STRUCTURE OF'LARGE SCALE POOL FIRES

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STRUCTURE OF’LARGE SCALE POOL FIRES

by Chang Qiaq Ghassan Tashtoush, Akihiko Ito, and Kozo Saito Combustion and Fire Research Laboratory Department of Mechanical Engineering University of Kentucky Lexingtonj KY 40S06

International Conference on Fire Research and Engineering, September 10-15,1995. Orlando, 1% Proceedings. Sponsored by National Institute of Standards and Technology (NIST) and Society of Fire Protection Engineers (SFPE). D. Peter Lund and Elizabeth A. Angell, Editors. Society of Fire Protection Engineers, Boston, M& 1995.

NOTE

This paper is a contribution of the National Institute of Standards and Technology and is not subject to copyright

STRUCTURE OF LARGE SCALE POOL FIRES Cheng Qian, Ghassan Tashtoush, Akihiko Ito, and

~OZO

Saito

Combustion and Fire Research Laborato~ Department of Mechanical E~”neering University of Kentuc& Lezington, KY ~0506

INTRODUCTION Combustion, as a tool to mitigate spilled oils on the ocean surface, turned out to be more feasible compared to other possibIe means by converting rapidly large quantities of oil into its primary combustion products, carbon dioxide and water, with a small percentage of other unburned and residue byproducts [I]. According to Evans et al.,

“In-situ burning of spilled oil has distinct advantage over other counter measures. It requires minimal equipment and less labor than other techniques. It can be applied in areas where many other methods can ‘t due to lack of response infra-structure and/or lack of alternatives.” To establish an effective combustion method which has a high burning rate and emit only environmentally acceptable products, we need to understand of large crude oil fires is not well understood.

the structure

Building and Fire Research Laboratory at the National Institute of Standards and Technology (NIST) conducted a series of pool fire experiments; radiation and smoke emissions from large scale crude-oil pool–fires were characterized [2]. To establish effective correlations among these different pool fires, a series of pool fire experiments were performed using different diameter pools. The main difficulty in this approach is that radiant energy emitted to surroundings changes as a function of the pool diameter affecting chemical and physical structures of pool fires as can be seen in the pioneer work by Blinov and Khudiakov [3]. Blinov and Khudiakov reported the first results of a series of pool fire experiments using four different liquid fuels for the diameter of the pool from less than 1 cm to 22 m. They showed that the regression rate of fuel did not depend on the pool size, and introduced two different scale lengths. One was associated with convective motion of fuel vapor, hot products and air, and the other was associated with the thickness of the hot layer formed on the fuel surface. In 1959, two years after the report of Blinov and Khudyakov [3], Hottel, by analyzing Blinov and Khudyakov data, concluded that in the turbulent pool fires, the dominant heat input to the fuel surface occurs by radiation [4]. Their experimental data [3] had been widely used by many fire researchers until the Japan Society for Safety Engineers conducted a series of larger scale pool–fire experiments [5]. In their 1979 experiments, light crude oil was burned using three different diameter (6.5m, 10.9m, and 30.8m) tanks. In their 1981 experiments, kerosene For both was burned using three larger diameter (30m, 50m, and 80m) tanks. experiments, they measured visible flame (smoke) height, burning rate, irradiance from the flame to ambient environment, and heat feed-back rate at the center of fuel surface. Unfortunately, the 80m pool fire could not spread over the entire fuel surface due to a However other five different diameter tank wind effect leaving that data incomplete, fires provided interesting data. Akita and Kashio [61 thoroughly studied those data and

