Scholarly article on topic 'Experimental Study of Water Drops with Additive Impact on Wood Surfaces'

Experimental Study of Water Drops with Additive Impact on Wood Surfaces Academic research paper on "Materials engineering"

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{"Water mist" / "Fire suppression" / "Drop impact" / Additive / "Wood fire"}

Abstract of research paper on Materials engineering, author of scientific article — Xianjia Huang, Pingping Chen, Meijuan Lan, Xishi Wang, Guangxuan Liao

Abstract Experiments of water drops with and without additives impact on wood surfaces were conducted. Three kinds of wood which are Paulownia, Fraxinus mandshurica and Jatoba, were considered, since they are commonly used for architecture and furniture in China. The dynamics of solution drop with additive impact on wood surfaces and the effects of the surface topography on drop spreading were investigated. Comparing to the collision dynamics of a pure water drop, the results show that the additives significantly alter the dynamics of the drop impact on wood surfaces. The maximum and the final spread factor increase as the surface tension decrease. In addition, the surface topography plays an important role on dynamics of liquid drop impact on a solid surface. The drop spreading on different wood surfaces except for Fraxinus mandshurica surface agrees well with the scaling results reported in literature. The grooves comprising in the wood surface can deform the shape of the liquid lamella when the spreading drop reaches the maximum diameter

Academic research paper on topic "Experimental Study of Water Drops with Additive Impact on Wood Surfaces"

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Procedía Engineering

ELSEVIER

Procedía Engineering 62 (2013) 852 - 858

www.elsevier.com/locate/procedia

The 9th Asia-Oceania Symposium on Fire Science and Technology

Experimental study of water drops with additive impact on wood

surfaces

Xianjia Huang, Pingping Chen, Meijuan Lan, Xishi Wang*, Guangxuan Liao

State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei 230026, China

Abstract

Experiments of water drops with and without additives impact on wood surfaces were conducted. Three kinds of wood which are Paulownia, Fraxinus mandshurica and Jatoba, were considered, since they are commonly used for architecture and furniture in China. The dynamics of solution drop with additive impact on wood surfaces and the effects of the surface topography on drop spreading were investigated. Comparing to the collision dynamics of a pure water drop, the results show that the additives significantly alter the dynamics of the drop impact on wood surfaces. The maximum and the final spread factor increase as the surface tension decrease. In addition, the surface topography plays an important role on dynamics of liquid drop impact on a solid surface. The drop spreading on different wood surfaces except for Fraxinus mandshurica surface agrees well with the scaling results reported in literature. The grooves comprising in the wood surface can deform the shape of the liquid lamella when the spreading drop reaches the maximum diameter. © 2013 InternationalAssociation for FireSafetyScience. Published byElsevierlnc.AllRightsReserved Selection and peer-review under responsibility of the Asian-Oceania Association of Fire Science and Technology

Keywords: Water mist; Fire suppression; Drop impact; Additive; Wood fire

1. Introduction

Liquid drop impact on a surface has been studied for more than a century, due to its relevance in many technical applications, such as fire suppression by sprinklers, ink-jet printing, spray cooling, etc. There are huge differences between liquid and solid surface for impact dynamics of the impinging drop [1]. The collision dynamics of a water drop interaction with liquid surfaces have been studied under various conditions [2-4]. The impact of drops onto solid surfaces, such as solid metallic surface, heated wax surface, solid and liquid coexist surface, textured surface, structured rough substrates with grooves, have also been widely studied [5-10].The impact dynamics of an additive solution drop on solid surface might be vastly different from that of the additive free drop. The effects of additives on the dynamics of drop impact upon a solid surface have been carried out in many areas [11-15].

