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Behavior of luminous zones appearing on plumes of large-scale pool fires of kerosene
Fire Safety Journal 33 (1999) 1}10
Behavior of luminous zones appearing on plumes of large-scale pool "res of kerosene Nobuhide Takahashi, Masataro Suzuki, Ritsu Dobashi*, Toshisuke Hirano Department of Chemical System Engineering, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan Received 26 October 1998; received in revised form 1 February 1999; accepted 15 February 1999
Abstract In order to understand characteristics of large pool "res, behavior of intermittent luminous zones appearing on the smoky plumes has been analyzed using a recently developed image processing technique. The images of large pool "res analyzed in this study are of 30 and 50-m diameter pool "res of kerosene recorded on videotapes at the large-scale experiments performed in Japan, 1981. The results indicate that the probability of a luminous zone appearing is the maximum on the central point of the average plume width and at a distance from its base, and decreases with the distance from the maximum point. It can be inferred that the luminous zone on the smoky plume of a pool "re likely appears at its concave boundary (in a side view). It ascends at an almost constant velocity while changing it's area at the same frequency as that of the `breathinga. 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction As the diameter of an oil-"lled pan increases, the structure of a #ame formed above the pan becomes unstable. For the pan diameter larger than about 1 m, the #ames are fully turbulent [1]. As the diameter increases further, heavy smoke is intensively generated. For a pool diameter of tens of meters, the #ame is almost covered by smoke and luminous zones appear intermittently on the smoky plume. Even in such a case, there remains a continuous luminous zone at the base of the smoky plume, i.e., near the rim of the pan. * Corresponding author. Tel.: 0081-3-5841-7304; fax: 0081-3-5841-7313. E-mail address: [email protected] (R. Dobashi) 0379-7112/99/$ - see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 3 7 9 - 7 1 1 2 ( 9 9 ) 0 0 0 0 9 - 0
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Thermal radiation from the "re plume often plays an important role in hazard evaluation of practical oil tank "res. Models to predict thermal radiation from the "re plume above a pan have been proposed in previous studies [2]. Many of them require a #ame height. However, as the pool diameter increases, it becomes harder to determine the #ame height due to the generation of smoke. Also, such models tend to overestimate thermal radiation from the "re plume in a large pool "re [3], nevertheless the protection of large-scale tanks from "res is extremely important. Thermal radiation from a pool "re strongly depends on the luminous zones on its plume. Therefore, in order to establish an appropriate model available for hazard evaluation of practical oil tank "res, an exact understanding of the behavior of luminous zones and smoke in large-scale pool "res is required. However, large-scale experiments of pool "res, if the pool diameters are larger than a few tens of meters, are very hard to carry out due to the vast cost and requirements of open space. Consequently, there are few available data on the behavior of luminous zones and smoke in large-scale pool "res. Thus, in the present study, we have analyzed the behavior of the luminous zones of pool "res at the large-scale experiments performed in Japan, 1981 [4]. The analysis of the images recorded on videotapes in the experiments has been performed using a recently developed image processing technique with a personal computer.
2. Large-scale experiments In 1981, large-scale pool-"re experiments were performed in Japan by Japan Society for Safety Engineering under the support of the Oil Union, Japan. The pans used in the experiments were 30, 50 and 80 m in diameter, and the fuel was kerosene. The following were examined: temperature distribution inside the plume and the liquid fuel, thermal radiation to the outside and to the fuel surface from the plume, burning rate, composition of the product gases, velocities of the ambient air #ows and shapes of the "re plume. More details about the experimental method and the results are presented in Refs. [4,5]. The experiment with the 80-m-diameter pan was carried out under a windy condition so that the "re plume leaned to the leeward signi"cantly and the burning area did not extend over the whole surface of the pan. The other two experiments were carried out under meteorological conditions with less in#uence of wind. The average wind velocities and directions before ignition were 0.9 m/s and northwest by west in the 30 m-diameter experiment, and 0.6 m/s and northeast by east in the 50 m-diameter experiment, respectively. The "re plume were observed to extend almost vertically above the fuel surface. In the present study, the "re plumes in the experiments with the pans of 30 and 50 m in diameter were analyzed. Assuming that results obtained from the analysis are independent of the direction from which the images of the "re plume were recorded, the behavior of the plume in each experiment has been analyzed based on the images recorded from a "xed point in the northeast of the pan in the 30 m-diameter experiment and in the west of the pan in the 50 m-diameter experiment.
