15
MAGNETIC FIELD CONTROL OF HEAT PIPES
Filip Cingroš
CZECH TECHNICAL UNIVERSITY IN PRAGUE
Faculty of Electrical Engineering
Department of Electrotechnology
Abstract – This paper deals with variable heat conductance heat pipes controlled by a static
magnetic field. We have ascertained the magnetic field effects on heat flow in a heat pipe
filled with pure oxygen. The cryogenic heat pipe with a special construction has been made
for that purpose and a testing device has been arranged as well. We have found out that the
heat transport efficiency of the heat pipe has rapidly decreased by magnetic field exposure.
Some interesting results of our experiment are presented in this paper.
1. Introduction
Heat pipe is an equipment which allows a very effective heat transport. They are
commonly used in electronic devices and in many other types of equipment as well. Some
applications require a heat flow controllability. For that purpose heat pipes are modified to
many different constructions with the variable heat conductance. Usual solution is e.g. gasloaded heat pipe, where the additional noncondensable gas makes a barrier for a working fluid
steam flow. According to the results of some experiments, implemented in our laboratory and
several another, it seems to be possible to control heat transport in chosen types of heat pipes
by a magnetic field exposure [2].
In numerous experiments implemented in our laboratory we have observed how the
magnetic field influences the flow of several gases patterns (e.g. water steam, nitrogen,
ethanol flame exhausts etc.). Candle flame has been pressed down under magnetic field or
even quenched (with a special magnetic field configuration). It is obvious that magnetic field
can disturb a gas flow [3].
According to these facts we decided to modify this mechanism to control heat transport in
heat pipes. For ascertaining these effects several gravitational heat pipes has been made and a
special testing and manufacturing device has been arranged. At the first time we have
ascertained the magnetic field effects on heat flow in heat pipes with common working fluids
for a normal temperature range (water, ethanol). In that case magnetic field barrier has
disturbed only a small part of heat flow, because of a very low magnetic activity of used
working fluids [4], [5], [6].
Presently we implement the similar experiments with cryogenic heat pipes. Oxygen has
been chosen as a working fluid because the magnetic properties of the oxygen molecule are
suitable for mentioned aims. We have observed that the heat flow efficiency of the heat pipe
has rapidly decreased with the magnetic field exposure. Some important results and
characteristics are specified in the following text.
2. Experimental Techniques
In our experiment we have been ascertaining how the heat transport efficiency of an
oxygen heat pipe changes by the static magnetic field exposure. The experiment installation is
shown in the Chyba! Nenalezen zdroj odkazů..
The top part of the heat pipe was chilled by liquid nitrogen and the rest was exposed to
the room temperature (about 295 K). The middle part was exposed to the static magnetic field
with the variable value of the magnetic induction B. The blocking rate of the magnetic barrier
was determined by measuring the temperature changes in two points on the heat pipe - above
and bellow the magnetic field zone. The used thermocouples type K have been calibrated by
the Pt-thermometer. The pressure changes inside the heat pipe were continuously measured
by the connected manometer too.
Fig. 1: The experimental installation with the testing device and the cryogenic heat pipe
For these experiments a special gravitational heat pipe system has been designed and
performed (Fig. 2). It consists of a heat pipe, filling and connecting capillaries and a
manometer. The heat pipe container is made from a copper tube (length 270 mm, outside
diameter 8 mm, wall thickness 1 mm). Pipe ends are closed by brassy plugs with hole
through, mechanically deformed and soldered. The both ends are connected to the copper
capillaries (outside diameter 2 mm, wall thickness 0,5 mm), which connect the heat pipe with
a filling device and the manometer. They are easily closeable by a mechanic deformation with
soldering (although the system pressure is very high). All other joints were hermetically
soldered.
filling capillary
liquid nitrogen
T2
magnetic field
heat pipe
T1
manometer
p
capillaries
Fig. 2: The experimental cryogenic heat pipe system
Oxygen has been chosen as a working fluid because of its paramagnetic properties. The
relative permeability μr of gaseous oxygen is 1,00053, but for liquid oxygen μr = 1,003 (at the
boiling point). Therefore liquid oxygen could be attracted by magnetic field.
