Group Title: 7th International Conference on Multiphase Flow - ICMF 2010 Proceedings
Title: 7.2.4 - Experimental Investigation of the Hydrodynamics of Non-Wetting Droplets
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 Material Information
Title: 7.2.4 - Experimental Investigation of the Hydrodynamics of Non-Wetting Droplets Particle Bubble and Drop Dynamics
Series Title: 7th International Conference on Multiphase Flow - ICMF 2010 Proceedings
Physical Description: Conference Papers
Creator: Chatzikyriakou, D.
Zeng, Y.J.
Hale, C.P.
Walker, S.P.
Hewitt, G.F.
Publisher: International Conference on Multiphase Flow (ICMF)
Publication Date: June 4, 2010
 Subjects
Subject: non-wetting droplets
disintegration
Leidenfrost
 Notes
Abstract: When a water droplet impinges on a high temperature solid surface it bounces off the surface without wetting it. That is due to the rapid formation of a vapour film between the droplet and the solid (Leidenfrost phenomenon). This phenomenon is believed to occur in the precursory cooling of the fuel rods in a Pressurised Water Reactor (PWR) after a Loss of Coolant Accident (LOCA). The cooling efficiency of such an interaction is of vital importance for the safety of the nuclear reactor. The heat removal ability of a non-wetting droplet is affected by the hydrodynamics of the collision. For high impact velocities and high wall temperatures, beyond the Leidenfrost temperature, the formation of secondary droplets can be observed as the droplets disintegrate upon impingement. The dynamics of such collisions can be described by the Weber (We) number which is the ratio of the gravitational and the surface tension forces. There is a critical value of that number above which the droplet is bound to disintegrate upon impingement. In the present study, the hydrodynamics of such phenomena are investigated using high speed photography. Depending on the material of the surface, the droplet velocity of approach and the droplet angle on approach different regimes are observed. The described experiments have been conducted using water droplets with diameter of 2mm and for moderate Weber numbers in the range of 0.5 to 88. Image processing is used to obtain quantitative results regarding the time of droplet proximity to the wall, the spreading of the droplet and the loss of momentum during the droplet-hot wall interaction for the cases of droplet rebound without break-up. The critical Weber number for normal collisions is found to be around 40 when the substrate is stainless steel whereas for aluminum the critical Weber number is found to be over 88.
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
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Resource Identifier: 724-Chatzikyriakou-ICMF2010.pdf

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


Experimental Investigation of the Hydrodynamics of Non-Wetting Water Droplets


D. Chatzikyriakou*, Y.J. Zengt, C.P. Halet, S.P. Walker* and G.F. Hewittt

Department of Mechanical Engineering, Imperial College, London, SW7 2AZ, UK
T Department of Chemical Engineering and Chemical Processes, Imperial College, London, SW7 2AZ, UK
d.chatzikyriakou05 @imperial.ac.uk


Keywords: non-wetting droplets, disintegration, Leidenfrost




Abstract

When a water droplet impinges on a high temperature solid surface it bounces off the surface without wetting it. That is due to
the rapid formation of a vapour film between the droplet and the solid (Leidenfrost phenomenon). This phenomenon is
believed to occur in the precursory cooling of the fuel rods in a Pressurised Water Reactor (PWR) after a Loss of Coolant
Accident (LOCA). The cooling efficiency of such an interaction is of vital importance for the safety of the nuclear reactor. The
heat removal ability of a non-wetting droplet is affected by the hydrodynamics of the collision. For high impact velocities and
high wall temperatures, beyond the Leidenfrost temperature, the formation of secondary droplets can be observed as the
droplets disintegrate upon impingement. The dynamics of such collisions can be described by the Weber (We) number which is
the ratio of the gravitational and the surface tension forces. There is a critical value of that number above which the droplet is
bound to disintegrate upon impingement. In the present study, the hydrodynamics of such phenomena are investigated using
high speed photography. Depending on the material of the surface, the droplet velocity of approach and the droplet angle on
approach different regimes are observed. The described experiments have been conducted using water droplets with diameter
of 2mm and for moderate Weber numbers in the range of 0.5 to 88. Image processing is used to obtain quantitative results
regarding the time of droplet proximity to the wall, the spreading of the droplet and the loss of momentum during the
droplet-hot wall interaction for the cases of droplet rebound without break-up. The critical Weber number for normal collisions
is found to be around 40 when the substrate is stainless steel whereas for aluminum the critical Weber number is found to be
over 88.


