Group Title: 7th International Conference on Multiphase Flow - ICMF 2010 Proceedings
Title: P3.14 - Refrigerant flow void fraction measurements by a capacitive measurement technique
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 Material Information
Title: P3.14 - Refrigerant flow void fraction measurements by a capacitive measurement technique Droplet Flows
Series Title: 7th International Conference on Multiphase Flow - ICMF 2010 Proceedings
Physical Description: Conference Papers
Creator: Canière, H.
Colman, G.
Colman, L.
De Paepe, M.
Publisher: International Conference on Multiphase Flow (ICMF)
Publication Date: June 4, 2010
 Subjects
Subject: flow boiling
macro-scale tube
HFC
evaporators
 Notes
Abstract: A capacitive void fraction sensor was developed to study the dynamic behaviour of refrigerant two-phase flows in horizontal macro-scale smooth tubes of evaporators used in air-conditioning or heat pump applications. Because the dielectric constant of liquid HFC refrigerants (εr = 6-10) is much smaller than the one of water (εr = 80), the capacitances to be measured are much smaller. Therefore a capacitance transducer was developed to make dynamic measurements of these HFC fluids possible and still achieve high signal-to-noise ratios. The sensor signal can thus be used for flow pattern detection and void fraction measurements of high pressure HFCs, like R410A. A macro-scale test facility for two-phase flow and heat transfer studies of HFCs, was designed and constructed. The adiabatic test section consists of the capacitive void fraction sensor and a sight glass with high speed camera (200fps) for flow visualization purposes. Two datasets of sensor signals were gathered using R410A and R134a respectively. The void fraction measurements are compared to the Rouhani-Axelsson (1970) drift-flux void fraction model. On average the difference is less than 5%.
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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Bibliographic ID: UF00102023
Volume ID: VID00515
Source Institution: University of Florida
Holding Location: University of Florida
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Resource Identifier: P314-Caniere-ICMF2010.pdf

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



Refrigerant flow void fraction measurements by a capacitive measurement technique


Hugo Caniere*, Guus Colmant, Luc Colman** and Michel De Paepe*


*Ghent University UGent, Department of Flow, Heat and Combustion Mechanics
St.-Pietersnieuwstraat 41, Gent, 9000, Belgium
Hugo.Caniere@UGent.be
tGhent University UGent, INTEC_design, Department of Information Technology
St.-Pietersnieuwstraat 41, Gent, 9000, Belgium
**Hogeschool Gent, Vakgroep Elektronica, Departement Toegepaste Ingenieurswetenschappen
Schoonmeersstraat 52, Gent, 9000, Belgium


Keywords: flow boiling, macro-scale tube, HFC, evaporators




Abstract

A capacitive void fraction sensor was developed to study the dynamic behaviour of refrigerant two-phase flows in horizontal
macro-scale smooth tubes of evaporators used in air-conditioning or heat pump applications. Because the dielectric constant of
liquid HFC refrigerants (E, = 6-10) is much smaller than the one of water (sr = 80), the capacitances to be measured are much
smaller. Therefore a capacitance transducer was developed to make dynamic measurements of these HFC fluids possible and
still achieve high signal-to-noise ratios. The sensor signal can thus be used for flow pattern detection and void fraction
measurements of high pressure HFCs, like R410A.

A macro-scale test facility for two-phase flow and heat transfer studies of HFCs, was designed and constructed. The adiabatic
test section consists of the capacitive void fraction sensor and a sight glass with high speed camera (200fps) for flow
visualization purposes. Two datasets of sensor signals were gathered using R410A and R134a respectively. The void fraction
measurements are compared to the Rouhani-Axelsson (1970) drift-flux void fraction model. On average the difference is less
than 5%.


Introduction

Several measurement techniques are used to study the
dynamic behaviour of two-phase flows. Drahos and Cermak
(1989) made a comprehensive review on these techniques.
Among them are X-ray photography, photon attenuation
techniques, optical methods, ultrasonic transmission
techniques, etc. The electric capacitance technique exploits
the difference in dielectric constant between the gas phase
and the liquid phase. The capacitance between electrodes is
dependent on the void fraction of the volume in between
them. It is a cheap technique, non-intrusive and relatively
simple in construction. The electrodes are positioned around
or flush with the tube wall. Different electrode
configurations are possible. Concave electrode pairs were
used by Canibre et al. (2007) in their capacitive void
fraction sensor, which was initially developed for air-water
flow.

