Effects of pool dimension on flame spread of aviation kerosene coating on a metal substrate

International Journal of Heat and Mass Transfer 84 (2015) 54–60

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International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt

Effects of pool dimension on flame spread of aviation kerosene coating on a metal substrate Manhou Li a,b, Shouxiang Lu a,⇑, Jin Guo a, Xiujuan Wu a, Kwok-Leung Tsui c a

State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei 230026, Anhui, China Department of Civil and Architectural Engineering, City University of Hong Kong, Hong Kong 999077, Hong Kong c Department of Systems Engineering and Engineering Management, City University of Hong Kong, Hong Kong 999077, Hong Kong b

a r t i c l e

i n f o

Article history: Received 4 December 2013 Received in revised form 8 May 2014 Accepted 1 January 2015 Available online 17 January 2015 Keywords: Flame spread Pool dimension Aviation kerosene Critical liquid depth Scaling analysis

a b s t r a c t Experimental investigations are undertaken to study flame spread over aviation kerosene coating on a metal substrate with various pool dimensions. According to the experimental data, flame spread over aviation kerosene is separated into shallow and deep pool for fuel depth of 4.0 mm, as well as narrow and wide pool for pool width of 12 cm. The variation of flame spread velocity with pool dimension indicates that flame spread accidents on wide or deep pools are potentially more hazardous. Meanwhile, the critical liquid depth is defined as the minimum fuel depth for supporting flame propagation. For pool narrower than 16 cm, the critical liquid depth declines inversely proportionally with an increase in pool width. For pool wider than 16 cm, however, the critical liquid depth stabilizes at 1.2 mm. At critical liquid depth, theoretical analysis verifies that surface deformation possesses a significant effect on flame extinction. The division criterion for deep and shallow pool is also validated by scaling analyses: the values of characteristic length scale ratio (ht/L) and Ra/Ma increase steeply under shallow pool conditions, but remain stable under deep pool conditions. Moreover, the variation trend of Ra/Ma confirms that Marangoni force dominates buoyancy force in driving flame spread at any fuel depths, whereas buoyant effect performs an increasingly importance as the fuel depth increases. Furthermore, the calculated Prandtl number demonstrates that the decrease of flame spread velocity for shallow pool is mainly caused by viscous shear on the metal substrate surface and secondarily caused by heat losses. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction With the development of large-scale naval vessels, a thin oil layer of jet fuel coating on a steel substrate is frequently formed in oil leakage accidents, such as fuel spilling from aircraft crashes, working sites or oil tankers of carrier-based aircraft. These accidents easily result in a typical phenomenon of flame spread over liquid fuel which may further cause many casualties and considerable economic losses. In addition, this phenomenon is a complicated heat and mass transfer process and is significantly influenced by pool dimension, owing to its intrinsical connection with physical laws governing this behavior. Experiments, theoretical analyses and numerical models have concentrated on the effects of pool dimension on flame spread behaviors [1–14]. For pre-mixed flame spread regime, where liquid-phase subsurface convection flow is uninvolved, the flame spread processes are unaffected by pool dimension [15]. For preheated flame spread ⇑ Corresponding author. Tel.: +86 551 63603141; fax: +86 551 63603449. E-mail address: [email protected] (S. Lu). http://dx.doi.org/10.1016/j.ijheatmasstransfer.2015.01.005 0017-9310/Ó 2015 Elsevier Ltd. All rights reserved.

