Experimental Study about the Influence of Heat Tightness of an Enclosure Fire on Ignition Risk of Unburnt Gases in a Connected Exhaust SystemManescau, Brady; Wang, Hui-Ying; Coudour, Bruno; Garo, Jean Pierre
doi: 10.1088/1742-6596/1107/4/042001pmid: N/A
This experimental study was performed from a reduced scale enclosure with a length/height and width of 2 m. In this work, we investigate whether heat tightness of enclosure can change the compartment fire dynamics and pose ignition risk in exhaust system. An external ventilation system provides an air supply rate ranging from 24 to 40 m3/h, corresponding respectively to 3 and 5 Air Change Per Hour (ACPH). A circular dodecane pan with a diameter of 40 cm is placed in the middle of the enclosure. Two situations with and without insulation of the enclosure are compared for several ACPH. The results show that heat tightness of enclosure leads to faster fire growth implying more important peak in heat release rate, and thus more dangerous fire with regarding the ignition risk. Heat tightness of enclosure makes the depression level of the compartment decrease, and consequently the air inlet supply rate decrease too, but the mass loss rate of liquid fuel increase. With heat tightness of enclosure, the fire becomes very-under-ventilated, and the vaporized fuel does not completely contribute to the heat release due to a significant formation of the unburnt gases as hydrocarbons, CO and H2. It is found that ignition of unburnt volatiles near the exhaust system occurs more easily when the compartment is more heat-tight. Oscillating flames in under-ventilated conditions were observed experimentally due to the ignition/extinction of the liquid fuel pan in a vitiated air enclosure. After the fire extinction, the remaining flames as ghosting flame in the vicinity of the enclosure ceiling were not observed under lowered oxygen vitiation with a mass fraction of around 7%. With a time delay of about 15 min in the current situation, the energy released per mass of oxygen consumed allows to raise the temperature of fuel-air mixture to about 300°C high enough for ignition of the unburnt gases. When the molar fraction of unburnt fuels is above the low flammability limit, any return of air from dilution duct after the fire extinction in a vitiated air enclosure could lead to spontaneous ignition near the extraction duct without needing a pilot flame. Note that heat-tightness of enclosure contributes to a reduction in the ignition delay time at entrance of the connected extraction duct due to a quick accumulation of unburnt fuel gases there. The experimental results from such reduced scale enclosure can be extended to a full scale one by imposing a preservation of the ratio (/L5) between heat release rate () and characteristic length (L). This relationship is derived from dimensionless variable of the energy equation.
Calculation of radiation transmittance through n-heptane in a pool fireIsojärvi, Teemu; Bordbar, Hadi; Hostikka, Simo
doi: 10.1088/1742-6596/1107/4/042002pmid: N/A
The radiative transfer mechanisms in a volatile hydrocarbon pool fire were investigated by solving the transmittance of infrared radiation through fuel (n-heptane) layers of different depths. The incoming radiation was assumed to have the same relative amounts of different wavelengths as a spectrum obtained experimentally for a 2 meter pool fire, and the transmittances were calculated by integrating either the single-ray Lambert-Beer formula, the two-flux (Schuster-Schwarzschild) method solution or the analytical plane-parallel monochromatic/gray solution of the radiative transfer equation over wavelength, using the liquid absorption coefficients from several sources. The obtained transmittances were compared to earlier heat flux measurements, and the possibility of calculating them with significantly less computation time by using a k-distribution method was investigated. The results managed to replicate the measured heat flux values at depths over 1 mm in the liquid with reasonable accuracy, and the use of the k-distribution, more known and used in gas phase thermal radiation calculations, significantly speeds up the calculations.