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3. Image processing method In order to investigate the motion of luminous zones and smoke, a sequence of images of the plume were sampled during the period of steady burning. For each experiment, the sampling time was 30 s, the sampling frequency was 10 frames/s. Sampling was started at 240 s after ignition for the 30 m-diameter case and at 180 s after ignition for the 50 m-diameter one. Fig. 1 shows an example of the sampled images for the 50 m-diameter case. Each sampled image is of 304 pixels (horizontal) ;228 pixels (vertical), i.e., 69,312 pixels in size. The images are on a scale of 0.33 m to a pixel for the 30 m-diameter case and of 0.54 m to a pixel for the 50 m-diameter case. The color images were analyzed in this study, although the image shown in Fig. 1 is in a gray-scale mode. The images recorded on videotapes were changed into digital images on a personal computer. Each pixel on the digital image has been represented by the three values, R(red), G(green) and B(blue). In a full-color mode (24 bit), the value of R, G or B is in the range 0}255. The larger the value of R, G or B, the brighter the color, red, green or blue, respectively. As shown in Fig. 1, the image is composed of luminous zone part, smoke part, and the other parts such as sky, mountain, ground, etc. In order to examine the behavior of luminous zones and smoke, it is indispensable to distinguish the object part, that is, the luminous zone part and the smoke part, from the other parts. In the present study, the R, G and B values were adopted for that purpose. The R, G and B values of pixels included in the luminous zone or smoke part were examined, and the conditions to distinguish the luminous zone or the smoke part were determined. As shown in Table 1, the condition represented by a combination of the values indicating three colors was adopted for distinguishing the luminous zone parts. The smoke parts were found to be distinguishable with either R or G value. Fig. 2(a) and (b) show the shapes of the luminous zone and smoke extracted from the image shown in Fig. 1, based upon the pre-determined image processing condition. The shape of the luminous zone in Fig. 2(a) is nearly distinguishable by assuming the limiting condition of R, G and B values.
Fig. 1. An example of sampled images (50 m-diam. pan).
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Table 1 Image processing conditions. Pan diameter Sampling time Sampling (m) (s) frequency (frames/s)
Extracting conditions
30
R#G!B'22 G(120 0 R#G!B'20 R(110 0
30
50
10
Luminous zone Smoke
Size of image (pixels)
Scaling parameter (m/pixel)
228;304
0.33 0.54
Fig. 2. Shapes of luminous zone and smoke extracted from the image shown in Fig. 1.
On the other hand, the image shown in Fig. 2(b) includes not only the smoke part but also other parts. The dense smoke part is exactly distinguishable using the predetermined image processing condition, but the smoke part near the ground is not distinguishable. In the processing, therefore, the ground part was removed by restricting the calculation domain. An original program to continuously process the images was developed. 4. Results and discussion 4.1. Probability of luminous zone appearing The probability of a luminous zone appearing was examined to clarify the characteristics of the luminous zones, which are closely related to thermal radiation from a pool "re [3]. The probability P of luminous zone appearing in a pixel is de"ned as
follows: x P " ,
N
(1)
N. Takahashi et al. / Fire Safety Journal 33 (1999) 1}10
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Fig. 3. Probability distributions of luminous zone appearing.
where x is the number of images where a speci"ed pixel is judged to be in a luminous
zone and N is the total number of sampled images. Subscripts i and j indicate horizontal and vertical coordinates of a pixel, respectively. Fig. 3 shows the probability distributions of the luminous zone appearing in the 30- and the 50 m-diameter pool "res. Several contour lines are drawn at an interval of 0.04 in Fig. 3(a) and at an interval of 0.02 in Fig. 3(b). In both cases, the probability of luminous zone appearing is the maximum on the central point of the average plume width and at a distance from the pool surface, and decreases with the distance from the maximum point. Moreover, Fig. 3 indicates that the luminous zone did rarely appear in the lower part of the plume up to H/D"0.3 in the 30 m-diameter case and up to H/D"0.5 in the 50 m-diameter case. In the 30 m-diameter case, the region of higher probability of luminous zone appearing is found to shift toward the left due to a slight wind. The value and location of the maximum probability are about 0.22 and 25 m in height, respectively, in the 30 m-diameter case, and about 0.14 and 60 m in the 50 m-diameter case. Also, the maximum probability of luminous zone appearing tends to decrease with the increase in diameter of the pan although the probability depends on the image processing condition, especially the threshold. The probability of the luminous zone appearing is considered to be closely related to a periodical motion of the "re plume. Such aspects of the probability distribution of the luminous zone appearing are much di!erent from those in smaller pool "res of pan diameters less than several meters. In small pool "res, the region where the probability of luminous zone appearing is equal to one exists near the rim of the pan and the probability decreases with the increase in height. The #ame height, which is often de"ned as the height with the probability of "nding #ames equal to 0.5, can be determined in small pool "res. Also in large pool "res, such as analyzed in the present study, there exists a continuous
