Dynamics of a Liquid Film Produced by Spray Impact
onto a Heated Target
Olympia KYRIOPOULOS1,*, Ilia ROISMAN1, Tatiana Gambaryan-ROISMAN2,
Peter STEPHAN2 and Cam TROPEA1
1
Darmstadt University of Technology, Chair of Fluid Mechanics and Aerodynamics, Petersenstrasse 30, 64287
Darmstadt, Germany, [email protected]
2
Darmstadt University of Technology, Chair of Technical Thermodynamics, Petersenstrasse 30, 64287 Darmstadt,
Germany
Abstract
The present study is focused on the dynamics of a liquid film produced by spray impact. It is aimed at better
understanding of the hydrodynamics and heat transfer associated with spray impact onto a heated target and at
providing a basis for the reliable modeling of spray cooling. In this paper the phenomena associated with spray impact,
observed under terrestrial and microgravity conditions, are compared. A robust method of film characterization is
proposed. We show that a thin liquid film is created by spray impact even under microgravity conditions. However the
film thickness increases as the gravity force reduces.
1. Introduction
A spray impacting onto a rigid wall creates on its
surface a thin fluctuating liquid layer. A large variety of
phenomena is associated with the flow in this liquid
layer initiated by single drop impacts and their
interactions. In a comprehensive review1) various
models of drop impact onto dry and wetted substrates
can be found.
Spray impact is often modeled as a simple
superposition of single drop impacts2). However, it has
been shown that such approach cannot lead to a reliable
model of spray impact. In particular, such models are
not able to predict well the characteristics of the
secondary spray produced by spray impact.
Due to the complexity of the problem, the
hydrodynamics of the films created by sprays are not
completely understood and no reliable model describing
entirely the behavior of the film produced by the spray
or depicting the spray cooling exists up to now. The
most
relevant
models
are
completely
or
semi-empirical4),5) and thus valid only for a narrow
range of parameters used in the corresponding
experiments. By investigating the influence of gravity
on spray impact and spray cooling, the understanding of
the studied phenomena can be improved.
By observing the dynamics of a film created by the
deposited drops during the experiments, three scenarios
of splash have been identified6): cusp formation and
jetting due to the rim transverse instability; sheet
destruction and the consequent rapid axisymmetric
capillary breakup of a free rim (see Fig. 1), and rim
merging. However, the phenomena of spray impact is
even more complicated and rich on various
phenomena4).
The main objective of the present study is the
experimental investigation of the hydrodynamics of the
fluctuating film produced by spray impact onto a
smooth, rigid target under normal gravity, microgravity
and hypergravity conditions. The specific aims of the
experiment are (i) to investigate the different splash
scenarios and their statistical properties, (ii) to develop a
robust method of characterization of the fluctuating film
and (iii) to measure the average film thickness and its
height fluctuations under the various spray parameters
and at various gravity levels.
Fig. 1 Splash produced after the breakup of a free sheet. A –
free upraising sheet; B – rim; C – free jet formed after
the rim detachment and sheet breakup; D – secondary
droplets formed after the capillary breakup of a jet.
2. Experimental Setup
A scheme of the experimental setup consisting of five
major subsystems is shown in the following:
As shown in Fig. 2 the liquid is stored in a reservoir in
form of a pressurized membrane tank (1) providing the
water spray. The flow rate is measured using the
measurement device (2) and can be varied (3) by
changing the injection pressure in the water supply line.
During ground and parabolic flight tests the flow rate is
varied in the range of 0.20 l/min to 0.65 l/min.
The test cell (5) is ventilated by a gas co-flow
provided by a gas supply system (4). The water spray is
generated by a full-cone pressure swirl atomizer
(Spraying Systems 30° series) (6) installed in the test
cell. The spray impacts onto a convex heated target (7)
positioned at a distance of 70mm from the atomizer. The
walls of the cell are transparent which allows
observation of the spray impact leading to the formation
of the liquid wall-film and generation of the secondary
spray.
The target surface is convex allowing one to observe
the profile of the liquid film at the target’s generatrix.