regimes, however, flame spread behaviors are distinctive at different pool dimensions for both alcohols and hydrocarbons. Mackinven et al. [2] detailedly researched flame spread over n-decane under a variety of pool dimensions. Later, White et al. [7] extended this research for flame spreading over JP-5, JP-8 and mixtures of these fuels with a variety of pool dimensions. Both research teams noted that flame spread velocity increased with an increase in fuel depth or pool width. Burgoyne and Roberts [1], Tashtoush et al. [10], and Miller and Ross [5,15,16] experimentally investigated the effects of fuel depth on alcohol flame spread. Takahashi et al. [12,17,18] examined pulsating flame spread over alcohols at several fuel depths, using a non-dimensional number, characteristic length scale ratio, to distinguish between thin and thick pool regimes. Burelbach et al. [8] researched flame spread over decane and dodecane at different fuel depths in a circular pan which eliminated pool’s sidewall effects. Some investigators have also focused on flame spread over hydrocarbons floating on water, with various pool widths [2,7,19]. Among the majority of these previous studies, no consistent conclusions were achieved about the influence of fuel depth on

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liquid flame spread behaviors. Most researchers noted that flame spread velocity was increased by fuel depth [1–3,5,7–16]. However, some investigators did not discover any variations of flame spread velocity with distinct liquid layer depths [4,20]. Moreover, most prior studies were performed using hydrocarbons floating on water, no investigators focused on flame spread over hydrocarbon films directly covering a metal substrate. Actually, the water is a fluid which can flow with the hydrocarbon films, while the solid metal substrate may inhibit the motion of subsurface flow by viscous sheer. In addition, the thermal conductive coefficient of steel is considerably larger than the water counterpart, so more energy is transferred to the steel substrate. Therefore, the characteristics of flame spread over a thin pool of hydrocarbon fuel coating on a metal substrate at different fuel depths deserve detailed research, since this knowledge determines oil fire protection methods of naval fleets and the optimal time to extinguish fires. On the other hand, dissimilar to the non-unified status of fuel depth affecting on flame spread process, the effects of pool width on flame spread velocity are analogous. To wit, most researchers verified that flame propagated more rapidly on wider pool, until the pool width reached 20 cm [2,3,7]. The present research focuses on revealing the mechanisms and critical phenomena of flame spread over a thin pool of aviation kerosene (RP5 with the flashpoint of approximately 66 °C) coating on a metal substrate. The minimum fuel depth for initiating flame propagation over liquid surface is a significant parameter in fire hazard assessment because it essentially determines the available time to extinguish such oil spilling fires. In the present research, the minimum fuel depth for supporting flame propagation is defined as the critical liquid depth (HC). The results of flame spread over thin pools are comprehensively compared with those of deep pools, which are obtained from previous and current research. Based upon numerous experimental results, scaling and theoretical analyses are employed to characterize the flame spread behaviors.

2. Experimental setup As sketched in Fig. 1, a steel plate (long by wide by thick, 100  20  1 cm), with the surface roughness less than 0.1 mm,

was put in an open stainless steel pool, 100 cm long, 20 cm wide and 10 cm deep. Four adjustable nuts were fixed at the four corners of the steel plate in order to ensure it in a perfectly horizontal direction and obtain a well-defined fuel depth. Meanwhile, a 100 cm long removable baffle board was placed on the steel plate surface so that the pool was separated into two parts with different pool widths (4, 8, 12, 16 and 20 cm). At the centre of each lateral wall, a Pyrex window was equipped, 40 cm long by 10 cm deep, to allow optical access to the subsurface flow and ignore the pool’s end wall effects. In our tests, aviation fuels were dispensed into the pan to form 1.2 to 8.0 mm thick sub-flash fuel layers above the steel plate. The liquids were transferred with a 100 ml injector which enabled the fuel depth to be accurately measured. A 4 cm long ignition region was partitioned with a removable insulator strip at one extremity of the channel and separated from the remainder of the pool. Aviation kerosene was ignited with a small volume of heptane (about 2 ml), sprayed on the fuel surface of the ignition region. The barrier was removed and the flame was allowed to spread forward until the pilot-ignition heptane burned up. Once the flame had spread over the full length of the pool, it was extinguished with a fire-retardant board. The experiment was repeated with different fuel depths and pool widths. By repeating each test three or even more times, the scatter of results was reduced to almost ± 10%. Three experimental instruments were applied simultaneously to characterize the flame spread behaviors. First, a lateral CCD (Charge-Coupled Device) camera was utilized to record the flame spread process, which was further used to deduce flame spread velocity by video processing. Second, six 0.1 mm diameter S-type micro-thermocouples were immersed from the bottom of the pool. Two (TC-1 and TC-4) were placed 15 mm above the aviation kerosene surface to monitor the arrival of flame front. Two others (TC-2 and TC-5) were bent toward the liquid surface to record the temperature evolutions of the fuel surface. Two remainders (TC-3 and TC-6) were disposed on the surface of the steel plate but non-contact, to explore the temperature profiles of the metal substrate surface. Every two thermocouples were in the same horizontal level, with the same interval distance of 5 cm. Third, an infrared thermal imager with the spectral range of 8 to 14 lm, which was

