Experimental Study of Combustion Characteristics of Circular Ring

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Experimental study of combustion characteristics of circular ring thin-layer pool fire Hao Sun, Changjian Wang, Haoran Liu, Manhou Li, Aifeng Zhang, Weiping Zhao, and Caibin Gao Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01504 • Publication Date (Web): 18 Aug 2017 Downloaded from http://pubs.acs.org on August 19, 2017

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Experimental study of combustion characteristics of circular ring thin-layer pool fire Hao Sun, Changjian Wang*, Haoran Liu, Manhou Li*, Aifeng Zhang, Weiping Zhao, Caibin Gao School of Civil Engineering, Hefei University of Technology, Hefei, Anhui 230009, China

*Corresponding authors: Changjian

Wang,

Tel:

86-13856917064;

Fax:

86-551-62905590;

Email

address:

[email protected]; Postal address: School of Civil Engineering, Hefei University of Technology, Hefei, Anhui 230009, China. Manhou Li, Tel: 86-13739249256; Fax: 86-551-62905590; Email address: [email protected]; Postal address: School of Civil Engineering, Hefei University of Technology, Hefei, Anhui 230009, China.

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Abstract For clarifying the combustion behavior of circular ring thin-layer pool fire, a series of experiments with varying pool diameters were carried out. The equivalent pool area of 0.071 m and an initial n-heptane thickness of 10 mm were fixed. Electronic balance, digital camera and K-type thermocouples were used respectively to measure burning rate, flame height as well as centerline temperature. The results show that more burning stages can appear with the increase of the inner and outer diameters of circular ring pool. Fire merging can occur at any one of initial growth, quasi-steady burning with surface boiling, transition to bulk boiling and bulk boiling burning stages, and moreover the maximum burning rate of fire merging is about 3.5 times the one of the ordinary pool fire with same area. The core hollow region plays an important effect in decreasing the burning rate, flame height and fire merging time etc. The evolutions of the characteristic parameters e.g. burning rate, flame height and centerline temperature were analyzed. New correlations for predicting these parameters are also proposed.

Keywords：： Circular ring thin-layer pool fire; Fire merging; Burning rate; Flame height; Centerline temperature


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1. Introduction In the process of production, storage, transportation and application of liquid fuel (gasoline, diesel oil and crude oil, etc.), the accidental leakage can lead to pool fire upon the ignition sources1-4. Pool fire has the characteristics of longer duration and stronger heat radiation hazard, which possibly brings serious damage to the surrounding environment5-10. Therefore, the study of the combustion characteristics of pool fire is beneficial to fire risk assessment and fire safety, and also promotes the safety application of energy. Previous researches have mainly focused on combustion characteristics of bottom sealed pool fire (e.g. rectangle, square or round pool fires) under varying conditions. The evolutions of key parameters and the correlations have been extensively investigated, involving burning rate11-14, flame height15-17 and its pulsation18-20, thermal radiation21, 22, soot formation23-25 etc. Nevertheless, in reality exists another typical pool fire which cannot cover the whole surface due to the obstacle inside or no fuel in its core region, such as floating roof oil tank fire. When a hazardous fire occurs in the floating roof tank, the burning region is usually the sealing ring at the top of oil tank, and the annulus band fire can be observed until fire merging appears. Many such fire accidents caused serious hazards, which have been reported all over the world. On August 29th, 2011, the floating roof tank fire in Dalian of China formed a serious fire merging, resulting in the collapse of oil tank and the economic loss of 8 million yuan. This pool fire can be taken as the circular ring pool fire, on which the studies have not much touched. There are only a few similar studies reported in the literatures. Chatris et al.26 studied the burning rates of gasoline and diesel oil pool fires by using three circular and concentric pools with diameters of 1.5, 3 and 4 m. Hamins et al.27 investigated the heat feedback in two types of pool


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fires, including a simple burner (0.30 m outer diameter) and two annular ring burners (0.30 m and 0.38 m outer diameters, composed of four and five annular rings respectively). Fischer et al.28 carried out the annular pool fire experiments with two geometries (inner diameter 12 ins, annular gap 0.45 ins; and inner diameter 15 ins, annular gap 1.0 ins) to study the burning states of mixtures of ethanol and water. Specially, Wang et al.29 studied on the effects of fire merging on the mass burning rate of hollow square pool fires under quasi-quiescent and longitudinal wind conditions. An extra stage of fire merging was observed besides the previously discovered five stages including initial growth, quasi-steady burning with surface boiling, transition to bulk boiling, bulk boiling burning and decay to extinction. Fire merging appears probably at any one of the first four stages and moreover leads to 50% to 100% increases of the mass burning rate and flame height. However, compared with the square hollow pool fire, what will happen during the combustion process of another circular ring pool fire (pool surface is circular and there is a hollow region in the core)? How does the pool scale affect fire merging? How do characteristic parameters of the pool fire evolve? In this paper, a series of experimental studies were performed to address the above questions. The effects of inner and outer diameters, fuel thickness and air entrainment in the core region were taken into account. The fire merging behavior and combustion characteristics of circular ring pool fires were discussed in detail. The correlations of key fire parameters were also developed.