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luminous zone at the rim of a pan. Koseki indicated that a large part of thermal radiation from the "re plume to the surroundings is emitted from the continuous luminous zone near the rim of the pan in large-scale pool "res, based upon the experiment of a pool "re with a pan of 15 m in diameter [3]. However, the ratio of the height of the continuous luminous zone to the pan diameter tends to decrease with increase of the pan diameter. Therefore, the contribution of the continuous luminous zone near the rim to total thermal radiation from the "re plume to the surroundings is expected to become smaller with the increase of pan diameter. Since the dimensionless height of the continuous luminous zone was very small, it could not be identi"ed on the images analyzed in the present study. Thus, in predicting thermal radiation from the "re in large-scale pool "res, we should consider only the probability distribution of the luminous zones, in the intermittent luminous zone. Luminous zones appear on the surface of a smoky plume as observed at the image of a large-scale pool "re. When the e!ect of wind is small, the appearance of the luminous zones seems to be controlled by a probabilistic mechanism. Under this situation, if the smoky plume would not #uctuate and could be assumed as a column, the probability of luminous zone appearing as observed from the pool "re, should be uniform in the direction across the plume. This is the underlying concept of a solid #ame radiation model to evaluate the radiation from the plume of a pool "re [2]. As seen in Fig. 3, however, the probability of luminous zone appearing is the maximum at the center of the average plume. This fact indicates that the method previously adopted for evaluating radiation from a pool "re on the basis of the data on small-scale experiments cannot be applied for evaluating radiation from a large-scale pool "re. A reasonable explanation for the probability distribution of luminous zone appearing at a large-scale pool "re is the likelihood of the luminous zone appearing at a point near the apparent center of the average plume, on the concave part (in a side view) of the plume boundary. 4.2. Ascending velocity of luminous zones As mentioned before, luminous zones appear intermittently among the smoke in large-scale pool "res of several tens of meters in diameter. Each individual luminous zone appears at a relatively low surface of the smoky plume and then ascends at a certain velocity. Some of the luminous zones swell to become "re balls and "nally disappear. Examples of the analyzed results on the behavior of intermittent luminous zones in the experiments of 30- and 50 m-diameter pool "res are shown in Fig. 4(a) and (b), respectively. In these "gures, the movement of each luminous zone is seen as a trace in the "eld represented by the distance from the fuel surface and time from ignition. The degree of darkness of the trace represents the horizontal width of the luminous zone at a height and time. Also, the slope of the trace represents the velocity of the ascending luminous zone, and the distance from one relatively long trace to the next one along the time axis corresponds to the time interval of the appearance of luminous zones.
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Fig. 4. Movement of luminous zones.
It is found that the traces in each "gure have almost the same slope at a nearly regular interval. This result indicates that the luminous zones rise at almost the same velocity and appear nearly periodically. The mean ascending velocities estimated from these "gures are about 15 m/s in the 30 m-diameter pool "re and about 16 m/s in the 50 m-diameter pool "re. McCa!rey [6] showed that there is a correlation between the upward gas velocity scaled by Q along the centerline of a #ame stabilized on a #at surface gas burner and the height scaled by Q from the burner surface, where Q is the heat release rate. Based on this relation, it was inferred that the "re plume is divided into three regimes, the continuous #ame region, the intermittent #ame region, and the buoyant plume region. According to McCa!rey, the scaled centerline gas velocity is independent of the height and is a constant value, 1.9 ms\ kW\ , for the intermittent #ame region. Assuming that this correlation derived from the experimental data obtained, using small gas burners can be applied to large pool "res, the centerline gas velocities for the cases examined in the present study have been predicted. The predicted centerline gas velocities in the intermittent region are about 36 and 44 m/s for the 30- and the 50 m-diameter pool "res, respectively. These values are much larger than the observed ascending velocities of luminous zones in the present study. 4.3. Pulsation frequency of the xre plume In previous studies, the pulsation frequency of a pool "re plume was measured in terms of the #uctuation frequency of thermal radiation from the #ame, smoke buoyancy, or the vortex shedding frequency [7,8]. In the present study, the pulsation frequency of the "re plume has been examined as a frequency of total area variation of luminous zones, which appeared intermittently among the smoke in each large pool "re, and as a #uctuation frequency of the smoke movement at the lower level of the "re plume. Fig. 5 shows the variation of the total area of luminous zones. Here, the total area means the summed area of several luminous zones on each image. Time-averaged
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Fig. 5. Time variations in total area of luminous zones and in width of "re plume at 5 and 8 m in height above the pan in the 30 case and 50 m-diam. case, respectively.
