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Comparison of flame propagation in a tube with a flexible/ rigid obstacle Quan Li, Shou Xiang Lu, Mingjun Xu, Yanming Ding, and Changjian Wang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b01594 • Publication Date (Web): 06 Sep 2016 Downloaded from http://pubs.acs.org on September 10, 2016

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Comparison of flame propagation in a tube with a flexible/ rigid obstacle Quan Li1, Shouxiang Lu1,*, Mingjun Xu1, Yanming Ding1，Changjian Wang2,* 1

State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei 230027, PR China 2

School of Civil Engineering, Hefei University of Technology, Hefei 230009, PR China

*

Corresponding author:

Shouxiang Lu, Tel.: +86 551 63603141, Fax: +86 551 63601669, E-mail: [email protected] Changjian

Wang,

Tel.:

+86-551-63606285,

Fax:

+86-551-63601669,

E-mail:

[email protected]

Abstract: The safety issues in hydrogen production, storage, transportation and utilization, have caused a high level of concern. Flame acceleration stimulated by the rigid obstacles has been much more addressed while little attention has been paid to the cases with the flexible obstacle which exists popularly in explosion scenarios. In this paper, a series of experiments was conducted to study the effect of flexible/rigid obstacle on flame front and pressure evolutions with varying equivalence ratios. Results show that the gas flow ahead of the flame pushes the flexible obstacle to tilt, which induces smaller gap inlet between the tube wall and the top of the obstacle, and more significant shear layer and vortex. These, together with its rough surface, influence the flame shape, the transition of the flame from laminar one to turbulent one, the instability in the flame interaction with shear layer, and so on. Consequently, flame acceleration is significantly less pronounced for the flexible obstacle than that

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for the rigid one. The flexible obstacle always decays the pressure in the upstream region of the obstacle with varying equivalence ratio. But, as to the pressure in the downstream region, the damping effect can be found for the equivalence ratio of 4.0, but it disappears for 0.6 and 1.0. 1 Introduction Hydrogen as a promising energy carrier has been becoming popular among industries and laboratories because of its high efficiency and clearness

1, 2

. However

the safety issue of hydrogen in production, transportation and storage etc., has caused a high level of concern, which is supposed to be the main barriers for its wide use. Any leakage of hydrogen into a confine space by vehicles, tunnels or storage container etc., can form premixed gas cloud with air. Due to the low ignition energy (0.019 mJ)3, even a weak ignition source may induce a potential flame hazard 4. If there are some repeated obstacles along the flame path, including rigid obstacles and flexible obstacles , the initial laminar flame transits to turbulence and even detonation in worse cases, which brings serious damage to people’s life and property 5. For the prevention and mitigation of accidental flame even explosions during hydrogen storage, transportation and utilization, the accelerating deflagration phenomena and mechanism need to be studied. Previous studies show that the flame acceleration process is mainly controlled by several parameters in rigid-obstacle-laden tube

6

such as blockage ratio (BR) 7,

equivalence ratio (Φ) 8 and mixture reactivity 9 etc. Zhang10 and Li 11 found that Φ has influence on the hydrodynamic and diffusional-thermal instabilities, leading to initial


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acceleration of spherical flame 12. Ciccarelli13, Tedorczyk 14 and Yu et al.15 found that flame speed increases with Φ and reaches the maximum value when Φ=1.0. With further increase of Φ, it starts to decrease. Besides Φ, BR is also a critically important parameter on flame speed. Moen 16 discovered that the flame speed increases with BR, however, there is a critical BR beyond which the finial flame speed does not increase with BR 5, 9. Gu et al. 17 attributed this phenomenon to the increase in momentum and heat losses across the obstacles with bigger BR, and found that the larger the BR is, the more significant the speed deficit is. After a good understanding of the effect of Φ and BR on flame speed, the main mechanisms of flame acceleration are summarized as follow: Lee 18 found the changes in flow field (including turbulence and flow speed gradients) ahead of flame can induce flame acceleration. The experimental results of Wingerden and Zeeuwen

19

showed that the flow speed gradients forming along the

obstacle, result in the flame stretch as the latter enters this zone, which is the main mechanism of facilitating flame acceleration. Moen 20 further pointed out that both the turbulence and flow speed gradients are important on flame acceleration, but the former is dependent on flame front speed and distance away from obstacle.