When low dielectric fluids are considered, the difference in
dielectric constant between the gas and liquid phase is much
smaller than that of water (sr 80). The dielectric constant
for liquid HFCs used in refrigeration varies between 5 and
10 (ASHRAE Handbook, 2005). The difference in
capacitance to be measured is thus also much smaller. In a
macro-scale tube with 8 mm inner tube diameter, the


difference in electric capacitance between fully liquid flow
and vapour only flow is then of the order of 1pF. To study
the dynamic behaviour of two-phase flow, a time dependent
(cross-sectional) void fraction measurement is desired. The
electrode length should therefore be short enough compared
to the size of typical flow phenomena. Shorter electrodes
again result in smaller capacitance differences to be
measured.

To track the frequencies of the void fraction changes, the
capacitance transducer should also respond fast enough.
Typical frequencies encountered in the two-phase flows
aimed at are located in the 0-100 Hz bandwidth.

To be able to measure void fraction changes of low
dielectric fluids, a new capacitance transducer is developed
which can measure small capacitance differences (in the
order of sub-picofarads) with a fast response time (in the
order of milliseconds). A dataset of capacitance signals was
gathered, using two HFCs. The measurements are further
compared to the drift-flux void fraction model of
Rouhani-Axelsson (1970).






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

the inner tube diameter constant and to minimize the
disturbances of the flow due to joints. The assembly is
placed in an inox casing. The remaining space is filled with
a resin. The total assembly is able to withstand inside tube
pressures of 4 MPa, corresponding to the design pressure of
condensers using R410A. The outer casing also acts as
shielding and is therefore grounded.


PVC parts
connection wires


flexible tube wall
inox casing


etched electrodes
fitting

copper tube

Figure 2: Schematic of the capacitance void fraction
sensor assembly

Capacitance measurement technique

A first possible measurement technique is the frequency
deviation method (Jaworek and Krupa, 2004). An oscillator
is thereby tuned to high frequencies (typically about 100
MHz, i.e. FM radio frequencies) using a resonant LC circuit,
where L, is the inductance of the resonant circuit and C the
total capacitance in the resonant circuit comprising the
capacitance of the sensor and the stray capacitance of the
circuit. Capacitance variations thus cause changes in
frequency. The frequency difference can be determined by
comparison of the actual frequency of the oscillator with a
reference frequency. These high frequencies have the
advantage of eliminating the effect of conductance of the
liquid under test.

A competing technique and also the most frequently used
technique for dynamic capacitance measurements is the
charge/discharge technique (Huang et al., 1988). The
capacitance under test is placed in a switch matrix where it
is constantly charged and discharged at a frequency up to a
few MHz. The current through the capacitor is then
integrated, amplified and transformed to a voltage signal.
The major advantage of this technique is that it is immune
to stray capacitance. Huang et al. (1988) introduced this
technique. The implementation of Yang and Yang (2002)
was used as basis for the design of an improved transducer
for use with HFCs. This technique was chosen because of
the low conductance of the HFCs and the high stray
capacitance in the measurement setup.

In Figure 3, the measuring circuit is illustrated. The output
voltage V,, Eq. (1), is proportional to the capacitance to be
measured C,.


Paper No


Nomenclature


capacitance (F)
inner tube diameter (m)
mass velocity (kg m~s')
frequency (s ')
inductance (H)
resistance (02)
temperature (K)
voltage (V)
vapour quality (-)


Greek letters
A difference
a void fraction (-)

Subsripts
abs absolute
source
exp experimental
f feedback
L liquid
o output
R-4 Rouhani and Axelsson
rel relative
sat saturation
I' vapour
xto be measured
2PH two-phase flow

Capacitance sensor

A capacitance probe with a concave electrode configuration
was developed for dynamic tivo-phase flow void fraction
measurements (Canibre et al., 2007 and Canibre, 2009). The
output of the probe is a voltage signal proportional to the
capacitance of the tivo-phase mixture between the electrodes.
To acquire (quasi)-local two-phase flow data, the electrode
width is equal to the diameter of the tube. In Figure 1 the
electrode configuration is illustrated. The capacitance
between the middle electrode pair is measured. The outer
electrode pairs are used for guarding purposes.


