Compartment fire phenomena under limited ventilation

Fire behavior of heptane pool fires were investigated in a small-scale 40 cm cubic compartment with wall vents at the ceiling (top vent) and the floor (bottom vent). The measurements included pressure, mass loss, temperature, heat flux, and gas mole fraction. Flame oscillations, ghosting, and burning at the air inlet were seen. The regime of limited ventilation was examined to study the effect of extinction and the influence of oxygen. A theory based on a critical flame temperature showed that extinction depends on heating as well as oxygen concentration. A complete uniform property model was developed and its solution agrees qualitatively with the measurements.     

On the maximum smoke temperature under the ceiling in tunnel fires

Maximum smoke temperature under the ceiling in a tunnel fire was studied experimentally and numerically. Full-scale burning tests in two vehicular tunnels of length 3.27 and 1.032 km with and without operating the longitudinal ventilation system were carried out. Smoke temperatures at selected positions under the ceiling were measured under different longitudinal ventilation velocities. Two different pool fires of 1.6 and 3 MW were set up. Computational Fluid Dynamics (CFD) simulations with Fire Dynamics Simulator (FDS) version 3.10 were carried out on those scenarios. CFD predicted smoke temperatures were firstly verified by comparing with the measured values at those selected positions, and then compared with the calculated values using the empirical equation due to Kurioka et al. Fairly good agreement was achieved, though the slope of the tunnel was not considered in this empirical equation.     

Full-scale experiment and CFD simulation on smoke movement and smoke control in a metro tunnel with one opening portal

Three full-scale model experiments were conducted in a unidirectional tube, which is a part of a metro tunnel with one end connected to an underground metro station and the other end opened to outside in Chongqing, PR China. Three fire HRRs, 1.35 MW, 3 MW and 3.8 MW were produced by pool fires with different oil pan sizes in the experiments. Temperature distributions under the tunnel ceiling along the longitudinal direction were measured. At the same time, CFD simulations were conducted under the same boundary conditions with the experiments by FDS 5.5. In addition, more FDS simulation cases were conducted after the FDS simulation results agreed with the experimental results. The simulation results show that the smoke temperature and the decay rate of the temperature distribution under the tunnel ceiling along the longitudinal direction increase as HRR increases. The smoke exhausts effectively from the tunnel under mechanical ventilation system, whether the emergency vent is activated as a smoke exhaust or an air supply vent. The operation mode of the mechanical ventilation system depends on the evacuation route.     

Heat feedback to the fuel surface of a pool fire in an enclosure

As a part of an effort to determine the energy balance at the pool fire surface in compartments, a series of fire experiments were conducted to study heat flux of the flame in a vitiated environment formed with air and combustion products gases. This paper presents experimental results of the burning behaviour of a heptane pool fire in a reduced scale compartment equipped with a mechanical ventilation network. Measurements of heat fluxes, fuel mass loss rate, oxygen concentration and temperature are performed for heptane fires of 0.26 and 0.3 m diameter pans at different ventilation flow rates. An original method to separate effects of the radiant heat flux of the flame and of the external heat feedback to the fuel surface is developed. This was achieved by using an additional heat flux measurement located under the pool fire. A correlation was also developed to determine the temperature rise on the plume centerline in the compartment as a function of the heat release rate. The results indicate a decrease in the fuel mass loss rate, flame temperature and heat fluxes to the fuel surface as the oxygen concentration measured near the fuel decreases by varying the air refresh rate of the compartment. The flame radiation fraction shows a similar behaviour, whereas the convective fraction of the flame heat flux increases when oxygen concentration decreases. Based on these experimental findings, it was discussed that any classification of the burning regime of a pool fire should consider both the effects of pan diameter and the burning response to vitiated air.     

A compartment burning rate algorithm for a zone model

A model is presented that dynamically predicts the mass loss rate of fuel in a compartment as a function of ventilation, thermal feedback, fuel type and scale. Without a loss of generality, a floor-based fuel is considered. The effect of ventilation is included in the model through the ambient oxygen concentration in the ambient surrounding the fuel at the floor. A mixing model associated with the inlet airflow at the vent is developed to determine this oxygen concentration. An extinction criterion for the flame is based on a critical flame temperature for a diffusion flame associated with the ambient conditions surrounding the flame at the floor. The model is executed in BRI2002, a zone model, capable of computing species and thermal conditions in the upper and lower compartment gas layers. Computations show good agreement with small-scale compartment data for heptane pool fires. The results can accurately portray many regimes of burning including extinction, combustion oscillations, reduction in the flaming area, and quasi-steady burning.     