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showed that radiation received at geometrically similar locations outside the pool decreased with the increase of the pool diameter. The regression rate of fuel remained approximate ely constant regardless of the pool diameter. They also reported that smoke production increased by increasing the pool diameter and a thick smoke layer absorbed radiation from the flame, thus radiant energy per unit surface horn the smoke covered flame decreased. This effect, called “smoke blockage” becomes important for large scale pool fires, whose diameter is larger 5 to 10 m depending on fuel [6]. Because radiant emission from large scale pool–fires is not well known, Koseki and Yumoto [7 measured irradiance from the flame to an ambient environment using a narrow ang 1e radiometer in order to characterize the smoke blockage effect. Hayasaka et al [8] applied a high+peed thermography to measure radiation from a large scale pool fire. Recently, two large scale pool fire experiments were conducted in the US: (1 Gritzo et al [9] conducted a 20 m pool fire experiment and measured radiative )eedback distributions on the fuel surface. Their measurements were com ared with calculations with fairly good agreement. (2) Koseki, Evans and Walton [107 applied a high+peed thermography to measure flame characteristics in a 15 m square crude oil fire which was conducted by NIST at the US Coast Guard Fire and safety Test Detachment in Mobile, Alabama, March, 1994. In the second experiment, the University of Kentucky team participated in measurement of a series of IR image ma s for the pool fire [2]. Our main objective was to measure the flame temperature rhopefully the maximum flame temperature in a luminous zone) by penetrating cold smoke and combustion by–product layers which surround the flame. Based on results from our exploratory tests which were carried out during 1991 through 1993, we used four different band pass–filters (5, 6, 8, and 10.6 P, each with a band pass width ~ 0.5 pm) with the IR camera. The idea to use these filters with the IR camera was to test the effect of each band—pass filter on the flame temperature measurement. A color photograph of the 15 m pool fire was taken and compared ‘to the corresponding IR images without filters (Fig 1), and that with a 10.6 pm band-pass filter (Fig. 2). That the IR image with the 10.6 band–pass filter shows somewhat higher temperature than the IR image without filters is interesting. The 10.6 pm filter was designed to eliminate emissions from smoke and combustion by–product layers [11], while the no-filtering IR image likely corresponds to the highest temperature. These test results indicate that if we can correctly understand the effect of filtering, we may be able to measure “flame temperature” by penetrating smoke and combustion by–product layers enveloping the luminous flame. Because we do not know the real maximum temperature in a luminous flame zone, these IR data need to be compared to a direct temperature measurement (e.g., by thermocouples). However, the thermocouple technique has limitations for such a large scale fire. Therefore, we proposed to apply both IR image and thermocouple techniques to measure temperatures for laboratory-scale pool fires. In addition, we are investigating the mechanism of air entrainment near the base of pool fires. From our previous small-scale pool–fire experiments [12–14 and other larg~cale pool–fire experiments [1,2,5] we learned that the structure of a k ame base is always laminar regardless of pool diamete~, at least for the pool fires with diameter between 5 cm and 50 m. This observation suggests that mechanisms of air entrainment and flame structure near the base for a large scale pool fire can be studied from a small-scale laboratory pool fire. Our recent study showed that air entrainment and fuel–air mixing which occur near the base of pool fires influence soot production rate, flame height, and temperature structure. Flame anchoring, an important issue related to flame extinguishment, also occurs at the

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base on the diffusion flame. Therefore, the investigation the large scale pool fires is very important.

for the flame-base

structure in

POOL FIRE EXPERIMENTS During 1987 through 1991, we participated in the NIST organized program under sponsorship from the Mineral Management Service on the investigation of boilover phenomena in relation to the In-site burning of spilled crude oil on the ocean surface. We applied a holographic interferometry technique and successfully obt ained a series of transient temperature profiles in liquids [15,16]. We developed pool fire apparatus with a temperature and fuel level control system. This apparatus was applied to our small scale pool fire experiments. THE IR IMAGE TEMPERATURE MEASUREMENT TECHNIQUE The significant advantage of the IR technique thermocouple one can be summarized as follows: * * *

compared

to the conventional

No sample preparation is required prior to the measurement; The technique is non–intrusive (the system can be controlled in a remote location away from the fire); and Nearly simultaneous temperature mapping in a relatively large area (easily, 20m x 20m) is available with high spatial and temperature resolution.

The IR image technique was proved to be an effective tool for studying large structure fires [11]. The IR image technique can be a very valuable tool for measuring a luminous flame temperature of large scale pool fires where the conventional thermocouple technique has limitations. To investigate this possibility, the following tasks have been conducted in our laboratory. Five different diameter (5 cm, 10 cm, 30 cm, 60 cm, and 1.2m) pool fires will be used and four different hydrocarbon fuels (hexane, toluene, diesel oil, and crude oil) will be tested. These pool diameters and fuels are selected based on Ref [3] and our previous studies. The 5 cm pool fire will provide a laminar diffusion flame and the 1.2 m diameter pool fire will provide a fully developed buoyancy-controlled turbulent+liffusion-flame. The 10 cm, 30 cm and 60 cm pool fires will be in a transition regime from laminar to turbulent. In addition, a series of IR images were already obtained for the 15 m diesel oil fire at Mobile, AL. The above four different fuels were chosen, since our previous tests showed that these four fuels were typical examples of four different types of liquid hydrocarbon fuels, i.e., hexane as a saturated aliphatic hydrocarbon, toluene as an aromatic hydrocarbon, diesel oil as a homogeneous multi-component hydrocarbon, and crude oil as non–homogeneous multi-component hydrocarbon. A series of transient IR images for the five different small-diameter-pool fires are planned and these IR image data will be compared to the thermocouple data explained below. THERMOCOUPLE TEMPERATURE MEASUREMENT Radial and axial temperature distributions for the five different small diameter pool fires will be measured by fine (Alumel-chromel) thermocouples using the same technique applied in our previous work. Six thermocouples will be placed equi+listantly along the center line. Other six thermocouples will be also placed equi+listantly in radial direction near the flame base and half way to the flame tip. Radial thermocouples are