5 cm

Real-time display

Ignition region

TC-1

TC-4

TC-2

TC-5

15 mm

Infrared thermal imager

Aviation kerosene layer TC-3

Oil layer

TC-6

40 cm

4 cm

10 cm

Data storage

Data collection system

Signal processing

Steel plate

Real-time record

Pyrex windows

CCD camera

Fig. 1. Schematic diagram of experimental apparatus.

Adjustable nut

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located 1 m above the top of the pool, was used to map the temperature profiles of the fuel surface ahead of the flame front. The emissivity of RP5 was determined to be 0.95 after calibration with a thermocouple.

the flame spread velocity stabilizes at approximately 3.0 cm/s. The rapid flame speeds at wider or deeper pools indicate that flame spread accidents on such pools might be potentially more hazardous. 3.2. Critical liquid depth

3. Results and discussions 3.1. Flame spread velocity Flame spread velocity is one of the most significant parameters in characterizing flame spread behaviors because it closely links with fire hazard assessment. For a given initial fuel temperature of 30 °C, the influence of fuel depth (h) and pool width (w) on flame spread velocity was investigated in detail, and the experimental results are illustrated in Fig. 2. The data presented in Fig. 2 identify that a variation in pool width leads to a significant change in flame spread velocity. At pool width of 4 cm, the flame spread velocity is approximately 1 cm/s, markedly smaller than wide pools. As the pool width increases, the flame spread velocity increases rapidly until the width approaches 12 cm, indicating increasing fire hazard with wider pools. For pool width from 12 to 4 cm, the decrease of flame spread velocity is principally caused by the pool’s lateral walls, attributing to viscous shear and non-negligible heat losses of the pool’s side walls. Meanwhile, as the pool width increases, flame front exhibits fingering feature, where the flame tip alternates between convex and concave to the direction of flame spread. This curving flame tip may enhance the amount of heat transfer per unit width, thus accelerating flame propagation. For pool width larger than 12 cm, on the other hand, radiant energy output from the flame region to the zone in front of flame’s leading edge increases, owing to a markedly increase in flame height. This radiant energy offsets the pool wall’s viscous effects and heat losses, so the flame spread velocity does not change significantly with an increase in pool width. In the present study, most of the experiments are conducted in a 20 cm wide pool, which eliminates the narrow pool effect and provides theoretical guidance for practical fire problems. Furthermore, similar to the influence of pool width on flame spread velocity, an increase in fuel depth also leads to a considerably increase in flame spread velocity. For narrow pools (4 6 w < 12 cm), the flame spread velocities increase slowly with the fuel depth. However, for wide pools (w P 12 cm), considerable rise in flame spread velocity is observed when the fuel depth increases from 1.2 to 4.0 mm. For pool depth larger than 4.0 mm,