total area of luminous zones is estimated at about 50 and 110 m in the 30- and the 50 m-diameter cases, respectively. In the latter case, it is found that the total area reached about 600 m at the maximum. Fig. 5 also shows the variation in the width of the plume, i.e., the `breathinga at the lower level of the "re plume, where luminous zones rarely appear. Here, the lower level is 5 m in height in the 30 m-diameter case and 8 m in height in the 50 m-diameter case. As seen from this "gure, the "re plume near the pan pulsates periodically in both cases. Frequency spectra of the variations in the total area of luminous zones and in the width of the "re plume have been examined. Fig. 6 shows the results. The broken and
N. Takahashi et al. / Fire Safety Journal 33 (1999) 1}10
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Fig. 6. Frequency spectra for time variations shown in Fig. 5.
solid lines represent the frequency spectra for the luminous zone area and the "re plume breathing, respectively. The spectrum for the "re plume breathing frequency has one remarkable peak in each case. The frequency where the peak exists is estimated at 0.31 and 0.23 Hz in the 30- and 50 m-diameter cases, respectively. In the 50 m-diameter case, the spectrum for the luminous zone area variation also has one remarkable peak at 0.23 Hz, where the spectrum for the "re plume breathing frequency has a peak. However, the spectrum for the luminous zone area variation has several peaks in the 30 m-diameter case. One of them appears at the same frequency, 0.31 Hz, where the peak of the spectrum for the "re plume breathing frequency exists. The behavior of luminous zones in this case seems to be of multiple frequencies. Although the luminous zone appearing may depend on many factors, it is inferred that the pulsation of the "re plume is closely related to the appearance of intermittent luminous zones on the smoky plume surface in large pool "res. Bejan proposed the following relation between a vortex shedding frequency and a pan diameter for 0.03 to 60 m [7,9]: 3.1 f , D
(2)
where f is a vortex shedding frequency (Hz) and D is a pan diameter (m). The frequencies predicted using this relation are 0.32 and 0.24 Hz in the 30- and 50 m-diameter pool "re, respectively. These frequencies agree well with the frequencies of the "re plume pulsation observed in the present study.
5. Conclusions The behavior of intermittent luminous zones appearing on the smoky plumes of large-scale kerosene pool "res has been analyzed and the following conclusions have been derived.
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N. Takahashi et al. / Fire Safety Journal 33 (1999) 1}10
1. The probability of a luminous zone appearing is the maximum at the central point of the average plume width at about 25 m from the fuel surface in the 30 m-diameter pool "re, and at about 60 m in the 50 m-diameter pool "re, and decreases with the distance from the maximum point. The intermittent luminous zones ascend at almost the same velocity during steady burning. The mean velocities of ascent estimated for the 30- and 50 m-diameter pool "res are 15 and 16 m/s, respectively. 2. The "re plume near its base pulsates with an almost constant frequency. The pulsation frequency is 0.31 Hz in the 30 m-diameter pool "re and 0.23 Hz in the 50 m-diameter pool "re. In the 50 m-diameter pool "re, the pulsation frequency of the "re plume near its bottom corresponds to the frequency of luminous zone appearing at higher points of the plume. The behavior of the "re plume depends on the frequency of pulsation near the fuel surface. 3. The luminous zone on the smoky plume of a pool "re can be inferred to appear probably at its concave boundary (in a side view) at a distance from its base, ascending at an almost constant velocity and changing its area at the same frequency as the breathing.
Acknowledgements All data used in this study were obtained from the experiments performed under the auspices of Japan Society for Safety Engineering. The authors are grateful to JSSE.
References [1] Hottel HC. Certain laws governing the di!usive burning of liquids by V.I. Blinov and G.N. Khudiakov. Fire research abstracts and reviews 1959;1:41}4. [2] Drysdale D. Di!usion #ames and "re plumes. In: An introduction of "re dynamics, Chapter 4. 1996 New York: Wiley, 1996. [3] Koseki H. Study on characteristics of oil tank "res. PhD Dissertation, The University of Tokyo, Tokyo, Japan, 1996. [4] Japan Society for Safety Engineering. The Report of the Oil Pool Fire Experiment. J. S. S. E, Yokohama, Japan, 1981. [5] Yamaguchi, T, Wakasa K. Oil pool "re experiment. In: Fire Safety Science, Proceedings of the "rst international Symposium. Hemisphere. New York, 1985, pp. 911}8. [6] McCa!rey B.J. Purely buoyant di!usion #ames: some experimental result, Natl. Bur. Stand. (US), NBSIR 79-1910, 1979. [7] Bejan A. Predicting the pool "re vortex shedding frequency. J Heat Transfer 1991;113:261}3. [8] Portscht R. Studies on characteristic #uctuations of the #ame radiation emitted by "res. Combust Sci Technol 1975;10:73}84. [9] Pagni PJ. Some unanswered questions in #uid mechanics. Appl Mech Rev 1990;43:153}70.