Heat pipe was filled from a pressure vessel through the filling capillary. After that the
capillary was mechanically deformed, cut off and soldered. Before filling the whole heat pipe
system was flushed out with an oxygen stream. The pressure in the heat pipe was measured
by the integrated manometer; at the normal temperature the pressure was 10 MPa.
The static magnetic field barrier was generated by an electromagnet or by a permanent
magnets array. The special two coils electromagnet was provided by two columnar poles (Fig.
3) in an edge form (diameter 60 mm, angle 120o and radius of the edges 3 mm). The distance
between magnetic poles (air-gap) was 10 mm. The value of the magnetic induction B in the
middle of the poles could be adjustable in the range from 0 T to 1,3 T. The inhomogeneous
magnetic field at the poles edges achieved a gradient B app. to 300 T/m in the direction
perpendicular to the pole axis. A digital gaussmeter Lake Shore 410 with transversal Hall’s
probe was used for all measurements of the magnetic induction.
Fig. 3: The magnetic pole of the electromagnet
Magnetic field was alternatively generated by two special Nd-Fe-B permanent magnets
(with dimensions in mm - 40x20x10) with magnetic circuit. Magnetic induction B in the
middle of the air-gap was 0,45 T and the magnetic field was in this case app. homogenous.
3. Results and Discussion
The results of the measurement are illustrated in the following figures. They represent
temperature dependences for different values of the magnetic field induction B in the air-gap.
From the temperature changes it is possible to determine how the magnetic field effects heat
transport in the heat pipe. Additionally the pressure curves describe the pressure changes
during the heat pipe operation.
300
10
T1
8
T2
6
T [K]
220
180
4
p [MPa]
260
p
140
2
100
0
0
50
100
150
t [s]
200
250
300
Fig. 4: The temperature characteristics and the pressure curve for the heat pipe
without magnetic field
The heat pipe behavior without magnetic field exposure is illustrated in the Fig. 4. Since
the time t = 0 the top of the heat pipe was chilled. The temperature T2 started to fall down
immediately, followed by the temperature T1. The temperature difference after stabilizing was
minimal, the heat pipe operated without any restrictions.
300
10
T1
260
8
B=0
T [K]
180
6
4
p
140
p [MPa]
T2
220
2
100
0
0
100
200
300
400
500
t [s]
Fig. 5: The temperature characteristics and the pressure curve for the heat pipe with the
magnetic field exposure B = 1,25 T, at the end magnetic field was switched off
As against to the previous, with the magnetic field exposure B = 1,25 T the heat transport
was disturbed as illustrated in the Fig. 5. In the time stable state the temperature difference
was about 60 K. When the magnetic field was turned off, the temperatures became equal,
even their order changed.
300
10
B=0,45
B=0,35
220
T [K]
8
B=0,55
T1
T2
180
6
4
p [MPa]
260
p
140
2
100
0
100
200
300
t [s]
400
500
0
600
Fig. 6: The temperature characteristics and the pressure curve for the heat pipe
with magnetic field exposure with variable B from 1,0 T to 0,35 T
300
10
260
8
T1
T [K]
220
B=0
6
B=0,45
T2
180
4
p [MPa]
The heat flow disturbance for the variable magnetic field induction B from 1,0 T to
0,35 T is represented in the Fig. 6. When the magnetic field induction decreased, the
temperature difference (in the stable state) decreased as well. By the magnetic field induction
B = 0,35 T the temperatures became equal and the heat flow was not disturbed any more.
p
140
2
100
0
200
400
600
800
0
1000
t [s]
Fig. 7: The temperature characteristics and the pressure curve for the heat pipe
with magnetic field exposure of permanent magnets B = 0,45 T
(without magnetic field for a time period)
The permanent Nd-Fe-B magnets have been alternatively used as a magnetic field source.
The magnetic induction B in the air-gap (15 mm) was 0,45 T. In this case the influence on the
heat transport is shown in the Fig. 7. With the magnetic field the temperature difference was
about 60 K. As soon as the permanent magnets were moved away, the temperature T1 rapidly
fell down. After returning permanent magnets the temperatures were the same as before.