Experimental Study of Fire Ventilation During Fire Fighting Operations

Fire ventilation measures taken by fire & rescue services, including positive pressure ventilation, were investigated. Fifteen tests were performed in a three-room apartment, with an attached staircase, on the first floor of a training facility. The fire source was a 0.5 m diameter pool of heptane. The temperature and pressure in the apartment, the weight of the fire source, and the flow through openings were recorded continuously. The tests showed that the rate of burning was increased by positive pressure ventilation. Also, positive pressure ventilation increases the temperature in rooms on the leeward side of the fire and reduces temperatures in rooms on the windward side of the fire. Safety and working conditions for fire fighters are improved by positive pressure ventilation, but it jeopardises the lives of anyone that might be trapped. The importance of command and control during fire fighting operations is prominent.     

Numerical Simulation of Fire Exposed Façades Using LEPIR II Testing Facility

The fire behavior of external wall insulation system on façades is assessed during LEPIR II testing. This facility involves a 600 kg wood crib fire in a 30 m³ lower compartment of a two levels high concrete structure. External flames develop in front of the façade from the fire compartment through windows with dimensions 1?×?1.5 m (W?×?H). In order to predict the fire exposure of a façade during the test, CFD simulations were carried out with the computational fluid dynamics code Fire Dynamics Simulator (FDS) for two full-scale experiments. The main objective of this study was to evaluate the ability of FDS to reproduce quantitative results in terms of gas temperatures and heat fluxes close to the tested façade. This is an important step before the fire performances of any insulation system can be predicted by numerical tools. A good repeatability was observed in terms of measured gas temperatures for experiments. Maximum heat release rate of the fire, close to 5 MW, was achieved after 5 min of test. When experimental results were compared with numerical calculations, good agreement was found for every quantity. The most critical zone on the facade is located above the fire room and is directly impacted by external flame outgoing from the fire compartment. Temperatures up to 500°C were observed in this zone. For the thermocouples located up to the second level opening, these probes were not located directly in the flames, but rather in the hot gases above the fire plume. The maximum temperature achieved was thus close to 400°C. The proposed model gives correct thermal loads and flames shape near the façade during calibration tests and can be used for further evaluation of combustible material on façade.     

An experimental study on the effect of ventilation velocity on burning rate in tunnel fires—heptane pool fire case

The 1/20 reduced-scale experiments using Froude scaling are conducted to investigate the effect of longitudinal ventilation velocity on the burning rate in tunnel fires. The n-heptane pool fires with heat release rate ranging from 3.71 to 15.6 kW are used in this study. A load cell is used to measure the mass loss rate of burning fuel and the temperature distributions are measured by K-type thermocouples in order to investigate smoke movement. The ventilation velocity in the reduced-scale tunnel is controlled by the wind tunnel through an inverter. The increases in ventilation velocity lead to enhance burning rate of n-heptane fuel. The reason is that the oxygen supply effect prevails rather than the cooling effect as the ventilation velocity increases. As a result, the heat release rates in experiment are larger than constant heat release rates by 4.45–11.3 times in the n-heptane pool fires. Also, it is found that non-dimensional critical ventilation velocity is proportional to one-third power of non-dimensional heat release rate.     

Factors Affecting the Make-Up Air and Their Influence on the Dynamics of Atrium Fires

In case of fire, constructive features of typical atria could favor the spread of smoke. This makes the design of their smoke control and management systems a challenging task. Five full-scale fire experiments in the literature have been analyzed and numerically compared in FDS v6 to explore the influence of the make-up air. However, these fire experiments cover only a limited number of set-ups and conditions, and require further numerical modeling to obtain a deeper understanding of the makeup air influence. Subsequently, 84 simulations with FDS v6 have been carried out, considering different vent areas (air velocity from 0.4 to 5.3 m/s) and configurations, two heat release rates (2.5 and 5 MW), and two pan locations. It is demonstrated that make-up air velocities lower than the prescribed limit of 1 m/s, by the international codes, may induce adverse conditions. Based on our results, we recommended fire engineers to numerically assess the fire scenario with even lower velocity values. The results also show that asymmetric configurations are prone to induce circulation around the flame which can contribute to the formation of longer flames and fire whirls. Thus, this numerical study links two fire types allowing the connection of pool fires to fire whirls, which completely differ in behaviour and smoke filling, for the sake of design of fire safety.     

