Transcript - ePrints Soton - University of Southampton

Boiling heat transfer of liquid nitrogen in the presence of electric fields
P Wang, P L Lewin , D J Swaffield and G Chen
University of Southampton, Southampton, UK
Experimental results and discussion
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0kV,increasing heat
0kV,decreasing heat
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20
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• Mesh-plane electrodes
• Electrode gap: 10 mm
• Heater power up to 450 W
• +/- 0-40 kV power supply for
both polarities
• 3 pt100s for temperature
measurement
• Vacuum insulates the copper
block from its surroundings
• High-speed camera for
observation and recording of
images
Fig.1 Schematic diagram of the experiment
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c
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0
0
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3
4
5
6
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9
2
∆T (K)
∆T (K)
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4
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8
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∆T (K)
Fig.4 Second hysteresis phenomenon: (a) 0kV, (b) +10kV, (c) +20kV.
 Effect of electric field on nucleate boiling
Fig.5 shows the nucleate boiling curves of LN2 obtained under different high-voltages with
both increasing and decreasing heat flux. It is seen that the EHD effect can enhance heat
transfer, i.e. the heat flux q increases with increasing voltage for a given ∆T. Especially, the
EHD enhancement is very obvious for applied voltages over 20 kV. When nucleate boiling
appears and with no electric field condition, the heat transfer mechanisms are governed by
three factors: heat conduction through the macrolayer surrounding the bubble interface,
evaporation of the microlayer located between the heated surface and the bubble bottom,
and convection within the LN2 adjacent to the bubble and the heated surface (Fig.6a). For
EHD nucleate boiling conditions (Fig.6b), as the appearance of volume electric force,
electroconvective movements are induced within the
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liquid region. Thus, the macrolayer thickness
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decreases and heat transfer within the macrolayer is
increased. The electric force tends to maintain the
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bubble against the heat transfer surface, causing
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intense vaporization of the microlayer between the
bottom of the bubble and the heating surface. The
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a
heat transfer enhancement through both the
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macrolayer and the microlayer encourages rapid
∆T (K)
growth of bubbles, increasing their departure volume.
More heat energy can be effectively transferred.
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Thus, effectively causing the electrode surface
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temperature to decrease.
V= 0kV increasing heat
V= +10kV increasing heat
V= +20kV increasing heat
V= +30kV increasing heat
Heat flux (W/cm2)
Experimental setup and electrode system
b
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a
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+20kV,increasing heat
+20kV,decreasing heat
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2
2
20
0
The results obtained may help the design of the LN2 related components for HTS device
cooling and also provide an initial perspective on the possible improvements for cryogenic
cooling of HTS equipment.
+10kV,increasing heat
+10kV,decreasing heat
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The effects of electric fields on boiling hysteresis, nucleate boiling and critical heat flux
have been analyzed and discussed.
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2
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Heat flux (W/cm )
A copper block electrode with temperature measurement and vacuum heat insulation was
designed and manufactured.
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Heat flux (W/cm )
An experimental study has been undertaken to determine the influence of a d.c.
uniform electrical field on boiling heat transfer of liquid nitrogen (LN2).
Heat flux (W/cm )
Introduction
V= +40kV increasing heat
V= 0kV decreasing heat
V= +10kV decreasing heat
Experimental tests were carried out using commercial grade LN2 at atmospheric pressure.
The LN2 was renewed after all data for each boiling curve had been obtained.
 Effect of electric field on the boiling hysteresis
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V= 0kV increasing heat
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Heat flux (W/cm )
V= +10kV increasing heat
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V= +30kV increasing heat
V= +40kV increasing heat
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10
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0
0
2
4
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8
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∆T (K)
Fig.3 First hysteresis of LN2 under high positive
voltages
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Fig.3 shows the effect of the electric
field on the first hysteresis of LN2. It
can be seen that the magnitude of the
first hysteresis reduces with increasing
voltage. Thereby decreasing the
degree of the superheat required to
start nucleate boiling. The reasons for
the decrease of the first hysteresis
under
electrohydrodynamic
(EHD)
conditions is due to the reduction of the
thermal boundary layer caused by an
electroconvective movement in the
presence of electric fields and then the
electrical activation of nucleation sites.
Fig.4a shows the second boiling hysteresis phenomenon that exists for increasing heat flux
and decreasing heat flux conditions under a zero-field. Fig. 4b and c show the effect of an applied
DC positive voltage of 10 and 20 kV on this hysteresis, respectively. It is seen that application of
an electric field can decrease the second hysteresis, particularly the hysteresis disappears for
high voltages in excess of 20 kV for this electrode arrangement. By analyzing the energy of a
bubble formation in an electric field, the effect of the electric forces on the nucleation process
shows the overpressure in the vapor phase as a function of the electric field can be expressed
as:
2
3 0 l ( l   v ) E  2
2
Pv  Pl   
r   (   ( E ))
r
4(2 l   v )
 r
where ε0 is the dielectric constant in vacuum, εl is the dielectric permittivity of liquid, εv is
the dielectric permittivity of saturated gas in bubble, E is the electric field strength, r is the
equivalent radius of the deformed bubble for EHD boiling, the pressure difference Pv-Pl is
the sum of capillary pressure, characterized by the fluid surface tension σ, and an
electrostatic pressure characterized by an apparent surface tension σ(E), which is
proportional to the square of the electric field strength. The electric field modifies the
surface tension so that the fluid has non-wetting fluid behaviour. According to this
analysis, for LN2, the ratio σ(E)/σ is about 48% and 108% for 20 and 30 kV conditions,
respectively. This shows that the pressure of the electric field on the bubble is substantial
in comparison with the capillary pressure. The increase of σ(E) with electric field gives a
reason for the reduction of the hysteresis phenomenon and result in the activation of
nucleation sites being easier under increasing heat flux conditions.
V= +30kV decreasing heat
V= +40kV decreasing heat
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a
b
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b
0
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∆T (K)
Fig.5 Nucleate boiling curve of LN2 under
different positive voltages. (a) increasing heat,
(b) decreasing heat
Fig.6 Nucleate boiling heat transfer
mechanisms. (a) under zero-field
conditions, (b) under EHD conditions
 Effect of electric field on the critical heat flux (CHF)
6 2
Fig.7 shows the electric field effect on CHF. The
d  2
2
(G  (G 2  3Bo 2 )1/ 2
CHF obtained in zero-field is about 24.5 W/cm
and for this experiment a 14% increase can be
where δ is the vapour film thickness that
obtained for an applied voltage of 40 kV. Very
covers the heating surface. The bond
similar results have been obtained for positive
number Bo and G, which represents the
and negative high voltages. A model of the liquidratio of the electric forces to the surface
vapour interface stability has been given to
tension forces, are given by
explain the increase under electric fields. Without
2
2

