Group Title: 7th International Conference on Multiphase Flow - ICMF 2010 Proceedings
Title: P2.85 - Effects of Droplets Impingement on a Heated Plate Composed of Hydrophilic Surfaces and Super-hydrophilic Surfaces
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 Material Information
Title: P2.85 - Effects of Droplets Impingement on a Heated Plate Composed of Hydrophilic Surfaces and Super-hydrophilic Surfaces Droplet Flows
Series Title: 7th International Conference on Multiphase Flow - ICMF 2010 Proceedings
Physical Description: Conference Papers
Creator: Yamagishi, T.
Morita, K.
Hagiwara, Y.
Kitagawa, A.
Publisher: International Conference on Multiphase Flow (ICMF)
Publication Date: June 4, 2010
 Subjects
Subject: droplets
bio-inspired surface
natural convection
 Notes
Abstract: Adding water droplets to air has been used to improve the cooling efficiency for warm surfaces in various manufacturing procedures and equipment. When the surfaces are upward-horizontal, a water film is generated by the droplets deposited onto the surfaces. If the heat flux is low, sensible heat transfer through the film is predominant. Thus, the film acts as thermal resistance. In order to reduce this thermal resistance, we have created a new functional surface made of glass plate. The surface consists of a hydrophilic bare part and a super-hydrophilic part covered with a nano-pin structure. We have observed the natural convection flow and measured temperature field on and above the plate. The results show that the droplets deposited onto the plate move promptly from the edge of the hydrophilic part to the super-hydrophilic part. Furthermore, it is found that the local, instantaneous heat transfer coefficient in the case of the new surface can be higher than that of the hydrophilic surface.
General Note: The International Conference on Multiphase Flow (ICMF) first was held in Tsukuba, Japan in 1991 and the second ICMF took place in Kyoto, Japan in 1995. During this conference, it was decided to establish an International Governing Board which oversees the major aspects of the conference and makes decisions about future conference locations. Due to the great importance of the field, it was furthermore decided to hold the conference every three years successively in Asia including Australia, Europe including Africa, Russia and the Near East and America. Hence, ICMF 1998 was held in Lyon, France, ICMF 2001 in New Orleans, USA, ICMF 2004 in Yokohama, Japan, and ICMF 2007 in Leipzig, Germany. ICMF-2010 is devoted to all aspects of Multiphase Flow. Researchers from all over the world gathered in order to introduce their recent advances in the field and thereby promote the exchange of new ideas, results and techniques. The conference is a key event in Multiphase Flow and supports the advancement of science in this very important field. The major research topics relevant for the conference are as follows: Bio-Fluid Dynamics; Boiling; Bubbly Flows; Cavitation; Colloidal and Suspension Dynamics; Collision, Agglomeration and Breakup; Computational Techniques for Multiphase Flows; Droplet Flows; Environmental and Geophysical Flows; Experimental Methods for Multiphase Flows; Fluidized and Circulating Fluidized Beds; Fluid Structure Interactions; Granular Media; Industrial Applications; Instabilities; Interfacial Flows; Micro and Nano-Scale Multiphase Flows; Microgravity in Two-Phase Flow; Multiphase Flows with Heat and Mass Transfer; Non-Newtonian Multiphase Flows; Particle-Laden Flows; Particle, Bubble and Drop Dynamics; Reactive Multiphase Flows
 Record Information
Bibliographic ID: UF00102023
Volume ID: VID00511
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: P285-Yamagishi-ICMF2010.pdf

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Paper No 7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30-June 4, 2010



Effects of Droplets Impingement on a Heated Plate Composed of Hydrophilic Surfaces and
Super-hydrophilic Surfaces


T. Yamagishi*, K. Morita*, Y Hagiwara* and A. Kitagawa*


Department of Mechanical and System Engineering, Kyoto Institute of Technology, Kyoto, 606-8585, Japan
m8623056 i edu kit ac jp. k6123078 7edu kit ac jp. olshi u kil ac ip and kiktL'\a u kitl c ip