在受限和通风防火分区内对燃烧速率特性的试验研究

在核安全研究框架内对在受限和通风防火分区内油池火燃烧速率进行了试验研究。在实体火灾试验基础上，此研究为在受限和通风火灾场景下的燃烧速率机理提供了新的信息。描述了在自由条件和空气受限条件下所进行的试验，对试验装置、仪器以及火源进行了详细叙述。在相同场景（0．4m^2TPH油池火）下，对自由条件和空气受限条件的试验情况下的燃烧速率进行了对比。在空气受限情况下，燃烧速率与时间的变化曲线显示出三个不同阶段：自由条件和受限燃烧速率相同；不稳定阶段，空气受限条件下燃烧速率高于自由条件下的燃烧速率;稳定阶段。从图像分析看，不稳定阶段显示，动荡和间歇火焰大大提高了燃烧速率。介绍了通风速率和油池面积对此现象的影响。试验结果为理解有限区域内燃烧速率提供了新的试验信息。     

Numerical studies on heat release rate in a room fire burning wood and liquid fuel

Heat release rates of burning gasoline and wood fires in a room were studied by computational fluid dynamics (CFD). Version 5.5.3 of the software Fire Dynamics Simulator (FDS), which is the latest one available, was selected as the CFD simulation tool. Predicted results were compared with two sets of reported data from full-scale burning tests. In the two sets of experiments, the scenarios were set at gasoline pool fire and wood chipboard fire with gasoline respectively. The input heating rate of gasoline pool fire based on experimental measurements was used in the first set of experiments. Three scenarios G1, G2 and G3 with different grid systems were simulated by CFD. The grid system of scenario G2 gave more accurate prediction, which was then used to study the second set of experiments on wood chipboard with gasoline. The combustion model in FDS was used in wood chipboard fire induced by gasoline pool. The wood chipboard was allowed to burn by itself using the pyrolysis model in FDS. The effects of the boundary conditions on free openings for the same set of experiments were studied by three scenarios SOB1, SOB2 and SOB3. Boundary condition SOB2 gave more reliable prediction among the three boundary conditions. Two other scenarios on the effect of moisture content of wood were also studied. The predicted HRR curve was found to agree better with experiment in using SOB2.     

Assessment of the Use of Fire Dynamics Simulator in Performance-Based Design

This paper discusses a procedure for the use of fire modelling in the performance-based design environment to quantify design fires for commercial buildings. This procedure includes building surveys, medium-and full-scale experiments and computer modelling. In this study, a survey of commercial premises was conducted to determine fire loads and types of combustibles present in these buildings. Statistical data from the literature were analysed to determine the frequency of fires, ignition sources, and locations relevant to these premises. Based on the results of the survey and the statistical analyses a number of fuel packages were designed that represent fire loads and combustible materials in commercial buildings. The fuel packages were used to perform medium- and full-scale, post-flashover fire tests to collect data on heat release rates, compartment temperatures and production and concentration of toxic gases. Based on the experimental results, input data files for the computational model, Fire Dynamics Simulator (FDS), were developed to simulate the burning characteristics of the fuel packages observed in the experiments. Comparative analysis between FDS model predictions and experimental data of HRR, carbon monoxide (CO), and carbon dioxide (CO₂), indicated that FDS model was able to predict the HRR, temperature profile in the burn room, and the total production of CO and CO₂ for medium- and large-scale experiments as well as real size stores.     

