Scholarly article on topic 'A case study on the dynamic process of water drop impacting on heated wood surface'

A case study on the dynamic process of water drop impacting on heated wood surface Academic research paper on "Nano-technology"

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Case Studies in Thermal Engineering
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{"Numerical simulation" / "VOF-based method" / "Drop impact" / "Drop deformation" / "Heat transfer"}

Abstract of research paper on Nano-technology, author of scientific article — Meijuan Lan, Xishi Wang

Abstract A preliminary case study of the impact of a water drop of 2.4mm diameter upon a heated wood surface is presented. The coupled problem of liquid and air flow, and heat transfer via wood surface was predicted by using a VOF-based method. The dynamic process of the water drop impacting the heated wood surface was visualized with a Photron Fastcam high speed video camera. The impact velocities of the drops were varied from 1.71m/s to 2.81m/s. The effects of several case parameters, such as wood surface temperature and basic density, liquid surface tension, and drop–wood contact angle were considered and discussed. The results show that an obvious deformation occurs on the hot wood surface, and the maximum non-dimensional rebound height increases as the collision velocity increases. The numerical simulated results agree well with the experiments.

Academic research paper on topic "A case study on the dynamic process of water drop impacting on heated wood surface"

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Case Studies in Thermal Engineering

journal homepage: www.elsevier.com/locate/csite

A case study on the dynamic process of water drop impacting on heated wood surface $ $$

Meijuan Lan, Xishi Wang*

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

ARTICLE INFO

ABSTRACT

Article history:

Received 16 October 2013

Received in revised form

10 November 2013

Accepted 11 November 2013

Available online 19 November 2013

Keywords:

Numerical simulation VOF-based method Drop impact Drop deformation Heat transfer

A preliminary case study of the impact of a water drop of 2.4 mm diameter upon a heated wood surface is presented. The coupled problem of liquid and air flow, and heat transfer via wood surface was predicted by using a VOF-based method. The dynamic process of the water drop impacting the heated wood surface was visualized with a Photron Fastcam high speed video camera. The impact velocities of the drops were varied from 1.71 m/s to 2.81 m/s. The effects of several case parameters, such as wood surface temperature and basic density, liquid surface tension, and drop-wood contact angle were considered and discussed. The results show that an obvious deformation occurs on the hot wood surface, and the maximum non-dimensional rebound height increases as the collision velocity increases. The numerical simulated results agree well with the experiments.

© 2013 The Authors. Published by Elsevier Ltd. All rights reserved.

1. Introduction

The fluid dynamic phenomena of a liquid drop during its impingement upon hot surfaces occur in many fields, such as in internal combustion engines, electronic circuits, refrigeration cycles, fire suppression, liquid coating and spray/mist cooling, etc. [1]. Numerous literatures refer to the impingement dynamics of a drop impacting on hot surfaces [2-8]. The results show that collapse, bouncing, splashing, vapor explosion and Leidenfrost phenomena may occur during the liquid drop impact. Fundamental studies also show that the phenomena of liquid drop deformation are affected by the drop impacting velocity, drop diameter, surface temperature, contact angle, surface roughness, surface density, etc. [9-12], where the well-known non-dimensional numbers, such as Weber number (We), Reynolds number (Re), and Ohnesorge number (Oh) were considered to describe the collision dynamics.

However, most of the above studies just focused on hot metallic surfaces or liquid surfaces. There are few studies that consider the drop impact on heated wood surfaces, although wood is one of the widely used materials for architecture and furniture, and wood fire is the typical type of class A fires. The effects of the wood surface characteristics on drop deformation had been studied by Chen et al. [13], but only the cases of wood surfaces at room temperature were considered.

Water mist has been regarded as a potential fire suppression technology that can replace the conventional means of fire suppression due to many merits [14-16]. But the dynamical processes of a water drop impacting on heated wood surfaces are still not clear. Therefore, in order to deepen the knowledge on the mechanism of wood fire suppression with water

☆ This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial-No Derivative Works License, which permits non-commercial use, distribution, and reproduction in any medium, provided the original author and source are credited. ☆☆ This work was partially presented at the 4th Asian Symposium on Computational Heat Transfer and Fluid Flow, Hong Kong, 3-6 June, 2013. * Corresponding author: Tel.: + 86 551 63606437; fax: + 86 551 63601669. E-mail address: wxs@ustc.edu.cn (X. Wang).