2. Experimental Circular ring pool fire experiments were carried out in a test room with the dimensions of around 4.0 m×3.0 m×5.0 m. The windows and doors were closed during the process of fuel combustion, for avoiding disturbance effects of external wind on pool fire. In experiment, five


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different circular steel trays with a same area of 0.071 m were employed, as shown in Fig. 1. Wherein, Tray A with a radius of 150 mm is an ordinary pool without hollow region in the core area. All tray wall heights are 60 mm, and the tested liquid fuel is n-heptane (C7H16) whose boiling point temperature and combustion heat are 371.7 K and 4806.6 KJ ∙ mol , respectively. Table 1 lists test cases in current study. The experimental setup is shown in Fig. 2. An electronic balance with the accuracy of 0.1 g and a measurement range of 0-6400 g was used to record the fuel mass loss in the combustion process. One Canon digital video camera with sampling frequency of 25 frames per second and a resolution of 5472×3648 was employed to record the flame images. 10 K-type thermocouples with diameter of 1 mm were distributed in center line above the pool. TC1-TC10 were set in order from the bottom to the top and the distance between adjacent two was 15 cm. The centerline temperature was measured by these thermocouples with a measurement range of 0-1100 K and a response time of 1 s. For investigating the effect of air entrainment into the core region of the circular ring pool, a holder with the height of 40 cm was placed above the electronic balance to support the pool. It allows the fresh air to be entrained freely into the core region of the pool. For the cases without air entrainment into the core region of the pool, a 30 mm thick fire-proof plate was inserted between the pool and the holder.

3. Heat transfer For a free burning pool fire, the mass burning rate is determined by the heat flux feedback Q from the flame into the fuel surface. It can be expressed as the sum of conductive heat flux Q , convective heat flux Q , and radiative heat flux Q 13: Q = Q + Q + Q


(1)

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Then, the heat flux per unit area can be written as:

∑ ( ! ) Q = = + h(T − T& ) + σF)T − T& *(1 − e - )

(2)

where T is the flame temperature; T& is the liquid fuel temperature; k and h are the conductive and convective heat transfer coefficient, respectively; σ , F , and τ are the Stefan-Boltzmann constant, the view factor and the radiative extinction coefficient, respectively. Based on the pool diameter, three basic regimes from burning mode of pool fire have been found by three different heat feedback dominant mechanisms14: (1) D < 0.07 m, conduction-controlled; (2) 0.07 m < D < 0.20 m, convection-controlled; (3) D > 0.20 m, radiation-controlled. In current experiment, the equivalent diameter of all circular ring pools is 0.3 m, so we only take radiation into consideration. For the radiation-controlled circular ring pool fire, Eq. (2) can be expressed as: Q = Q = σF)T − T& *(1 − e - )

(3)

Then the mass burning rate can be calculated as2, 14:

m = 3

012

4 56,! (!,8 !,9

= )

:;? =@? A( B CDE ) FG 5HI,@ (=@,J =@,9 )

= K L (1 − M NO )

where h is the latent heat of vaporization; cQ,& is the specific heat of liquid fuel;

(4) T&,R and

T&,S are the boiling point and initial temperature of liquid fuel, respectively；K L is the burning rate for pool fire with an infinite diameter. For a heptane pool fire, the values of K L and T are found out to be 0.101 kg ∙ m ∙ s and 1.1 m , respectively14.

4. Results and discussions 4.1 Burning behavior For a limited or relatively thin heptane pool fire, Wang et al.29, Chen et al.30 and Hayasaka31


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found five stages including initial growth(Ⅰ), quasi-steady burning with surface boiling(Ⅱ), transition to bulk boiling(Ⅲ), bulk boiling burning(Ⅳ), and decay to extinction(Ⅴ). If the fuel thickness is further decreased, the fuel is consumed completely before the bulk-boiling occurs, which leads to the disappearance of two stages of transition to bulk boiling(Ⅲ) and bulk boiling burning(Ⅳ). So only three stages are left: initial growth (Ⅰ), quasi-steady burning with surface boiling(Ⅱ) and decay to extinction(Ⅴ). As shown in Fig. 3, the 20-mm-thick pool fire undergoes five stages while the 10-mm-thick pool fire evolves in three stages. It should be noted that the average burning rate is around 0.0189 kg·m-2·s-1 for 10-mm pool fire. However, for 20-mm-thick pool fire, the maximum average burning rates are 0.0141 kg·m-2·s-1 at the quasi-steady burning stage and 0.0245 kg·m-2·s-1 at the bulk boiling burning stage, respectively. Fire images in Fig. 4 also show the evolution difference. When the fire burned on the circular ring Tray E, as shown in Fig. 5, fire rapidly spreads following ignition. After a short time or at 10 s, it covers the entire pool surface and forms an annular band fire. Then the fire falls in the steady combustion stage, and the flame height reaches a relatively steady value for a certain period. With the liquid fuel transition from surface boiling to bulk boiling, the fire merging occurs, which causes a circular ring fire to be transited as a merged one just as that on the tray with no hollow part inside. The flame height rapidly increases to a maximum value at around 420 s. After some time, the fire decays and extinguishes after the liquid fuel burn-out. Fig.6 presents the typical burning rate profiles of circular ring pool fires with the same pool area of 0.071 m2 and fuel thickness of 10 mm. Two kinds of burning rate profiles can be observed. For Trays A and B, only three stages exist while five stages are on Trays C, D and E, respectively.