1 Introduction

After a Design-Basis Loss of Coolant Accident (LOCA) in a
Pressurised Water Reactor (PWR), cold water is introduced
to the core by bottom-up reflooding in order to cool down
the increasingly hot fuel rods. At the rewetting front, boiling
generates slugs of liquid, which are swept upwards, break
up and form drops. Above the rewetting front, the
conditions are characterized by a flow of superheated
vapour between even hotter metal surfaces, with a
population of small saturated droplets entrained in the
vapour flow. Cooling of the fuel by this droplet-steam
mixture above the rewetting front ("precursory cooling") is
very important in preventing fuel failures before the final
quenching by the rising liquid water.
Cooling is mainly achieved by convective heat transfer to
the vapour, with the vapour in turn being cooled by the
evaporation of the entrained saturated droplets. The
cladding temperatures are such that wetting of the metal by
the entrained droplets does not occur, with droplets instead
rebounding from a cushion of vapour generated between the
droplet and the surface while they are close together (the
Leidenfrost phenomenon, Leidenfrost (1966)). Whilst these
interactions are brief and involve only a small area, it may
be that in total the multiple droplet-wall interactions are able


to extract a worthwhile amount of heat.
In order to characterise the behaviour of the droplets under
such circumstances, understanding of phenomena such as
the momentum change and the heat transfer between the
droplet and the solid surface has to be gained. The processes
accompanying the interaction of a non-wetting droplet with
a hot surface (spreading, recoiling, bouncing and
evaporating) are undoubtedly complicated and depend on a
number of parameters. These include the droplet velocity,
droplet angle of approach, droplet temperature, solid
temperature, ambient conditions and surface roughness
(Figure 1).
In the present study the hydrodynamics of such phenomena
are investigated using high speed photography. Depending
on the material of the surface, the droplet velocity of
approach and the droplet angle on approach different
regimes are observed. Initially, a critical threshold is
identified (in the form of a non-dimensional number, the
Weber number) beyond which droplets disintegrate. Then,
the droplet dynamics for the case of vertical impingement
on a very hot stainless steel dry surface are investigated.
In section 2, a review of previous experimental work in the
field of non-wetting droplet dynamics is presented with an
emphasis on the parameter of droplet approach velocity. In
section 3, the experimental arrangement is described.









Nomenclature

D Diameter (mm)
1 Characteristic length (m)
R Radius (mm)
T Temperature (C)
V Droplet velocity (ms-1)

Greek letters

0 angle of droplet approach (0)
p density (kgm 3)
a surface tension coefficient (Nm 1)
T droplet resident time (ms)

Subscripts

drop referring to the droplet
max maximum
o initial
solid referring to the solid subsrate


In section 4, the results of the present study are presented
and discussed. In section 5, there is some discussion about
the conclusions of the current research project and future
work plans are proposed.


Droplet Ambience

Temperature Liquid Vapour
material presence
Angle --\
4- Velocity Air Droplet
Size -impinging
Son very hot
wall and
Temperature bouncing
Hydrophobicity Roughness off

Surface

Figure 1: Cause and effect diagram for the case of the
interaction of a non-wetting droplet with a dry, solid surface.