2214-157X/$ - see front matter © 2013 The Authors. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.csite.2013.11.004

Table 1

Properties of the test materials.

Materials Surface tension (N/m) Viscosity (Pa s) Density Contact angle Contact angle Contact angle

(kg/m3) (at 80 C) (deg) (at 100 C) (deg) (at 120 C) (deg)

Water 0.072 0.001 998 78 81 82

Wood - - 520 78 81 82

0 ms 1 i ms

l -¿i*

4 ms 6 ms

* 1" 1 ¿i

9.5 ms 13 ms

18 ms 30 ms

Fig. 1. Comparison of the simulated drop impact on wood surface with the experimental results. (V0=1.71 m/s, We=97, D0=2.4 mm).

Dimensionless time(t*=tV0/Do)

Fig. 2. Comparison of the simulated drop spreading factor with the experimental results.

mist/spray, the collision dynamics of a water drop impacting on heated wood surfaces has been studied in this work. Current results are limited to the cases of coarse spray.

2. Methods and experimental apparatus

2.1. VOF methodology

Harlow and Shannon [17] developed the method to numerically solve the problem of fluid flow during a drop impact, where the "Marker-and-Cell" (MAC) finite difference method was used to solve the Navier-Stokes equation. However, the MAC method neglected the effects of liquid surface tension and viscosity. Lately, the "Volume-of-Fluid" (VOF) method was used by Liu et al. [18] and Pasandideh-Fard et al. [19] to simulate the spreading and simultaneous solidification of molten droplets on a substrate during plasma spraying.

The dynamic of drop impact on heated wood surface involves liquid, gas and solid phases, and focuses on the free surface of the drop. In order to identify each phase separately, a volume fraction, denoted by a, is introduced following the VOF method as [20]

Volume of liquid phase

= Total volume of the control volume (

where a equals 1 means inside the liquid, 0 means in the gas phase, and values between 0 and 1 mean in the cells containing the interface area.

2.2. Initial and boundary conditions

In the case of this study, the flow induced by the impacting drop on the hot wood surface is considered as two-dimensional and axisymmetric. Typically, the solution domain for simulation of the 2.4 mm diameter water drop impacting the flat wood surface is 20 mm x 20 mm. The considered drop temperatures are in the range of 80-120 °C. Viscosity and surface tension of the drop are regarded as constant with a certain temperature. The drop spreading movement is considered as a laminar flow and the equilibrium contact angle is set at the boundary conditions.

2.3. Experimental apparatus

The experimental setup is similar as described elsewhere [21]; it consists of a drop generator system, illumination system, a high speed video camera and a heating equipment. The wood surface temperature and contact angle are measured by a TM550 infrared thermometer and a SL200B contact angle meter, respectively. Chen et al. [13] reported that, on wood surface, a rebound jet occurred as We=42 (V0= 1.13 m/s), while the jet disappeared as We=98 (V0 —1.71 m/s). Considering such a critical Weber number and the comparison between wood surface with room temperature and the heated wood surface, the impact velocity of the water drop is adjusted from 1.71 m/s to 2.81 m/s in this work.

The wood thickness is 50 mm and its surface area is 100 mm x 100 mm. It is initially heated to 120 °C by an alcohol burner. The detailed information of the drop and wood surface is listed in Table 1.

3. Results and discussion

3.1. Comparison of the experimental results with the numerical simulation

Fig. 1 shows the time-elapsed images of the water drops impacting a 120 1C Betula costata wood surface both by numerical simulation and experiment with impact Weber number of 97; here the impact Weber number can be defined as We = (pV 2D)/ s, where D is the drop diameter, V is the drop impact velocity,p is the liquid density, and s is the drop liquid surface tension. The equilibrium contact angle is about 821. As the drop impacts on the hot wood surface, a liquid crown is

Dimensionless time(t*=tV0/Do)

Fig. 3. Non-dimensional liquid rebound height under different velocities.

obviously formed at the charring wood surfaces at about 4 ms and ultimately produced a big liquid column on the hot surface. A few milliseconds later, the water drop changed to a small liquid column and existed on the wood surface at about 30 ms. It shows that the simulated results agree well with the experimental data.