Keywords: Droplets, measurements, bio-inspired surface, natural convection




Abstract

Adding water droplets to air has been used to improve the cooling efficiency for warm surfaces in various manufacturing
procedures and equipment. When the surfaces are upward-horizontal, a water film is generated by the droplets deposited onto
the surfaces. If the heat flux is low, sensible heat transfer through the film is predominant. Thus, the film acts as thermal
resistance. In order to reduce this thermal resistance, we have created a new functional surface made of glass plate. The surface
consists of a hydrophilic bare part and a super-hydrophilic part covered with a nano-pin structure. We have observed the
natural convection flow and measured temperature field on and above the plate. The results show that the droplets deposited
onto the plate move promptly from the edge of the hydrophilic part to the super-hydrophilic part. Furthermore, it is found that
the local, instantaneous heat transfer coefficient in the case of the new surface can be higher than that of the hydrophilic
surface.


Introduction

Recently, many highly effective methods for heat transfer
have been developed as part of various manufacturing
procedures and equipment from the viewpoint of saving
energy. The natural convection of air, for which energy input
is unnecessary, is not an exception. Adding water droplets to
air is one promising method for improving the cooling
efficiency for the natural convection of warm surfaces.

In the case of horizontal surfaces, heat transfer associated
with deposited droplets on the surfaces is extremely
important for evaluating the heat transfer enhancement and
thus the cooling efficiency. If the surface temperature is
high, the latent heat transfer due to evaporation from the
droplets is predominant. Takimoto(') et al. conducted
experiments by using an aluminum-nickel alloy plate with
super-minute fins, and showed that the plate enhanced the
evaporation of droplets onto it. On the other hand, if the
surface temperature is low, sensible heat transfer in a liquid
film is predominant. The film is formed by the deposited
droplets. As far as the present authors know, no study has
been carried out for the enhancement of sensible heat
transfer.

The present authors have paid attention to functional
surfaces, which control the motion of deposited droplets or
film generated by the droplets. We have obtained hints from
the tenebrionid beetle living in the Namib Desert, which
collects drinking water from early-morning fog by using its
special surface(2). The elytra of the beetle are covered with
an array of bumps, the peaks of which are smooth and


hydrophilic. The fog water droplets settle on the peaks,
forming big drops. Other areas, including the sides of the
bumps, are covered by a microstructure coated in wax.
These areas are hydrophobic. The fog water droplets are
blown from near these areas to the hydrophilic peak regions.
Eventually, the drops at the peaks are detached and roll
down the tilted surface to the beetle's mouth. Zhai et al.'3
mimicked the structure by creating a hydrophilic surface
with super-hydrophobic coatings. They referred the
potential applications of such surfaces to a controlled drug
release coating and lab-on-chip devices. However, as far as
we know, there have been no attempts at creating functional
surfaces to control droplet motion and thus enhance the
natural convection of horizontal plates.

In the present study, we prepare a flat heating plate and a
new heating plate with a functional surface. We supply
water droplets to enhance heat transfer. We conduct natural
convection flow visualization above the heating plates and
temperature measurement of the air and plate surfaces. The
effects of the functional surface on the motion of the
deposited droplets, and thus also the heat transfer, are
discussed.


Nomenclature


gravitational acceleration (in iS-)
local heat transfer coefficient (W/m2K)
thermal conductivity (W/mK)
length of plate (mm)
number of samples





Paper No


Nux local Nusselt number
q, surface heat flux (W/m2)
Ra ~ local modified Rayleigh number
T, instantaneous temperature (C)
T, time-averaged temperature of (C)
Trms RMS value of fluctuating temperature (C)
T, surface temperature (0C)
T, ambient temperature (C)
t time (s)
W width of plate (mm)
x longitudinal distance from the plate edge (mm)
y vertical distance from the plate surface (mm)
z spanwise distance from the centre of the plate edge
(mm)

Greek letters
a thermal diffusivity (m2/s)
/p coefficient of volume expansion (1/K)
v kinematic viscosity (mr/s)


Experimental method

The apparatus is shown in Fig. 1. The heating plate made of
glass is set at the bottom of an acrylic-resin container
(160mm X 300mm X 240mm) in order to reduce the
influence of disturbance on the natural convection as much
as possible. The plate is W=26mm in width, L=76mm in
length, and 1mm in thickness. The Peltier device is used as a
heat source and is located below the plate. It is thought that
the air is heated by the device through the plate under
uniform heat flux condition. In order to restrict the area of
natural convection, we arranged two parallel guide plates,
which do not touch the heating plates and the Peltier device.

