09-05-2014, 04:46 PM
The experimental study on flat plate heat pipe of magnetic working fluid
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ABSTRACT
An effective thermal spreader can achieve uniform heat flux distribution and thus enhance heat dissipa-
tion of heat sinks. Flat plate heat pipe is one of the highly effective thermal spreaders. Magnetic fluid is
liquid and can be moved by the force of magnetic field. Therefore, the magnetic fluid is suitable to be used
as the working fluid of flat plate heat pipes which have a very small gap between evaporation and con-
densation surfaces. We prepared a disk-shaped wickless flat plate heat pipe, and the distance between
evaporation and condensation surfaces is only 1 mm. From experimental study, the effect of heat flux
and working fluid ratio on the performance of flat plate heat pipe is presented. Also we compared the
experimental results between the performance of water and magnetic fluid as working fluids.
Introduction
The magnetic fluid is a colloidal solution of many ultrafine mag-
netic particles coated with surfactants and dispersed in a carrier
liquid. The sizes of the particles are among 10–100 nm. The carrier
liquid can be water, hydrocarbon, fluorocarbon etc. Therefore, the
magnetic fluid possesses both magnetic and fluid properties. Mag-
netic fluid consists of magnetic particles, carrier liquid and surface
active agent. Magnetic fluids have many practical engineering
applications, such as sealing, bearing, magneto gravimetric separa-
tion, damper and energy conversion. Water-based magnetic fluid is
preferred for heat transfer applications due to its higher latent heat
over hydrocarbon liquids.
Results and discussion
Fig. 5 shows the temperature responses of magnetic fluid flat
plate heat pipe at different heat flux. The working fluid of the flat
plate heat pipe is magnetic fluid and the working fluid ratio is
53.5%. We use the volume ratio of working fluid and cavity of heat
pipe to define the working fluid ratio. The room air temperature is
28 °C and the air velocity that impinges on the heat sink is 9.7 m/s.
At the beginning of experiment, we set the heat flux as 20.9 W/
cm2, and then we increased the heat flux at a certain time to see
the performance at different heat flux. We can see that the temper-
ature difference between evaporation and condensation surfaces
increased with time before the startup of the flat plate heat pipe.
Then the flat plate heat pipe starts up suddenly at a certain tem-
perature difference.
CONCLUSION
We used magnetic fluid as the working fluid of the flat plate
heat pipe. With the presence of magnetic field, the effect of heat
flux and working fluid ratio on the performance of wickless flat
plate heat pipe is studied. From the experimental results and the
above discussions, it is possible to draw the following conclusions:
The wickless flat plate heat pipe starts up suddenly at a certain
temperature difference when the working fluid ratio is at high
level. After the startup of heat pipe, the temperature difference be-
tween evaporation and condensation surfaces decreases suddenly
and the temperature values fluctuate at a certain range. The opti-
mal working fluid ratio of this magnetic fluid flat plate heat pipe is
53.5% and the flat plate heat pipe only can work normally at the
proper working fluid ratio range. The water flat plate heat pipe
can not start up versus gravity at all kinds of working fluid ratio,
because there are not any capillary structures in this flat plate heat
pipe. On the present of magnetic field, the magnetic fluid flat
plate heat pipe could work versus gravity at high working fluid ra-
tio. The liquid magnetic fluid can be attracted by magnetic field
and the circulation of working fluid can be improved, so the mag-
netic fluid flat plate heat pipe can achieve more uniform heat flux
distribution on the condensation surface than water flat plate heat
pipe.