Towards the prediction of the convective and the radiative heat fluxes in turbulent buoyant flamesBoyer, Germain
doi: 10.1088/1742-6596/1107/4/042003pmid: N/A
The present paper aims at describing the flow dynamics of turbulent buoyant flames encountered in gaseous or liquid pool fires or horizontally oriented solid materials. Indeed, the predictive simulations of liquid pools or solid fuel fires requires the prediction of the thermal stress received by the evaporating liquid or the pyrolysing material, and therefore a fine description of the surrounding flame. Yet, reactive Large Eddy Simulation is a powerful tool in fluid mechanics to correctly simulate turbulent flows such as large-scale buoyant flames, so long as the discretization and the numerical schemes allow to resolve the large scale turbulent structures which determine the global flame shape.Thus, this study is divided into two steps. In the first step, Large Eddy Simulations of the SANDIA FLAME methane pool fire are performed using the CALIF3S-ISIS software, for different grid refinements and convective schemes. The ability of the different numerical choices in correctly describing this flame is compared in terms of velocity variance-to-average ratio, puffing frequency and turbulent structures resolution. In the second step, the flame above a 40×40 cm2 PMMA slab is simulated with the best numerical parameters found in the first step. The simulation of the flame is decoupled from pyrolysis aspects assuming by imposing the fuel mass flow rate observed at a particular stage of the experiment. The average temperature axial profile and the transverse profile of convective, radiative and total heat flux at the sample surface are compared to their experimental counterparts.It is shown that, at least for large scale pool fires such as the SANDIA methane flame, the increasing the mesh refinement permits to reach a good balance between the average velocity and the turbulent kinetic energy and to correctly recover the characteristic flame puffing frequency, whereas using non-dissipative centred schemes allows to catch a wider range of turbulent scales in the flowfield. Unfortunately, for smaller fuel sources such as the 40×40 cm2 PMMA slab, the present Large Eddy Simulations do not exhibit fully developed turbulence in most of the combustion zone, in spite of the use of centred schemes and of a fine grid. This uncertainty on the behaviour of the turbulence in such a configuration may be responsible for the observed anomalies, namely the overprediction on the average temperature and the radiative-to-convective heat flux ratio. Further experimental investigations would be useful to better understand the behaviour of turbulence on such mid-scale fires and prescribe relevant numerical model modifications.
Assessment of an insulating air layer model of façade external system: contribution to fire simulation of facade performance fire testDrean, Virginie; Schillinger, Renaud; Auguin, Gildas
doi: 10.1088/1742-6596/1107/4/042004pmid: N/A
Ventilated façades as a part of external thermal insulation are useful systems when applied to building design, especially in bioclimatic construction. In the context of the fire safety engineering, well taking into account the air layer arranged in such systems is of importance to assess the fire behavior of a building. In fire engineering studies, CFD codes as FDS (Fire Dynamics Simulator) are widely used. However, mesh sizes used in simulation are generally greater than 100 mm. Thus, the air layer cannot be modeled accurately because of typical thickness lower than mesh size.In the present study, a method based on an analytical formulation of the thermal resistance for building components is evaluated to approach the local air layer behavior. Radiative, convective and conductive mechanisms in the layer are then accounted. The proposed method could be implemented in a numerical model when a ventilated layer must be considered through an equivalent thermal conductivity for the air layer.CFD simulations are performed with FDS for fire exposed façade during LEPIR II performance tests where the ventilated façades with the larger air layer thicknesses are modeled. Ambient conditions in terms of flow temperature and velocity inside the air layer are evaluated in several locations of the façade.These façade models are then modified to replace the air layer by an equivalent thermal conductivity analytically assessed. Its local internal temperature is then compared with the one predicted with CFD, using 2D thermal calculations with the FEM code SAFIR. Extrapolations for smaller thicknesses of air layers that cannot be model with FDS can then be assessed numerically.This preliminary work shows encouraging application for air layer reserved in façades during a fire. Further development and validation are in progress to apply this method to other configurations of air layering.
Study on Smoke Behavior in Under-viaduct Stations by using Scale Reduced Model.Ozumi, Hideki; Mafune, Susumu; Suzuki, Osamu; Park, Hyun woo; Ohmiya, Yoshifumi
doi: 10.1088/1742-6596/1107/4/042005pmid: N/A
In the event of fire at concourse of rail stations under elevated platforms, smoke may emit through stairways leading from the concourse to platforms as well as standard natural smoke discharge outlets decreed by the Building Standards Law. However there is little knowledge about smoke discharge from the concourse via the stairway. In this study, Experiments regarding smoke flow in the case of fire in concourse of rail stations below elevated platforms were carried out compare volume of smoke emitted through stairway opening with that emitted through standard natural smoke discharge outlets as decreed by the Building Standards Law, with a simple scale reduced model which stairway was placed at right angle to the concourse. As a result, in these experiments, relationship of smoke discharge between stair openings and standard natural smoke discharge outlet was found.