Using the high-speed video system (HS) the wall film
motion is observed and the film thickness is measured at
various time instants. The drop size distribution is
analyzed with the high-resolution camera system (HR).
The film pattern is characterized with the help of the
high-speed infrared camera (IR).
The deposited liquid and the gas are exhausted from
the cell using the extraction system (8). During
parabolic flights there is the vent-line provided by the
aircraft whereas for ground tests a vacuum pump is used.
The pressure in the experimental cell is kept constant in
the range of 0.8bar-1bar using a pressure controller (9).
3. Image Processing
In our experiments the interaction between a spray
and a convex heated target has been observed. The
target has a shape of a spherical cap with a radius of
20 mm.
The experiments have been performed on ground and
in parabolic flights. During the ground experiments the
images of the fluctuating film produced by the spray
impact have been captured using a CMOS camera with
a frame rate of 36,000 fps.
During the ground experiments the spray parameters
(drop diameter, two components of the drop velocity
and the local flux densities) have been characterized
using the phase Doppler instrument2). During the
parabolic flight experiments the images of the
spray/wall interaction have been captured with a frame
rate of 8,000 fps using a high-speed video system
consisting of a light source and a CMOS camera.
Simultaneously, the images of the film/wall interaction
have been collected at a frame rate of about 2 fps using
a high resolution camera. With the help of a high-speed
infrared camera the film temperature pattern has been
also captured.
The high frame rates have allowed observation of
several stages of the same splash event. The convex
shape of the target provides the possibility to observe
the contour of the free film surface at the generatrix. By
processing these images important characteristics
relevant to splash and film dynamics such as rim
diameter, inter-finger distances, as well as diameters of
secondary drops have been evaluated. With the help of
the edge detection technique the collected images have
been also processed to delineate the liquid film contour
of the wetted target, to evaluate its total length, to
compare it with the contour of the dry target and to
define the averaged minimal distance from the liquid
film to the dry target. The present work is focused on
the determination of the wetted target’s liquid film
contour and on the classification of the film thickness
characteristics. Both characteristics have been
investigated before the parabolic flights on the ground
as well as during 1g-, micro- and hypergravity phase of
a parabolic flight campaign.
Fig. 3 exemplifies the processing of a typical image
at a water flow rate of 0.30 l/min with the general aim to
(a)
(b)
(c)
Fig. 2 Schematic of the experimental setup
Fig. 3 Image processing of the results (1g): (a) image of the
film with corona, jets, upraising sheets, craters; (b)
liquid film contour line; (c) liquid film thickness
distribution: spherical dry target projected in white,
wetted target shown in grey scale values.
identify the liquid film contour used for computation of
the contour length, for characterization of crowns and
jets and for determination of the film distance
distribution.
In order to characterize the film thickness distribution
the image processing procedure detects the contour of
the wall film including the crowns and jets. The distance
from a point of the liquid film contour to the
corresponding nearest point at the surface of the dry
target has been calculated (Fig. 4).
4. Results and discussion
4.1. Liquid film contour
To characterize the film pattern and to compare the
film distribution resulting from the drops impacting on
the heated target pictures were taken by the high-speed
infrared camera.
It is seen from the infrared images that at the lower flow
rate (Fig. 5) the drops impacting onto the heated target
are larger but spatially sparser than at the higher flow
rate (Fig. 6). This is evident from the drop impact
temperature ‘footprint’ apparent in the images, where
the darker shade represents a cooling of the surface
upon drop impact.
dmin
dry target
Fig. 4
Film contour-to-target distance
Fig. 5 Water flow rate: 0.25 l/min, μg
Fig. 6
Water flow rate: 0.45 l/min, μg
Fig. 7 Film contour: flow rate is 0.30 l/min.
Fig. 8
Film contour: flow rate is 0.45 l/min.
This effect of the spray parameters on the liquid film
contour is illustrated in Figs. 7 and 8. The images
presented in these figures have been collected for
different water flow rates but both taken during
microgravity phases of the parabolic flight.
