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International Journal of Applied Engineering Research ISSN 0973-4562 Volume 11, Number 22 (2016) pp. 10832-10837

© Research India Publications. http://www.ripublication.com

10832

High Speed Visualization and Analysis of Maximum Spreading of Water

Droplet during Impact on Hot Horizontal Surface

Suhaimi Illias1,*

, Mohamad Shaiful Ashrul Ishak1, Suhaila Hussain

1 and Khairul Azwan Ismail

1

1School of Manufacturing Engineering, Universiti Malaysia Perlis,

Pauh Putra Campus, 02600 Arau, Perlis, Malaysia.

Abstract

This paper discusses about droplet spreading phenomena

during impact with hot surface in the film boiling region. In

this experiment, aluminum material was selected as a test

surface. Degassed and distilled water was used as the test

liquid. The water droplet diameter, Do was 4.0 mm and the

droplet temperature was fixed at 16.0 °C. The droplet falling

height was 65.0 mm corresponding to droplet impact velocity,

v of 1.129 m/s. This unique droplet impact phenomenon was

recorded at 10,000 frames per second (fps) by using a high

speed camera. Results show that variation in surface

temperature does not affect the spreading dynamics of water

droplet in the film boiling region. The measured residence

time also agrees closely with the theoretical calculation.

Keywords: Droplet spreading, droplet impact, hot horizontal surface, high

speed camera

INTRODUCTION

In material manufacturing process, spray cooling can be

considered as one of the important factors that can affect the

quality of finish material. If the hot material is being cooled

too fast or too slow, it will give enormous effect on the

strength, hardness and ductility of the final material product.

In order to study this complex phenomenon of liquid-solid

contact, drop impact test have been studied widely by

numerous researchers using high speed camera [1-10].

These droplet behaviors are very unique due to its complexity

upon impact on hot surface. Furthermore, this droplet impact

experiment can give us better understanding on how to

enhance the heat transfer rate between the liquid and the solid

surface. Although it has been studied for decades, there are

still many new interesting findings that can be understood

through droplet impact experiments.

Richard et al. [1-2] studied the droplet bouncing phenomena

on solid surface without wetting it. From the experimental

work, they hoped that their findings could help to quantify the

efficiency of water repellent surfaces (super-hydrophobic

solids) and to improve water-cooling of hot solids. Tuan Tran

et al. [3] studied the spreading dynamics of droplets in the

film boiling region. They focused on the deformation of

droplets impacting on superheated surfaces in the film boiling

region. From their report, they suggested that the maximum

spreading diameter of impacting droplets on superheated

surfaces in the film boiling region is not strongly influenced

by the geometry of the structure and is higher than that on

unheated surfaces.

Takata et al. [4-7] also performed several experimental studies

regarding droplet impact on hot surface using high speed

camera. They conducted many experimental works in order to

investigate the behavior of small water droplets impinging

onto a hot surface and water spray cooling using a micro-jet.

They measured the solid-liquid contact time, maximum

spreading ratio on a hot surface and cooling time until the

surface temperature drops from 500 °C to 100 °C. In their

report, they concluded that the maximum spreading ratio gets

bigger with increase in the impinging velocity. They also

reported that the cooling rate increases with decrease in

droplet diameter mainly in low temperature regions [4].

Other researchers such as Araki et al. [8] studied theoretically

the deformation behavior of a liquid droplet impinging on a

hot metal surface. From their report, they have developed a

new correlation to evaluate the maximum extent of the spread,

Rmax, and time-integral of the heat transmitting base area.

Fujimoto et al. [9-10] studied the collision of single water

droplets using hot Inconel 625 alloy surface. They reported

that at the temperature of less than or equal to 300 °C, the

blowout of vapor bubbles occurred at the early stages for a

large droplet. At a surface temperature of 500 °C, the two

dimensionless deformation behaviors of the droplets were

very similar to each other.

Skripov et al. [11] reported that if the liquid temperature

reaches the limit of superheat due to abrupt heating, both

heterogeneous nucleation and homogeneous nucleation occur,

resulting in the nucleation of numerous vapor bubbles.

Wachters and Westerling [12] conducted an experimental

study of water droplet impacting on a polished gold surface.

The surface temperature was heated up to 400 °C. They

reported that when the surface temperature was increased to

about 400 °C, the fully spheroidal state appears to occur and

no bubbles were seen in the drop.

Suhaimi et al. [13] also performed several experimental works

regarding droplet impact experiment. In their report, they

concluded that the surface inclination angle, degree of liquid

subcooling and the initial temperature will give an effect on

the vapor film generation time in the film boiling region.

Other researchers [14-22] also have conducted several

experimental studies on droplet impact using high speed

camera in order to understand the boiling phenomena and

droplet impact characteristics on hot surfaces.

In this experiment, we studied and investigated the

hydrodynamic behaviors of impacting droplets on hot

horizontal aluminum surface. We focused our study on

spreading dynamics of water droplet until it reached

maximum spreading during the impact with a hot horizontal

surface in the film boiling region. We also compared the



10833

residence time of droplet using both experimental and

theoretical values.