Determinations of the fire smoke layer height in a naturally ventilated room

According to the case-based reasoning of natural ventilation designs in recommended Green Buildings, an investigated model space was proposed in this study. FDS simulations and full-scale experiments were carried out to measure the impact of natural ventilation conditions and the installation of a natural ventilation shaft on smoke layer descent during different fire scenarios. The feasibility of using the N-percentage rule to determine the fire smoke layer height in a naturally ventilated space was also investigated.In a non-fire room, the smoke descent curve determined from the FDS simulated temperatures is consistent with the experimentally measured temperatures and visual observation of the smoke layer. However, the thermocouples in the fire room are affected by direct burning and fire radiation, and the experimentally measured temperatures cannot be used to determine the smoke height. Under these conditions, FDS simulations can be used to compensate for the lack of experimental measurements. In fire scenarios without outdoor winds blowing into the building's interior, FDS simulations can reliably model the fire smoke layer height. When outdoor air blows into the interior, it causes the smoke layer temperature to become unstable. Thus, the temperature will not be thermally stratified, and the use of the N-percentage rule is not recommended.     

Investigation of the effect of tunnel ventilation on crib fires through small-scale experiments

This paper adopts a series of 1:20 scale tunnel experiments based on a series of large-scale tunnel experiments to study the influence of forced ventilation on fires. The small-scale tunnel has dimensions of 0.365 m (W)×0.26 m (H)×11.9 m (L). Cribs using a wood-based material provide the fuel source and forced ventilation velocities from 0.23 to 1.90 m/s are used. From the study of the measured heat release rate (HRR) and mass loss rate data it is found that the forced air velocity affects the fire spread rate and burning efficiency and further affects peak HRR values at different air velocities. A simple model to describe these influences is proposed. This model is used to reproduce the enhancement of peak HRR for cribs with different porosity factors noted by Ingason [1] and to assess the effects of using different length of cribs on peak HRR. The results from these analyses suggest that different porosity fuels result different involvement of burning surface area and result different changes in peak HRR. However, no significant difference to the enhancement on fire size is found when the burning surface area is similar. It is also found that the trend in the enhancement on fire size by using sufficiently long crib and available ventilation conditions matches the predictions of Carvel and Beard [2] for two-lane tunnel heavy goods vehicle fires.     

Predicting the heat release rates of liquid pool fires in mechanically ventilated compartments

In this paper we perform predictive simulations of liquid pool fires in mechanically ventilated compartments. We show that steady state burning rates are accurately predicted using a detailed model for the liquid phase heat transfer. The effect of lowered oxygen vitiation on the burning rate of pool fires is correctly captured. Simulations were done using the Fire Dynamics Simulator and the experiments considered were conducted in the OECD PRISME project. The main difference between the present study and previous simulation studies is the use of a detailed liquid evaporation model and the direct calculation of the vitiation and thermal environment interactions through the CFD solver.     

Simulating the effects of fuel type and geometry on post-flashover fire temperatures

This paper discusses the effect of fuel type and geometry on predicted compartment temperatures derived from computer modelling of post-flashover compartment fires. Many previous studies have investigated post-flashover fires with either wood crib or liquid pool fuels, but very few analytical or experimental studies have considered realistic wood-based fuels with different ratios of surface area to volume, combined with plastic-based fuels. A simple single zone fire model was used to calculate the temperatures in post-flashover compartment fires. The program includes a catalogue of furniture items, each with fuel mass loss rate evaluated on the basis of a constant regression rate on all exposed surfaces. The program also includes a pool-burning model and considers wood fuels and thermoplastic fuels burning together inside a compartment. Use of the model shows that the total fuels load alone is not sufficient to characterise a post-flashover fire. The fire temperature is highly dependent on the fuel type and geometry. For given ventilation and total fuel load, the resulting temperature depends greatly on the average thickness of the wood fuels and the presence of thermoplastic fuels. The ratio of the available fuel surface area to the ventilation opening is particularly important. Several fire scenarios involving different fuel types and characteristics are simulated and compared with Eurocode parametric fires.     

Effects of Leakage in Simulations of Positive Pressure Ventilation

The fire service uses a number of tactics to reduce hazards for fire-fighters and civilians within a structure on fire. One offensive fire-fighting tactic that has potential for rapidly improving or degrading conditions within the structure is ventilating the structure. Positive pressure ventilation is a tactic in which a fan is used to push hot products of combustion out of a burning structure. While a recent body of work has been produced on the effects of positive pressure ventilation in a number of fire systems, there is still widespread uncertainty on how the tactic affects the fire environment. Computational tools will play an important role in exploring the impact of positive pressure ventilation in various fire scenarios. In many simulations of structure fires, the impact of leakage on the evolution of the fire is not addressed. We find in this study that ad hoc models of leakage have significant impact on the evolution of the fire. Several ad hoc leakage models are proposed and these are studied in terms of their impact of the fire. We show that one particular leakage geometry is able to best model leakage effects in a series of fire simulations that are compared to experiments. Simple, first-order analysis is used to understand how these leakage flows affect the predictions.     