However, most of the above researches just focused on metallic surface or liquid surface. There are few studies considering the drop impact on wood surface in literatures except for the work conducted by Chen et al. [16]. In fact, wood is one kind of the widely used material for architecture and furniture, and wood fire is the typical type of class A fires. In general fire-fighting sprays, the efficiency of wood fire-fighting is compromised since the majority of water droplet remains in the liquid phase and form runoff [17]. Splashing and bouncing limit fire extinguishment efficiency of drop deposition from spray [15].

Water mist fire suppression technology has been developed to replace conventional means for fires suppression [18]. In order to improve the efficiency of water based fire suppression technologies by enhancing the wetting and spreading on wood surface, additives can be mixed into water for reducing the equilibrium surface tension [19]. Cong et al. [20] studied

* Corresponding author. Tel.: +86 551 6360 6437; fax: +86 551 6360 1669. E-mail address: wxs@ustc.edu.cn.

1877-7058 © 2013 International Association for Fire Safety Science. Published by Elsevier Inc. All Rights Reserved Selection and peer-review under responsibility of the Asian-Oceania Association of Fire Science and Technology doi:10.1016/j.proeng.2013.08.135

the pool-fire suppression with and without additives. Zhou et al. [21] studied the improvement of water mist's fire-extinguishing efficiency with multi-composition additives. Wang et al. [22] studied the optimization of fire suppression with multi-component foam agents. Jiang et al. [23] studied the suppression chemistry of water mists on poly (methyl methacrylate) flames.

All of the above studies indicate that the efficiency of fire suppression of water mist can be obviously improved by adding additives with optimized concentration, especially to wood crib fires. However, the reasons of such improvement and the dynamics of a multi-component droplet impact on wood surface are still not known in detail. Therefore, the major objectives of this work are to deepen the knowledge on fire suppression by water mist with additives through investigating the dynamic process of an additive solution drop impacting on wood surface.

2. Experimental apparatus and materials

The experimental setup is the same as described in the literature [16] which consists of a drop generator system, illumination system, and a high speed video camera. Wood surface roughness was measured by a TR240 surface roughness measuring instrument with accuracy of 0.001°. Wood surface microscope feature was imaged by a Sirion200 FESEM. The initial diameter of the pure water drop and additive solution water drop with 5% NaCl is about 2.4 mm±0.1 mm, while the drop with 4% AFFF is about 1.8 mm±0.1 mm. The impact velocity of the drop is determined by V0 = -Jlgh and varying by the change of the injector height which is less than 40 cm in this work [16, 24].

The detail information of the drop is listed in Table 1. The wood surfaces of Paulownia, Fraxinus mandshurica and Jatoba are considered since they are used commonly in home and office area as wood materials. The measured data of their basic density and average surface roughness (Ra ) are listed in Table 2. And the microscope features of these wood surfaces are shown in Fig. 1.

Table 1. Initial diameter, viscosity, surface tension of different drops

Drop solution Diameter Viscosity (mm2/s) Surface tension

(mm) (at 20 °C) (mN/m)

Pure water 2.4±0.1 1.004 72.0

With 5% NaCl 2.4±0.1 1.043 59.4

With 4% AFFF 1.8±0.1 1.205 20.1

Table 2. Basic density and average surface roughness of the woods [16]

Wood type Basic density (g/cm3) Average Ra (m)

Paulownia 0.24 3.185

Fraxinus mandshurica 0.56 3.635

Jatoba 0.82 8.347

Fig. 1. FESEM image of the three kinds of wood surfaces.

3. Results and discussions

3.1. On spread factor

The non-dimensional method is adopted to analyze the experimental results. The spread factor is defined as non-

dimensional film diameter, d*=D/D0, D is determined as D = 2^NP , where Np is pixel number of the drop spread area

and a is the area of each pixel [16]. D0 denotes the initial diameter of the impacting drop. The time t of drop spread is made non-dimensional with initial impact velocity, V0 and initial diameter, D0 , i.e., (t*=t(V0/D0)) [25]. Figs. 2-4 show the evolution of spread factor of various cases.