EXPERIMENTAL APPARATUS

Figure 1 shows a schematic diagram of the experimental

apparatus which was also used in previous reports [15-16].

The heated surface was a disk material made of aluminum.

The disk material was of 8.0 mm in height and 30.0 mm in

diameter. The heated surface was retained levelly and was

heated from its periphery by an electric cartridge-type heater.

In the experiment, degassed and distilled water was used as a

test liquid. The water droplet diameter was retained at 4.0

mm. Meanwhile, the falling height was maintained at 65.0

mm corresponding to the impact velocity, v of 1.129 m/s. This

height is in the range that the droplet itself does not

disintegrate by the collision energy of the droplet. The

halogen lamp was used as a lighting system. The temperature

of the water droplet was kept at 16.0 ºC. On the other hand,

the temperature of the heating surface was measured by

attaching the thermocouple with ceramic adhesive at two

points on the hot aluminum surface. The high speed camera

was in horizontal position. The droplet spreading phenomena

was photographed in real time and was recorded by a frame

rate of 10,000 fps.

1. High speed video camera 2. Main memory 3. TV monitor 4. TV monitor controller 5. Computer 6. Pen recorder 7. Degassed

and distilled water 8. Circulating water at constant temperature 9. Halogen lamp 10. Droplet 11. Shutter 12. Cartridge heater 13.

Thermocouple 14. Material disc

Figure 1: Schematic diagram of the experimental apparatus

RELATED FORMULA

In this experiment, degassed and distilled water was used as a

test liquid. Water drop is released at the height of 65.0 mm.

This height is in the range that the droplet itself does not

disintegrate by the collision energy of the droplet. Neglecting

the viscosity and friction, one can estimate the velocity before

the impact using Eq. (1), where h and g are the droplet falling

height and gravity, respectively. The property of the

√ or thermal inertia is an important parameter related to

the interfacial temperature at the moment of contact between

two bodies of different temperatures (ρ, Cp and k, are the

density, specific heat and heat conductivity, respectively). The

interfacial temperature, Ti at the moment of contact is given

by Eq. (2) ;

√ (1)

(2)

where Twi is the initial surface temperature of the heated

surface, Tf is initial temperature of the droplet, the subscripts

w and f of the thermal property β indicates the case for the

heated surface and the droplet, respectively. The Weber



10834

number, We of the droplets, which is known as the ratio of the

inertia force to the surface tension force, is very important and

often regarded as the key parameter in droplet impact research

is given by Eq. (3) ;

(3)

where ρ, v, Do and σ are the liquid density, impact velocity of

the droplet, initial diameter of the droplet, and the surface

tension force, respectively. Meanwhile, for the residence time

or liquid-solid contact time , it can be estimated using Eq.

(4) [4,12,21] ;

(

)√

(4)

where ρ, Do and σ are the liquid density, initial diameter of the

droplet, and the surface tension force, respectively. From the

theoretical calculation, the liquid-solid contact time , for

this experiment was approximately 23.0 ms.

RESULTS AND DISCUSSION

Figures 2 and 3 show the images of boiling conditions at two

different initial surface temperatures, Twi of 300 °C and 320

°C during impact with hot surface at the same impact velocity,

v of 1.129 m/s. The numerical value under each photograph is

the elapsed time from the droplet impact. From Fig. 2, it is

observed that the liquid dispersion can still be seen after the

droplet impact on the hot surface at 4.0 ms up to 12.0 ms. This

droplet is then split into several number of smaller droplets as

shown in Fig. 2 at 14.0 ms up to 22.0 ms.

In contrast, a fully developed film boiling took place after the

impact at the initial surface temperature, Twi of 320 °C as

shown in Fig. 3. At the initial surface temperature, Twi of 320

°C, a stable vapor film layer was formed beneath the liquid. In

other words, the vapor film was developed between the liquid

and the solid surface. In this film boiling period, the droplet

does not touch the solid surface. This condition is also known

as film boiling. During this film boiling region, the existence

of vapour film between liquid and hot surface hinders heat

transfer from the surface. As can be seen in Fig. 3, after the

first impact, the droplet keeps spreading on the hot surface

without any liquid dispersion. Then, the mother droplet

reaches its maximum spreading on the hot surface.

From the experimental analysis, the maximum spreading of

the droplet was approximately 12.30 mm in diameter at about

7.0 ms after the first impact. Furthermore, this maximum

spreading situation seems to take place for about 1.0 or 2.0 ms

on the hot surface. Then, after the maximum spreading

occurred, the droplet begins to retract or recede on the hot

surface as shown in Fig. 3 beginning from 8.0 ms up to 22.0

ms. Finally, the mother droplet begins to bounce from the hot

surface at the time of 23.0 ms as shown in Fig. 3. This unique

bouncing phenomena continues at the time of 24.0, 26.0, 28.0

and 30.0 ms after the first impact. This bouncing phenomenon

occurs due to the vapour pressure released by the hot surface.