The air temperature is measured with eight K-type
thermocouples whose diameter is 0.1mm. These
thermocouples are fixed onto one of the guide plates at a
distance of 2mm in the vertical direction and 20mm in the
horizontal direction. The electromotive force of each
thermocouple is recorded simultaneously with a PC via a
data logger. The plate surface temperature Tw is measured
with an infrared camera (FLIR Systems, SC640). A
calibration is carried out between the temperature measured
with the infrared camera and that measured with a
thermocouple attached to the plate surface. This is because
the emissivity of the surface is not known a priori. The
accuracy of the surface temperature increases due to this
calibration.

The heat flux from the plate surface to the air is measured
by using a heat flux meter (Kyoto electronic industry,
HFM-201). The measurement was carried out three times
for a period of five minutes using 1Hz sampling. The
average values of heat flux were 356 and 910 W/m2.

A micro syringe is used for supplying water droplets. In
order to minimize the influence of droplet supply on the
natural convection, only the micro needle of the micro
syringe is inserted into the inside of the container through a
small hole in its sidewall.


7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30-June 4, 2010

We adopt the following two plates as the heating plates: one
is a bare plate whose surface is hydrophilic (see Fig. 2(a)),
and the other is a partially-modified plate (see Fig. 2(b)).
The chemical-bath deposition method was adopted for the
surface modification. Approximately 30 % of the surface is
covered with numerous pins of nano-order 4). This modified
part is super-hydrophilic. Droplets on this part move
immediately after their deposition.

The natural convection flow is visualized with smoke. The
images of smoke were recorded with a high-speed CMOS
camera (Photron, FASTCAM-1024PCI) controlled by a PC.


--W


160mm


~~Ktrn


300mm

1:Test plate 2:Peltier device 3:Acrylic-resin container
4:Acrylic-resin guide 5:Micro syringe 6:Thermocouple

Fig. 1 Apparatus


hydrophilic part


76
(a) Plate (a)


hydrophilic part superhydrophilic part


26 10
I 59
76
(b) Plate (b)


Fig. 2 Glass plates


_______S


14^~----
I L1I


I T &00


240mm


T/.






7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30-June 4, 2010


Natural convection without droplet
The local heat transfer coefficient h., the local Nusselt
number Nu. and the local modified Rayleigh number Ra *
are defined as follows:


S10


h qw
(T. T-)
Nu= (.
k
S* -'' ,.. C
aa k


where the physical properties are evaluated from the film
temperature. Figure 3 shows the relationship between the
local Nusselt number and the local modified Rayleigh
number. The experimental result obtained by Kimura et al.(4
is also shown in this figure. Nux in the present study is
higher than that in reference (4) in the whole range of Ra x.
This is due to the difference in the experimental conditions:
the size and heat capacity of the heating plate and the heat
flux.

Figure 4 illustrates a schematic drawing of flow field
obtained from the video images of the visualized flow over
Plate (a) in the case of q, = 356 W/m2. The local flow can be
categorized as follows:

(I) Intruding flow from the edge of the plate along the
plate's surface
(II) Approaching flow (II) from above the plate edge to flow
(I)
(III) Confluence of flow (I) and flow (II)
(IV) Rising flow due to the buoyancy effect near the centre
of the plate
(V) Induced flow by the rising flow

The average temperature distribution above Plate (a) in the
case of q, = 356 W/m2 is indicated in Fig. 5. The origin of
the Cartesian coordinate is located at the middle of the
left-hand edge. The result is shown in the range of 0 < x <
38 mm due to the symmetry of the flow.