Behavior of luminous zones appearing on plumes of large-scale pool "res of kerosene Nobuhide Takahashi, Masataro Suzuki, Ritsu Dobashi*, Toshisuke Hirano Department of Chemical System Engineering, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan Received 26 October 1998; received in revised form 1 February 1999; accepted 15 February 1999
Abstract In order to understand characteristics of large pool "res, behavior of intermittent luminous zones appearing on the smoky plumes has been analyzed using a recently developed image processing technique. The images of large pool "res analyzed in this study are of 30 and 50-m diameter pool "res of kerosene recorded on videotapes at the large-scale experiments performed in Japan, 1981. The results indicate that the probability of a luminous zone appearing is the maximum on the central point of the average plume width and at a distance from its base, and decreases with the distance from the maximum point. It can be inferred that the luminous zone on the smoky plume of a pool "re likely appears at its concave boundary (in a side view). It ascends at an almost constant velocity while changing it's area at the same frequency as that of the `breathinga. 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction As the diameter of an oil-"lled pan increases, the structure of a #ame formed above the pan becomes unstable. For the pan diameter larger than about 1 m, the #ames are fully turbulent [1]. As the diameter increases further, heavy smoke is intensively generated. For a pool diameter of tens of meters, the #ame is almost covered by smoke and luminous zones appear intermittently on the smoky plume. Even in such a case, there remains a continuous luminous zone at the base of the smoky plume, i.e., near the rim of the pan. * Corresponding author. Tel.: 0081-3-5841-7304; fax: 0081-3-5841-7313. E-mail address: [email protected] (R. Dobashi) 0379-7112/99/$ - see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 3 7 9 - 7 1 1 2 ( 9 9 ) 0 0 0 0 9 - 0
2
N. Takahashi et al. / Fire Safety Journal 33 (1999) 1}10
Thermal radiation from the "re plume often plays an important role in hazard evaluation of practical oil tank "res. Models to predict thermal radiation from the "re plume above a pan have been proposed in previous studies [2]. Many of them require a #ame height. However, as the pool diameter increases, it becomes harder to determine the #ame height due to the generation of smoke. Also, such models tend to overestimate thermal radiation from the "re plume in a large pool "re [3], nevertheless the protection of large-scale tanks from "res is extremely important. Thermal radiation from a pool "re strongly depends on the luminous zones on its plume. Therefore, in order to establish an appropriate model available for hazard evaluation of practical oil tank "res, an exact understanding of the behavior of luminous zones and smoke in large-scale pool "res is required. However, large-scale experiments of pool "res, if the pool diameters are larger than a few tens of meters, are very hard to carry out due to the vast cost and requirements of open space. Consequently, there are few available data on the behavior of luminous zones and smoke in large-scale pool "res. Thus, in the present study, we have analyzed the behavior of the luminous zones of pool "res at the large-scale experiments performed in Japan, 1981 [4]. The analysis of the images recorded on videotapes in the experiments has been performed using a recently developed image processing technique with a personal computer.
2. Large-scale experiments In 1981, large-scale pool-"re experiments were performed in Japan by Japan Society for Safety Engineering under the support of the Oil Union, Japan. The pans used in the experiments were 30, 50 and 80 m in diameter, and the fuel was kerosene. The following were examined: temperature distribution inside the plume and the liquid fuel, thermal radiation to the outside and to the fuel surface from the plume, burning rate, composition of the product gases, velocities of the ambient air #ows and shapes of the "re plume. More details about the experimental method and the results are presented in Refs. [4,5]. The experiment with the 80-m-diameter pan was carried out under a windy condition so that the "re plume leaned to the leeward signi"cantly and the burning area did not extend over the whole surface of the pan. The other two experiments were carried out under meteorological conditions with less in#uence of wind. The average wind velocities and directions before ignition were 0.9 m/s and northwest by west in the 30 m-diameter experiment, and 0.6 m/s and northeast by east in the 50 m-diameter experiment, respectively. The "re plume were observed to extend almost vertically above the fuel surface. In the present study, the "re plumes in the experiments with the pans of 30 and 50 m in diameter were analyzed. Assuming that results obtained from the analysis are independent of the direction from which the images of the "re plume were recorded, the behavior of the plume in each experiment has been analyzed based on the images recorded from a "xed point in the northeast of the pan in the 30 m-diameter experiment and in the west of the pan in the 50 m-diameter experiment.