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located in six different locations. TC1 is located outside the luminous flame zone. TC2 is approximate ely in the flame sheet location, TC3 – TC5 are in the flame interior, and TC6 in the center of the flame. Soot de~osit may occur on t hermocoutie beads located in soot formimr zone. Some thermocouples-located In a high temperatu~e flame zone may cause radiati& loss. The first cause is related to thermophoretic effect, which will be experimentally estimated ikom our previous data for a small scale hydrocarbon–air diffusion flames. The IR image and the thermocouple data will be compared in order to find effective correlations between these two as a function of pool diameter and fuel kind. If an effective engineering correlation is found among these laboratory-scale pool fires, a scaling law analysis will be conducted to apply these correlations to large-scale pool fires. VELOCITY MEASUREMENT BY A PARTICLE-TRACK LASER-SHEET

TECHNIQUE

A 5 cm diameter hexanc+pool-fire was used for this study. The flame was seeded by talc particles and two-component velocity profiles were measured by a particle track laser sheet technique. We focused on the following two mechanisms: (1) Anchoring and stabilization of the pool fire. Much work has been conducted on the stability and lift+f of laminar and turbulent jet diffusion flames; whereas, in a pool fire the fuel and oxidizer velocities near the base are much smaller, and insufficient to produce shear+tress—induced circulation zones which were observed in the jet diffusion flames. Therefore, the mechanism for fuel and oxidizer mixing in pool fires and jet diffusion flames is expected to be different and will be investigated. (2) Why and how much (magnitude) air entrainment occurs near the base? The air entrainment mechanisms near the base and in the intermittent regions are quite different. In the intermittent region of the flame, air entrainment is mainly by large-scale buoyancy-induced mixing. near the base, by cent rast, air is connectively entrained to satisfy mass conservation because of the rapidly accelerating buoyant gases in the flame interior. ACKNO~EDGEMENTS This work was supported by the Building and Fire Research Laboratory, National Institute of Standards and Technology under Grant 60NANB4D1674. We wish to acknowledge Dave Evans and Doug Walt on for their guidance in this research program. REFERENCES [1]

Evans, D. D., Walton, W.D., Baum, H.R., Notarianrri, K. A., Lawson, J. R., Tnag, H. C., Keydel, K. R., Rehm, R. G., Madrzykowski, D., Zile, Koseki, H., and Tennyson, E. J., “In-Situ Burning of Oil Spills: Mesoscale Experiments,” Proc. the FijZh Arctic and Marhw Oil Spill Program Technical Seminar, June 10–12, 1992, Edmonton, Alberta.

[2]

Walto:, W. D., “In Situ Burning of Oil SpiIls: Mesoscale Experiments and Analysls,” AUSTIR 5192, Building and Fire Research Laboratory, National Institute of Standards and Technology, Gaithersburg, MD 20899, September, 1993.

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.

[3]

131inov,V.I. and Khudyakov, G. N., “Certain Laws Governing Diffusion Burning of Liquids,” Academiia Nauk, USSR, Doldady 113,1094.

[4]

Hottel, H.C., A Review of Blinov, V.I. and Khudyakov, Rev., 1:41 (1959).

[5]

Japan Society for Safety Engineers, Report on Tank Fire Experiments, Report (January 1979); Second Report (December 1981) (both in Japanese).

[6]

Akita, K., and Kashio, T., Study of Disaster, 19:231 (1988) (in Japanese).

[7]

Koseki, H. and Yumoto, T., Fire Technology,2433

[8]

Hayasaka, H., Koseki, H., and Tashiroj Y., Fire Z’echnoZogy,28:110 (1992).

[9]

Gritzo, L.A., Nicolette, V. F., Tieszen, S.R., Moyaj J. L., and Holen, J., “Heat Transfer to the Fuel Surface in Large Pool Fires,” submitted to the Eighth Intemutional Symposium on Transport Phenomena in Combustion, 1995.

[10]

Koseki, H., Evans, D.D. and Walton, W. D., “Application of High–Speed Thermography in Large Crude Oil Fires,” submitted to the Forth International Symposium on Fire safety Sciences, Ottawa, Canada, June, 1994.

[11]

Arakawa, A., Saito, K., and Gruver, W.A., Combustion and Flamq 92:222 (1993); and Qian, C., Arakawa, A., Ishida, H., Saito, K., and Cremers, C.J., ThermaZ Conduhity 22, Edited by T.W. Tong, Technomic Pub., 1994, pp. 973–984.

[12]

Emori, R.I. and Saito, K., Combust. Sci. Tech 36:285 (1984).

[13]

Saito, K-, Chap. IV, Section 2, in Fire Research Handbook K. Kishitani Tokyo, Japan, Kyoritsu Pub. Corp., 1984.

[14]

Arai, M., !%ito, K., and Altenkirch, R.A., Comhst. Sci. Tech., 71:25 (1990).

[15]

Inamura, T., Saito, K. and Tagavi, K. A., Combust. Sci. Tech., 86:105 (1992).

[16]

Ito, A., Saito, K., and Inamura, T., J. Heat Transfkr, 114944

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G.N., Fire Res. Abst. First

(1988).

(1992).

Ed.,

-

Figure 1

An IR image without square pool fire.

Figure 2

An IR image with 10.6 micro-meter band-pass filter for a 15 m square pool fire.

filters for a 15 m