Fire Spread between Industrial Premises

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Fire Spread between Industrial Premises HAUKUR INGASON and ANDERS LÖNNERMARK SP Technical Research Institute of Sweden Fire Technology Borås, Sweden
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  Fire Spread between Industrial Premises   HAUKUR INGASON and ANDERS LÖNNERMARK SP Technical Research Institute of Sweden Fire Technology Borås, Sweden ABSTRACT The study focuses on investigating models for calculating the risks of fire spread from an industrial  building to adjacent buildings. The basic parameters necessary to determine the risk of fire spread are the flame height and the incident heat flux. There is limited information found about flame heights from industrial buildings where the fire breaks through the ceiling. Calculation methods for flame heights and incident heat fluxes are discussed and compared to model-scale data and large-scale data. A series of model-scale tests with flames through openings in a building where flashover has occurred are presented and compared to flame height correlations. This paper provides new data and a better understanding of the necessary input for such calculations. The model-scale tests show a good correspondence between a simple method to calculate heat flux from a point source and experimental data. KEYWORDS:  industrial fires, flame height, radiation, fire spread, model-scale tests. NOMENCLATURE LISTING  A  area (m 2 ) T  a  ambient temperature (K)  D  diameter (m) T   f   radiation temperature of flame (K)  D eff   effective diameter (m) W   width (m)  E  b  blackbody emissive power (kW/m)  X ratio  H   f   /r     E  f   average emissive power of the flame (kW/m) Y ratio W   f   /r     H   f   flame height (m) Greek     L  distance (m)    emissivity  ̇   heat release rate (kW)     Stefan-Boltzmann constant (kW/m 2 K  4 ) comb Q   chemical heat release rate (kW)    atmospheric transmissivity rad  Q   radiative fraction of the heat release rate (kW)    geometric view factor inrad  Q ,     thermal radiation intensity (kW/m 2 ) r   distance between radiating and receiving surface (m) INTRODUCTION In buildings where there are no sprinklers, the basic idea is that the building should be located and arranged so that the fire cannot spread to nearby buildings. Fires can spread to adjacent building by flying brands, direct contact of flames or convective and radiative heat transfer (incident heat flux) from the fire plume or some combination of these mechanisms. Ignition due to incident heat flux is the most common mode of fire spread between buildings and can occur at much greater distances than direct flame impingement and convection. When the flames break through the building of srcin, the risk of fire spread by radiation is increased significantly. At the time when the flames penetrate through the ceiling, the flame heights can  become large, resulting in a huge increase in thermal radiation to the surroundings. The shape and height of the flames penetrating through the ceiling may vary considerably, and in windy conditions the risk for fire spread increases when the flames are projected towards the neighbouring  buildings. If the width of the opening becomes very large, the flame height may decrease and at a certain ratio of diameter (  D ) of the fire base and the flame height (  H   f  ), there is a risk of a breakup of the flame into many smaller fires. This phenomena has been investigated by Heskestad [1] , but the tests in Heskestad‟s investigation were carried out to investigate „ mass fires ‟ . Mass fires are not expected to generate high flames relative to their base dimension. This type of fires may be related to large industrial fires where FIRE SAFETY SCIENCE-PROCEEDINGS OF THE TENTH INTERNATIONAL SYMPOSIUM, pp. 1305-1318 COPYRIGHT © 2011 INTERNATIONAL ASSOCIATION FOR FIRE SAFETY SCIENCE / DOI: 10.3801/IAF S S.FSS.10-1305  1305  large portions of the ceiling have collapsed. There is, however, very little information concerning flame heights and shapes of industrial fires, mainly because this type of information is usually not registered. This information is, however, critical if one would like to calculate the risk for fire spread between buildings. In the Swedish building regulations [2], there is a regulation concerning the risk of fire spread between adjacent buildings. The requirement is as follows: the incident radiation should not exceed 15 kW/m 2  during a period of 30 min. This regulation is only applicable when the building is not sprinklered. There are numerous engineering methods available to calculate flame heights, using view factors or simpler methods (point source), to calculate the risk of fire spread [3  –  6]. These correlations are based on tests in a laboratory environment, and work well for pool fires and wood crib fires. Fire spread in an industrial building, in  particular with flames penetrating through the ceiling, may not give similar results using traditional fire engineering methods. Other related works of interest are the EN 13501-5 on external fire behaviour and also the large-scale tests carried out within the framework of the FLUMILOG project. A 860 m² warehouse was built and deliberately set on fire, on 26 September 2008, on the site of the future European Environmental and Safety Technology Research Center (CERTES), in Oise (France). More detailed