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3. Image processing method In order to investigate the motion of luminous zones and smoke, a sequence of images of the plume were sampled during the period of steady burning. For each experiment, the sampling time was 30 s, the sampling frequency was 10 frames/s. Sampling was started at 240 s after ignition for the 30 m-diameter case and at 180 s after ignition for the 50 m-diameter one. Fig. 1 shows an example of the sampled images for the 50 m-diameter case. Each sampled image is of 304 pixels (horizontal) ;228 pixels (vertical), i.e., 69,312 pixels in size. The images are on a scale of 0.33 m to a pixel for the 30 m-diameter case and of 0.54 m to a pixel for the 50 m-diameter case. The color images were analyzed in this study, although the image shown in Fig. 1 is in a gray-scale mode. The images recorded on videotapes were changed into digital images on a personal computer. Each pixel on the digital image has been represented by the three values, R(red), G(green) and B(blue). In a full-color mode (24 bit), the value of R, G or B is in the range 0}255. The larger the value of R, G or B, the brighter the color, red, green or blue, respectively. As shown in Fig. 1, the image is composed of luminous zone part, smoke part, and the other parts such as sky, mountain, ground, etc. In order to examine the behavior of luminous zones and smoke, it is indispensable to distinguish the object part, that is, the luminous zone part and the smoke part, from the other parts. In the present study, the R, G and B values were adopted for that purpose. The R, G and B values of pixels included in the luminous zone or smoke part were examined, and the conditions to distinguish the luminous zone or the smoke part were determined. As shown in Table 1, the condition represented by a combination of the values indicating three colors was adopted for distinguishing the luminous zone parts. The smoke parts were found to be distinguishable with either R or G value. Fig. 2(a) and (b) show the shapes of the luminous zone and smoke extracted from the image shown in Fig. 1, based upon the pre-determined image processing condition. The shape of the luminous zone in Fig. 2(a) is nearly distinguishable by assuming the limiting condition of R, G and B values.
Fig. 1. An example of sampled images (50 m-diam. pan).
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N. Takahashi et al. / Fire Safety Journal 33 (1999) 1}10
Table 1 Image processing conditions. Pan diameter Sampling time Sampling (m) (s) frequency (frames/s)
Extracting conditions
30
R#G!B'22 G(120 0 R#G!B'20 R(110 0
30
50
10
Luminous zone Smoke
Size of image (pixels)
Scaling parameter (m/pixel)
228;304
0.33 0.54
Fig. 2. Shapes of luminous zone and smoke extracted from the image shown in Fig. 1.
On the other hand, the image shown in Fig. 2(b) includes not only the smoke part but also other parts. The dense smoke part is exactly distinguishable using the predetermined image processing condition, but the smoke part near the ground is not distinguishable. In the processing, therefore, the ground part was removed by restricting the calculation domain. An original program to continuously process the images was developed. 4. Results and discussion 4.1. Probability of luminous zone appearing The probability of a luminous zone appearing was examined to clarify the characteristics of the luminous zones, which are closely related to thermal radiation from a pool "re [3]. The probability P of luminous zone appearing in a pixel is de"ned as
follows: x P " ,
N
(1)
N. Takahashi et al. / Fire Safety Journal 33 (1999) 1}10
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Fig. 3. Probability distributions of luminous zone appearing.
where x is the number of images where a speci"ed pixel is judged to be in a luminous
zone and N is the total number of sampled images. Subscripts i and j indicate horizontal and vertical coordinates of a pixel, respectively. Fig. 3 shows the probability distributions of the luminous zone appearing in the 30- and the 50 m-diameter pool "res. Several contour lines are drawn at an interval of 0.04 in Fig. 3(a) and at an interval of 0.02 in Fig. 3(b). In both cases, the probability of luminous zone appearing is the maximum on the central point of the average plume width and at a distance from the pool surface, and decreases with the distance from the maximum point. Moreover, Fig. 3 indicates that the luminous zone did rarely appear in the lower part of the plume up to H/D"0.3 in the 30 m-diameter case and up to H/D"0.5 in the 50 m-diameter case. In the 30 m-diameter case, the region of higher probability of luminous zone appearing is found to shift toward the left due to a slight wind. The value and location of the maximum probability are about 0.22 and 25 m in height, respectively, in the 30 m-diameter case, and about 0.14 and 60 m in the 50 m-diameter case. Also, the maximum probability of luminous zone appearing tends to decrease with the increase in diameter of the pan although the probability depends on the image processing condition, especially the threshold. The probability of the luminous zone appearing is considered to be closely related to a periodical motion of the "re plume. Such aspects of the probability distribution of the luminous zone appearing are much di!erent from those in smaller pool "res of pan diameters less than several meters. In small pool "res, the region where the probability of luminous zone appearing is equal to one exists near the rim of the pan and the probability decreases with the increase in height. The #ame height, which is often de"ned as the height with the probability of "nding #ames equal to 0.5, can be determined in small pool "res. Also in large pool "res, such as analyzed in the present study, there exists a continuous