The average temperature decreases with an increase in y at
any x. Furthermore, the temperature increases with x in the
range ofx = 0 13 mm at any y. This is due to the heating of
flow (I). The temperature decreases with x in the range of x
= 13 19 mm particularly near the surface. This is due to the
approach of the cool flow (II) to the heated flow (I). The
temperature increases with x in the range of x = 19 34 mm
at any y. This is caused by the heating of flow (III). The
temperature decreases slightly with x in the range of x = 34 -
38 mm particularly far from the surface. This is due to the
cool flow (V) being integrated into warm the upward flow
(IV).

Figure 6 shows the RMS value T,, of the fluctuating
temperature. The definition of Tms is as follows:

T,, 1 (T, T, (4)


1 10 10' 10' 10' 10o 10 10s
Rax*
Fig. 3 Nusselt number as a function of Rayleigh number


ambient fluid

(V)

( I)


S9 ")

X


38 76


Fig. 4 Pattern diagram of natural convection


55

50

45

S40
- 35

30

25

20


E y=2mm
Ay=4mm
*y=6mm *E E
y=8mm


*e
If *EE. E *" A A A

AA *





0 10 20 30 40


x(mm)
Fig. 5 Temperature profile of natural convection flow


2
1.8
1.6
1.4
1.2
C,
1
0.8
0.6
0.4
0.2


0 10 20 30 40
x(mm)
Fig. 6 RMS of temperature in natural convection flow


Paper No


Result and discussion


A f9l10 W/i
* q.=356W/m'

- imura




I






7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30-June 4, 2010


The RMS values do not change so much in the range of x =
0 13 mm at any y. This is because the flow (I) is still
laminar. The RMS values increase with x in the range ofx =
13 30 mm at any y. This is due to the confluence of flow
(I) and flow (II) and the confluence of flow (III) and flow
(V). The RMS values drop sharply in the range of x = 30 -
34 mm. This is due to the development of the upward flow
(IV). The RMS values increases with x in the range of x =
34 38 mm. This is caused by the confluence of flow (IV)
and flow (V).

Influence of deposited droplets on natural convection
Pure water 70pl in total volume and 21 C in temperature
was deposited at x =41mm and z =0mm of Plate (a). Figure
7 shows a typical image of visualized natural convection
over the plate. Figure 8 illustrates a schematic drawing of
flow field obtained from video images of the visualized flow
over Plate (a) with deposited droplets. It is found by
comparing Fig. 8 with Fig. 4 that the flow along the plate
surface is more predominant in the wetted case than that of
the dry. Furthermore, the region of upward flow (IV) due to
buoyancy is more restricted in the vicinity of the wetted
region than that above the dry surface.

The average temperature distributions at y = 2, 4 mm above
Plate (a) in the period of 30s to 130s are compared with
those without droplets in Fig. 9. These distributions in the
range ofx < 21 mm are similar to those without droplets. On
the other hand, the distributions in the range of 21 mm < x <
38 mm are lower than those without droplets.

The RMS values of the temperature fluctuation
corresponding to Fig. 9 are compared with those without
droplets in Fig. 10. The RMS values in the range of x < 19
mm are similar to those without droplets. On the other hand,
the distributions in the range of 21 mm < x < 32 mm are
lower than those without droplets.


0 10 20 30 40
x(mm)
Fig. 9 Temperature profile of natural convection
with droplet
2
1.8 -- y=2_with droplet
1.6 -A-- y=4_with droplet
1.4 -0--y=2
1.2
-A- y=4

0.8
0.6
0.4
0.2

0 10 20 30 40
x(mm)
Fig. 10 RMS of temperature in natural convection flow
with droplet

The results shown in Figs. 9 and 10 indicate that the flow
(IV) in the natural convection is attenuated by the stationary
droplets deposited on the plate. Thus, the control of droplet
mobility on the plate is important for enhancing the natural
convection heat transfer.

The natural convection flow above Plate (b) was visualized
in the case where droplets 350pl in total volume were
deposited at x = 15 mm and z = 0 mm at an interval of 2
seconds. It was observed that the deposited droplet moves
immediately after the part of the droplet comes into contact
with the super-hydrophilic part.