scientific information in English from these tests is found in Ref. [3]. The work presented here focuses on flames coming out of openings in the ceiling and how this affects the incident heat flux towards a neighbouring building. A flashover situation was created inside a model of an industrial building, and the flames that burst out were documented. The heat flux from the fire was then measured and compared to different calculation models available. A comparison to large scale pool fire tests using different type of radiation models was also explored. These large scale pool fire tests have not  been used for such comparison before. The model-scale results and the comparisons between experiments and correlations are presented in this paper. THEORETICAL ASPECTS It is very difficult to obtain reliable flame height data from industrial buildings on fire. Therefore, Ingason et al. [4] made an attempt to estimate the flame heights from photos of real industrial fires to provide methodological guidance. One difficulty methodologically is the fact that the heat release rate in a real fire is never measured and therefore not known a priori . Flame Height In order to estimate the heat release rate, the flame height correlation given by Heskestad [5] is assumed to  be a reasonable starting point. The main reason is that Heskestad ‟s  equation takes into account the effects of the fuel base through its diameter and it has been used for numerous types of applications. Estimation of the flame heights was done by observing photos from real accidents and by comparing an object with a known height in the photo. By using the flame height correlation given by Heskestad the heat release rate could be calculated: 52 235.002.1  Q D H   f      (1) where    is the flame height (m),  D  is the diameter of the fuel base (m) and  ̇  is the heat release rate (kW). Equation 1 applies well for well-ventilated pool fires in open space and has been validated for rack storages and other types of fuels [6] using an effective diameter of the fuel base. This equation has, however, not  been applied to the estimation of flame heights in buildings with relatively large ceiling openings. For this the effective diameter,    , can be used, defined by the following equation:   √    (2) where  A  (m 2 ) is the area of the ceiling opening estimated from a photo of the actual fire. By rearranging Eq. 1 Ingason et al. were able to estimate the heat release rate between 6 GW and 21 GW in four out of five cases investigated. Corresponding flame heights varied from 15 m to 50 m and the ratio      varied between 0.1 and 0.4. The heat release rate  per unit projected area  (horizontal base area of 1306  the fire) of the fire was found to be in the range of 0.5  –  1.3 MW/m 2 . For comparison, the heat release rates  per unit exposed fuel area  (total surface of the fuel that is burning) for mixture of solid materials in large scale is found to be in the range of 0.1 MW/m 2  to 0.5 MW/m 2  [7]. For pure plastics such as polystyrene foam and polyurethane foam in bench-scale tests show that these numbers can vary from about 0.4 MW/m 2  to 1.5 MW/m 2  [8]. Usually industrial fires consist of mixture of different materials and not only one type of material. Further, a projected area is usually less than or equal to the exposed fuel area, which means that the estimated numbers for heat release rates per unit projected area should be higher than when using exposed fuel area. In other words, the estimated heat release rate per unit projected area is expected to be close to the estimated heat release rate per unit exposed fuel area. This indicates that the heat release rates estimated from real industrial fires are reasonable, and therefore Eq. 1 should be useful for this type of calculations. Further, Heskestad [1] has discuss his flame height correlation in relation to the ratio    . According to Heskestad, low flame height data exhibits a transition from coherent flaming to distributed flamelets when the ration     becomes less than about 0.5. When this transition occurs, the air induced or entrained by combustion, if shared by all the fuel vapours, will dilute the vapours below their ability to burn. Based on this consideration, Heskestad speculates that mass fires in sufficiently large homogenous fuel beds may only be possible as distributed localized fires. Experiments carried out by Heskestad [1], using wood fibreboard arranged to produce a square array measuring 7.32 m on each side, confirmed that luminous flames exhibited an initial tendency to break up into distributed flamelets near     = 0.52, being fully  broken up near     = 0.34. Heskestad was able to go down to     = 0.04 in his experiments, where the entire burner surface showed flickering blue flamelets racing back and forth. These ratios are in the same range as the values obtained from real cases (0.1  –  0.4). Therefore, Heskestad ‟ s observations from fires with low     values are of great interest for the study presented here, and indicate that Eq. 1 can be used for industrial buildings after the flames have penetrated through a large portion of the ceiling. Therefore, it was decided to investigate the validity of Heskestad Eq. 1 further for industrial buildings by using model-scale experiments. Incident Heat Flux Mudan and Croce [9] present different methods for the calculation of the incident heat flux to an adjacent