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luminous zone at the rim of a pan. Koseki indicated that a large part of thermal radiation from the "re plume to the surroundings is emitted from the continuous luminous zone near the rim of the pan in large-scale pool "res, based upon the experiment of a pool "re with a pan of 15 m in diameter [3]. However, the ratio of the height of the continuous luminous zone to the pan diameter tends to decrease with increase of the pan diameter. Therefore, the contribution of the continuous luminous zone near the rim to total thermal radiation from the "re plume to the surroundings is expected to become smaller with the increase of pan diameter. Since the dimensionless height of the continuous luminous zone was very small, it could not be identi"ed on the images analyzed in the present study. Thus, in predicting thermal radiation from the "re in large-scale pool "res, we should consider only the probability distribution of the luminous zones, in the intermittent luminous zone. Luminous zones appear on the surface of a smoky plume as observed at the image of a large-scale pool "re. When the e!ect of wind is small, the appearance of the luminous zones seems to be controlled by a probabilistic mechanism. Under this situation, if the smoky plume would not #uctuate and could be assumed as a column, the probability of luminous zone appearing as observed from the pool "re, should be uniform in the direction across the plume. This is the underlying concept of a solid #ame radiation model to evaluate the radiation from the plume of a pool "re [2]. As seen in Fig. 3, however, the probability of luminous zone appearing is the maximum at the center of the average plume. This fact indicates that the method previously adopted for evaluating radiation from a pool "re on the basis of the data on small-scale experiments cannot be applied for evaluating radiation from a large-scale pool "re. A reasonable explanation for the probability distribution of luminous zone appearing at a large-scale pool "re is the likelihood of the luminous zone appearing at a point near the apparent center of the average plume, on the concave part (in a side view) of the plume boundary. 4.2. Ascending velocity of luminous zones As mentioned before, luminous zones appear intermittently among the smoke in large-scale pool "res of several tens of meters in diameter. Each individual luminous zone appears at a relatively low surface of the smoky plume and then ascends at a certain velocity. Some of the luminous zones swell to become "re balls and "nally disappear. Examples of the analyzed results on the behavior of intermittent luminous zones in the experiments of 30- and 50 m-diameter pool "res are shown in Fig. 4(a) and (b), respectively. In these "gures, the movement of each luminous zone is seen as a trace in the "eld represented by the distance from the fuel surface and time from ignition. The degree of darkness of the trace represents the horizontal width of the luminous zone at a height and time. Also, the slope of the trace represents the velocity of the ascending luminous zone, and the distance from one relatively long trace to the next one along the time axis corresponds to the time interval of the appearance of luminous zones.
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Fig. 4. Movement of luminous zones.
It is found that the traces in each "gure have almost the same slope at a nearly regular interval. This result indicates that the luminous zones rise at almost the same velocity and appear nearly periodically. The mean ascending velocities estimated from these "gures are about 15 m/s in the 30 m-diameter pool "re and about 16 m/s in the 50 m-diameter pool "re. McCa!rey [6] showed that there is a correlation between the upward gas velocity scaled by Q along the centerline of a #ame stabilized on a #at surface gas burner and the height scaled by Q from the burner surface, where Q is the heat release rate. Based on this relation, it was inferred that the "re plume is divided into three regimes, the continuous #ame region, the intermittent #ame region, and the buoyant plume region. According to McCa!rey, the scaled centerline gas velocity is independent of the height and is a constant value, 1.9 ms\ kW\ , for the intermittent #ame region. Assuming that this correlation derived from the experimental data obtained, using small gas burners can be applied to large pool "res, the centerline gas velocities for the cases examined in the present study have been predicted. The predicted centerline gas velocities in the intermittent region are about 36 and 44 m/s for the 30- and the 50 m-diameter pool "res, respectively. These values are much larger than the observed ascending velocities of luminous zones in the present study. 4.3. Pulsation frequency of the xre plume In previous studies, the pulsation frequency of a pool "re plume was measured in terms of the #uctuation frequency of thermal radiation from the #ame, smoke buoyancy, or the vortex shedding frequency [7,8]. In the present study, the pulsation frequency of the "re plume has been examined as a frequency of total area variation of luminous zones, which appeared intermittently among the smoke in each large pool "re, and as a #uctuation frequency of the smoke movement at the lower level of the "re plume. Fig. 5 shows the variation of the total area of luminous zones. Here, the total area means the summed area of several luminous zones on each image. Time-averaged
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Fig. 5. Time variations in total area of luminous zones and in width of "re plume at 5 and 8 m in height above the pan in the 30 case and 50 m-diam. case, respectively.