Spread and burning behavior of continuous spill fires

Spill fire experiments with continuous discharge on a fireproof glass sheet were conducted to improve the understanding of spill fire spread and burning. Ethanol was used as the fuel and the discharge rate was varied from 2.8 mL/s to 7.6 mL/s. Three ignition conditions were used in the experiments; no ignition, instantaneous ignition and delayed ignition. The spread rate, regression rate, penetrated thermal radiation and the temperature of the bottom glass were analyzed. The experiments clearly show the entire spread process for spill fires. Further, the regression rate of spill fires at the quasi-steady burning was lower than that of pool fires and the ratio of the spill fires’ regression rate to the pool fires’ regression rate was found to be approximately 0.89. With respect to the radiative penetration and the heat conduction between the fuel layer and the glass, a regression rate expression for spill fires was developed based on some modifications on existing expressions for pool fires. In addition, a complete phenomenological model for spill fires was developed by combining the characteristics of spread and burning. The model was verified by the experimental data and found to predict the spread process for spill fires with reasonable accuracy.     

Simulation of water mist suppression of small scale methanol liquid pool fires

The focus of this paper is on numerical modeling of methanol liquid pool fires and the suppression of these fires using water mist. A mathematical model is first developed to describe the evaporation and burning of liquid methanol. The complete set of unsteady, compressible Navier–Stokes equations are solved along with an Eulerian sectional water mist model. Heat transfer into the liquid pool and the metal container through conduction, convection and radiation are modeled by solving a modified form of the energy equation. Clausius–Clapeyron relationships are invoked to model the evaporation rate of a two-dimensional pool of pure liquid methanol.The interaction of water mist with pulsating fires stabilized above a liquid methanol pool and steady fires stabilized by a strong co-flowing air jet are simulated. Time-dependent heat release/absorption profiles indicate the location where the water droplets evaporate and absorb energy. The relative contribution of the various suppression mechanisms such as oxygen dilution, radiation and thermal cooling is investigated. Parametric studies are performed to determine the effect of mist density, injection velocity and droplet diameter on entrainment and suppression of pool fires. These results are reported in terms of reduction in peak temperature, effect on burning rate and changes in overall heat release rate. Numerical simulations indicate that small droplet diameters exhibit smaller characteristic time for decrease of relative velocity with respect to the gas phase, and therefore entrain more rapidly into the diffusion flame than larger droplet. Hence for the co-flow injection case, smaller diameter droplets produce maximum flame suppression for a fixed amount of water mist.     

Correlation equations on fire-induced air flow rates through doorway derived by large eddy simulation

Air flow rates through a doorway are important in modelling compartment fires. The ventilation factor is regarded as a key parameter and numerous efforts have been made on deriving the correlation of air flow rates with it. Most of the correlation expressions reported in the literature were derived empirically from experiments. The results might be different if the fire geometry, fuel type and ambient conditions are changed. Further, the heat release rates measured in most of the experiments were based on the mass loss rate of fuel, not by the oxygen consumption method. There might be some deviations from the actual heat release rates.Computational fluid dynamics (CFD) is now a practical tool in fire engineering. Aerodynamics through a doorway induced by a compartment fire can be simulated accurately. Factors which are difficult to control in experiments but affecting the doorway flow can be studied.The Fire Dynamics Simulator (FDS) developed by the National Institute of Standards and Technology, USA, is one of such CFD software. This is a product achieved from long-term research on developing a CFD model capable of carrying out fire simulations. This model is different from the others based on the Reynolds Averaging Navier–Stokes equations method. Physical processes occuring at small length and time scales were modelled by large eddy simulation (LES). Larger length scale on buoyancy-induced turbulence flow structure was computed directly from the set of equations with acoustic waves filtered out. The new version of this CFD package, FDS version 3.01, is now applied to derive equations on doorway flow rates induced by a fire. Results will be compared with those reported in the literature.