Closed Loop Heat Pipe
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Transcript of Closed Loop Heat Pipe
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International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, Volume 2, Issue 10, October 2012)
202
State of The Art on Closed Loop Pulsating Heat PipeRahul S. Borkar1, Pramod R. Pachghare2
1M Tech Student, Thermal Engineering, Government College of engineering, Amravati2Asst. Professor, Department of Mechanical Engineering, Government College of engineering, Amravati
Abstract The closed loop Pulsating Heat Pipes (CLPHPs)
are two-phase passive heat transfer devices that enhances
large amount of heat which works on the principle of
evaporation and condensation of a working fluid. CLPHP
consists of a copper capillary tube bent in many turns, which
is firstly evacuated, then partially filled with an appropriate
working fluid and finally sealed. At one end heat is absorbed
by evaporation and heat rejected through another end by
condensation of working fluid. Selection of working fluids are
depends on the desired performance from the device and theperformance of device depends on the thermo physical
properties of working fluids i.e. Saturation temperature,
viscosity, surface tension, sensible heat, latent heat etc.
Working fluids which having lower saturation temperature,
lower latent heat, high specific heat and low dynamic viscosity
gives better thermal performance. Different input parameters
are internal tube diameter, input heat flux, filling ratio,
number of terns, device orientation, size and capacity of
condenser and evaporator also important parameters for
thermal performance of CLPHP.
Keywords Pulsating heat pipe, heat transfer, working
fluid, saturation temperature, surface tension sensible heat.
I.
INTRODUCTION
In the 1990s, Akachi et al. [1] proposed a new type of
heat pipe known as Pulsating Heat Pipe (PHP). As a
passive two-phase heat transfer device that has been used
for thermal management of electronic devices to remove
heat without any electrical power input. Heat pipe is able to
dissipate substantial amount of heat with relatively small
temperature drop. Closed loop pulsating heat pipe
(CLPHP) is made from long capillary tube bent into many
U-turns, closed in an endless loop, with the evaporator and
adiabatic section is optional depends on the locations of
evaporator and condenser [1]. The diameter of the tube
must be small enough such that liquid vapor plugs andslugs exist. The unique feature of PHPs compared with the
conventional heat pipe is that there is no wick structure to
return the condensate to the heating section. Therefore,
there is no countercurrent flow between the liquid and
vapor [2].
Figure 1: Schematic of Closed Loop Pulsating Heat Pipe
II.
WORKING PRINCIPLES OF CLPHP
A.
Fluid Dynamic Principle
Initially the tube is evacuated and then filled partially (as
per required filling ratio) with working fluid, whichdistributes itself naturally in the form of liquid vapor plugs
and slugs inside the capillary tube. The liquid Plugs are
able to completely bridge the tube, because surface tension
forces overcome gravitational forces. There is a meniscus
region on either end of each slug caused by surface tension
at the solid/liquid/vaporinterface. The slugs are separated
by plugs of the working fluid in the vapor phase. The vapor
plug is surrounded by a thin liquid film trailing from the
slug.
B. Thermodynamic PrincipleWhen one end of the bundle of turns of the undulating
capillary tube is subjected to high temperature, the working
fluid inside evaporates and increases the vapor pressure,which causes the bubbles in the evaporator zone to grow ,
and extinction of vapor bubbles drive the flow in a PHP
[3].
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International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, Volume 2, Issue 10, October 2012)
203
C. Heat Transfer Principle
This vapor bubbles pushes the liquid column toward thelow temperature end (condenser). The temperature and
pressure decrease in the condenser due to condensation.
Therefore a constant, unsteady internal pressure difference
exists in the system which is the driving force. Because of
the interconnection of the tubes; motion of liquid slugs and
vapor bubbles at one section of the tube toward the
condenser also leads to the motion of slugs and bubbles in
the next section toward the high temperature end
(evaporator), this works as the restoring force.
The inter-play between the driving force and the
restoring force leads to oscillation of the vapor bubble and
liquid slugs in the axial direction. The frequency and the
amplitude of the oscillation are expected to be dependenton the shear flow and mass fraction of the liquid in the
tube.
In PHPs, heat is transferred from the evaporator to the
condenser through sensible and latent heat transfer, which
is a result of the working fluid oscillations and phase
changes.
