1. 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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2. 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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3. 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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4. 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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5. !
2. Methodology:
2.1 Experiment
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6. 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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7. #
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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8. 3. Results
3.1 Leidenfrost Temperature
!
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.
Temperature
0
63
125
188
250
Silicone Oil Water IPA
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9. 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
SpreadDiameter(mm)
0
4.25
8.5
12.75
17
Height (cm)
0 4.5 9 13.5 18
200C
240C
280C
320C
360C
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10. 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
SpreadDiameter(mm)
0
4
8
12
16
Droplet Release Height (cm)
0 4.5 9 13.5 18
160C
192C
224C
256C
288C
Silicone Oil
SpreadDiameter(mm)
0
4.5
9
13.5
18
Height (cm)
0 4.5 9 13.5 18
245C
294C
343C
392C
446C
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11. 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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12. 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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