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In other word, with increasing the circular ring pool diameter, more burning stages appear. Chen et al.30 and Kang et al.32 analyzed the temperature differences between the fuel surface and wall of a pool fire, which indicated that the convective heat is transferred from the liquid fuel to the pool wall at the first combustion stage, but this tendency is reversed during the latter combustion stages. In current study, with increasing the circular ring pool diameter, since more heat is lost from the larger outside and inside wall surfaces at the stage of quasi-steady burning with surface boiling, less fuel evaporates and smaller burning rate can be observed. With fire evolution, more heat is transferred by radiation from the fire to the fuel and the steel walls. Since the steel walls can be heated faster than the fuel, when the wall temperature is higher than fuel surface temperature, the heat transfers from the steel wall to the liquid fuel. Simultaneously, lower burning rate at the steady burning stage with surface boiling and higher thermal transmission from the pool walls to the liquid fuel also provide an opportunity for the fuel in the circular ring tray with larger diameter to boil entirely and an additional bulk boiling stage appears. For the annular fire, an important phenomenon, fire merging occurs. Wang et al.29 found that the fire merging appears at the stages of initial growth (Ⅰ) and bulk boiling burning (Ⅳ), and deduced that the fire merging appears probably at any one of the first four stages (Ⅰ-Ⅳ) on the rectangular tray with hollow part inside. For current circular ring pool fire, fire merging can be observed at these stages of initial growth (Tray B), transition to bulk boiling (Trays C and D) and bulk boiling burning (Tray E) in cases without air entrainment in the core hollow region. With air entrainment, fire merging appears at stages of quasi-steady burning (Tray B), transition to bulk boiling (Tray C) and bulk boiling burning (Trays D and E). So current study provides a stronger proof for Wang et al.‘s conclusion29. With increasing the inner and outer diameters of circular ring


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pool fire, the burning rate is increased more. In current experiment, the maximum burning rate is about 3.5 times the one of the ordinary pool fire with same area. Therefore, for controlling the hazardous fire, the safety precautions should be taken before fire reaches the state of merging. Fig. 6 and Fig.7 also show the effect of air entrainment in the core hollow part on the fire. It can be found that, in cases with air entrainment in the core hollow region, more fresh air is entrained and more space is used for heat losses, so the burning rate decreases and therefore the fire lasts for longer time. Furthermore, the fire merging is also delayed. Compared with Figs. 3, 6 and 7, it can be found that the three factors, including fuel thickness, pool diameter and air entrainment in the core region, have many significant effects on the burning rate of circular ring pool fire. For thicker n-heptane fuel and larger pool diameter, the maximum burning rate is increasing and more burning stages appear. Moreover, the increased pool diameter and air entrainment in the core hollow region make fire merging become more difficult. According to Eq. (4) in Section 3, in case with circular ring pool fire, the equivalent pool diameter DXY = D = 0.3 m. Therefore, the calculated value of K is a constant rather than an exponential form. However, we found that the burning rate is changed with time in the experimental measurement, as shown in Fig. 3 and Fig. 6. So, for circular ring pool fire, the only factor that influences the burning rate is the characteristic ratio of the inner diameter (D]) and the outer diameter (D^_). Fig.8 plots the average burning rate K à,bb at quasi-steady burning stage with surface boiling (Ⅱ) and the maximum burning rate K c`d of fire merging as a function of the ratio of inner diameter to outer diameter of circular ring pools (efg /eijk ). It can be seen that, with an increase in efg /eijk , the maximum burning rate rises linearly while the average burning rate at steady combustion stage decreases. Obviously, the difference between K c`d and K à,bb


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rises with increasing pool diameter. The fitting correlations can be obtained: Without air entrainment in the core hollow region: O

K c`d = 0.078 O no − 0.003 pqr

O

K à,bb = −0.034 O no + 0.032 pqr

(5) (6)

With air entrainment in the core hollow region: O

K c`d = 0.069 O no − 0.006 pqr

K à,bb = −0.024

Ono

Opqr

+ 0.023

(7) (8)

Where the unit of the burning rates K c`d and K à,bb is kg·m-2·s-1, efg = 2wfg , eijk = 2wijk . When D] /D^_ increases, It can be speculated that there must be a critical ratio for fire merging. In other words, fire merging does not appear as D] /D^_ is larger than the critical ratio. So the correlations (5) and (7) are limited to the cases with fire merging. When D] /D^_ is infinitely close to 1, the maximum burning rate K c`d will not grow exponentially to infinity due to the limitation of non-fire merging. Moreover, when D] /D^_ ≈ 1, the fire maybe burn with very small burning rate or completely extinguish, so the steady burning rate K à,bb approaches a very small value or even zero, which is consistent with the correlations (6) and (8).

4.2 Flame height In fire accidents, the flame height may affect the characteristics of the flame thermal radiation, the heat balance between flame and liquid fuel as well as other parameters. SFPE defines flame height as the vertical distance between burning surface and the frontal line of flame33. The experimental data was processed by using MATLAB software to extract flame image per frame, and then the color flame images were transformed into binary ones. Using the pool diameter as reference, the transient flame height was obtained according to the ratio of fire height to pool


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diameter in the image. Following Zukoski et al.’s concept of intermittent function y(z{ )34, I=0.5 and I=0 are used to calculate the average flame height z{,à at the steady-state combustion stage and the maximum flame height z{,c`d of fire merging, respectively. Fig. 9 presents the profiles of flame height as a function of time for circular ring pools. It is found that flame heights provide further proof that only three stages exist for Trays A and B while five stages are on Trays C, D and E, respectively. Table 2 presents z{,à,bb and z{,c`d for circular ring pool fires. In cases without air entrainment in the core hollow region, z{,à,bb decreases from 85.6 cm for Tray B to 32.2 cm for Tray E. However z{,c`d increases from 87.7 cm for Tray B to 120 cm for Tray E. In cases with air entrainment, z{,à,bb decays from 75.2 cm for Tray B to 22.1 cm for Tray E while z{,c`d rises from 81 cm for Tray B to 110 cm for Tray E. It is also observed that air entrainment in the core hollow region leads to the significant decrease of both z{,à,bb and z{,c`d . Two important mechanisms may affect the flame height significantly. Firstly, more air entrainment in the core region can result in more heat loss by convection and radiation from the flame. Secondly, when fire merging appears, the flame will be dragged down by air flow due to the air entrainment below the flame center. Resultantly, the flame height decreases. Following Thomas’s correlation of flame height35, one can re-plot the non-dimensional flame height

|>

O}~

as a function as non-dimensional

c

/

S.