D. Flow Pattern
During the startup period the working fluid oscillate with
large amplitude, after this period continuous circulation can
in the working fluid occurs. The direction of circulation for
working fluid is consistent once circulation is obtained but
the direction of circulation can be different for same
experimental run [3].
III.
INFLUENCE PARAMETERS
From the available literature, following major thermo-
mechanical parameters have emerged as the primary design
parameters, that affecting on the thermal performance of
CLPHP. These include The internal tube diameter is the
most important geometrical parameter because it
essentially manifests the fundamental definition of
CLPHPs. The slug flow pattern inside the tube is a
fundamental working condition because pumping force is
generated by the growing bubbles in the evaporator and the
collapsing bubbles in the condenser area. Such condition isensured only if the tube inner diameter is smaller than a
critical diameter.
A. Internal diameter of the CLPHP tube
Figure 2: Thermosyphon mode of operation when D>>Dcri [4]
The critical Bond number (or Etvs) criterion gives thetentative design rule for the diameter. The theoretical
maximum inner diameter of capillary tube can be
calculated as-
2
(l v
Dcrig
(1)
( ) 0.5[ ]g
l vBo Dcri
(2)
2[ ] 4E Bo (3)
If D < Dcri, surface tension forces dominate and stable
liquid plugs are formed. However, if D > Dcri, the surface
tension is reduced and the working fluid will stratify by
gravity and oscillations will cease. The CLPHP may
operate as an interconnected array of two-phase
thermosyphon [4].
The influence characterization has been done for the
variation of internal diameter, number of turns, of the
device by Charoensawan P. et al. [5, 6 &7] made CLPHPs
of copper tube with internal diameters 2.0 and 1.0 mm,
heated by constant temperature water bath and cooled by
constant temperature water-ethylene glycol mixture (50%
each by volume). The number of turns in the evaporator is
varied from 5 to 23.
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International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, Volume 2, Issue 10, October 2012)
204
The working fluid employed is water and found that
doubling the diameter did not double the performance. Atthe same internal diameter and evaporator length, the
performance is higher with increasing the number of turns.
The performance can be increased by increasing the tube
inner diameter and/or the number of meandering turns.
The performance of uniform channel CLPHP is more
sensitive to inclination especially when the inclination
angle is small and it is not functional at a horizontal
configuration.
On the other hand, the proposed non-uniform channel
CLPHP is functional to all inclinations provided the charge
ratio is sufficient (above 50%) [8].
B.
Input heat flux.
For a defined CLPHP geometry of the device, the input
heat flux is directly responsible for the type of flow pattern
which will exist in the channel. The operating heat flux
may also affect the level of perturbations inside a CLPHP
thereby affecting the thermal performance of the device.
The applied heat flux affects the following:-
1.Internal bubble dynamics, sizes and agglomeration,
2.Breaking patterns,
3.
Level of perturbations and flow instabilities, and
4.Flow pattern transition from capillary slug flow to Semi-
annular and annular.
CLPHPs are inherently suitable for high heat flux
operations. Since the input heat provides the pumpingpower, below a certain level no oscillations commence [9].
Cai Q. et al. [10] presented an experimental investigation
of heat transfer characteristics of CLPHP versus operating
temperatures. The CLPHP with 12 turns was made of
copper and charged with water at three charge ratios: 40%,
55%, and 70% and observed the minimum temperature
difference and fluctuation appear at temperatures between
120C and 160C.
Low input heat fluxes are not capable of generating
enough perturbations and the resulting bubble pumping
action is extremely restricted. The bubbles only oscillate
with a high frequency and low amplitude. There are periods
of no action intermission stage followed by some small
bulk activity phase. Overall, this scenario results in a poor
performance (i.e. very high thermal resistance).
As the heat input is increased, slug flow oscillations
commence whose amplitudes increase with increasing heat
flux and become comparable to the length of the device.
This improves the heat transfer coefficient to a marked
degree. As the heat flux is further increased, the oscillating
flow tends to take a fixed direction and thermal resistance
further reduces [11].
C. Volumetric filling ratio
The filling ratio (FR) of a CLPHP is defined as the ratioof working fluid volume actually present in the device to
that of the total volume of the device (say at room
temperature). Thus, a given CLPHP has two operational
extremities with respect to the filling ratio, an empty device
without any working fluid i.e. FR = 0, half filled device i.e.