- Pure water -O- With 5% NaCl ST- With 4% AFFF

V=1.13 m/s Wood: Paulownia

0 5 10 15 20 25 Dimensionless time (t*=tVo/Do)

'o 10-

Ll_ 4-

Ш 0-1

-A- Pure water

- With 5% NaCl -V- With 4% AFFF V=2.21m/s Wood: Paulownia

0 5 10 15 20 25 Dimensionless time (t*=tVQ/DQ)

°10-|

-A- Pure water -O- With 5% NaCl With 4% AFFF V=2.8m/s Wood: Paulownia

0 5 10 15 20 25 Dimensionless time (t*=tVQ/DQ)

Fig. 2. Spread factor of different solution drop impacting on Paulownia surface with different velocities.

Ll_ 4-

-A- Pure water -O- With 5% NaCl -V- With 4% AFFF

V=1.13 m/s Wood: Fraxinus mandshurica

0 5 Dimensionless

10 15 20 25

time (t*=tV0/D0)

-A- Pure water -O- With 5% NaCl With 4% AFFF V=2.21 m/s Wood:Fraxinus mandshurica

0 5 10 15 20 25 Dimensionless time (t*=tVQ/DQ)

■R 6

- Pure water -O- With 5% NaCl -V- With 4% AFFF

V=2.8 m/s Wood:Fraxinus mandshurica

0 5 10 15 20 25 Dimensionless time (t*=tVQ/DQ)

Fig. 3. Spread factor of different solution drop impacting on Fraxinus mandshurica surface with different velocities.

-A- Pure water -O- With 5% NaCl With 4% AFFF V=1.13 m/s Wood:Jatoba

0 5 10 15 20 25 Dimensionless time (t*=tVQ/ÜQ)

0 5 10 15 Dimensionless time (t*

20 25 Wq/DQ)

-A- Pure water -O- With 5% NaCl With 4% AFFF V=2.21 m/s Wood: Jatoba

'л 10-

Ll_ 4-

-A- Pure water

- With 5% NaCl -V- With 4% AFFF V=2.8 m/s Wood: Jatoba

0 5 10 15 20 25 Dimensionless time (t*=tV0/Do)

Fig. 4. Spread factor of different solution drop impacting on Jatoba surface with different velocities.

Rioboo et al. [25] divided the time evolution of the spread factor into four distinct phases: the kinematic phase, the spread phase, the relaxation phase and the wetting/equilibrium phase, and concluded that the kinematic phase is independent on the surface tension, viscosity etc., but completely influenced by the impact velocity and initial diameter. For these three kinds of drops, the kinetic phase is not observed in all cases because this phase relates to small non-dimensional time, i.e., (t* < 0).

Although surface tension, viscosity and initial diameter of the drop with 4% AFFF are significantly different from the drop with 5% NaCl, there is a phase that the drop spread is similarly before it reaches the maximal spread factor during the spreading phase. But obvious spreading difference exits between the drops with or without additive. It indicates that the effects of wettability may be dominant to this phase.

To the relaxation phase and wetting/equilibrium phase, for a given velocity and wood surface, an inverse proportion occurs between the spread factor and the surface tension of the drop. Comparing Fig. 3 with Fig. 2 and Fig. 4, the evolution of drop spreading on Fraxinus mandshurica surface is quite different from that on Paulownia and Jatoba surface, the reasons will be discussed in section 3.3.

3.2. On the effect of surfactants on maximal spread factor and final spread factor

As shown in Fig. 5, the variation of the maximal spread factor increases as a function of the drop impact velocity with or without additive. In accordance with the results reported earlier in the literature for additive-free and additive load liquid [13], the maximal spread factor increases monotonically with drop impact velocity.