（O}~ ）

, where z{ , eB , K , L and g

are flame height, equivalent pool diameter, the burning rate of liquid fuel, the ambient air density under standard condition and the local acceleration of gravity, respectively. As shown in Fig. 10, both

|>,G, O}~

and

|>, O}~

increase linearly with

c

/

（O}~ ）

S.

. The fitting correlations are

expressed as follows. Without air entrainment in the core hollow region:


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|>, O}~

= 20.05

|>,G, O}~

= 51.26

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S.

c

/

c G,

/

c

c G,

9 （O}~ ）

+ 1.70

(9)

− 0.30

(10)

/

+ 1.81

(11)

/

− 0.81

(12)

S.

9 （O}~ ）

With air entrainment in the core hollow region: |>, O}~

= 18.53

|>,G, O}~

= 66.60

S.

9 （O}~ ）

S.

9 （O}~ ）

Substitute Eqs. (5), (6), (7) and (8) into Eqs. (9), (10), (11) and (12), respectively, and we can get: Without air entrainment in the core hollow region: |>, O}~

|>,G, O}~

= 20.05 = 51.26

Eno S.SS Epqr / 9 （O}~ ）

S.S

S.

Eno 5S.S Epqr / 9 （O}~ ）

S.S

+ 1.70

S.

− 0.30

(13)

(14)

With air entrainment in the core hollow region: |>, O}~

|>,G, O}~

= 18.53

Eno S.SS Epqr / 9 （O}~ ）

S.S

= 66.60

S.

Eno 5S.S Epqr / 9 （O}~ ）

S.S

+ 1.81

S.

− 0.81

(15)

(16)

where the equivalent pool diameter eB = eijk − efg .

4.3 Centerline temperature profile Centerline temperature above pool fire reflects the degree of damage to the surrounding environment36-38. Centerline temperature is one of the important characteristic parameters of pool fire. Fig. 11 and Fig. 12 show that the typical profiles of centerline temperature vs. time for the ordinary tray A and the circular ring tray E. It can be found that there is a significant difference between them. For Tray A, centerline temperature rises sharply from the ambient temperature, and then fluctuates at a certain temperature value until it finally drops to the surrounding temperature.


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The tendency of temperature at different heights is basically same. The temperature gradually decreases with the increase of the monitored height, and the measured maximum temperature which is about 1089 K at the lowest height of TC1. For Tray E without air entrainment in the core hollow area, due to a largest hollow size, the circular ring fire burns at the initial growth and quasi-steady burning stages. Resultantly the initial centerline temperature increases relatively slowly and the average temperature at the quasi-steady burning stage is nearly below 373 K. After the quasi-steady burning stage, the fire transition to bulk boiling burning and fire merging occur, which causes the centerline temperature to rise sharply. The maximum temperature of 962 K is around 2.6 times that at the quasi-steady burning stage. There are similar evolution trends in cases with air entrainment in the core hollow region, but its maximum centerline temperature is about 1085 K, which is 3 times that at the quasi-steady burning stage, and the emergence time is delayed around 60 s. Based on the analysis of experimental data, Heskestad39 found that centerline values of excess temperature above pool fire obey the following correlation: ∆S = 9.1(

=

HI

) ⁄ H

⁄

( − S ) ⁄

(17)

where ∆S = − L is centerline value of excess temperature; and L are the plume temperature at height Z and the ambient temperature; is the specific heat of air, and H is the convective heat-release rate. It can be expressed as33 : H = 0.7

(18)

Q = βm ∆zH

(19)

where Q is the total heat-release rate; β is the combustion efficiency factor; m is the mass loss rate; ∆zH is the heat value of combustion. is the height above fire source, and S is the height


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of the virtual origin above fire source. On the contrary, the virtual origin lies below the pool fire if S is negative. For standard atmospheric conditions, S can be written as40: S = 0.083 ⁄ − 1.02D

(20)

So, the dimensionless vertical centerline temperature in the buoyant plume region (Eq. (17)) can be further expressed as the following formula41: ∆=9 /= ⁄ ∗

¡ ¡9 ⁄ ) O

∝(

where ∗ is the non-dimensional heat release rate, ∗ =

(21)

¢I = £O¤

. The equivalent diameter

eB is defined as D, then it can be arranged as: ∗

⁄

Φ = (∆= ⁄= )⁄ ∝ ( − S ) 9

(22)

Fig. 13 plots typically the dimensionless temperature profiles against the pool-source height. For no air flow entrainment in the core region, as shown in Fig.13a, there is a linear relationship between Φ and Z for different trays. The non-dimensional temperature rises up with the increase of vertical height above fire source. Generally, Φ increases as the circular ring pool diameter increases at a given height. Under the effect of air entrainment in the core region, as illustrated in Fig.13b, Φ evolves much differently. As Z is smaller than 0.6 m, the larger circular ring pool fire has larger Φ. For Tray B, Φ keeps increasing with Z while it always decreases for Tray E. For the other two trays, the difference can be observed that Φ first decreases and then increases. As Z is larger than 0.6 m, the evolution of Φ shows more simply. It slightly decreases with increasing circular ring pool diameter, and increases with Z linearly. These differences can be attributed to the two aspects: (1) in cases with air entrainment in the core hollow region, more fresh air is entrained and more space is used for heat losses, so the centerline temperature at a low height is reduced by the convection and radiation heat transfer


