Investigation of New Cooling Methods for

Hot Solid Surfaces Using Droplets

Ahmad A. Khashogji

Under the direction of Prof. Sigurdur Thoroddsen

And Dr. Erqiang Li

King Abdullah University of Science and Technology

Saudi Research Science Institute 2014

20 July, 2014

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1. Introduction:

1.1 Background

The absence of contact between water and hot solid surfaces was first observed by

Herman Boerhaave. However, this phenomenon was named after Johann Gottlob

Leidenfrost, as he was the first one to explain and describe this phenomenon in

chapter 15 of his "Treatise on the Properties of Common Water". The Leidenfrost

phenomenon states that when you splash droplets of water on hot solid surfaces, a

layer of vapor is created, and that will make the droplets slide on this layer without

evaporating or cooling the surface. People who tried using liquids to cool hot solid

surfaces did not realize that until this phenomenon was studied and discovered.

However, the temperature at which Liedenfrost effect occurs is different for different

liquids.

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Figure1: Leidenfrost droplet. Image attribution: Vystrix Nexoth at the English language

Wikipedia.

1.2 High-Speed Photography

High-speed photography is used to capture rapidly occurring phenomena. The

fundamental characteristics of photography are the same at high speed. Aperture,

shutter speed, frame rate, and international standards organization (ISO) are the basic

settings of any image. Aperture sets the depth of field and focal length of the image,

while shutter speed sets the speed at which the image is taken. Moreover, frame rate

sets the number of frames taken per second, and the ISO sets the light sensitivity of

the image.

High-speed photography contains certain drawbacks in image quality. With high

shutter speed, the amount of light entering the camera will significantly decrease,

creating a darker image. One method used to improve such images is to increase the

ISO light sensitivity level. By doing so, some quality increase can be achieved.

Methods of high-speed photography such as stroboscopy and laser visualization are

used in studying events including explosions, chemical reactions, and combustion.

Taking high-speed photographs of frames produced by different methods allows

scientists to analyze, process, and study various aspects of the drops, and its

properties and applications.

1.3 Summary

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The dynamics of this method were studied in droplets contacting hot solid surfaces

through high-speed visualization. Droplet's diameters and structures were analyzed

using image processing tools. Moreover, different velocities of drops were calculated,

And to see different impacts when the contact happens between the drop and the hot

surface.

1.4 Study Aim:

The aim of this project is to process a set of high speed image frames of droplets

from different liquids. Moreover, we analyze the behavior of the droplets in terms of

the drop diameters and structure when it falls on the hot solid surface. Different types

of liquids were studied using high speed imaging to compare how different

temperatures and velocities affect the Leidenfrost phenomenon.

Spray cooling is widely used in industry, for example to cool computer chips. It is

also relevant to fire-suppression. Heat is extracted from the surface, by heating and

evaporating the droplet. However, if the surface is too hot the liquid forms a stable

vapor layer between the droplet and the surface and it simply bounces off the surface.

Here we want to observe the detailed dynamics of the drop impact process to find the

most efficient cooling method.

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!

2. Methodology:

2.1 Experiment

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Figure 4: The setup used to experiment the Leidenfrost phenomenon. Figure done by: Professor Erqiang Li

Gravity-driven pinch-off mechanism is used to produce droplets from minuscule

needles, which are made of a glass puller of diameter in the range 5–100 µm. As

shown in Fig. 1, a syringe pump will be used to push the liquid from a silicone tube to

the glass needle. Moving the slider on the railing can control the height of the needle.

Different liquids will be used such as water, ethanol, perfluorohexane, silicone oil of

different viscosities and other liquids for the experiments.

The substrate is heated using a Thermo Scientific digital stirring hot plate to

different temperatures (range from room temperature to a value higher than the

Leidenfrost temperature; which defers depending on the liquid. For example water

starts from 375 C). The temperature of the substrate is monitored by using an infrared

thermometer. The droplet is then released onto the substrate once the temperature

reaches the targeted value. The rapid motions of the droplets require high-speed video

at frame rates up to 150 kfps acquired with a Photron SA-3 video camera. The

elongated geometry allows frame rates up to 50 kfps at resolution of 128 × 1024

pixels. Backlighting is accomplished with a 350 W metal-halide lamp (Sumita Optical

Glass, Inc.), which is shown onto a diffuser.

Experiments will be carried out with changing liquids, substrate temperatures and

droplet release heights. The recorded videos is analyzed to investigate the effects of

surface temperature, liquid surface tension, liquid viscosity, and release height on

spreading and splashing of the droplets.

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#

Figure 5: Steps of the Experiment.