The critical liquid depth, repeatedly measured at least ten times to minimize experimental errors, is explored at different pool widths and plotted in Fig. 3. Guo et al.’s [13] experimental result, which was obtained by flame spread over RP5 floating on water, is also added into this figure. Apparently, this curve involves two regimes: for pool width from 4 to 16 cm, the critical liquid depth declines inversely proportionally with an increase in pool width. For pool width greater than 16 cm, however, the critical liquid depth remains constant. Some extra tests were performed in order to verify this conclusion, and it is discovered that the critical liquid depth remained 1.2 mm under the pool width of 18 cm. This result indicates that the aircraft areas should be paved porous or heavily ridged. In the case of fuel leakage, the flame cannot spread forward even though some oil layer lies on the top of the ridges where the fuel depth is shallower than the critical liquid depth. Moreover, at the pool width of 4 cm, the critical liquid depth for flame spreading across RP5 layer coating on a steel substrate is 4.8 mm, while it is approximately 2.0 mm for the fuel floating on water [13]. It is widely accepted that the thermal conductive coefficient of steel is greatly larger than the counterpart of water, thus more energy is transferred to the steel plate under the same fuel depth. Second, during the preheating process by the subsurface flow, the fluid water can flow with liquid fuel and preheat the bulk of cold fuels ahead of flame front. By contrast, viscous shear by the solid steel substrate constricts the preheating effects. Therefore, the initiation fuel depth for supporting flame propagation across RP5 coating on a steel substrate is greatly larger than that floating on water. At the critical fuel depth, a noticeable surface deformation was observed in our test, due to rapid interfacial flow in the shallow fuel layer. This surface depression phenomenon, driven by surface tension flow, appears predominant in flame extinction. In fact, at such a shallow liquid film, the preheating effects to the bulk of cold fuels are greatly inhibited because the subsurface vortex is broken into a number of small roll cells [15], which greatly enhance the total heat transfer area. The relative magnitude of the surface deformation may be estimated by the equation as follows [21]:

5 4.0

w = 4 cm w =8 cm w =12 cm w =16 cm w =20 cm

Critical liquid depth (mm)

3.0 2.5 2.0 1.5 1.0 0.5

Coating on steel substrate Floating on water-Guo

4

No Flame Spread

Flame spread velocity (cm/s)

3.5

3

2

1

No flame propagation 0

0.0 0

1

2

3

4

5

6

7

8

Fuel depth (mm) Fig. 2. Flame spread velocity as a function of fuel depth and pool width.

9

2

4

6

8

10

12

14

16

18

Pool width (cm) Fig. 3. Critical liquid depth as a function of pool width.

20

22

M. Li et al. / International Journal of Heat and Mass Transfer 84 (2015) 54–60

Dh 3 r T ¼ ðT f  T 0 Þ h 2 gh2 q

ð1Þ

where Dh is variation of fuel depth, and T f is flashpoint of RP5. For 1.2 mm thick RP5, Dh=h ¼ 0:69. Therefore, flame cannot spread forward across a shallow aviation kerosene layer covering on a metal substrate when the amount of surface deformation is larger than 69%.

57

approaches 4.0 mm. Also, the characteristic length scale ratio gradually increases with an increase in fuel depth. This ratio approximately equals to 0.21 over the fuel depth h from 4.0 to 8.0 mm. This identifies that flame spread could be divided into two regimes: a shallow pool flame spread (h < 4.0 mm), and a deep pool flame spread (h > 4.0 mm). This conclusion is similar to that of alcohol flame propagation [12], so the division of deep- and thinpool for flame spreading over RP5, using the characteristic length scale ratio, ht =L, is reasonable.