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mechanism; (2) the hollow area causes the core to form a non-flame zone at the bottom of fire, and moreover this effect is more significant with the increase of the inner and outer diameters, which leads to lower temperature. As described above, the outer diameter eijk and the inner diameter efg have a great influence on the centerline temperature. Thus, one can further formulate the relationship between centerline temperature and the height. It should be noted that, due to more complicated evolutions shown for the cases with air entrainment in the core region, here we only focus on those cases without air entrainment. As shown in Fig.14, a new correlation can be obtained as: Opqr Ono ⁄ ) Opqr

Φ(

= 4.88 + 0.80

(23)

4.4 Fire merging 4.4.1 Fire merging mechanism Merging behavior of multiple pool fires has been studied by many previous researchers42-45, the corresponding fire merging mechanisms were reported. Liu et al.42-43 investigated square fire arrays ranging from 3×3 to 15×15, and found that heat feedback enhancement and air supply restriction have a competitive effect on multiple fire interactions. Weng et al.44 studied merged flame height from multiple fires, and found that the asymmetry of entrained air leads to the asymmetrical merged flame. Fan et al.45 reported the effect of cross wind on flame interaction of two heptane pool fires, and concluded that the two flames tilt towards each other due to the competition of air entrainment. Moreover, the cross wind can lead to the disappearance of fire merging at a smaller distance between two pool fires. Current circular ring pool fire can be considered as being composed of multiple adjacent small-scale pool fires, so the fire interaction mechanism can be roughly interpreted by two major


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reasons: (1) The thermal radiation is the main mode of heat transfer from the flame to the fuel surface. The liquid fuel receives heat feedback from not only its own flame but also the adjacent flame, and then the mass burning rate increases while more heat flux is obtained to accelerate the evaporation of liquid fuel. Furthermore, the enhanced fire releases much heat to the adjacent fuel. Therefore, the positive feedback between the flame and the fuel is formed. (2) Air entertainment in the hollow region plays a very significant role in fire merging. In comparison with enough air from the outer side of the flame, there is relatively little air in the inner surface of the flame. For supporting the combustion of the inner surface, more air flow is required from the hollow space. Resultantly, due to different air entrainment, the differential pressure appears between the inside and outside surfaces of the flame, which leads to the flame tilt toward the centerline. With an increase in the mass burning rate, the flame height increases. As the flame lengths reach the critical state of fire merging or the flame tips touch together, the fire merging appears. Generally, the above two mechanisms both contributes to the fire merging occurrence.

4.4.2 Fire merging time Fire merging time is an important parameter for circular ring pool fire, showing how fast the more hazardous situation appears. Fig. 15 presents fire merging time as a function of be observed that the fire merging time linearly increases with increasing

Ono

Opqr

Ono

Opqr

. It can

. In other words, fire

merging is delayed with increasing the ratio of inner to outer diameter of circular ring pool. Moreover, air entrainment in the core region can result in larger fire merging time. The fitting correlations can be obtained as follows.


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With air entrainment in the core hollow region: = 706.2

Ono

− 132.2

(24)

= 667.4 O no − 169.3

(25)

Opqr

Without air entrainment in the core hollow region: O

pqr

5. Conclusions This paper presents an experimental study on circular ring thin-layer pool fire under a quasi-quiescent condition. Several critical fire parameters including burning rate, flame height as well as centerline temperature were measured and analyzed. The main conclusions can be summarized as: (1) With increasing the circular ring pool diameter, more burning stages appear. For Trays A and B, only three combustion stages (initial growth, quasi-steady burning with surface boiling and decay to extinction) can be observed while two extra stages (transition to bulk boiling and bulk boiling burning) exist on Trays C, D and E. (2) Fire merging can occur at any one of initial growth, quasi-steady burning with surface boiling, transition to bulk boiling and bulk boiling burning stages, which provides the direct and strong proof to Wang et al.’s conclusion29. However, in current experiment, the maximum burning rate of fire merging is about 3.5 times the one of the ordinary pool fire with same area. This increment ratio is larger than that in Wang et al.’s experiment29. Moreover, fire merging is delayed as circular ring pool diameter is increased. The fire merging time is linearly correlated with the ratio of inner to outer diameter. (3) The core hollow region plays an important effect in fire evolution. Since more fresh air is entrained and more space is used for heat losses, the burning rate and flame height decrease.


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Resultantly the fire lasts for longer time and fire merging is delayed. (4) With increasing the ratio of inner to outer diameter, the average burning rate at the stage of quasi-steady burning with surface boiling decreases linearly while the maximum one at bulk boiling burning stage increases linearly. (5) For the circular ring pool fires, the normalized flame height

c

/

（O}~ ）

S.

|>

O}~

increases with the increase of

. A correlation between flame height and the ratio of inner diameter to

outer diameter (efg /eijk ) was found by linear fitting. (6) For no air flow entrainment in the core region, the non-dimensional centerline temperature ⁄

∗

Φ = (∆= ⁄= )⁄ increases with the height Z for varying trays. Moreover, a linear correlation 9

was formulated between Φ and Z. Under the effect of air entrainment in the core region, Φ evolves more complicated. Generally, the fire evolution and burning behavior analysis provide a better prediction model for the fire safety of oil leakage accidents. In the practical scenarios of circular ring pool fire, the maximum burning rate and maximum flame height increase with the increase of efg /eijk once the fire merging appears, which exhibits increased hazard to the surroundings. Therefore, for controlling the hazardous circular ring pool fire, the safety precautions should be taken to avoid the occurence of fire merging.