-A- Pure water -O- With 5% NaCl -V- With 4% AFFF Wood: Paulownia

1.5 2.0 2.5

Velocity (m/s)

-A- Pure water -O- With 5% NaCl -V- With 4% AFFF Wood:Fraxinus mandshurica

1.5 2.0 2.5

Velocity (m/s)

-A- Pure water -O- With 5% NaCl -V- With 4% AFFF Wood: Jataba

1.5 2.0 2.5

Velocity (m/s)

Fig. 5. Maximal spread factor of the different solution drops impacting on different wood surfaces.

According to Table 1, the surface tension of the drop is reduced significantly by adding NaCl and AFFF. The effects of adding additives on maximal spread factor is significant. The maximal diameters of the impact drop with additive are bigger than that with pure water. These results are consisted with the conclusions stated by previous study [6], which confirms that the maximal spread factor should be increased by adding additive.

As shown in Fig. 6, the final spread factor of an impact drop is determined when the drop approaches to an asymptotic state in which all observable oscillation of interface shape and any motion of the contract line completely die out. It indicates that the final spread factors are enhanced by adding additive to the liquid of the drops, which consists with the conclusions stated in the literature [6]. For the given velocity and the wood surface, the final spread factor increases as the surface tension decreases. The wood surface topography affects the maximal spread factor, so the evolution of drop colliding on Fraxinus mandshurica surface is different from that on Paulownia and Jatoba surface.

-A- Pure water -O- With 5% NaCl -V- With 4% AFFF Wood: Paulownia

Velocity (m/s)

-A- Pure water -O- With 5% NaCL -V- With 4% AFFF Wood:Fraxinus manshurica

1.5 2.0 2.5 Velocity (m/s)

-A- Pure water -O- With 5% NaCL -V- With 4% AFFF Wood: Jatoba

Velocity (m/s)

Fig. 6. Final spread factor of the different solution drops impacting on different wood surfaces.

3.3. The effects of wood surface topography on drop spreading

From Fig. 1, we can see that the surface topographies of the considered woods are obviously different. Especially, there are many pore grooves in Fraxinus mandshurica surface. So the different spreading processes occur to the drops impact on Fraxinus mandshurica surface as shown in Figs. 5 and 6. Therefore, it is necessary to discuss the effects of wood surface topography on drop spread on wood surface. • on the maximal spread factor

Figure 7 shows that maximal spread factor of drop increases as the velocity increases. It is interesting that in low-speed experiments (V = 1.13 m/s), the maximal spread factor achieved by the drop impacting on Fraxinus mandshurica is no less than that achieved by the drop impacting on Paulownia and Jatoba surface. However, when the initial impacting velocity increases(V > 2.21 m/s), the maximal spread factor of the drop spreading on Fraxinus mandshurica surface is less than that of drop spreading on other two kinds of wood surfaces.

12-i 1086 4 2

-A- Paulownia -V- Fraxinus mandshurica -O— Jatoba Pure water

1.5 2.0 2.5 Velocity (m/s)

12-, 10 864 2-1 0

-A- Paulownia -V Fraxinus mandshurica -O— Jatoba With 5% NaCl

1.5 2.0 2.5 Velocity (m/s)

12 108642 0

-A- Paulownia

Fraxinus mandshurica —O— Jatoba With 4% AFFF

1.0 1.5 2.0 2.5 3.0 Velocity (m/s)

Fig. 7. Maximal spread factor of different drops spreading on different wood surfaces.

Fig. 8. Images of water drop spreading impact on different wood surfaces with different velocities (a) on Paulownia surface, V = 1.13 m/s (b) on Paulownia surface, V = 2.8 m/s (c) on Fraxinus mandshurica surface, V = 1.13 m/s (d) on Fraxinus mandshurica surface, V = 2.8 m/s (e) on Jatoba surface, V=1.13 m/s (f) on Jatoba surface, V = 2.8 m/s.