Figure 1: Electrode configuration of the capacitive void
fraction sensor

A flexible circuit material was used to construct the tube
wall. By etching the copper cladding, precise sizing and
positioning of the electrodes is accomplished. In Figure 2
the assembly of the sensor is shown. The flexible tube wall
with the three pairs of electrodes is enclosed and glued in
machined PVC parts. These parts make the connection
possible with the copper tubing and ensure the structural
stability of the tube wall. Special attention was paid to keep


v, ~2fl' C,R ,


To obtain an output voltage which is high enough to get
acceptable signal-to-noise ratios in the case of small C,






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

thus only performed to V,. After this compensation, all
measurements of V, and V1-fell within f4mV. There was no
significant difference in VL or I1- between the start and the
end of the experiments. Drift from the electronic transducer
can therefore be neglected. The noise level of both liquid
only and vapour only flow is 10mV (peak to peak). The
corresponding uncertainty evaluated as 20 is f4mV or
f0.3% of AV, resulting in signal-to-noise ratios SNR higher
than 300. The step response of the transducer on a change in
capacitance of 1pF was faster than the sample frequency
(1kHz).

Experimental Facility

In Figure 4, a schematic of the refrigerant test facility is
shown. A pump provides subcooled refrigerant to the
preheater. This preheater consist of six tube-in-tube heat
exchangers with a total length of 15m. The length of the
preheater can be altered between 1m and 15m in steps of 1m.
The refrigerant in the central tube is heated and evaporated
to the desired vapour quality x, by hot water flowing in the
annuli. A boiler system heats a 2n? tank to provide hot
water at a stable temperature during the experiments. The
conditioned vapour-liquid mixture is fed into the test
sections after which it is dumped into a plate condenser. The
condenser transfers the heat from the refrigerant to an ice
water flow and provides subcooled liquid to the pump. The
ice water is supplied from a In? tank which is cooled by a
chiller system. In contrast with a traditional compressor
l00p, there is only one working pressure. The pump only
bridges the pressure losses. By controlling the frequency of
the pump the mass velocity G, in the refrigerant loop is set.
The reservoir is submerged in a water bath. By changing
that water temperature, the saturation pressure can be
altered.
C-sensor ,cmera



Ice water CondenserII
II

Pump Flow meter~ rhae




Figure 4: Schematic of refrigerant test facility

A horizontal adiabatic test section is used for flow
visualization and characterization purposes (Figure 5). It
consists of a sight glass with a camera, the capacitive void
fraction sensor and a second sight glass. The second sight
glass after the capacitance sensor is required to ensure
electrical separation between the set-up and the sensor. This
is absolutely necessary to prevent noise pick-up in the
capacitance sensor by the antenna effect of the copper
tubing. To eliminate disturbances from bends or valves, a
minimum entrance and exit length of 60D was ensured
upstream and downstream of the test section. The flow in
the test section is then fully developed and a constant tube
diameter is assured over the full section with as little
disturbances as possible.


Paper No


three parameters can be optimized for. First, the
amplification can be increased by using a larger feedback
resistance Rf. This is limited by the stability of the op-amp
feedback. It is therefore necessary to also increase the
voltage signal before amplification. This can be done by
increasing the switch frequency fat one hand and increasing
the voltage source 11 at the other hand. But, increasing the
voltage source results in an increase in power through the
switches. They have to be able to handle this to avoid
overheating. Increasing the voltage source also implies
longer charge and discharge times. These have to remain
shorter than half of the clock period.


1 2 2 1 2 1 2




Figure 3: Charge transfer measuring circuit and switching
wave forms ('1' = ON, 'O' = OFF) (Yang and Yang, 2002)

Stability of the voltage source is another important issue to
avoid signal drift and obtain an acceptable uncertainty in the
output voltage signal. For the same reason, a stable switch
frequency is required. A crystal has to be used. The use of
well-chosen components enables this circuit to measure
capacitances in the (sub)-picofarad range.

Performance of the capacitance transducer

The output voltage signals are gathered by a DAQ system at
a sample frequency of 1kHz and are made dimensionless
according to Eq. (2). VL and I1- are the voltage levels of
liquid only and vapour only flowing in the tube.


v 7 y r


The transducer gain is 1.16V/pF. At 150C, the difference
between V, and V1-was measured AT =1.32V for R410A and
AT =1.31V for R134a. The difference in electric capacitance
between liquid flow and vapour flow is thus 1.14pF and
1.13pF respectively. A temperature variation of f0.50C
results in variations of V, of f6mV or f0.44% of AV. The
slope of the 11-T curve is -0.0117 V/oC for R410A and
-0.0099 V/oC for R134a. The negative slope of the T-11
calibration curve corresponds to the decreasing dielectric
constant of liquid HFC in function of temperature. The
influence of temperature on the dielectric constant of the
vapour phase is negligible. A temperature compensation was

































































G=300kg/ms, x=0.675












0012345


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

into the vapour phase and vapour bubbles to appear into the
liquid phase. The two-phase flow becomes fully chaotic.
This results in a higher frequency spectrum content at
frequencies higher than 5Hz. The tube perimeter remains
fully wetted. The amplitude of the wave patterns diminishes
and the liquid content in the upper film increases gradually.