Acknowledgements The authors gratefully acknowledge the National Key Research and Development Program of China (No. 2016YFE0113400).


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References (1) Mudan, K. S. Thermal radiation hazards from hydrocarbon pool fires. Prog. Energy Combust. Sci. 1984, 10 (1), 59-80. (2) Siddapureddy, S.; Wehrstedt, K. D.; Prabhu, S. V. Heat transfer to bodies engulfed in di-tert-butyl peroxide pool fires-Numerical simulations. Journal of Loss Prevention in the Process Industries 2016, 44, 204-211. (3) Guiberti, T. F.; Cutcher, H.; Roberts, W. L.; Masri, A. R. Influence of Pilot Flame Parameters on the Stability of Turbulent Jet Flames. Energy Fuels 2016. (4) Saisirirat, P.; Foucher, F.; Chanchaona, S.; Mounaïmrousselle, C. Spectroscopic Measurements of Low-Temperature Heat Release for Homogeneous Combustion Compression Ignition (HCCI) n-Heptane/Alcohol Mixture Combustion. Energy Fuels 2010, 24 (10), 1-5. (5) Schälike, S.; Chun, H.; Mishra, K. B.; Wehrstedt, K. D.; Schönbucher, A. Mass Burning Rates of Di-tert-butyl Peroxide Pool Fires—Experimental Study and Modeling. Combust. Sci. Technol. 2013, 185 (3), 408-419. (6) Hu, L.; Wang, Q.; Delichatsios, M.; Lu, S.; Tang, F. Flame radiation fraction behaviors of sooty buoyant turbulent jet diffusion flames in reduced-and normal atmospheric pressures and a global correlation with Reynolds number. Fuel 2014, 116, 781-786. (7) Bouhafid, A.; Vantelon, J. P.; Souil, J. M.; Bosseboeuf, G.; Rongere, F. X. Characterisation of thermal radiation from freely burning oil pool fires. Fire Saf. J. 1989, 15 (5), 367-390. (8) Ditch, B. D.; de Ris, J. L.; Blanchat, T. K.; Chaos, M.; Bill Jr, R. G.; Dorofeev, S. B. Pool fires – An empirical correlation. Combust. Flame 2013, 160 (12), 2964-2974. (9) Schälike, S.; Mishra, K. B.; Wehrstedt, K. D.; Schönbucher, A. Limiting distances for flame


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merging of multiple n-heptane and di-tert-butyl peroxide pool fires. Chemical Engineering Transactions 2013, 32, 121-126. (10) Hu, L.; Liu, S.; Wu, L. Flame radiation feedback to fuel surface in medium ethanol and heptane pool fires with cross air flow. Combust. Flame 2013, 160 (2), 295-306. (11) Mishra, K. B.; Wehrstedt, K. D. Diffusive burning characteristics of peroxy-fuels. Fuel 2013, 113, 158-164. (12) Blinov, V. I. and Khudiakov, G. N. Certain Laws Governing Diffusive Burning of Liquids. Academiia Nauk, SSSR Doklady 1957, 113, 1094-1098. (13) Hu, L.; Tang, F.; Wang, Q.; Qiu, Z. Burning characteristics of conduction-controlled rectangular hydrocarbon pool fires in a reduced pressure atmosphere at high altitude in Tibet. Fuel 2013, 111, 298-304. (14) Babrauskas, V. Estimating large pool fire burning rates. Fire Technol. 1983, 19 (4), 251-261. (15) Schalike, S; Wehrstedt, K. D.; Schönbucher, A. Flame Heights of Di-tert-butyl Peroxide Pool Fires–Experimental Study and Modeling. Chemical Engineering Transactions 2012, 26, 363-368. (16) Gao, Z. H.; Liu, Z. X.; Ji, J.; Fan, C. G.; Li, L. J.; Sun, J. H. Experimental study of tunnel sidewall effect on flame characteristics and air entrainment factor of methanol pool fires. Appl. Therm. Eng. 2016, 102, 1314-1319. (17) Tang, F.; Hu, L. H.; Delichatsios, M. A.; Lu, K. H.; Zhu, W. Experimental study on flame height and temperature profile of buoyant window spill plume from an under-ventilated compartment fire. Int. J. Heat Mass Transfer 2012, 55 (1–3), 93-101. (18) Tang, F.; Hu, L.; Wang, Q.; Ding, Z. Flame pulsation frequency of conduction-controlled rectangular hydrocarbon pool fires of different aspect ratios in a sub-atmospheric pressure. Int. J. Heat