object. They presented two thermal radiation models. The first is the point source radiation model, and the other is the solid flame radiation model using view factors. The first model assumes that the flame can be represented by a small source of thermal energy, that the energy radiated from the flame is a specified fraction of the energy released during combustion and that the thermal radiation intensity varies  proportionately with the inverse square of the distance from the source. Expressed mathematically, radiant intensity at any distance from the source is given by [9]: 2, 4  LQQ  combinrad           (3) where combrad  QQ      is the fraction of total heat release that is radiated away and  L  is the distance from the flame centre to the observer in metres. This means the distance  L  is dependent on the flame height. Mudan and Croce say that while the model is elegant in its simplicity, two important limitations should be recognized. The first limitation involves the modelling of radiative output and the second is the description of the variation of the intensity as a function of the distance from the source. There is a considerable variation in the fraction of radiated energy from flames, due to smoke obstruction and combustion conditions, everything from 0.2  –  0.4. A value that is often quoted is 30 % (       0.3) for many fuels [10]. This value is an average value for the flames, and not a local one. In reality it may vary depending on the test set-up, and if it is carried out at large-scale or model-scale. The second model is a solid flame radiation model. The solid flame model is based on the postulation that the entire visible volume of the flame emits thermal radiation and the non-visible gases do not emit much radiation. The thermal radiation intensity, inrad  Q ,   , can be obtained using the following equation [9]: 1307        f  inrad   E Q ,   (4) where    is the atmospheric transmissivity (    = 1 assumed here but it may vary depending on the relative humidity and path length),  E  f   is the average emissive power of the flame and    is the geometric view factor which is a measure of the decrease of the radiation at different distances. Mudan and Croce give different correlations for the view factor. In Ösesik [11] or Siegel and Howel [12] a very simple expression for the configuration factor of a parallel rectangular radiator and a remote receiver is given:            212212 1tan11tan121  y x y y x y x x     (5) where  X = H   f   /r , Y = W   f   /r, H   f   is the height of rectangular (m)  , W   f   is the width of rectangular (m)   and r   is the distance between radiating and receiving surface (m). Equation 5 determines the configuration factor in one of the corners of a rectangle representing the flame volume. The distance r   must be at right angles to the rectangle. Configuration factors are additive, given the configuration factors of each contributing part are calculated from the same receiver [13]. The total configuration factor    is a sum of the configuration factors of each rectangle. The emissive power of a large turbulent fire may often be approximated by the following expression [9]:      b f    E  E   (6) where  E  b  is the blackbody emissive power, kW/m 2 , and    is emissivity. The emissive power can vary considerably depending on the fuel type and the diameter of the flame volume. If the mean radiation temperature of the fire is known it can be converted to irradiance using the Planck‟s law of radiation. Thus, the blackbody emissive power,    is given by [9]   44 a f  b  T T  E       (7) where T   f   is radiation temperature of flame (K), T  a  is ambient temperature (K) and    is Stefan-Boltzmann constant, (kW/m 2 K  4 ). Mudan and Croce [9] reported that for large fires, the numerical value of the emissivity of flames approaches unity. Therefore, the emissive power can be determined using the mean radiation temperature. There is, however, a lack of experimental data although they do report some, e.g. the emissive power  E   f of gasoline is 60  –  130 kW/m 2  (maximum) for pools varying between 1 m and 10 m, of JP-5 is 30  –  50 kW/m 2  for pool fires varying between 1 m and 30 m and of ethylene 130 kW/m 2  for a 2.5 m pool fire. Mudan and Croce reported that most hydrocarbon fuel fires become optically thick when the diameter is about 3 m or larger. Substantial parts of hydrocarbon fires are obscured by a thick black smoke on the outer  periphery. This smoke layer absorbs a significant part of the radiation and results in very little emission to the surroundings. In fact, the smoke layer occasionally opens up, exposing the hot flame, and a pulse of radiation is emitted to the surroundings. Although the thermal radiation from black soot is low, the hot spots appearing on the flame surface due to turbulent mixing have a higher emissive power. Large eddies within the flame bring fuel to the outer edges of the fire plume and a more efficient combustion takes place on the flame surface. These luminous spots have an emissive power of about 110  –  130 kW/m 2 . It is not  possible to calculate the radiation field surrounding a fire with intermittent luminous spots. For example Hägglund and Persson [14] observed that the emissive power of the black smoke for pool fires with 10 m diameter is 20 kW/m 2  when the temperature is about 800 K [9]. 1308
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