A Large Eddy Simulation of LNG Pool Fire on Board a Chemical/Oil TankerBolek, Abdullah; Ergin, Selma
doi: 10.1088/1742-6596/1107/4/042006pmid: N/A
The air pollution from maritime transportation has become one of the major environmental concerns. The liquefied natural gas (LNG) has better environmental performance compared to conventional ship fuels. Therefore, the use of LNG as ship fuel has recently gained more attention in the maritime industry. On the other hand, LNG as a fuel can have high risks of explosion and fire on ships. Among the various LNG ship fires, the pool fire is the most common phenomenon. Therefore, this study focuses on the LNG pool fire on board a chemical/oil tanker with different pool aspect ratios. Also, the effects of wind speed on the flame characteristics are investigated. The LNG pool fire on board a chemical/oil tanker is studied by using large eddy simulation approach and Fire Dynamic Simulator (FDS) code, numerically. The results show that the flame characteristics are affected markedly by the pool aspect ratio and wind speed. While the aspect ratio increases, the mean flame height reduces, whereas the heat flux values increases. It is also found that the heat flux values increase with the wind speed.
Fire Growth Rate Strategies in FDSCiani, Francesco; Capobelli, Monica
doi: 10.1088/1742-6596/1107/4/042007pmid: N/A
The HRR curve is characterized by four phases: the incipient phase, the growth phase, the fully developed phase and the decay phase. A central parameter in the design of buildings and the provision of fire protection measures, is the rate at which the fire develops. Therefore the speed at which a space becomes untenable is mainly dependent on the fire growth rate, which is also closely related to the risk profile selection. To reproduce the fire growth phase, it is generally recommended the use of the t-squared fires [1].Fire Dynamics Simulator (FDS) 6.6.0 [2] enables us to reproduce the t-squared fire in several ways, by using different “fire growth functions”, i.e. RAMP-function and SPREAD_RATE function. These functions use different approaches. While the RAMP function associates the time history of the HRR curve with the entire surface, the SPREAD_RATE function is similar to a cellular automaton approach and it mimics a radially growing fire along the surface with a constant speed.In this paper the behavior of the dimensionless heat release rate Q*, during the fire growth phase, has been studied for both the fire growth rate functions. Furthermore, a numerical study of the “growth function dependency” of some important outputs has been carried out, in order to quantify the implications on safety design. If on one hand, in accordance with the Swedish Best Practice [3], the SPREAD RATE should be preferred, seeing that the burning surface is kept small when the HRR is low and therefore it is possible to maintain applicable values of the ∗during the fire growth, on the other the case studies performed, using the RAMP function, have shown to be the worst case scenario analysing the gas temperature and the visibility in order to evaluate the Available Safe Egress Time.
Design of fire safety equivalence of building open arcades to exterior spacesGissi, Emanuele; Cancelliere, Piergiacomo; Caciolai, Mauro
doi: 10.1088/1742-6596/1107/4/042008pmid: N/A
An arcade is a succession of arches, each counter-thrusting the next, supported by columns, piers, or a covered walkway enclosed by a line of such arches on one or both sides. In warm or wet climates regions, exterior arcades provide shelter for pedestrians. A building open arcade is sometimes called a porch. Arcades usually border a covered section of a building and sometimes they support roofs. They are very old architectural elements that date back thousands of years.Building open arcades (“Portici”) are traditionally employed in cities to provide shelter for pedestrians along public and private streets. Along the closed side of the arcades, commercial activities of all sort often offer their main entrance and exhibition spaces. Other activities as private dwellings or any other kind of civil activity may communicate directly through doors or other openings towards the covered space under the arcade.According to current prescriptive fire safety regulations, open arcades cannot be considered equivalent to exterior open spaces, and all the activities that have openings towards these covered spaces should be presumed in mutual communication from a fire safety design point of view. In fact, it is hypothesized that closed enclosures allow for the faster spread of fire effluents between the communicating spaces. Nevertheless, according to experiences and engineering judgments, under certain conditions and constraints, the open arcades allow for evacuation of smoke and heat towards the exterior spaces through the open side. When these conditions are met, all the activities that have openings towards these covered spaces could be assumed mutually independent from a fire safety design perspective, because, in case of fire, the fire effluents do not spread easily towards the adjoining activities.In this paper a fire safety engineering calculation is performed to assess in which circumstances an open arcade can be considered as equivalent to an exterior space for deemed-to-satisfy solutions of the Italian Fire Code (IFC 2015).The study proposes an arcade parametric fire model, whose pre-processor has been developed in Python, for assessing conditions, safety measures and geometric constraints that allow to recognize building open arcades as exterior safety places. The open arcade can be considered equivalent to exterior space for deemed-to-satisfy solutions when occupants can safely escape from the open arcade towards a safe place, occupants of other activities abutting in the same open arcade are not harmed by the fire effluents coming from the activity involved in the fire, and when fire and its effluents do not spread to other abutting activities. Furthermore, the paper assumes a simplified equivalent model structure of the building arcade where the parametric study conducted with FDS is aimed at setting some basic rules for the fire safety design of building open arcades. The results of the parametric study show that under certain conditions and constraints, the open arcades allow for evacuation of smoke and heat towards the exterior spaces through the open side. Therefore, when these conditions are met, all the activities that have openings towards these covered spaces could be assumed mutually independent from a fire safety design perspective, since in case of an outbreak of a fire the fire effluents do not spread easily towards the adjoining activities.