0 19 38 X


Fig. 7 Snapshot of visualized natural convection flow
with droplet (x =41mm)
ambient fluid
(V)


\ y x
L_


19 38


76


Fig. 11 Natural convection flow above Test plate (b)


Fig. 8 Pattern diagram of natural convection with droplet


Paper No


~.=. ._.





7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30-June 4, 2010


Figure 11 displays a typical image of visualized natural
convection over Plate (b). The natural convection flow in
this figure is similar to that shown in Fig. 7 for Plate (a)
except for the wider area for flow along the surface. This
modification is due to the motion of deposited droplets.
However, this modification is not so noticeable.

Surface temperature measurement
Figure 12 shows the time changes in the surface temperature
of Plates (a) and (b) in the case of q, = 356 W/m2. 15
droplets of 350pl in total volume were dropped at x = 15
mm and z = 0 mm every two seconds. The arrows in this
figure depict the instants of dropping. Note that the surface
temperature is uncertain because the emissivity of the water
surface is not well-known. The surface temperature reaches
the lowest value when the first droplet collides with the
surface. In the case of Plate (a), the temperature increases
with time, though it drops slightly at the instant when the
following droplets are added. This shows that the liquid film
formed by the droplets is heated regardless of the deposition
of cool successive droplets (See Fig. 13). Thus, the liquid
film becomes a form of thermal resistance.

On the other hand, the temperature recovers more noticeably
until t = 22s in the case of Plate (b). This is because the
droplet moves immediately after the part of it comes into
contact with the super-hydrophilic part. Even during this
period, however, the peak temperature decreases gradually.
This suggests that a small amount of liquid still remains
after the droplet moves. In the period of t = 22 36s, the
temperature does not change remarkably. This shows that
the amount of remaining liquid is still little and does not
become a form of thermal resistance during the period.

55 ----

50

45

40
I .

30 *

25 4 :tttttttt : t


Fig. 12


(b) Plate (b)






(c) Plate (b)

Fig. 14 Thermal images with temperature measuring
positions

The temperature of a surface, whose emissivity is known a
priori, can be measured accurately through the air with a
infrared camera. The emissivity is evaluated from the
calibration, in which the surface temperature is measured
simultaneously with the other technique. Thus, any error in
the measurement is small for the temperature of the plate
surface, while it is bigger for the temperature of the water
surface. Considering this, we measured the temperature at
several positions on the peripheral of the wetted area on the
plate. Figures 14(a), (b) and (c) demonstrate these points
superimposed on the thermal images. The positions are
shown by the symbol of x in the figure. In the case of Plate
(a), the wetted area increases with time due to the
accumulation of droplets supplied successively onto the area.
Thus, the measurement positions move from their original
locations as time proceeds. The positions of the symbol at a
certain time are shown in Fig. 14(a). On the other hand, in
the case of Plate (b), the wetted area becomes smaller
quickly when it reaches its maximum. This is due to the fact
that some fraction of the deposited water always moves on
the super-hydrophilic part of the surface. As shown in Figs.
14(b) and (c), the measurement positions are unchanged.


0 10 20 30 40 50 The time changes in the average temperatures at the
t(s) measurement positions are indicated in Fig. 15. The average
Time changes in temperature of wetted surfaces temperature for Plate (a) is found to rise to t = 15s. This
shows that the deposited droplets are warmed and become
forms of thermal resistance. The average temperature is
nearly constant during the period until t = 30s. This shows
a ._ that the cooling by droplets is balanced with the heating.
S Eventually, the average temperature increases gradually.


(a) Early stage
(a) Early stage


(b) Later stage


IC
25we


Fig. 13 Thermal images with droplets on Plate (a)


The average temperature for Plate (b) decreases until it
reaches t = 30s and increases afterwards. It is higher than
that for Plate (a) until t = 22 s. This is because the
measuring locations are almost always dry due to the quick
motion of the droplets. After t = 22 s, the temperature is
lower than that for Plate (a). This is due to the small amount
of liquid remaining on the surface.