2 Literature Review

Extensive research work has been carried out on the effect
of the droplet velocity on the droplet impingement. The
effect of the droplet velocity has been studied in conjunction
with the diameter and the angle of impingement effects
since these parameters are inherently connected. The
roughness of the solid surface has also been studied quite
extensively.
Wachters and Westerling (1966), conducted experiments
with droplets impinging on a 300 inclined surface (or 600 to
the horizontal), and showed that the component of velocity
parallel to the wall did not change significantly during the
impingement. The normal component of the velocity
seemed to characterise the behaviour of the droplet. This led
them to the definitions of a Weber number in terms of this
normal velocity, used for the classification of the droplet
impingement behaviour, or in other words impact regimes.
In the same study, they found that the residence time of the


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

droplet on the hot surface decreased with increasing
velocity and that larger impact velocity resulted in thicker
vapour layer between the droplet and the solid substrate.
Pedersen (1970) found that in the non-wetting state there is
a minimum velocity below which the droplets deform
considerably without break-up upon impingement. Pedersen
(1970) also showed that the approach velocity has a
significant influence on heat transfer; when the approach
velocity increases then heat transfer increases.
Yao and Cai (1988) showed experimentally that, for large
angles of impingement, the critical Weber number (based on
the normal velocity component) was found to be around 50,
i.e. rather lower than the values obtained by Wachters and
Westerling (1966). This, however, did not seem to hold for
small impingement angles, for which the critical Weber
number was demonstrated to be in the range of 20-30.
Beyond these values, disintegration of the droplets was
observed.
Anders et al. (1993) studied the effect of the variation of
velocity on the collision process using ethanol droplets.
They examined several droplet diameters (91-207ptm) with
a wall temperature of 2800C, compared to the Leidenfrost
temperature of 78.50C for ethanol, defined always on a
thermodynamic basis. The temperature of the droplets was
250C and the angle of approach was approximately 45.
Keeping all other experimental conditions the same and
altering only the velocity of approach of the stream of
droplets led to a change in the rebound behaviour. For a low
Weber number (again based on the normal velocity
component) the droplet simply rebounded; at a higher
Weber number the consistent formation of a small
secondary droplet was observed.
Hatta et al. (1994) obtained a value for the critical Weber
number regarding droplet break-up. That number was
approximately 50, clearly lower than that obtained by
Wachters and Westerling (1966). Ko and Chung (1996)
revealed that the break-up probability increases with
increasing velocity.
Kang and Lee (2000) studied the effect of the impingement
angle on the droplet impact behaviour. They found that
when the angle of approach was 300 then behaviour similar
to that of a normal impact was observed with more
disintegration of the droplet being present. When the
impingement angle was changed to 600 no disintegration
was observed due to lower impact momentum in the normal
direction. By increasing the velocity, a satellite droplet has
started being apparent.
The effect of droplet initial velocity on the different stages
of the droplet-hot wall impingement process was studied by
Moita and Moreira (2002). They found that low velocity
droplets move periodically, spreading and recoiling without
splash or break-up. On the other hand, droplets with initial
large velocity spread almost without recoiling and the
droplet breaks-up to form smaller droplets during the first
phase of the impingement process. Inertial forces overcome
surface tension and shear forces for a longer period when
velocity is larger. At high impingement velocities, the
wetted area is larger and the energy dissipated against
progression of the liquid film on the surface is larger.
Biance et al. (2006) supported the findings of Moita and
Moreira (2002) and demonstrated the dependence of the
base radius (spreading factor) on the Weber number (based
on the normal velocity component).









