical results. It is also indicated that these combined models can be used to predict the partially premixed combustion and fire phenomena under ventilation-controlled conditions.     

The Florida palm coast fire: An analysis of fire incidence and residence characteristics

A quantitative analysis of the relationship between residential fire incidence, fire intensity, house characteristics, and location is presented. Fire intensity (ground vs. crown fire) was shown to be the most significant factor. Brush clearance and type of soffit vent were also shown to be related to fire incidence. Logit analysis was used to analyze the joint effect of the various factors. Five factors allowed the model to correctly predict whether a house burned in 83 percent of the observations. Reference: Robert Abt, David Kelly, and Mike Kuypers, The Florida Palm Coast Fire: An Analysis of Fire Incidence and Residence Characteristics,Fire Technology, Vol. 23, No. 3, August 1987, pp. 186–197.Florida Agricultural Experiment Station Journal Series No. 8241     

Large eddy simulation of the backdraft phenomenon

A sub-grid scale model for partially premixed combustion has been adapted and applied to simulate the backdraft phenomena. A fast deflagration or backdraft is produced when into a hot, fuel-rich compartment an inflow of fresh air is suddenly allowed through an opening. It is essentially a violent combustion process involving both premixed and non-premixed regimes. The present model is based on the coupling of independent approaches for non-premixed and premixed turbulent combustion. The ‘flame index’ concept was used to separate the two different combustion regimes. This index describes the structure of the flame based on fuel and oxygen gradients. Due to the lack of detailed experimental measurements, the results were largely analysed qualitatively. The predictions have provided valuable insight into the backdraft phenomenon suggesting that the development of backdraft can be divided into five phases, i.e. initial condition, free “spherical propagation, “plane” front propagation, stretching of the flame front through the opening and fireball outside the container. Quantitatively, the experimentally measured and predicted lapse of time between the maximum over- and under-pressure at the opening of the container is found to be in reasonably good agreement.     

地铁火灾中回燃现象研究初探

回燃是在通风受限的建筑火灾进入缺氧燃烧甚至闷烧后,由于新鲜空气的突然大量补充引起热烟气急剧燃烧的现象.目前腔体回燃的研究已取得很多有价值的成果.在总结前人研究成果的基础上,结合地铁火灾的特点,指出地铁火灾中也存在回燃现象,并简略地提出了针对地铁火灾中回燃现象应进行的研究内容.     

The Capabilities and Limitations of Total Flooding, Water Mist Fire Suppression Systems in Machinery Space Applications

Water mist fire suppression systems are being seriously considered as replacements for Halon 1301 total flooding systems in machinery space applications. A total flooding water mist system is designed to discharge mist uniformly throughout the entire enclosure/compartment. The tests conducted to date form a substantial database for water mist systems installed in machinery spaces with volumes from 100 m³ to 1,000 m³ and varying degrees of ventilation. These tests have also identified the strengths and limitations of water mist in these applications. This report describes the capabilities of total flooding water mist fire suppression systems in machinery space applications.     

Collapsed spring observations in arson investigations: A critical metallurgical evaluation

Observation of collapsed coiled (steel) furniture springs has been utilized for several decades or more by arson investigators as an indicator of whether an accelerant or smoldering source (such as a cigarette) caused a fire. This paper cites the contradictory literature, synopsizes metallurgical phenomena operative when coiled steel springs are subjected to fires, and presents empirical data from U.S. Federal Bureau of Investigation (FBI) Laboratory testing. It is concluded that observation of the collapsed state of coiled furniture/bedding springs is not a reliable indicator of whether a fire was initiated by a smoldering cigarette or accelerated by the presence of a hydrocarbon.     