Paper No


V






Paper No


60

50 *****

40 *oo

a 30 -
I-
20
S (a)
10 i (b)
A ______ i _____ i ______ i ____


7th International Conference on Multiphase Flow
ICMF 2010, Tampa, FL USA, May 30-June 4, 2010

functional glass plate whose surface consists of a
hydrophilic part and a super-hydrophilic part. The main
conclusions obtained are as follows:

(1) The flow visualization for natural convection with
smoke and temperature measurement on and above the
heating plate with an infrared camera and the rake of
thermocouples are effective for examining the effect of
deposited droplets on the flow and heat transfer of
natural convection.


0 10 20 30 40 (2) The flow field and temperature field are modified by the
t(s) presence of deposited droplets. In particular, the upward
Fig. 15 Time change in the temperature at the peripherals of flow due to buoyancy is restricted to the area near the
wetted area droplets; in addition, the temperature fluctuation due to
the flow is attenuated. Furthermore, the flow along the


surface is predominant.


(3) Even if the cool droplets are supplied successively, a
thin liquid film, formed by the deposited droplets,
becomes a form of thermal resistance on the hydrophilic
surface. The new functional surface, composed of the
hydrophilic part and the super-hydrophilic part, is
effective for the motion of droplets immediately after
their deposition. This motion enhances the heat transfer.

Acknowledgements


10 20 30 40 The authors acknowledge Dr. Hosono at AIST for his kind
t (s) instruction for producing the nano-pin surfaces.


Fig. 16 Local heat transfer coefficient

We estimate the local heat transfer coefficient by assuming
that the wall temperature T, at the center of the wetted area
on the heating surface is equal to the averaged temperature
measured at the points on the peripheral of the area. Figure
16 depicts the time changes in the local heat transfer
coefficient hx for Plate (a) and Plate (b). The coefficient in
the case without droplets is also shown in this figure.

In the case of Plate (a), hx decreases with an increase in time.
This clearly shows that the liquid film formed by the
deposited droplets is effective when the number of droplets
is low and that the film becomes a form of thermal
resistance as time proceeds. On the other hand, in the case
of Plate (b), h, increases with an increase in time until t =
28.5s. In the early stage (t < 10s), the small amount of liquid,
remaining on the surface after the movement of the droplet,
is the thermal resistance. However, the successive droplet
carries this remaining warm liquid when it moves as time
proceeds. Since the droplet supply is used up at t = 28s, h,
decreases afterwards. If the droplet supply is not used up,
the heat removal process caused by the motion of the
droplet continues. Thus, it is expected that the local heat
transfer coefficient further increases or at least maintains
high values.

Conclusions

We carried out experiments on the effects of deposited
droplets on the flow and temperature of natural convection
above small horizontal heating plates. We created a new


References

(1) Takimoto, A., Matsukawa, M. and Kosaka, A.,
Effectiveness of ultra-fine structure surface for
enhancement of mist cooling heat transfer (Evaporation
experiment of a single droplet) (in Japanese), Trans.
Japan Soc. Mech. Engr Series B, vol. 67 (2001), 1445 -
1450.
(2) Parker, A. R, and Lawrence, C. R., Water capture by a
desert beetle, Nature, vol. 414 (2001), No. 6859, 33 34.
(3) Zhai, L. et al., Patterned superhydrophobic surfaces:
toward a synthetic mimic of the Namib Desert beetle,
Nano Letters, vol. 6 (2006), 1213 1217.
(4) Kimura, F, Joba, Y., and Kitamura, K., Heat transfer and
fluid flow natural convection over heated, horizontal
plates (effect of Prandtl numbers) (in Japanese), Trans.
Japan Soc. Mech. Engr Ser B, vol.68 (2002),
1515-1522.
(5) Hosono, E., Fujihara, S. Honma, I. and Zhou, H.,
Superhydrophobic perpendicular nanopin film by the
bottom-up process, Journal of American Chemical
Society, vol. 127 (2005), 13458 13459.


***

* *

******* o O *


(a) with droplet
(b) with droplet
without droplet




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