Figure 8 shows some images of the drop shapes when it reaches the maximal diameter during the spreading on different wood surfaces. When the drop reaches the maximal diameter, the kinetic of drop impact will be partially converted into the surface energy associated with the increased free-surface area and partially dissipated by the viscous [26, 27]. The large

sized rim is developed in the periphery of lamella on the three kinds of wood surfaces. On Fraxinus mandshurica surface as shown in Fig. 8(c), a tiny droplet is ejected from the rim of the drop due to the influence of the grooves in the wood surface, which is similar to the situation reported in previous study [24]. The result caused by high roughness can be seen in Fig. 8(f), there are several daughter droplets are shaped along the spreading rim, which is similar as the results reported in the literature [28]. These results indicate that the low impacting velocity of the drop dominants the maximal diameter of the drop spread, while the surface topography plays little effect on drop spread on wood surface.

When the velocity of impacting drop is increased (V> 2.21 m/s), the influence of wood surface topography plays more important role on maximal spreading diameter. The most striking difference among Fig. 8(b), Fig. 8(d) and Fig. 8(f) is the shape of lamella attained by the drop. The appearance of lamella with maximal diameter on Fraxinus mandshurica surface is a circle with gaps due to pore grooves comprising in the wood surface. Nevertheless, the shapes of lamella are relative circle reached by the drop spreading on Paulowina and Jatoba surface. During the spread of liquid drops on rough surface, more perturbation is met by the lamella on the grooved surface comparing to that on the smooth surface. These perturbations are caused by micro asperities of irregular surface texture for the lamella spreading on rough surfaces [29]. In a similar manner, because of the grooves comprising in the surface the perturbations are observed on the lamella spreading on Fraxinus mandshurica surface. The images of the droplet shapes further reveal that the intensity of these perturbations increases with increasing of the impact velocity.

3.4. Maximal spreading factor scaled by weber number (We)

Clant et al. [30] found that maximal spreading diameter of the drop, Dmax could be scaled as D0 We114 for low viscosity and low wettability liquid, and it was confirmed by Mounir et al. [15]. Our data for the drops with different impact speeds and with or without additives partly confirm this scaling (as shown in Fig. 9).

12-, 10 864 2-1

A Paulownia + Fraxinus mandshurica O Jatoba

-Linear Fit

Drop with pure Water

2.5 3.0 3.5 4.0 4.5 5.0 5.5

A Paulownia

O Jatoba

-Linear Fit

Drop with 5% NaCl

2.5 3.0 3.5 4.0 4.5 5.0 5.5

12 10 o 8

A Paulownia O Jatoba

-Linear Fit

Drop with 4% AFFF

2.5 3.0 3.5 4.0 4.5 5.0 5.5

Fig. 9. Maximal spread factor of different drops scaled as We14.

To the pure water drop impacting upon the different wood surfaces, the experiment results agree well with the scaling one. Nevertheless, to the drop with additive, it is found that the data of drop spreading on wood surfaces agree with the scaling except for Fraxinus mandshurica surface.

4. Conclusions

The drop with and without additives impacting onto wood surfaces has been investigated experimentally. Based on the experimental results, following conclusions can be drawn: (1) the evolution of pure water drop spreading on wood surface is obviously different from that of the additive solution drop. For water drops containing different additives, similar phases occur before the maximal spread factor is reached, although their surface tensions are quite different. (2) The maximal and final spread factor increase almost linearly as the impact velocity increases, while decrease when the surface tension increases. So it can be concluded that the fire extinguishing efficiency of water based technologies can be improved by reducing surface tension of the agent and enhancing momentum of the drops. (3) The surface topography, especially the grooves comprising in the wood surface, plays an important role on the dynamics of drop impact interaction with wood surface.

Acknowledgements

The authors appreciate the support of the China National Key Basic Research Special Funds project (Grant No. 2012CB719704), The National Key Technology R&D Program (Grant No. 2011BAK03B02), the Fundamental Research Funds for the Central Universities (Grant No. WK2320000002), and the National Natural Science Foundation of China (Grant No. 51028401).

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