Okumura et al. (2003) showed that spreading increases with
velocity and Wang et al. (2005) have demonstrated that, as
long as the surface temperature is well above the
Leidenfrost temperature, an increase of Weber number,
which in other words means an increase of the impingement
velocity, results in the complete rebound of the impinging
droplet with the formation of secondary droplets.
In all those previous studies, the materials used and the
experimental conditions were different. Therefore, the
critical value of Weber number above which disintegration
of the droplet was observed varied significantly. Even the
definition of the angle of approach was different in most
cases.
Additionally, in most reported studies, the Weber number
was used for the quantification of the impingement
parameters. However, its definition has not always been
plV21
consistent. The Weber number is defined as We= -
o-
where a is the surface tension coefficient, V is the
perpendicular droplet velocity, p is the droplet density and I
is a characteristic length which is usually defined as either
the droplet radius or the droplet diameter. This variation in
the definition of characteristic length causes some confusion
in the literature. In the present wok, the droplet diameter is
used consistently.
All relevant previous work is presented here using a uniform
definition for the Weber number as well as for the droplet
angle of approach. These cases and the experimental
conditions corresponding to each case are presented
collectively in Table 1.
Regarding the effect of surface roughness, early work by
Engel (1955) suggested that surface roughness promotes
droplet break-up. Cumo et al. (1969), Baumeister et al.
(1970) and Nishio and Hirata (1977) observed that the
Leidenfrost threshold is higher when the roughness of the
surface is higher. Fujimoto and Hatta (1996) and Hatta et al.
(1997) confirmed the fact that the critical Weber number,
above which the droplet disintegrates, depends also on the
surface material. However, for low impact velocity, the
dynamics of the water droplet are almost independent of the
surface materials when the surface temperature is above that
temperature that would give a dry droplet-wall interaction.
Bernardin et al. (1997) used three different surface finishes
and found that the temperature required for dry droplet-wall
interaction exhibited a high sensitivity to the surface finish.
In the study presented here, two surface materials were used
(stainless steel and aluminum) for various droplet velocities.
The droplet size was kept the same throughout the
experimental procedure.


3 Experimental Arrangement

This series of experiments is part of an ongoing
experimental project aimed at determining the heat transfer
attributable to non-wetting droplets and at correlating the
amount of heat transferred with quantities such as droplet
velocity and angle of approach. To obtain such
measurements, an infrared camera was used. Despite the
high significance of these measurements, the scope of the
present paper is to describe the experimental procedure and
the results regarding the hydrodynamics of non-wetting


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

droplets and not the cooling effectiveness of such droplets.
The experimental arrangement consists of a heating facility,
a droplet generator and a high speed camera (Figure 2). The
heating facility comprises a stainless steel plate in good
thermal contact with an electric resistance heater. A
microprocessor-based controller adjusts the electrical supply
to the heater in order to maintain a temporally uniform plate
temperature.

Table 1: Collective table with previous work relevant to our
work on Critical Weber number using the droplet diameter
as the characteristic length

0 (o) Wecr Ddop (mm) Solid TS
(oC)
Wachters
and 60 80 2.3 Gold 400
Westerling
(1966)
Hatta et al. Inconel
(1995) 90 50 0.3-0.6 alloy 500
(1995) alloy
Jeda Stainless
e(1 ) 30 70 2.0-3.0 Stles 300
(1978) Steel
Yao and Over
90 45 0.6-3.5 Brass
Cai (1988) 0 4 260

Droplets of de-ionised water fall onto the plate under gravity.
The distance from which they fall and hence their approach
velocity can be adjusted. The entire heater and plate
assembly can be angled from a horizontal to a nearly
vertical orientation, allowing both normal and oblique
impingement to be studied. Consistent droplet sizes are
generated using a fine hypodermic needle. Heating of the
water occurs whilst it is in the needle, and water
temperature at the needle mouth is measured using a
thermocouple.
A high-speed optical camera is used both to observe the
hydrodynamics of the droplet impingement and as a means
of determining the droplet size and velocity on release as
well as before the collision. The high speed camera used
here is the Olympus i-SPEED 3. It is capable of capturing
150,000 fps. At frame rates up to 2000 fps, as used in the
present experiments, the resolution is 1280x1024 pixels.


DROPLET GENERATOR


OPTICAL HIGH SPEED CAMERA


EATING SECTION


Figure 2: Experimental arrangement


The temperature of the hot surface upon which the droplets
fall is measured by a combination of thermocouples and
infrared camera for high accuracy (+20C). The droplet
diameter is known with an accuracy of 4%. The droplets are
spherical before impinging on the hot disk. That is
confirmed by determining several diameters of the same
droplet using the high speed camera software tool. The


I~
Ct~i~j_









droplet mass is thus known with an accuracy of 12%. The
droplet velocity uncertainty varies depending on the velocity.
It is calculated by using three successive frames from the
infrared camera and it is approximately 6%. The Weber
number uncertainty varies but it is generally between
10-12%. The temperature of the water at exit is measured
(48+0.50C).


