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)

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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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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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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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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

Closed Loop Heat Pipe

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