FR = 0.5 and a fully filled device i.e. FR = 1.
Complete stop-over is in the loop occurs more frequently
for FR < 50% coupled with low heat input power. Stop-
over phenomenon has also been observed for higher filling
ratios. The self-sustained oscillating character is then lost
(Fig.3). Such a behavior has never been reported for multi-
turn CLPHPs because of alternating periods in which
bubble plugs are moving rapidly (activity phase) andstopping (static phase) [12].
Khandekar S. et al. [13] conducted experiments on a
CLPHP made of copper capillary tube of 2.0 mm inner
diameter for three different working fluids viz. water,
ethanol and R-123. The CLPHP was tested in vertical
(bottom heat mode) and horizontal orientation, and found
that a 100% filled CLPHP (not working in the pulsating
mode but instead as a single-phase buoyancy-induced
thermosyphon) is thermally better performing than a
partially filled pulsating mode device under certain
operating conditions. The CLPHP not operate in the
horizontal mode for small number of turns and too low
operating pressures.Pulsating heat pipes are deterministic chaotic systems,
not periodic or random systems. Song Y. and Xu J. [14]
found that Autocorrelation function coefficients for both
FC-72 and water CLPHPs are decreased with respect to
time, indicating that the prediction ability of the system is
finite and the optimal charge ratios are about 60- 70% for
both FC-72 and water CLPHPs with four, six, and nine
turns.
PHPs have better thermal performance at such a narrow
range of charge ratio. Filling ratio (FR) of 70% has better
performance compared with other FRs (30% and 50%)
[15].
D.
Total number of turns and Inclination AngleAs the inclination angle is varied from vertical to
horizontal then gravity play a significant role, the thermal
performance of CLPHPs degraded, and some did not
operate at all. Other CLPHPs, often with many turns, were
able to perform satisfactorily independent of orientation. If
the inner diameter of the CLPHP is decreased, it may also
aid in the CLPHPs ability to perform at low inclination
angles.
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International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, Volume 2, Issue 10, October 2012)
206
Larger diameter will allow improved performance, but
an increased pressure drop will require greater bubblepumping and thus a higher heat input to maintain pulsating
flow. The filling ratio (FR) of a CLPHP is defined as the
ratio of working fluid volume actually present in the device
to that of the total volume of the device.
B.
Latent heat
A low latent heat will cause the liquid to evaporate more
quickly at a given temperature and a higher vapor pressure;
the liquid slug oscillating velocities may be increased and
the heat transfer performance of the CLPHP also improved,
on the other hand the dry-out phenomenon may occur at
lower heat input levels.
C.
Specific heatA high specific heat will increase the amount of sensible
heat transferred. Because in most of the cases a great
percentage of the total heat transfer in a CLPHP is due to
sensible heat, a fluid with a high specific heat is desirable.
D.
Latent heat vs. sensible heat transferred
The net heat transfer in a device is a combination of the
sensible heat of the liquid plugs and the latent heat of the
vapor bubbles. If the internal flow patterns in the liquid
plug form then latent heat will not play a dominant role in
the overall heat transfer. If transition to annular flow under
the imposed thermomechanical boundary conditions then
the power of latent heat increases leading to betterperformance [17, 18].
E.
Viscosity
A low dynamic viscosity will reduce shear stress along
the wall and will consequently reduce pressure drop in the
tube. This will reduce the heat input required to maintain a
pulsating flow.
Long vapor plugs are only for the methanol CLPHP, not
in the water CLPHP, due to the vapor plug deformation and
breakup mechanism observed by J.L. et al. [19]. The bulk
circulation flow and the local flow direction switch flow
are induced by the combined effects of bubble nucleation,
coalescence and condensation and bulk circulation flow
sustains longer while the local flow direction switch flowshorter.
The cycle periods and the oscillating amplitudes are
increased with increasing the heating powers.
Higher heating powers result in more severe local
random oscillating nature with short time periods and smallamplitudes superimposed, due to the complicated local
flow direction switch process.
No measurable difference has been recorded between the
CLPHP running with the azeotropic mixture and the
CLPHP running with pure ethanol, in terms of overall
thermal resistance [20].
Dadong Wang [21] observed thermal resistance of pure
working fluids PHP at different power inputs with the
filling ratio of 60% and different power inputs. The thermal
resistances have the results of Racetone< Rmethanol< Rethanol