From Figs. 2 and 3, it can be concluded that the

hydrodynamics behaviour and deformation mechanics of the

droplet during evaporation, spreading and bouncing are

strongly dependent on the local or initial temperature of the

solid surface.

Figure 4 shows the diameter D of the deforming droplet

normalized by the droplet diameter Do versus time t measured

for droplets impacting upon hot surface for three different

surface temperatures of 320, 340 and 360 °C. As mention

before, the impact velocity, v was 1.129 m/s. From Fig. 4, it

is observed that the maximum spreading of droplet took place

at approximately 7.0 ms (12.30 mm) for initial surface

temperature, Twi of 320 °C as shown by the red broken line.

Meanwhile, for higher initial surface temperatures, Twi of 340

°C and 360 °C, the maximum spreading of droplet took place

at about 6.5 ms (11.60 mm) and 6.3 ms (12.00 mm) which is

very close to each other as shown by the black and blue

broken line in Fig. 4. From Fig. 4 also, it is observed that the

droplet keeps spreading smoothly on the hot surface without

any liquid dispersion until it reaches maximum spreading.

After that, the droplet begins to recede until it begins to

bounce from the surface. Furthermore, it is observed that the

time that it takes for the droplet to recede until it bounces

from the surface is almost double compared to the time that it

takes for the droplet to reach maximum spreading. The details

of initial surface temperature, droplet impact time and

maximum spreading measurement were shown in Table 1 for

easy understanding.

From the high speed images, it is observed that after the

mother droplet reaches the maximum spreading on the hot

surface, the droplet will enter the receding process or

retraction process and the droplet diameter will decrease

again, very close to its original initial diameter. Finally, the

mother droplet will start bouncing on the hot surface at about

23.0 ms (Twi = 320 °C), 22.8 ms (Twi = 340 °C) and 23.6 ms

(Twi = 360 °C) as shown by the red, black and blue arrows

respectively in Fig. 4. These unique bouncing phenomena

only took place at very high temperatures or in the film

boiling region.

In addition, based on theoretical calculation of Eq. (4), it was

estimated that the liquid-solid contact time, also known as

residence time, was approximately 23.0 ms. Therefore, this

experimental data for residence time shown in Fig. 4 proved

that the experimental data agrees closely with the theoretical

calculation in Eq. (4).



10835

4.0 mm

0.0 ms

2.0 ms

4.0 ms

6.0 ms

8.0 ms

10.0 ms

12.0 ms

14.0 ms

16.0 ms

18.0 ms

20.0 ms

22.0 ms

Figure 2: Boiling situation for initial surface temperature, Twi of 300 °C at the impact velocity, v of 1.129 m/s.

4.0 mm

0.0 ms

2.0 ms

4.0 ms

6.0 ms

8.0 ms

10.0 ms

12.0 ms

14.0 ms

16.0 ms

18.0 ms

20.0 ms

22.0 ms

24.0 ms

26.0 ms

28.0 ms

30.0 ms

Figure 3: Boiling situation for initial surface temperature, Twi of 320 °C at the impact velocity, v of 1.129 m/s.



10836

Figure 4: Diameter D of the deforming droplet normalized by the initial droplet diameter Do versus time t measured for droplets

impacting upon three different surface temperatures. The diameter of mother droplet Do = 4.0 mm, droplet impact velocity v =

1.129 ms-1

, and Weber number We = 70.

Table 1: Initial surface temperature, droplet impact time and maximum spreading

Initial surface

temperature, Twi (°C)

Time, t

(ms)

Maximum spreading of droplet, D

(mm)

Initial droplet diameter,

D0 (mm)

D/D0

320 7.00 12.30 4.00 3.10

340 6.50 11.60 4.00 2.90

360 6.30 12.00 4.00 3.00

CONCLUSION

From the present study, the maximum spreading of droplets

on heated surfaces will occur at almost the same period even

when the initial surface temperatures are different. From the

high speed images, it was observed that after the droplet

achieved its maximum spreading value, the droplet will enter

the receding or retraction process and the droplet diameter

begins to decrease again, very close to its initial diameter. The

droplet spreading data also shows a similarity pattern even at

different surface temperatures. Finally, the droplet will start to

bounce from the hot surface. These unique bouncing

phenomena only occur in the film boiling region where there

is no contact between liquid and solid surface. At a very high

temperature, a stable vapor film was formed beneath the

liquid which hindered heat being transferred from the surface.

From the experimental results, it shows that variation in

surface temperature will not give a significant effect to the

spreading dynamics of water droplet in the film boiling

region. In addition, the measured residence time agrees

closely with the theoretical calculation.

ACKNOWLEDGEMENT

The authors would like to thank to Heat Transfer Laboratory,

Gunma University, Japan for providing research facilities and

space to undertake this work. Special thanks also to Universiti

Malaysia Perlis (UniMAP) and Ministry of Education,

Malaysia for providing scholarship during the author’s

graduate study at Gunma University, Japan.

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10837

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