Fig. 4. Simulated temperature field of the drop spreading and recoiling impacting on 120 °C wood surface.

The non-dimensional method is also adopted to analyze the experimental and simulated results. The spread factor is defined as non-dimensional film diameter, | = Ds/D0, Ds is determined as Ds = 2^JNpA/n, where Np is the pixel number of the drop spreading area, and A is the area of each pixel which can be determined with the image scale. In addition, a dimensionless time of the drop spread, t*, which is defined as t* = t(V/D0) [22] is also used. Fig. 2 shows the comparison of the simulated drop spreading factor with the experimental results. Though the variation tendency is almost consistent, differences are still obvious, especially when t* is within the range of 1-6. This may be mainly caused by the fact that the effects of wood surface roughness and the vaporization are not considered in the current numerical simulation.

3.2. The effects of the initial velocity on liquid rebound height

The liquid rebound height above the wood surface is defined as a non-dimensional height, i.e., H* = H/D0, where H is the maximum height of the ejected liquid column. Fig. 3 displays the variation of the non-dimensional rebound height under three different initial velocities. The results show that the non-dimensional liquid rebound height firstly decreases and then increases. This may be caused by the liquid evaporation or the heat transfer between the solid and liquid. After that, the maximum H* fluctuates with time, which can be explained with the contact temperature, Tc, as suggested by Castanet et al. [23]:

where Tw is the wood wall temperature, Td is the temperature of drop, ew is the wall emissivity, and ed is the drop emissivity. As the drop impacts on the hot wood surface, the heat may transfer from the wood surface to the drop. Therefore, the contact temperature would be changed with time which may cause the fluctuation. In addition, the curve of 1.71 m/s velocity displays a relatively different trend. The reason is, at this velocity, the Weber number (We=97) is less than the critical Weber number of drop splash.

Fig. 3 also shows that the maximum H* increases as the drop initial velocity increases. This can be explained by the fact that the drop initial velocity has a big effect on We and Re, and it also directly relates to the drop impact kinetic energy. The drop impact kinetic energy increases as the initial velocity increases, thus the larger impact velocity will give the larger kinetic energy.

3.3. Temperature field distribution and its effect on total heat transfer

Fig. 4 gives time-elapsed images of spreading and recoiling of the drop impacting the 393 K Betula costata wood surface by numerical simulation with an impact velocity of 0.5 m/s. The liquid drop temperature is the same as the ambient air. At 2 ms, the drop's internal temperature field undergoes no change and only the heat transfer from the solid surface occurs. Thus, the air temperature close to the wall begins to rise up. At 10 ms, the liquid drop temperature close to the wall begins to rise up. At 40 ms, the whole drop temperature rises up and the temperature outside the liquid drop also increases. Fig. 5 shows that the wall total heat transfer rate almost increases as the wood surface temperature increases and it fluctuates with time

TwSw + Td£d £w + £d

T=353K

Hj- T=373K -A- T=393K V0=0.5m/s, D0=2.4mm

Dimensionless time(t*=tV0/D0)

Fig. 5. Comparison of wall total heat transfer rate with different temperatures.

4. Conclusions

Experimental and numerical simulations have been conducted to study a drop impacting a hot wood surface; the following conclusions can be drawn: (1) the behavior of drop impact upon hot wood surface shows that an obvious deformation occurs on the hot wood surface, and the maximum non-dimensional rebound height increases as the collision velocity increases, while at low velocity splash will not occur since the Weber number is less than the critical one. (2) The maximum non-dimensional H* fluctuates with time due to the liquid evaporation or the heat transfer between the solid and liquid. (3) The numerical simulation results indicate that the heat transfer among liquid-solid-air is obvious as the drop impacts the hot wood surface. The wall total heat transfer rate almost increases as the wood surface temperature increases.

Acknowledgments

The authors appreciate the support of the China National Key Basic Research Special Funds project (Grant no. 2012CB719704) and the National Key Technology R&D Program (Grant no. 2011BAK03B02).

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