A further increase in vapour quality results in the
development of an annular film. The thickness of the film
always remains larger at the bottom of the tube. The
transition from intermittent to annular flow is very gradual.
In fully developed annular flow, the interface between the
liquid annulus and the vapour core is disturbed by small
amplitude waves. Droplets may be dispersed in the vapour
core but these are hard to notice due to the limited visual
access to it. The annular film thickness gradually diminishes
with increasing x. The average signal values are low
because of the high vapour content, the variance of the
signal values is low as well, but the frequency content at
high frequencies is high instead.

Void fraction

If the capacitance sensor has a uniform sensitivity
distribution in the space between the electrodes, then a
one-to-one correlation between the capacitance and the
cross-sectional void fraction exists. Unfortunately the
concave electrode configuration has an inherently
non-uniform sensitivity distribution. The contribution to the
measured capacitance is higher near the electrode ends
compared to the midsection of the tube and is lower near the
tube perimeter in between the electrodes (Xie et al., 1990).
To eliminate this effect other electrode configurations have
to be used. The double helix configuration is more suitable
(Tollefsen and Hammer, 1998). But because local
measurements are preferred to spatially averaged
measurements, concave electrodes were chosen.

Nevertheless, a strong relationship exists between the
cross-sectional void fraction and the capacitance in the
concave electrode configuration. The time averaged voltage
signal is a measure for the liquid hold-up, so the
experimentally determined void fraction 8, can be defined
(Eq. 3).


Paper No


G=300kg/m's, x=0.05


G=300kg/m~s, x=0.20


2345


time [s] time [s] time [s]
(a) (b) (c)
Figure 6: Capacitive void fraction signals of R410A at La, = 150C (a) slug flow (b) intermittent flow (c) annular flow


Figure 5: Adiabatic test section

The sight glasses are made of smooth quartz glass (100nun
x 8mm I.D./10mm O.D.) which is mounted in the 7.91nun
copper tube. Nylon ferrules are used as sealing in the fittings.
The glass tube was annealed and hardened to prevent
fracture caused by micro cracks at higher pressure. Two bolt
connections are used to absorb the axial forces. To ensure
the electrical separation of the tubing, the supports for these
bolts are made of electrically insulating material. The
construction was successfully pressure and leak tested with
nitrogen up to 4 MPa. To capture images of the refrigerant
flow, a monochromatic high speed camera was used which
could capture images at 250 frames per second.

Results and Discussion

Capacitance sensor signals

Three typical sensor signals are shown in Figure 6, i.e. a
slug flow, an intermittent flow and an annular flow signal
obtained with R410A at T,,=150C. At low vapour qualities
slugs frequently fill the entire cross section with liquid. The
slugs are often aerated with vapour bubbles. The
concentration of these bubbles is higher near the top of the
tube. Each liquid slug causes a peak in the voltage signal
that approaches V* =1. This results in a high variance in the
signal values. The slug frequencies dominate the frequency
spectrum. The average signal values of slug flows are high
due to the large liquid content.

At transition from slug flow to intermittent flow, the vapour
content in the slugs is that high, that the liquid bridges break
up. The interfacial waves are more turbulent in the
intermittent flow regime, causing liquid droplets to swing




































































measurements with R134a
absolute [%] relative [%]

G As lAs RMS AE IAsl RMS
200 4.63 5.05 5.64 5.15 6.09 7.16
300 2.52 3.12 4.36 2.84 3.97 5.89
400 2.89 3.62 4.61 3.43 4.60 5.96
500 3.38 3.92 4.70 4.13 5.15 6.21
All 3.39 3.96 4.90 3.89 4.95 6.38

The difference between the mass velocities in void fraction
prediction by Rouhani-Axelsson is rather small. The
dependency on G in the measurements is higher. The
relative deviation Aere (Eq. 6) is higher for lower mass
velocities. This dependency is related to the non-uniform
sensitivity distribution over the cross-section. Therefore
different flow regimes have different sensor responses even
though the void fraction is the same. At low mass velocities,
the flow is less symmetric because of gravity, which
explains this difference. The deviation for annular flow is


absolute [%] relative [%]

G As las RM/S As As RMS
200 4.40 5.86 6.44 3.45 8.36 12.9
300 3.74 4.85 5.65 3.50 6.68 9.11
400 2.98 3.90 4.91 2.80 5.50 8.39
500 1.35 2.47 3.23 1.18 3.54 5.47
All 3.12 4.27 5.19 2.74 6.02 9.33


Some statistical parameters are listed in Table 1 and Table 2
for R410A and R134a respectively.