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Mass Transfer 2014, 76, 447-451. (19) Chen, X.; Lu, S.; Wang, X.; Liew, K. M.; Li, C.; Zhang, J. Pulsation Behavior of Pool Fires in a Confined Compartment with a Horizontal Opening. Fire Technol. 2016, 52 (2), 515-531. (20) Ding, Y.; Wang, C.; Lu, S. The effect of azeotropism on combustion characteristics of blended fuel pool fire. J. Hazard. Mater. 2014, 271, 82-88. (21) Hamins, A.; Fischer, S. J.; Kashiwagi, T.; Klassen, M. E.; Gore, J. P. Heat Feedback to the Fuel Surface in Pool Fires. Combust. Sci. Technol. 1994, 97 (1-3), 37-62. (22) Chun, H.; Wehrstedt, K. D.; Vela, I.; Schönbucher, A. Thermal radiation of di-tert-butyl peroxide pool fires—Experimental investigation and CFD simulation. J. Hazard. Mater. 2009, 167 (1–3), 105-113. (23) Yao, W.; Zhang, J.; Nadjai, A.; Beji, T.; Delichatsios, M. A. A global soot model developed for fires: Validation in laminar flames and application in turbulent pool fires. Fire Saf. J. 2011, 46 (7), 371-387. (24) Murphy, J. J.; Shaddix, C. R. Soot property measurements in a two-meter diameter JP-8 pool fire. Combust. Sci. Technol. 2006, 178 (5), 865-894. (25) Liu, H. F.; Bi, X. J.; Huo, M.; Lee, C. F. F.; Yao, M. F. Soot Emissions of Various Oxygenated Biofuels in Conventional Diesel Combustion and Low-Temperature Combustion Conditions. Energy Fuels 2012, 26 (3), 1900-1911.

(26) Chatris, J. M.; Quintela, J.; Folch, J.; Planas, E.; Arnaldos, J.; Casal, J. Experimental study of burning rate in hydrocarbon pool fires. Combust. Flame 2001, 126 (1–2), 1373-1383. (27) Hamins, A.; Fischer, S. J.; Kashiwagi, T.; Klassen, M. E.; Gore, J. P. Heat Feedback to the Fuel Surface in Pool Fires. Combust. Sci. Technol. 1994, 97 (1-3), 37-62.


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(28) Fischer, I.; Buckmaster, J.; Lozinski, D.; Matalon, M., Vapor diffusion flames, their stability, and annular pool fires. Modeling in Combustion Science 1970, 449, 249-257. (29) Wang, C.; Guo, J.; Ding, Y.; Wen, J.; Lu, S. Burning rate of merged pool fire on the hollow square tray. J. Hazard. Mater. 2015, 290, 78-86. (30) Chen, B.; Lu, S. X.; Li, C. H.; Kang, Q. S.; Lecoustre, V. Initial fuel temperature effects on burning rate of pool fire. J. Hazard. Mater. 2011, 188 (1–3), 369-374. (31) Hayasaka, H. Unsteady Burning Rates of Small Pool Fires. Bulletin of Japan Association for Fire Science and Engineering 1996, 45 (5), 19-25. (32) Kang, Q.; Lu, S.; Chen, B. Experimental study on burning rate of small scale heptane pool fires. Chin. Sci. Bull. 2010, 55 (10), 973-979. (33) Hurley, M. J.; Gottuk, D. T.; Hall, J. R.; Harada, K.; Kuligowski, E. D.; Puchovsky, M.; Torero, J. L.; Watts, J. M.; Wieczorek, C. J. SFPE Handbook of Fire Protection Engineering. 2016; 29, 487-500. (34) Zukoski, E. E.; Cetegen, B. M.; Kubota, T. Visible structure of buoyant diffusion flames. Symposium (International) on Combustion, 1985, 361-366. (35) Thomas, P. H. The size of flames from natural fires. Ninth Symposium (International) on Combustion, 1963, 844-859. (36) Planas-Cuchi, E.; Casal, J. Flame temperature distribution in a pool-fire. J. Hazard. Mater. 1998, 62 (3), 231-241. (37) Li, Q.; Lu, S.; Xu, M.; Ding, Y.; Wang, C. Comparison of Flame Propagation in a Tube with a Flexible/Rigid Obstacle. Energy Fuels 2016, 30 (10), 8720-8726. (38) Jiotode, Y.; Agarwal, A. K. Endoscopic Combustion Visualization for Spatial Distribution of Soot and Flame Temperature in a Diesohol Fueled Compression Ignition Engine. Energy Fuels 2016, 30 (11),


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9850-9858. (39) Heskestad, G. Engineering relations for fire plumes. Fire Saf. J. 1984, 7 (1), 25-32. (40) Heskestad, G. Virtual origins of fire plumes. Fire Saf. J. 1983, 5 (2), 109-114. (41) Hu, L.; Wang, Q.; Tang, F.; Delichatsios, M.; Zhang, X. Axial temperature profile in vertical buoyant turbulent jet fire in a reduced pressure atmosphere. Fuel 2013, 106, 779-786. (42) Liu N., Liu Q., Lozano J. S., Shu L., Zhang L., Zhu J., Deng Z., Satoh K. Global burning rate of square fire arrays: Experimental correlation and interpretation. Proceedings of the Combustion Institute 2009, 32 (2), 2519-2526. (43) Liu, N.; Liu, Q.; Lozano, J. S.; Zhang, L.; Deng, Z.; Yao, B.; Zhu, J.; Satoh, K. Multiple fire interactions: A further investigation by burning rate data of square fire arrays. Proceedings of the Combustion Institute 2013, 34 (2), 2555-2564. (44) Weng, W. G.; Kamikawa, D.; Fukuda, Y.; Hasemi, Y.; Kagiya, K. Study on flame height of merged flame from multiple fire sources. Combust. Sci. Technol. 2004, 176 (12), 2105-2123. (45) Fan, C. G.; Tang, F. Flame interaction and burning characteristics of abreast liquid fuel fires with cross wind. Exp. Therm Fluid Sci. 2017, 82, 160-165.