300
T1
260
220
T [K]
T2
180
140
100
0
2
4
6
8
10
t [min]
Fig. 8 Temperature characteristics for the empty heat pipe
In the last Fig. 8 there are temperature characteristics for the empty heat pipe system
without working fluid. In that case the temperature changes were due the longitudal heat
conduction of the copper wall only. Of course this reality influenced also the measurement
results of the heat pipe with working fluid.
The blocking efficiency of the magnetic barrier depends on the magnetic field intensity.
Over the magnetic induction B = 0,4 T the heat transport in the heat pipe was rapidly
disturbed. When the magnetic induction was more increased, the disturbing effects slowly
increased too. We have observed that heat transport was successfully influenced by using
permanent Nd-Fe-B magnets as well.
In the time stable state the temperature difference of the empty heat pipe was about 60 K,
which corresponds to the reality of the filled heat pipe influenced by magnetic field. Therefore
we assume the heat transport in the heat pipe with magnetic field exposure was provided in
this case mostly by the heat conduction of the copper wall.
How does this control mechanism work? A working fluid in gaseous and liquid state
circulates in the heat pipe by the operation. The liquid oxygen, flowing down on the wall by
the gravity, can be captured by magnetic field. When the magnetic field intensity is large
enough, the most of oxygen is trapped. The lower part of the heat pipe is cut off from the
working fluid and is apparently inactivated.
The gas streams very fast in the heat pipe, consequently to the continual evaporation and
condensation of working fluid. Therefore it is improbable to disturb it by magnetic field.
Without magnetic field exposure the working fluid can circulate with no restrictions.
The pressure curves represent the state changes of the working fluid – oxygen. It is
noticeable an interruption of the pressure decreasing at the moment when the heat pipe started
to operate.
4. Conclusion
In our experiments we have observed the magnetic field influence on the heat transport in
the heat pipe filled with oxygen as a working fluid. Heat flow was clearly disturbed with the
magnetic field exposure over 0,4 T. When the magnetic field induction increased, the
blocking effects slowly increased too.
The magnetic barrier has been generated by the electromagnet and alternatively by the
Nd-Fe-B permanent magnets as well. In addition to the temperature dependences, the pressure
changes in the heat pipe system have been observed.
For that purpose the experimental cryogenic heat pipe has been made and the testing
arrangement has been realized. This arrangement will be used for the other research works
and for educational aims at our department as well.
The reality described in this paper has been also observed at the other cryogenic heat
pipes filled with oxygen but with another construction, shapes etc.
This paper is based on the research program no. MSM 6840770012 “Transdisciplinary
Research in the Area of Biomedical Engineering II” of the CTU in Prague, sponsored by the
Ministry of Education, Youth and Sports of the Czech Republic.
Research described in this paper has been supervised by Doc. Ing. Jan Kuba, CSc.,
Department of Electrotechnology, FEE CTU in Prague.
5. References
[1] JELÍNEK, J., MÁLEK, Z.: Kryogenní technika, SNTL, Prague (Czech Republic), 1982
[2] UENO, S., IWAKI, S., TAZUME, K.: Control of Heat Transport in Heat Pipes by
Magnetic Field, J. Appl. Phys. 69 (8), p. 4925 - 4927, 1991
[3] KUBA, J.: Ascertaining of the Effects of Magnetic Curtain, in Proceedings of the
International Conference Diagnostika2005, Plzeň (Czech Republic), 2005
[4] CINGROŠ, F., HRON, T., KUBA, J.: Tepelné trubice a ovlivňování transportu tepla
magnetickým polem, in Proceedings of the Conference on Electrical Engineering
ELEN2006, Prague (Czech Republic), 2006
[5] KUBA, J., HRON, T., CINGROŠ, F.: Vliv magnetického pole na transport tepla, in
Proceedings of the International Conference Diagnostika2007, Plzeň (Czech Republic),
2007
[6] CINGROŠ, F.: Transport tepla tepelnými trubicemi, the Master‘s Thesis at the Dept. of
Electrotechnology FEE CTU in Prague, Prague (Czech Republic), 2007