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


Paper No


In Figures 7 and 8, the Rouhani-Axelsson void fraction
model (Rouhani and Axelsson, 1970) SR-A (Eq. 4) is plotted
for G=200-500kg/m2S as well as the experimentally
determined void fraction ee and the relative deviation from
the Rouhani-Axelsson model.


Sa~bsR-A exp

Ar = R-A exp
R-A


R134a, D=8mm, Tst= 150C


0.9


0.7
0.6
S0.5
0.4


1]


SR-A = -V [1+10.1x- +
+x +


Gp 5


R410A, D=8mm, Tsat = 150C


0 9
0 8
0.7
0.6
'05
04
0.3
02


i 0.3 0.4 0.5 0.6 0.7 0.8 0.9
x [-]

(a)
R134a, D=8mm, T ~= 150C


I~


-0.3
-0.4
-0.5


R410A, D=8mm, Tsat = 15"C


-0.7 '
00.1 0 2 0.3 0.4 0.5 0 6 0 7 0.8 0 91
x [-]

(b)
Figure 7: (a) Void fraction measurements of R134a and
the Rouhani-Axelsson (1970) model (b) Relative deviation
from the Rouhani-Axelsson model
(o: G=200kg/m2S, x: G=300kg/m2S, V: G=400kg/m2S,
: G=500kg/m2S)


-0.2Fe


-0.s a

-0. 0. 2 0.3 0.4 0.5 0.6 07 0.8 0.9 1


Table 2: Statistical analysis


of void fraction


Figure 6: (a) Void fraction measurements of R410A and
the Rouhani-Axelsson (1970) model (b) Relative deviation
from the Rouhani-Axelsson model
(o: G=200kg/m2S, x: G=300kg/m2S, V: G=400kg/m2S,
: G=500kg/m2S)


Table 1: Statistical analysis
measurements with R410A '


of void fraction


nO






Paper No


smaller than the deviation for stratified and intermittent
flows. The deviation also reduces when the vapour quality
increases. Then the flow becomes annular and thus more
symmetric towards gravity. The transition from intermittent
flow to annular flow occurs at lower vapour qualities for
higher mass velocities. This also explains the dependency of
the deviations on the mass velocity.

-1 n
As = A (7)


As = Asl (8)


RALS = 1 ~(Aef2 (9)



The average deviation As (Eq. 7) tells how centered the
data is compared to the prediction. The average deviation is
positive for every mass velocity. On average, the measured
void fraction is smaller than the predicted void fraction. For
R410A, the average deviation drops with increasing mass
velocity from A6abs = 4.40% at G=200kg/m's to A~as =
1.35% at G=500kg/m's. For R134a the average deviation
also drops with increasing G. But because for G=400kg/m's
and G=500kg/m's the dataset is not complete, the average
deviation is slightly higher compared to G=300kg/m's. On
average the difference is less than 5%.

At low vapour qualities, As is negative. Positive and
negative deviations counteract when averaging. Therefore
the average deviation is an optimistic comparison. The mean
deviation As (Eq. 8) and the root mean square (RALS)
deviation (Eq. 9) do not have this counteracting effect.
These figures are thus slightly higher.

Conclusions

A capacitance probe and transducer was developed for use
with low dielectric fluids, like HFC refrigerants. The
transducer is able to measure differences in the sub
picofarad range with a fast response and excellent
signal-to-noise ratios. Sensor signals are gathered with
R410A and R134a in an 8mm I.D. smooth tube at a
saturation temperature of 150C in the mass velocity range of
200 to 500kg/m~s and vapour quality range from 0 to 1 in
steps of 0.025. When comparing the time-averaged sensor
signal measurements to the void fraction obtained with the
drift-flux model of Rouhani and Axelsson (1970), the
deviation from the measurements is less than 5% on
average.

Acknowledgements

The authors would like to express gratitude to the BOF fund
(B/06634) of the Ghent University UGent which provided
support for this study. Also special thanks to Josse Van
Buggenhout for the help during the development of the
transducer.


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

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http://hdl.handle.net/1 854/LU-8 12984

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Tollefsen, J. and Hanuner, E.A., Capacitance sensor design
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Xie, C.G, Stott, A.L., Plaskowski, A. and Beck, M.S.,
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