Simulations of a Turbulent Line Fire with a Steady Flamelet Combustion Model and Non-Gray Gas Radiation ModelsVan Minh Le, ; Marchand, Alexis; Verma, Salman; White, James; Marshall, Andre; Rogaume, Thomas; Richard, Franck; Luche, Jocelyn; Trouvé, Arnaud
doi: 10.1088/1742-6596/1107/4/042009pmid: N/A
The general objective of this project is to develop an accurate combustion and radiation modeling framework for high-fidelity large eddy simulations (LES) of well-controlled turbulent laboratory-scale fires for which the fuel composition and fuel oxidation chemistry are known. This modeling framework is aimed at providing a solid basis for the development and validation of engineering-level models used in simulations of real-world fire problems for which the sources of fuel are diverse, complex, and in many cases, poorly characterized. The combustion model features a library of flamelet solutions corresponding to one-dimensional, steady, laminar, counterflow diffusion flames simulated with specialized software, a chemical kinetic mechanism and an equi-diffusive molecular transport model (i.e., unity Lewis numbers). Two different flamelet libraries are considered here: a first library generated with a solver called libOpenSMOKE and a detailed chemical kinetic mechanism developed for C1-C3 combustion chemistry and a second library generated with a solver called FlameMaster and the GRI-Mech v3.0 chemical kinetic mechanism developed for methane combustion chemistry. The radiation model features a banded Weighted-Sum-of-Gray-Gases model but (so far) no description of subgrid-scale turbulence-radiation interactions (TRI). This modeling framework is incorporated into a LES solver developed by FM Global and called FireFOAM, and is evaluated in simulations of a two-dimensional, plane, buoyancy-driven, methane-air, turbulent diffusion flame experimentally studied at the University of Maryland. The configuration corresponds to an intermediate validation step in our model development strategy without the complications of flame extinction. The flame structure is characterized by new micro-thermocouple measurements of the temporal mean and root-mean-square gas temperatures. Comparisons between simulated and measured temperatures show significant discrepancies that are explained by the large values of the width of the presumed probability density function (PDF) representing subgrid-scale variations of mixture fraction and by the absence of a model for subgrid-scale TRI.
An Experimental Study of on the Combustion Properties of Lobby Chairs in Open Space and in ISO Room CompartmentLee, Jaeyoung; Ohmiya, Yoshifumi; Saito, Fumiaki; Shintani, Yusuke; Harada, Kazunori
doi: 10.1088/1742-6596/1107/4/042010pmid: N/A
Performance-based design should reflect initial fire phenomena properly. For this purpose, necessary data are being accumulated through combustion tests. It is reported that the maximum heat release rate and the fire growth rate are significantly increased by thermal radiation feedback effects from a smoke layer and a wall in a compartment in comparison with burning in an open environment [1-2]. In this study, a series of burning tests were carried out using lobby chairs to compare burning behavior in open environment and in a compartment. The measured items were heat release rate, mass loss rate, heat flux and vertical distribution of temperature in the compartment and above the center of combustibles. The cause of flame spread to adjacent chair was different between in an open space and in a compartment. In an open space, flame spread was mainly caused by direct contact of flame with adjacent chair. However, the main cause of the flame spread in a compartment was thermal radiation. As a result, heat release rates in open space and in a compartment significantly differ in shape. In a compartment, heat release rate slowly decreases after it reaches at the maximum level. In open space, heat release rate increased and decreased as flame spread to adjacent chairs successively. Heat flux showed similar patterns with that of heat release rate. Both maximum heat release rate and maximum heat flux increased as the number of chairs were increased. Measured values in a compartment are 2-4 times larger than that in an open space. Fire growth rate was increased by hot smoke layer. However, the relationship with the number of chairs was not clear. In addition, it was confirmed that the air temperature around chairs was higher than that in an open space.