3.3. Structure and temperature profile of subsurface convection flow The subsurface convection flow plays a role of preheating the bulk of cold liquids ahead of the flame’s leading edge and it possesses the same order of magnitude as the flame spread velocity [14]. Therefore, specifying the structure and the temperature profile of the subsurface convection flow is essential to understand the controlling mechanisms of flame spread across a thin pool. As proposed by Takahashi et al. [12,22], flame spread over alcohols was separated into thin and thick pool regime, using a non-dimensional number, characteristic length scale ratio: ht =L, where ht is thermal boundary layer thickness, and L is subsurface convection flow length. Similarly, we analyze the experimental results of flame spread over RP5 with a variety of fuel depths. The thermal boundary layer thickness, ht, is usually determined by liquid-phase visualization diagnostics, such as a holographic interferometry [23], a particle image velocimetry [10], or a schlieren measurement [24,25]. In the present research, however, the 20 cm wide pool disables these precise visualization techniques from directly observing the subsurface circulation flow, owing to adverse effects by light refraction. Subsequently, we developed an alternative method to measure ht, employing several S-type micro-thermocouples arranged as plotted in Fig. 1. The vertical temperature distributions at different fuel depths were obtained from the temperature evolutions of thermocouples TC-1 to TC-3, and are presented in Fig. 4. The temperature evolutions of TC-4 to TC-6 are very similar to those of TC1 to TC-3, thus they are not displayed here. The starting time for each test is selected for clarity in these figures, so the horizontal axis shows only relative time. The data shown in Fig. 4 (a)-(c) identify that, for fuel depth lesser than 3.0 mm, the temperature of fuel surface starts to rise as the subsurface convection flow front arrives. Accompanied by the temperature rise on the oil surface, the temperature on steel substrate surface (bottom of oil layer) also increases. Thus, the thermal boundary layer thickness equals to the oil layer depth under this condition [12]. For fuel depth of 4.0 mm, Fig. 4 (d) indicates that the temperature at the bottom of oil layer does not rise until the flame tip approaches. For fuel depth larger than 4.0 mm, the data are very similar and not shown here. Therefore, the thermal boundary layer thickness is hypothesized to be 4.0 mm, which remains constant for deeper pools. With regard to L, it is determined by the data of the infrared thermal imager. Two fuel surface temperature profiles corresponding to the longitudinal centre line of the 4.0 mm pool, with a time interval of 2s, are plotted in Fig. 5. The subsurface convection flow front is easily determined because a noticeable temperature gradient exists between the subsurface flow region and the bulk of cold fuels. Moreover, according to the temperature distributions shown in Fig. 4, the fuel surface temperature in the flame’s leading edge is measured to be approximately 110–125 °C. Here, the liquid surface temperature of 110 °C is selected as a criterion for distinguishing the flame tip. At the fuel depth of 4.0 mm, the subsurface convection flow length is discovered to be approximately 20.1 cm. Fig. 6 shows that the subsurface convection flow length, L, as well as characteristic length scale ratio, ht =L, varies with the fuel depth. The data shown in Fig. 6 confirm that the subsurface convection flow length increases with the fuel depth until the depth

3.4. Mechanisms of flame spread The subsurface convection flow is driven by combined effects of surface tension (Marangoni force) and buoyancy force [9,18,26]. Since a temperature gradient generates along the fuel surface, the subsurface flow moves forward, pulled by the surface tension. Meanwhile, warm liquid is elevated and stabilized near fuel surface by the stratification effects of buoyancy force. This behavior affects both subsurface convection flow rate and fuel surface temperature profiles that further govern flame propagation [9]. On the other hand, a gas recirculation cell just ahead of the flame tip is formed by the no-slip condition at the pool surface, coupled with the buoyant-induced natural gas flow in the opposite direction as the flame spreads [27]. Using a holographic interferometry instrument, Ito et al. [23] divided subsurface convection flow into two zones: a thin layer of surface-tension-driven flow (STF) and a round circular convective zone (CCZ), driven by coupling effects of surface tension and buoyancy. Schematic illustrations of flame spread mechanisms are shown in Fig. 7. The relative magnitude of buoyancy force and surface tension can be estimated by the ratio of Rayleigh number (Ra) to Marangoni number (Ma). Buoyancy force dominates surface tension when the ratio is larger than 1 [18]. 3