The

unburned gas flow field for three BRs was visualized by Johansen 7 , who concluded the turbulence behind the obstacle governs the flame propagation. The flame acceleration phenomena and mechanism in a rigid obstacle-laden tube have been well understood. So far, however, few reliable reports are yet found in literatures on the flame propagation in a flexible obstacle-laden tube. In order to fill the gap in knowledge, a typical flexible obstacle (polyurethane sponge) was applied in


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our research, which is placed along the flame path. The aim of this paper is to study the effect of the flexible obstacles on unburned gas flow field and flame acceleration, and then obtain insight into the flame propagation mechanism. Furthermore, it is also expected to compare the flame propagation in flexible and rigid obstacle-laden tube. 2 Experimental Experiments on premixed hydrogen-air flame interaction with an obstacle are performed in a steel combustion tube with the length of 1 m and cross section of 70 mm×70 mm, as shown in Figure 1. The channel is comprised of one optical module and one non-optical module with a gas injection port at the end flanges. Two 30 mm-thickness rectangular quartz glass windows are mounted on the front and back sides of optical module to provide a 235 mm×70 mm optical path for view. As shown in Figure 1, the flame visualization is available by placing the optical module vertically to the parallel light generated by the schlieren system. The schlieren system mainly consists of a 50 Watt point light source, a focusing lens, two 350 mm-diameter concave mirrors with the focal length of 3.5 m and a digital high-speed camera (NAC HX-3). In experiment, the camera is operated at a 30000 frames per second and a 10 µs shutter. Three PCB piezoelectric pressure transducers (model 102B16) are used to record pressure evolution in the tube. PT1 is installed upstream of the obstacle and 110 mm away from the ignition end. PT2 is located at the top up of the obstacle and 100 mm away from PT1. PT3 is placed downstream of the obstacle and 100 mm away from PT2. Flame acceleration is facilitated by a rigid obstacle or a cubic flexible obstacle. The density of flexible obstacle is 10 kg/m3 and its pore diameter is 0.1 mm,


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as shown in Fig.2. All the obstacles have the same size of 70 mm(long)× 70 mm(width)× 50 mm(height) and are placed at about 160 mm away from the ignition end. Thus the BR is fixed to be around 0.71 in current study. It should be noted that the flexible obstacle is stuck on the bottom surface for avoiding unexpected movement. All experiments are carried out using a combination of a 99.99% pure hydrogen and dry air with three equivalence ratios （0.6, 1.0 and 4.0）. After 20-min evacuation of the tube to a vacuum state, the hydrogen and air are filled into the combustion vessel. The initial pressure and temperature are 1atm and 298 K, respectively. The quiescent mixture is ignited by a spark plug with high voltage pulse from a well-designed high-voltage AC power (ignition energy of approximately 7 Joules). For well capturing the flame images, both the ignition system and high speed camera are controlled by the synchronization system.


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Figure 1. Sketch of experimental apparatus. 1-computer 2-data recorder

3-vacuum pump

6-spark igniter 7-flexible obstacle 10-schlieren mirror

a

4-focusing lens

8-quartz glass windows

5-light source

9- high-speed camera

11- synchronization controller 12-gas mixing device

b

Figure 2. Obstacle a: flexible; b: rigid.


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3 Results and discussion 3.1 Unburned gas flow visualization The interaction of flame and unburned gas flow ahead of the flame has an effect on flame acceleration, flame structure evolution and combustion mode

21, 22

. In order to

get a better understanding of flame tip speed evolution, flame fine structure changes and even flame propagation mechanisms, it is imperative to get insight into the unburned gas flow field. However, it is difficult to visualize the unburned gas flow field by experimental technology, except Johansen

7

who observed the evolution of

unburned gas flow field by a novel technique that a small amount of helium as the tracer gas was injected into the rigid-obstacle-laden tube. In present experiments, the unburned gas flow field including shear layer and vortex structure was observed in the tube with a flexible obstacle without any tracer, as shown in Figure 3. However, the vortex structure was not observed in a rigid-obstacle-laden tube. In other words, the unburned gas flow field is affected by the obstacle type. t

t +0.33ms Shear layer Vortex

t +0.60ms

t +0.90ms

t +1.13ms

t +1.30ms

Figure 3. Schlieren photographs of the unburned gas flow field in a flexible-obstacle-laden tube with Φ=1.0.