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Figure Captions Fig. 1. Five circular ring trays with different sizes. Fig. 2. Experimental setup. Fig. 3. Burning rate profiles for tray A with different fuel thickness: (a) 20mm (b) 10mm. (Ⅰ) initial growth; (Ⅱ) quasi-steady burning with surface boiling; (Ⅲ) transition to bulk boiling; (Ⅳ) bulk boiling burning; (Ⅴ) decay to extinction. Fig. 4. Fire images for tray A with different fuel thickness: (a) 20mm (b) 10mm. Fig. 5. Fire images for tray E with fuel thickness of 10 mm. Fig. 6. Burning rate curves for circular ring trays: (a) without air entrainment and (b) with air entrainment in the core region. Fig. 7. Selected burning rate profiles: (a) Tray B; (b) Tray E. Fig. 8. Burning rate vs. efg /eijk : (a) without air entrainment and (b) with air entrainment in the core region. Fig. 9. Flame height profiles for circular ring trays: (a) without air entrainment and (b) with air entrainment in the core hollow region. Fig. 10. Linear fitting of the dimensionless height

|>

O}~

and

c

S.

/

（O}~ ）

: (a) without air

entrainment and (b) with air entrainment in the core region. Fig. 11. Centerline temperature profiles for Tray A at different heights. Fig. 12. Centerline temperature profiles for Tray E at different heights: (a) without air entrainment and (b) with air entrainment in the core region. Fig.13. Φ vs. Z: (a) without air entrainment and (b) with air entrainment in the core region. Opqr Ono ⁄ ) Opqr

Fig.14. Φ(

vs. Z for cases without air entrainment in the core region.


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Fig. 15. Fire merging time vs.

Ono

Opqr

.

Table Captions Table 1 Test parameters. Table 2 Flame heights for circular ring pool fires.


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(a) Tray A

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(b) Tray B

(d) Tray D

(c) Tray C

(e) Tray E

Fig. 1. Five circular ring trays with different sizes.


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Fig. 2. Experimental setup.


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Fig. 3. Burning rate profiles for tray A with different fuel thickness: (a) 20mm (b) 10mm. (Ⅰ) initial growth; (Ⅱ) quasi-steady burning with surface boiling; (Ⅲ) transition to bulk boiling; (Ⅳ) bulk boiling burning; (Ⅴ) decay to extinction.


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Ⅰ

Ⅱ

Ⅱ

Ⅲ

Ⅳ

Ⅴ

(a) 20mm

Ⅰ

Ⅰ

Ⅱ

Ⅱ

Ⅴ

(b) 10mm Fig. 4. Fire images for tray A with different fuel thickness: (a) 20mm (b) 10mm.


Ⅴ

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Ⅰ

Ⅱ

Ⅱ

Ⅲ

Page 30 of 42

Ⅳ

Fig. 5. Fire images for tray E with fuel thickness of 10 mm.


Ⅴ

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Fig. 6. Burning rate curves for circular ring trays: (a) without air entrainment and (b) with air entrainment in the core region.


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Fig. 7. Selected burning rate profiles: (a) Tray B; (b) Tray E.


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Fig. 8. Burning rate vs. efg /eijk : (a) without air entrainment and (b) with air entrainment in the core region.


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Fig. 9. Flame height profiles for circular ring trays: (a) without air entrainment and (b) with air entrainment in the core hollow region.


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Fig. 10. Linear fitting of the dimensionless number

|>

O}~

and

c

/

（O}~ ）

S.

entrainment and (b) with air entrainment in the core region.


: (a) without air

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Fig. 11. Centerline temperature profiles for Tray A at different heights.


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Fig. 12. Centerline temperature profiles for Tray E at different heights: (a) without air entrainment and (b) with air entrainment in the core region.


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Fig.13. Φ vs. Z: (a) without air entrainment and (b) with air entrainment in the core region.


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Opqr Ono ⁄ ) Opqr

Fig.14. Φ(

vs. Z for cases without air entrainment in the core region.


Energy & Fuels

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Fig. 15. Fire merging time vs.

Page 40 of 42

Ono

Opqr

.


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Table 1 Test parameters.

Case

Tray

1 2 3 4 5 6 7 8 9 10

Tray A Tray A Tray B Tray C Tray D Tray E Tray B Tray C Tray D Tray E

Size(mm) wfg =0 wfg =0 wfg =50 wfg =100 wfg =150 wfg =200 wfg =50 wfg =100 wfg =150 wfg =200

Notes: efg = 2wfg ; eijk = 2wijk .

wijk =150 wijk =150 wijk =160 wijk =180 wijk =212 wijk =250 wijk =160 wijk =180 wijk =212 wijk =250

Pool area （m²）

Fuel thickness （mm）

Air entrainment in the core region

0.071 0.071 0.071 0.071 0.071 0.071 0.071 0.071 0.071 0.071

20 10 10 10 10 10 10 10 10 10

No No No No No No Yes Yes Yes Yes


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Table 2 Flame heights for circular ring pool fires. Type

Air entrainment in the core hollow region

z{,à,bb (cm)

z{,c`d (cm)

Tray A Tray B Tray C Tray D Tray E Tray B Tray C Tray D Tray E

No No No No No Yes Yes Yes Yes

79.7 85.6 49.6 39.1 32.2 75.2 32.7 23.6 22.1

85 87.7 98 117 120 81 94 107 110


Experimental Study of Combustion Characteristics of Circular Ring

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