4 Results and Discussion

4.1 Critical Weber Number

One important quantity for the description of such
droplet-wall interactions is the loss of momentum of the
droplet during collision. For high impact velocities and high
wall temperatures, beyond the Leidenfrost temperature, the
formation of secondary droplets can be observed as the
droplets disintegrate upon impingement. The dynamics of
such a collision can be described, as already seen, by the
Weber (We) number which is the ratio of the gravitational
and the surface tension forces. There is a critical value of
that number above which the droplet is bound to
disintegrate.
By using the high speed optical camera the vertical
impingement of a 2mm water droplet on the hot disk is
observed.
A series of experiments was conducted in order to determine
the critical Weber number for which the impinging droplets
start breaking-up. These cases as well as the experimental
conditions are summarised in Table 2.

Table 2: Experimental conditions and results for the vertical
impingement case.

Weber .
Conditions ber Behaviour
number
9.48
15.31
15.31 No break-up
Plate material: 22.10 observed
stainless steel 31.75
35.38
TSold: 310C 43.22 Break up
incipience
Droplet material: 47.14
water 49.61
53.43
Tdrop: 480C 57.23
64.39 Break-up
64.39
drop: 2mm 72.42
78.58
88.49


It is found in particular that the impact is more elastic when
the impingement speed is small. In the limit of very small
velocities, the use of non-wetting droplets leads to a regime
of quasi-elastic rebounds. For larger velocities, the shock
can be much less elastic.
A large range of Weber numbers was covered
experimentally. The cases observed range from those with
no break-up at all to those where complete break-up is


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

observed and formation of many tiny satellite droplets
occurs.


t=o.0 ms t=2.5ms t=5.0mns t=7.5ms t=10.0 ms

Figure 3: A 2mm diameter droplet impinging vertically onto
a 3100C stainless steel plate. The droplet bounces off the disk
after approximately 10ms. In the first case (upper row of
photos) the Weber number is almost 10 whereas in the
second case, where break-up occurs, the Weber number is
approximately 80.

However, between these two extremes, many regimes were
observed. Droplets may break up upon impingement or after
impingement. In other cases, droplets are on the verge of
breaking up but manage to hold together. The incipience of
droplet break-up upon impingement is found to occur for a
Weber number of 43. This is very close to the observations
of Yao and Cai (1988) and Hatta et al. (1995). However, it
would seem that the critical Weber number for break up is
sensitive to the nature of the solid surface.


4.2 Surface Effect

The results for the experimental conditions summarised in
Table 2 and shown in Figure 3 and Figure 4 were for an
unpolished stainless steel surface. However, when
impingement of droplets on the very smooth aluminum
covered disk (aluminum deposited via vapour deposition) is
investigated it is found that there is no droplet disintegration
up to the maximum value of Weber number investigated (i.e.
We= 88).
This observation is in good agreement to previous
experimental studies. When a rough solid surface is used as
the substrate where the droplet impinges on (stainless steel)
then the droplet is prone to disintegration for a much lower
Weber number then in the case of a polished, smooth
surface.


4.3 Angle of Approach Effect

The critical Weber number will be affected by the angle of
droplet approach to the surface. The experimental
arrangement enables the observation of the phenomenon for
several droplet impingement angles. Here, however, only
the cases for a vertical droplet impingement are presented. It
was found difficult to obtain reliable data for very small
angles of inclination mainly due to poor resolution. These
experiments have to be repeated. However, there are
encouraging indications for the accuracy of these
experiments since the critical Weber number for the oblique
angle cases seems to follow the trend presented by Yao and









Cai (1988). Some results were obtained for an angle of 600
to the horizontal. For this case, droplet break up occurred
for a Weber number of 35 which is lower than the value of
43 obtained for vertical impacts. This reduction in Weber
number is consistent with the results of Yao and Cai (1988)
who found that, the smaller the angle of approach the more
prone the droplet becomes to breaking-up and hence the
Weber number for break up is smaller. So, for the same
droplet size high tangential speeds, for a given normal speed,
make break up more likely.


t=O.0 ms





t=1.5 ms


t=3.0 ms


t=4.5 ms


t=6.0 ms


t=7.5 ms


t=9.0 ms


(a) (b)
Figure 4: Sequence of photographic images. (a) Weber
number is 35 and (b) Weber number is 43. In the first case
the droplet nearly breaks but finally gets back together. In
the second case, the droplet breaks up and finally forms two
satellite droplets.