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The main differences between the flexible and rigid obstacle are the top-surface roughness of two types of obstacles and the deformation of flexible obstacle as shown in Figure 4. Coceal

22

and Lee

23

pointed out that the structure of unburned gas flow

field in the near wall and wake region is significantly influenced by the surface roughness. The deformation is another significant distinction between flexible and rigid obstacles, which may contribute to the formation of vortex and shear layer. However, the vortex structure was also clearly observed in the tests with the equivalence ratio of 0.6 and without the obstacle deformation, as shown in Figure 5.This indicates that the obstacle deformation can induce the vortex and shear layer, but it is not the key factor. So it can be concluded that the surface roughness is more responsible for the vortex formation in the unburned gas flow field because of thicker boundary layer induced by it. Schlieren Images 68.41mm

Schematic

Shear layer

Flexible obstacle Laminar flame front

Vortex

40.39mm

Laminar flame front

Rigid obstacle

Figure 4. Schlieren photographs and schematic of unburned gas flow field.

Vortex

Figure 5. Schlieren photographs of the unburned gas flow field in a flexible-obstacle-laden tube with Φ=0.6.


Vortex

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From t+0.33ms to t+1.30ms in Figure 3, the expansion of the flame ball pushes the unburned gas flow to the right. The mass flow rate of the unburned gas experiences continually growth, due to the unceasingly increase of the flame surface. As the unburned gas approaches the obstacle and flows through the gap, it experiences a faster acceleration due to abrupt decrease of flow area and roughly forms an asymmetrical parabolic axial speed profile as shown in Figure 4. Besides at t+0.33ms, this unburned gas travels fast enough to induce shear layers at left-top and right-top corners of the obstacle. It is also found that a vortex forms downstream of the obstacle at t+0.33ms, as shown in Figure 3. At t+1.30ms, the shear layer at left-top corner grows in length extending beyond the obstacle and the vortex grows in size as more undisturbed unburned gas is drawn into it 7. The vortex nearly has a well-defined core and edge in its whole formation process. This edge is important to the flame acceleration downstream of the obstacle, which will be discussed in detail in the following section. It is noted that the unburned gas flow field in the flexible obstacle-laden tube is different from that in the rigid one. For the latter, there is a fully turbulent recirculation zone without discernible core and edge 7. 3.2 Flame front evolution and obstacle shape changes A series of high-speed schlieren photographs during premixed hydrogen-air flame propagation with Φ=1.0 in an obstructed tube is presented in Figure 6a and Figure 6b. Figure 6a presents the flame propagation in a flexible obstacle-laden tube and Figure 6b exhibits the one in a rigid one. Each case was repeated three times in both flexible/rigid obstacle-laden tubes. It should be noted that the number marked in the


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schlieren photographs is the flame location (L) which is defined as the distance between the flame leading edge and the left end of the field of view. Combustion is initiated at the ignition end and a flame propagates in all directions forming a spherical shape at L=19.32 mm in Figure 6a until it approaches the tube sidewalls. From L=19.32 mm to 41.16 mm in Figure 6a, the flame travels as a laminar mode and its surface can be supposed to experience a monotonic growth that is roughly proportional to the square of the flame ball diameter. Correspondingly, the increase of the flame surface can also induce a similar increase of volumetric burning rate, which in turn gives feedback to flame speed. The enhanced flame speed facilitates the growth of flame surface, and so on

7, 16, 19

. However this positive

feedback mechanism has a relatively weak effect on flame acceleration. The flame tip speed before 41.16 mm can provide a strong proof，as shown in Figure 7. The initial flame propagation in rigid obstacle-laden tube shows similarity with that in flexible obstacle-laden tube. For example, at L=17.97 mm in Figure 6b the flame front also spreads as a spherical shape with a smooth surface. It is worth noted that flame speeds in flexible and rigid obstacle-laden tubes nearly show the similar trend at the initial stage, which is attributed to the propagation of spherical shape flame nearly unaffected by the tube sidewalls and obstacle.