Development and Application of the Fire Brigade Intervention Model

A performance based building code [1] was introduced in Australia in 1996. In order that fire brigades could ensure that their functional role was maintained in the building code, a method of quantifying fire brigade roles was required. In response to this issue, the Australasian Fire Authorities Council (AFAC) formed a Performance Based Fire Engineering Committee. This committee developed a model that determines the time taken by a fire brigade to undertake its activities at a fire scene.The Fire Brigade Intervention Model [2] is an event-based methodology, which quantifies fire brigade responses employed during a structure fire from time of notification through to control and extinguishment. It has been primarily developed for use in fire engineering design in a performance based regulatory environment so that the functional role of a fire brigade can be effectively incorporated into the building design process. It establishes a structured framework necessary to both determine and measure fire brigade activities on a time-line basis. It interacts with the output of other sub-systems, which model such events as fire growth, smoke spread, fire spread, detection and suppression as well as occupant avoidance.This paper describes the Fire Brigade Intervention Model, now available for use by fire brigades and fire engineers. The model has been developed for specific case and site analysis and is applicable to most structural fire scenarios. As the expertise of the local fire brigade will be incorporated inthe input parameters, it is valid for most brigade types, crew sizes and resource limitations.This paper also describes ongoing developments including a training package and computer program.The terms fire brigade and fire department are synonymous.     

Fire tests on window assemblies protected by automatic sprinklers

Full-scale fire tests on wired and tempered glazing in steel and aluminum frames are described. These assemblies achieved fire resistance ratings when exposed to a standard fire of 45 min to 2 h. The maximum radiant heat flux transmitted through the glass was reduced by more than 90%. Reference: J. K. Richardson and I. Oleskiewicz, Fire Tests on Window Assemblies Protected by Automatic Sprinklers,Fire Technology, Vol. 23, No. 2, May 1987, pp. 115–132.This paper is a contribution from the Institute for Research in Construction, National Research Council of Canada.     

Sensitivity of fire size to fireline construction rates in a simulation model

An initial attack model was sensitive to changes in production rate of suppression forces, as shown by changes in final fire size.The adequacy of fireline construction rates used in fire planning models has been questioned. A fire containment model was tested over a range of suppression force productivities to show the effect on final fire size. The percentage of fires that escaped was sensitive to the productivity of line-building units. The degree of sensitivity depended on the severity of environmental conditions. Final fire size was most affected by fireline construction rates under the most severe environmental conditions tested. Using distributions of production rates from weighted averages of the results produced final fire sizes similar to those obtained using point estimates in the simulation model. Planners may be able to use graphs like those presented, in place of iterations of a fire containment model. Reference: Eric L. Smith, Sensitivity of Fire Size to Fireline Construction Rates in a Simulation Model,Fire Technology, Vol. 22, No. 2, May 1986, p. 136.     

Magnetometric Method of Investigating Fire Sites

Magnetic susceptibility (MS) measurements were taken to determine fire exposure patterns on noncombustible structural members and compartment boundaries. These measurements led on-site fire investigators to the cause of combustion and the point of fire origin. Thermal damage to walls and to ceilings was estimated quantitatively by measuring MS and two-valent Fe ion concentration in building materials such as concrete and plaster. It was shown that these effects are preconditioned by thermochemical transformations in the mineral biotite, which is present as a component of common building materials.For the first time, the effect of a substantial and irreversible increase in biotite magnetization exposed to powerful flows of N- and P-rays was found. It is hoped that the presence of this magnetic trace of radiation in building materials will allow investigators to use the magnetometric method at nuclear power plant catastrophes such as that in Chernobyl. It was also shown that the influence of N- and P-rays on biotite's magnetic properties greatly differs from the thermal influence on biotite.     

Smoke Control in Hospitals

Full scale fire tests have been carried out in order to study the influence of different ventilating principles on the time point of fire detection and the smoke filling of a four-bed room. Using conventional mechanical ventilating systems as smoke exhaust systems the time difference left for evacuation of the fire room can be positively influenced. With the conventional ventilating system operating there is a significant difference between time points of detection of the ionization and optical smoke detectors, for both flaming and smoldering fire. Using the low momentum displacement ventilation this difference is reduced, resulting in possibilities for the ionization smoke detector to be optimized for both flaming and smoldering fires. Reference: Øystein Meland and Eimund Skåret, Smoke Control in Hospitals,Fire Technology, Vol. 22, No. 1, February 1986, p. 33.     