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

been calculated using the droplet diameter and the data from
Biance et al. (2006) have been transformed accordingly in
order to allow direct comparison).
The duration of droplet proximity to the hot surface is
presented in Figure 5 as a function of the droplet initial
velocity. It becomes quite clear that the time the droplet
spends in close proximity to the surface is not strongly
affected by the droplet velocity.
As it impinges on the solid, the drop deforms: it first spreads
until it reaches its maximum extension. Then, it retracts and
elongates in the vertical direction before leaving the surface
in the opposite direction to that at which it impinged. The
upwards velocity V' of the droplet in the vertical direction
after impact is less than the downwards velocity (V, ) before
impingement. The ratio V /1V is plotted as a function of
Weber number in Figure 6. The present data indicate
somewhat higher relative velocities after impact than do the
data of Biance et al. (2006).

.0,


0 0.5 1 1.5 2
Vo (m/s)

Figure 5: Droplet time of proximity to the hot surface as a
function of the droplet velocity. Comparison between the
present experimental data and the data of Biance et al.
(2006).


000.o 0
0 0 0

Soo
0




Experimental data by Biance et al. (2006) 0
Experimental data from this work
00

0
0
0


o experimental data by Biance et al. (2006) 00
* experimental data from this work 0


0.1


4.4 Droplet Dynamics


In this section a study of droplet dynamics during
impingement is presented for the vertical impingement
cases. The results are compared with those of Biance et al.
(2006), which were obtained for similar droplet size,
velocity and solid plate temperature (the Weber number has


Figure 6: Ratio of droplet velocity (after and before
impingement) as a function of the Weber number.

After the impingement, the droplet spreads laterally from its
initial radius Ro to form a "disc" of radius Rmax which then
contracts to form a drop which is ejected from the surface at
a velocity V' (see above).


o experimental data by Biance et al. (2006)
* experimental data from this work



0 0 0 4 4 OF4- d- +- 0
0
o o F o -
o
























We
Figure 7: Maximum droplet spread as a function of the
Weber number.

The video pictures obtained in the present experiments were
analysed to obtain values of Rmax/Ro and the results are
compared in Figure 7 with those obtained by Biance et al
(2006). The present data for Rmax/Ro are seen to be in good
agreement with the earlier data and indicate that the drop
spreads on the surface to a radius of approximately twice the
original drop radius before springing back and departing the
surface in the vertical direction.


5 Conclusions

An experimental study of the hydrodynamics of non-wetting
droplets was presented here. The focus was on droplets
impinging vertically on the surface. The critical Weber
number for which droplet break-up occurs was determined
experimentally and more detailed studies were carried out of
the hydrodynamics of (non-breaking) droplets impinging at
Weber numbers lower than this critical value.
A sensitivity of the critical Weber number on the solid
surface roughness was observed. This aspect of the
phenomenon could be the subject of future studies as the
roughness of the surface seems to affect the phenomenon of
droplet break-up very significantly.
Several angles of droplet impingement were studied
experimentally. However, these results were not reliable
enough to be presented. Despite this fact, encouraging
agreement was found with the results of Yao and Cai
(1988). Further experimental studies should be conducted
focusing mainly on small angles of approach, in other words
on near-horizontal approaches as these are the cases of
interest to reflood conditions in a nuclear reactor.
The maximum droplet spreading, the droplet velocity and
time of proximity were investigated experimentally and
were found to confirm experimental findings of previous
researchers.


Acknowledgements

This work was carried out as part of the TSEC programme
KNOO and as such the authors are grateful to the EPSRC for
funding under Grant EP/C549465/1.


o experimental data by Biance et al. (2006)
-- best fit (exp. data by Biance et al. (2006))
- analytical correlation proposed by Biance et al.
* experimental data from this work


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

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

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