Ra ¼

bqght ðT c  T 0 Þ

Ma ¼

al

rT ðT c  T 0 ÞL al

ð2Þ

ð3Þ

Therefore, the ratio of Ra to Ma is expressed as follows: 3

Ra bqght ¼ Ma rT L

ð4Þ

q dV where b is volumetric expansion coefficient, b ¼ VdT ¼  qddT , q is

RP5 density (0.81 kg/m3), g is gravity acceleration, T c is fuel surface temperature under flame tip, a is thermal diffusivity coefficient, rT is surface tension coefficient (0.147  103 Nm1°C1), and l is dynamic viscosity (2.4 mPas). Fig. 8 shows the ratio between Ra and Ma as a function of fuel depth, deriving from the calculation results of Eq. (4). Evidently, the Ra/Ma increases steeply with the fuel depth under thin pool condition, indicating buoyancy force performing an increasingly importance in driving subsurface convection flow. An explanation is that: for thin pool, flame spread velocity possesses a cubic dependency on fuel depth by buoyancy force, while a first-order dependency on fuel depth by surface tension [14]. Flame spread velocities are higher than those from the linear dependency on fuel depth, which is consistent with the experimental results as shown in Fig. 2. However, for deep pool, this ratio is approximately 0.22 and independent of the fuel depth. This indicates that buoyancy contributes at most 22% to the movement of subsurface convection flow. In other words, the motion of subsurface convection flow attributes in large part to Marangoni force and in small part to buoyancy force.

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M. Li et al. / International Journal of Heat and Mass Transfer 84 (2015) 54–60

160

160

TC-1 TC-2 TC-3

140 120

120 100

(a) h = 1.2 mm

Temperature (°C)

Temperature (°C)

100 80 60

Subsurface flow front

40 20 0 54

58

60

62

64

66

68

70

72

60 40

4

6

8

10

12

14

16

18

20

22

24

Time (s) 160

TC-1 TC-2 TC-3

TC-1 TC-2 TC-3

140

120

120

Temperature (°C)

100

Temperature (°C)

Flame tip

0

74

160

(b) h = 2.0 mm

80 60

Subsurface flow front

40 20 0 30

Subsurface flow front

20

Flame tip 56

(c) h = 3.0 mm

80

Time (s)

140

TC-1 TC-2 TC-3

140

Flame tip 32

34

36

38

40

42

44

46

48

100

(d) h = 4.0 mm

80 60

Subsurface flow front

40 20

50

Flame tip

0 14

16

18

20

22

24

26

28

30

32

34

Time (s)

Time (s)

Fig. 4. Temperature evolutions of thermocouples TC-1 to TC-3 for different fuel depths.

0.22

110

w=20 cm h=4.0 mm T0=30 °C

Flame tip

90

2s

80 70

L

18

ht/L

0.20

w=20 cm T0=30 °C

16

L (cm)

0s

60

Subsurface flow front

50

0.18

h t /L

Fuel surface temperature (°C)

100

20

0.16 14 0.14

40 12

30

0.12

L

20

deep pool regime

thin pool regime

10

10

0.10 1

0 10

15

20

25

30

35

Fig. 5. Temperature profiles corresponding to the longitudinal centre line of the fuel surface.

According to boundary layer theory [28], the relationship between velocity boundary layer thickness (hm ) and thermal boundary layer thickness (ht ) can be expressed as follows:

3

4

5

6

7

8

9

h (mm)

40

Position (cm)

2

Fig. 6. Subsurface convection flow length and characteristic length scale ratio as a function of fuel depth.

hm ¼ Prn ht

ð5Þ

where Pr is Prandtl number, Pr ¼ cp l=k, cp is specific heat and k is thermal conductive coefficient. For most applications, it is accept-

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M. Li et al. / International Journal of Heat and Mass Transfer 84 (2015) 54–60

Vf

Buoyancy induced flow Flammable layer

Flame tip

Evaporation Gas

ht

Liquid CCZ

L

STF

Fig. 7. Schematic illustrations of flame spread mechanisms.

able to assume a value of n ¼ 13 in Eq. (5). For RP5, Pr is calculated to be equal to 43.4, so