Slide rule estimates of Fire Growth

A series of prediction methods has been assembled to provide an analytical basis for estimating fire growth in compartments. Solutions for each prediction method can be made using programmable scientific calculators. Prediction methods are presented for: fire size and growth rates, mass loss rates, radiant heat flux, flame height, radial flame impingement, heat flux to a ceiling, smoke filling of a room, carbon monoxide hazard with smoldering fires, temperature rise in a compartment, ventilation flow rate, flashover occurrence, corridor smoke transfer and filling, smoke concentration, visibility, flame spread rates, and fire burn time.These predictive methods are useful for estimating many of the critical elements related to fire behavior and help provide a better understanding of this complex phenomenon.This report appears as Appendix B inFire Growth in Combat Ships by J. G. Quintiere, H. R. Baum and J. R. Lawson, NBSIR 85-3159. Reference: J. R. Lawson and J. G. Quintiere, Slide Rule Estimates of Fire Growth,Fire Technology, Vol. 21, No. 4, November 1985, p. 267.This paper is a contribution of the National Bureau of Standards and is not subject to copyright.     

Fire Tests of Packaged and Palletized Computer Products

When an intense fire rage through a major computer distribution and storage warehouse in Northern Europe, it destroyed the complete stock of computer products. The products had been stored in high racks in the unsprinklered building of approximately 157,000 square feet (14,585 square meters). This research demonstrates how and why this fire grew from a single point of origin to a complete burnout of the building. Extensive fire testing of the major commodities stored, including packaged and palletized computer products, is central to this analysis. These tests focused on obtaining heat release rate data to be used as input for enclosure fire models and for algorithms used to predict fire spread between commodity arrays. We've described the fire modeling and fire spread analyses in other publications.^1,2,3 Test results show that all packaged computer pallet loads produce heat release rates consistent with fast or ultra-fast t² fire growth, as defined in fire models such as FPETOOL⁴ and FAST.⁵     

Numerical and experimental gravity currents related to backdrafts

In order to clarify the mixing that occurs in a gravity current which precedes a backdraft, a two-dimensional simulation, a series of salt water experiments, and backdraft experiments were performed. A compartment in a ratio width/height/length of 1/1/2 is used in the experiments and computations. Two different openings were used in the salt water experiments and numerical computations: a fully open end wall and a h₁/3 horizontal slot centred in an end wall, where h₁ is the compartment height. For the backdraft experiments only the h₁/3 horizontal slot was used. The visual observations from the salt water experiments compare well with the numerical simulation. Both show a small mixed region at the gravity current shear interface for the fully open wall and mixing throughout the entire current for the h₁/3 horizontal slot. Quantitative comparisons are made in terms of the normalised density differences, β=(ρ₀−ρ₁)/ρ₁, where ρ₀ is the higher density and ρ₁ is the lower density. The transit time results, i.e., time between opening the compartment and the time the gravity current reaches the wall opposite the opening, for the computations compared favourably with the transit times from the salt water experiments, over the range 0.003<β<0.100. Velocity measurements from the opening of the backdraft compartment, prior to ignition, also are favourable with the numerical simulation.     

The effect of aggregation on the output multipliers in input-output models

This paper compares the output multipliers of the 1967 National input output model (367 industries) with the multipliers of the aggregated version (81 industries). The results show that, generally, the set of industries in the full model that were aggregated together for the reduced model, display a wide range of output multiplier values. Therefore, the output multiplier of an aggregated industry may not be truly representative of any of the industries that were joined together. Finally we show how information for a particular firm or disaggregated industry can be incorporated in an aggregated input-output model to obtain a close estimate of the actual output multiplier for that firm or industry.     

A Fire Safety Assessment System for Existing Buildings

In Hong Kong which is characterized by its tall, reinforced-concrete buildings with multiple owners, most of the high-rise buildings constructed before the mid-1970s are dilapidated. In order to address this problem, the Hong Kong Government intends to introduce mandatory safety assurance requirements with its so-called Building Safety Inspection Scheme, under which building owners would be required to appoint building professionals to assess the safety level of their buildings and to recommend appropriate corrective actions. In order to ensure uniform standards and easy administration, the Hong Kong Government intends to publish assessment guidelines. One of the major safety aspects the guidelines will addresses is fire safety, since, over the past few years, several large, tragic fires have occurred in the city's high-rises. This article proposes a fuzzy fire safety assessment approach based on fire risk ranking techniques that may form part of the safety evaluation tool for existing buildings. The basic principles of the system are briefly described, and its implementation is outlined.     

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.