g


E

electric field conditions, near the CHF, the vapour
2
2
l
v
Bo 
,G 
columns flow adjacent to the liquid along an


interface that is unstable under the action of For LN , λ is about 3 mm for an electric
2
d
inertia and surface tension forces. A maximum field strength of 20 kV/cm. This shows
relative vapour rate exists, above which a small electric fields can decrease the λ and
d
disturbance is amplified and causes the break the vapour film and consequently
distortion of the flow. The wavelength of the increase the CHF.
disturbance with the largest growth rate is called
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the most dangerous wavelength, λd. For a plate
Negative
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Positive
surface, λd is defined as follows:
1/ 2



d  2 3 

(



)
g
v
 l

Critical heat flux, q CHF (W/cm2)
Experimental results and discussion
Heat flux (W/cm2)
A mesh-plane electrode configuration was used. The
high-voltage electrode is brass mesh (1 mm wire in
diameter and 2.36 mm in width mesh). The grounded
electrode is a special cylindrical copper block with a
heater and a vacuum jacket. The boiling process takes
place on the top centre surface of the copper block
(Fig.2).
Fig.2 A photograph of boiling of LN2
V= +20kV decreasing heat
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where ρl is the density of liquid and ρv is the
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density of vapour. For LN2, λd is about 11mm in
zero-field conditions. The presence of an electric
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0
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40
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field can modify the liquid-vapour interface
Applied voltage (kV)
stability. In this case, the wavelength λd taking
into account the electric field effects is given by Fig.7 CHF as a function of the high voltage
Conclusions
The boiling curves of LN2 have been obtained with and without electric fields. The results
show: 1) the electric field is able to reduce the first hysteresis of LN2. i.e. the higher the
electric field the easier to active nucleate boiling with a lower heat input. 2) A second
hysteresis phenomenon of LN2 exists for increasing heat and decreasing heat flux test
conditions. A higher electric field tends to eliminate the hysteresis phenomenon. 3) Electric
fields can enhance nucleate boiling heat transfer of LN2. 4) The CHF can be enhanced by the
electric field, for this experiment a 14% increase is obtained under an applied 40 kV high
voltage. 5) There is no noticeable polarity effect on boiling heat transfer.