Flame spread over RP5 is divided into shallow pool for fuel depth smaller than 4.0 mm, and deep pool for fuel depth larger than 4.0 mm. Similarly, flame spread is divided into narrow and wide pool for pool width of 12 cm. Scale effects on flame spread velocity indicate that flame spread accidents on wide or deep pool might be potentially more hazardous. Moreover, flame cannot spread forward unless the fuel depth is larger than the critical liquid depth. When the pool width is narrower than 16 cm, the critical liquid depth is inversely proportional to pool width. For pool width wider than 16 cm, the critical liquid depth stabilizes at approximately 1.2 mm. At critical liquid depth, theoretical analysis identifies that surface deformation is a significant factor in flame extinction. The division for deep and shallow pool is verified by the values of characteristic length scale ratio (ht/L) and Ra/Ma which both increase steeply with fuel depth and peak at 4.0 mm deep pool. Marangoni force is major mechanism in driving subsurface convection flow and buoyancy force is of secondary significance, even though buoyant effect performs an increasingly important role as the fuel depth increases. Finally, the decrease of flame spread velocity for shallow pool is mainly due to viscous shear effects of the metal substrate, while the heat losses is relatively lesser important.

hm  ht

Conflict of interest

250

w=20 cm T0=30 °C

200

(Ra/Ma) 10-3

150

100

50

0 0

1

2

3

4

h (mm)

5

6

7

8

9

Fig. 8. Ratio of the buoyancy force and Marangoni force as a function of fuel depth.

ð6Þ

This equation demonstrates that the velocity boundary layer thickness is significantly larger than the thermal boundary layer thickness. Therefore, as the fuel depth decreases, the velocity boundary layer is firstly destroyed by viscous shear of the metal substrate, while the thermal boundary layer is gradually restricted. In other words, the decreasing of flame spread velocity for shallow pool is mainly due to the suppression of the dynamic motion by viscous retarding on the metal substrate surface, while the conductive heat losses from the hot liquid layer to the cold metal substrate is relatively lesser important. When the fuel depth is shallower than 1.2 mm, both the velocity boundary layer and the thermal boundary layer are nearly completely destructed. The solid metal substrate serves as a momentum and heat sink which destroys the subsurface convection flow. The stagnation of the flame front is due to the combined effects of fluid drag and heat loss on the metal substrate.

4. Conclusions Flame spread across a thin pool of aviation kerosene covering on a metal substrate is investigated with various pool widths and fuel depths. Based upon numerous experimental results, scaling and theoretical analyses are developed to reveal the mechanisms of flame spread on shallow RP5 layers. Some useful conclusions have been reached:

None declared. Acknowledgement The authors would like to thank the National Natural Science Foundation of China (Project Nos. 51036007 and 51206157) and the Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20123402110048) for their support. The authors also thank Ms. Geraldine Carton for her help with writing assistance. Reference [1] J.H. Burgoyne, A.F. Roberts, The spread of flame across a liquid surface II, Steady-state conditions, Proc. Royal Soci. London Series A Math. Phys. Sci. 308 (1968) 55–68, http://dx.doi.org/10.1098/rspa.1968.0207. [2] R. Mackinven, J.G. Hansel, I. Glassman, Influence of laboratory parameters on flame spread across liquid fuels, Combust. Sci. Technol. 1 (4) (1970) 293–306, http://dx.doi.org/10.1080/00102206908952209. [3] I. Glassman, F.L. Dryer, Flame spreading across liquid fuels, Fire Safety J. 3 (2) (1981) 123–138, http://dx.doi.org/10.1016/0379-7112(81)90038-2. [4] Y. Matsumoto, T. Saito, Flame propagation over liquid fuel at sub-flash temperature, Bulletin of JSME 24 (187) (1981) 160–167, http://dx.doi.org/ 10.1299/jsme1958.24.160. [5] F.J. Miller, H.D. Ross, Further observations of flame spread over laboratoryscale alcohol pools, Proc. Combust. Institute 24 (1) (1992) 1703–1711, http:// dx.doi.org/10.1016/s0082-0784(06)80199-2.

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