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19.32mm

41.16mm

53.92mm

68.41mm

113.06mm

Not quenching

143.40mm

163.88mm

211.04mm

231.52mm

231.52mm+d1

Quenching

Figure 6a. Schlieren photographs of premixed flame evolution in a flexible obstacle-laden tube with Φ=1.0. 17.97mm

71.99mm

40.39mm

Not quenching

90.44mm

111.90mm

142.04mm

163.88mm

217.99mm

235.00mm

235.00mm+d2


Quenching

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Figure 6b. Schlieren photographs of premixed flame evolution in a rigid obstacle-laden tube with Φ=1.0. From L=41.16 mm to 68.41 mm in Figure 6a, the spherical flame front transforms to a sloping one, and this transformation in the rigid-obstacle-laden tube occurs from L=40.39 mm to 71.99 mm as shown in Figure 6b. In Figure 4, the obstacle presence generates disturbance on the moving unburned gas ahead of the flame and a sloping speed gradient forms just in front of the obstacle. When the flame front enters this flow field, it follows the unburned gas flow and therefore the transformation occurs. Accompanied with the shape changing, the flame tip speed experiences an exponential growth as show in Figure 7. This can be attributed to the fast growth of the flame surface, resulted from the effect of severe stretch of sloping speed gradient on the flame front. Wingerden and Zeeuwen

19

derived a simple model for flame tip

speed as a function of the flame surface and laminar flame tip speed:

 ∆S  U = U S 1 +  S  

(1)

U S = αU L

(2)

where U is the flame tip speed; Us is the flame tip speed at the beginning of the finger-shaped flame; ∆S/S is the relative increase in flame surface area during flame propagation; α is the coefficient that can be expressed as α=σ·∆S/S, whereσis the expansion ratio of the mixture ; UL is the laminar flame tip speed. It is clear that a growth of flame surface by flow speed gradient gives rise to an increase in the flame tip speed. Figure 7 shows that the flame acceleration is more pronounced in rigid obstacle than that in flexible obstacle. As discussed above, the


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growth on U is mainly determined by ∆S/S, which is strongly dependent on the BR 16, 19

. For the flexible obstacle, the unburned moving gas puts the pressure on left side of

the flexible obstacle and thus causes the latter’s deformation, which leads to smaller BR than its initial value of 0.71. So the flexible obstacle results in a smaller growth on flame surface ∆S/S and therefore a smaller flame tip speed U.

Rigid obstacle Flexible obstacle

Rigid obstacle

240

Stage 1

200

Flame Tip Velocity (m/s)

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Stage 2

Stage 3

160

120

80

Flexible obstacle

40

Stage 2

Stage 1

Stage 3

0 0

40

80

120

160

200

Position (mm)

Figure 7. Flame tip speed vs. position for premixed hydrogen-air mixture with Φ=1.0 and BR=0.71.

From L=71.99 mm to 142.04 mm in Figure 6b, the flame passes through the gap between the obstacle and the tube wall, accompanied with the flame shape changing from the sloping one to the jetting one. However there is a delay for the flame passing the flexible obstacle, which occurs from L=113.06 mm to 163.88 mm in Figure 6a. The flame tip speed in both obstacles shares the similar trend that it decreases first and then increases at stage 2, as shown in Figure 7. However this trend displays disagreement at the evolution of flow speed gradient. At this stage, the unburned gas flow follows the iso-entropic correlation:


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dU u − 1 dA = Uu 1 − Ma 2 A

(3)

where Uu , A and Ma are the unburned gas flow speed, cross area, and Mach number of the unburned gas, respectively. Uu can be expressed as the function of flame tip speed U and expansion ratio σ 24.

Uu =

(σ − 1) U

(4)

σ

According to Equation (4), the subsonic flame speed U leads to a subsonic unburned gas flow speed Uu (Ma

Rigid

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