Fig. 7 shows the image of the film together with the
digitalized film contour line for the water flow rate of
0.30 l/min, whereas Fig. 8 presents the data collected at
the flow rate of 0.45 l/min. The crowns produced by
drop impacts are significantly higher and wider for the
lower water flow rate. The size of the rim, the emerging
jets and of secondary drops decrease significantly by
increasing the water flow rate.
This behavior is also shown in the following sample
images of the spray-wall interaction (Fig. 9). The same
image processing technique has been used for the
investigation of the influence of gravity on the dynamics
film produced by the spray impact.
4.2. Film thickness characteristics
A series of images collected at constant experimental
parameters together with the image of the dry target
(reference image) have been used for characterization of
the film produced by spray impact. The flow rate has
varied in the range of 0.20 l/min to 0.65 l/min.
For each image the minimal film contour-to-target
distance dmin has been calculated. These data have been
then averaged over a series of images with the same
parameter. In Fig. 10 the averaged values <dmin> of the
minimal film contour-to-target distance dmin for a series
of 5000 images taken on ground as well as for a series
of 512 to 2048 images from parabolic flights for the
given water flow rate are shown. The resulting averaged
value <dmin> can be taken as the characteristic thickness
of the liquid layer not accounting the uprising crowns
and jets.
There are several tendencies that can be observed in
Fig. 10 By increasing the flow rate starting at
approximately 0.35 l/min the minimal film contourto-target distance <d min > significantly increases
independent of the gravity level. It varies insignificantly
with further increasing of the flow rate for ground (1g)
and microgravity (μg). By comparing the results only of
1g and μg we see a large difference between the
averaged values. The microgravity values are 180-360
Flow
rate,
l/min
It has been shown that several characteristics of the
liquid film dynamics essentially depend on the
parameters of the impacting spray. The characteristics of
the film contour line significantly change with an
increase of the volumetric water flow rate.
The film thickness is influenced not only by the
parameters of the impacting spray. It increases under
microgravity and decreases under hypergravity
conditions. The change is significant, indicating that the
average inertial, capillary and viscosity effects in the
film are comparable with the gravitational force.
While the crown development and film formation
have been shown to depend on spray flow rate and the
gravity level, it is not yet conclusive from these
experiments how these effects are to be modeled. In
particular, the flow rate and gravity level can also
influence the spray formation (drop size) and impact
velocity, and not just the impact drop density. Therefore
future studies must also investigate these parameters as
a function of flow rate and gravity to identify the major
physical effects being influenced; hence suggesting the
most effective modeling strategies.
The data collected in our experiments can be used to
analyze the hydrodynamics of the film flow generated
by spray impact. In particular, the effects associated
with drop impacts, their interactions and gravity effects
can be compared. This is the topic of our further studies.
Sample Image
0.20
0.25
0.30
0.35
0.45
0.55
0.65
Contour-to-target distance <dmin>, μm
Fig. 9 Spray-wall interactions at different water flow rates, μg
ground tests
1g, parabolic flight
2g, parabolic flight
μg, parabolic flight
800
600
400
200
0
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
water flow rate (l/min)
Fig. 10 Averaged minimal film contour-to-target distances
μm larger than the ones for ground conditions.
Regarding the results of hypergravity phase, the values
tend to be much smaller but in order to evaluate those
results further observations focusing only on 2g phase
are necessary in the future.
Acknowledgements
This work has been performed in the framework of the
research grant of BMWI "Sprayaufprall auf beheizte
Oberflächen unter Mikrogravitation/Spray impact onto a
heated surface under microgravity conditions". The
microgravity experiments have been performed during
several parabolic flight campaigns (44th ESA-PFC, 9th
DLR-PFC and 10th DLR-PFC).
The authors would like to thank the European and the
German Space Agencies ESA and DLR for financial
support. They would also like to extend special thanks to
the German Science Foundation (DFG) for a financial
support in the framework of the Emmy Noether Program
(GA 736/2-3) and in the framework of the Collaborative
Research Center 568 (TP A1).
References
1)
2)
3)
4)
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5. Concluding remarks
An experimental method has been developed to
investigate the dynamics of a liquid film produced by
spray impact onto a heated target. This method includes
the film observation, image processing and development
of a robust algorithm for detailed film characterization.
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