2. Lab Equipment

We use a high speed camera "Photron SA-3" to capture droplets falling, the contact

between the droplet and the surface, and changes that occur to the droplet in air. We

also use a light generator because using a high frame rate requires high light.

Moreover, we used water, ethanol, isopropanol, and silicone oil as different liquids to

do the experiment. We chose these liquids because they have different boiling points

and different viscosities. We also use thermo digital heater to heat the solid surface.

• Gravity-driven pinch-off mechanism produce droplets from minuscule needles.

• As shown in Fig. 1, a syringe pump is used to push the liquid through a silicone tube, to

• The droplet is then released onto the substrate once the temperature reaches the

•

• Experiments is carried out with changing liquids, substrate temperatures and droplet

• The recorded videos are analyzed to investigate the effects of surface temperature,

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3. Results

3.1 Leidenfrost Temperature

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Figure 2: The minimum surface temperature for the Liedenfrost effect to take place for each

liquid tested.

Figure 2 indicates different liquids that were imaged with high speed cameras to

figure out the minimum Leidenfrost temperature for each liquid. We discovered that

the Leidenfrost phenomenon is related to the boiling temperature of the liquid. We

find that the higher boiling temperature is, higher the Leidenfrost temperature will be.

The figure above states that isopropanol can reach up to 160C till the Leidenfrost

temperature occurs. However, the Leidenfrost temperature for ethanol can reach up to

170C. Similarly, the Leidenfrost temperature for water is 200C. The liquid that has

the highest Leidenfrost temperature was silicone oil, which can reach up to 245C.

Tem

pera

ture

0

63

125

188

250

Silicone Oil Water IPA

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3.2.Spread Diameters

We multiplied the Leidenfrost temperature for each liquid by 1.2, 1.4, 1.6, 1.8; so

we can get different temperatures with constant ratio. After that, we calculated the

highest spread diameter of each liquid to see how does different heights and

temperatures affect the spread diameter of the droplet.

Figure 6: The spread diameters of the water droplets, in different temperatures and heights.

Water Droplets

Sp

read

Dia

mete

r (m

m)

0

4.25

8.5

12.75

17

Height (cm)

0 4.5 9 13.5 18

200C240C280C320C360C

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Figure 7: The spread diameters of the water droplets, in different temperatures and heights

Figure 8: The spread diameters of the water droplets, in different temperatures and heights.

Isopropanol

Sp

read

Dia

mete

r (m

m)

0

4

8

12

16

Droplet Release Height (cm)

0 4.5 9 13.5 18

160C192C224C256C288C

Silicone Oil

Sp

read

Dia

mete

r (m

m)

0

4.5

9

13.5

18

Height (cm)

0 4.5 9 13.5 18

245C294C343C392C446C

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Acknowledgements:

I would like to thank my mentor professor Sigurdur Thoroddsen, and my co-mentor

Dr. Erqiang Li for helping me in lab. Also, I would like to thank my tutor Gunjan

Lakhlani for working with me on my papers and helping me out, and Ibrahim Al

Saffar my lab mate. Moreover, I would like to than Khalid Al Turki, Noorah Al

Faddagh, Saeed Al Surkhi, Saleh Al Hamad, Shaiza Sinha for their comments, advices

to my project, and their great help. I would also like to thank my counselor

Mohammed Al Senani for all his hard effort in helping me. Also, I thank Osama Al

Othman, Khalid Al Khumayis, Jumanah Al Sawaf, Hajar Al Reefi, Nawaf Al

Howaish, Reem Al Rabiah, and Husam Khawaja for all their help. I would also like to

acknowledge the help I got from the tutors and staff here at SRSI, and my mates;

Ahmed Tashkandi, Yousif Al Manaa, Dalal Bima.

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References:

1. Chen, R. et al. Nanowires for enhanced boiling heat transfer. Nano Lett. 9, 548–553

(2009).

2. Kim, H., Truong, B., Buongiomo, J. & Hu, L. On the effect of surface roughness

height, wettability, and nanoporosity on Leidenfrost phenomena. Appl. Phys. Lett. 98,

083121 (2011).

3. Photron - High Speed Camera Products (Photron - High Speed Camera Products)

http://www.photron.com/?cmd=product_general&product_id=7

4. Vakarelski, I. U. et al. Stabilization of Leidenfrost vapour layer by textured

superhydrophobic surfaces. Nature 489, 274–277 (2012).

5. Wang, C. H. & Dhir, V. K. Effect of surface wettability on active nucleation site

density during pool boiling of water on a vertical surface. J. Heat Transfer 115, 659–

669 (1993).

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