total area of luminous zones is estimated at about 50 and 110 m in the 30- and the 50 m-diameter cases, respectively. In the latter case, it is found that the total area reached about 600 m at the maximum. Fig. 5 also shows the variation in the width of the plume, i.e., the `breathinga at the lower level of the "re plume, where luminous zones rarely appear. Here, the lower level is 5 m in height in the 30 m-diameter case and 8 m in height in the 50 m-diameter case. As seen from this "gure, the "re plume near the pan pulsates periodically in both cases. Frequency spectra of the variations in the total area of luminous zones and in the width of the "re plume have been examined. Fig. 6 shows the results. The broken and
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Fig. 6. Frequency spectra for time variations shown in Fig. 5.
solid lines represent the frequency spectra for the luminous zone area and the "re plume breathing, respectively. The spectrum for the "re plume breathing frequency has one remarkable peak in each case. The frequency where the peak exists is estimated at 0.31 and 0.23 Hz in the 30- and 50 m-diameter cases, respectively. In the 50 m-diameter case, the spectrum for the luminous zone area variation also has one remarkable peak at 0.23 Hz, where the spectrum for the "re plume breathing frequency has a peak. However, the spectrum for the luminous zone area variation has several peaks in the 30 m-diameter case. One of them appears at the same frequency, 0.31 Hz, where the peak of the spectrum for the "re plume breathing frequency exists. The behavior of luminous zones in this case seems to be of multiple frequencies. Although the luminous zone appearing may depend on many factors, it is inferred that the pulsation of the "re plume is closely related to the appearance of intermittent luminous zones on the smoky plume surface in large pool "res. Bejan proposed the following relation between a vortex shedding frequency and a pan diameter for 0.03 to 60 m [7,9]: 3.1 f , D
(2)
where f is a vortex shedding frequency (Hz) and D is a pan diameter (m). The frequencies predicted using this relation are 0.32 and 0.24 Hz in the 30- and 50 m-diameter pool "re, respectively. These frequencies agree well with the frequencies of the "re plume pulsation observed in the present study.
5. Conclusions The behavior of intermittent luminous zones appearing on the smoky plumes of large-scale kerosene pool "res has been analyzed and the following conclusions have been derived.
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1. The probability of a luminous zone appearing is the maximum at the central point of the average plume width at about 25 m from the fuel surface in the 30 m-diameter pool "re, and at about 60 m in the 50 m-diameter pool "re, and decreases with the distance from the maximum point. The intermittent luminous zones ascend at almost the same velocity during steady burning. The mean velocities of ascent estimated for the 30- and 50 m-diameter pool "res are 15 and 16 m/s, respectively. 2. The "re plume near its base pulsates with an almost constant frequency. The pulsation frequency is 0.31 Hz in the 30 m-diameter pool "re and 0.23 Hz in the 50 m-diameter pool "re. In the 50 m-diameter pool "re, the pulsation frequency of the "re plume near its bottom corresponds to the frequency of luminous zone appearing at higher points of the plume. The behavior of the "re plume depends on the frequency of pulsation near the fuel surface. 3. The luminous zone on the smoky plume of a pool "re can be inferred to appear probably at its concave boundary (in a side view) at a distance from its base, ascending at an almost constant velocity and changing its area at the same frequency as the breathing.
Acknowledgements All data used in this study were obtained from the experiments performed under the auspices of Japan Society for Safety Engineering. The authors are grateful to JSSE.
References [1] Hottel HC. Certain laws governing the di!usive burning of liquids by V.I. Blinov and G.N. Khudiakov. Fire research abstracts and reviews 1959;1:41}4. [2] Drysdale D. Di!usion #ames and "re plumes. In: An introduction of "re dynamics, Chapter 4. 1996 New York: Wiley, 1996. [3] Koseki H. Study on characteristics of oil tank "res. PhD Dissertation, The University of Tokyo, Tokyo, Japan, 1996. [4] Japan Society for Safety Engineering. The Report of the Oil Pool Fire Experiment. J. S. S. E, Yokohama, Japan, 1981. [5] Yamaguchi, T, Wakasa K. Oil pool "re experiment. In: Fire Safety Science, Proceedings of the "rst international Symposium. Hemisphere. New York, 1985, pp. 911}8. [6] McCa!rey B.J. Purely buoyant di!usion #ames: some experimental result, Natl. Bur. Stand. (US), NBSIR 79-1910, 1979. [7] Bejan A. Predicting the pool "re vortex shedding frequency. J Heat Transfer 1991;113:261}3. [8] Portscht R. Studies on characteristic #uctuations of the #ame radiation emitted by "res. Combust Sci Technol 1975;10:73}84. [9] Pagni PJ. Some unanswered questions in #uid mechanics. Appl Mech Rev 1990;43:153}70.