study pipes

8/18/2019 study pipes

1/13

REVIEW ARTICLE

Xin TANG, Lili SHA, Hua ZHANG, Yonglin JU

A review of recent experimental investigations andtheoretical analyses for pulsating heat pipes

© Higher Education Press and Springer-Verlag Berlin Heidelberg 2013

Abstract Pulsating heat pipe (PHP), or oscillating heat pipe (OHP), a novel type of highly ef cient heat transfer

component, has been widely applied in many elds, suchas in space-borne two-phase thermal control systems, inthe cooling of electronic devices and in energy-savingtechnology, etc. In the present paper, the characteristics andworking principles of the PHPs are introduced and thecurrent researches in the eld are described from theviewpoint of experimental tests, theoretical analyses aswell as practical applications. Besides, it is found that thestate-of-the-art experimental investigations on the PHPsare mainly focused on the ow visualization and theapplications of nanouids and other functional uids,aiming at enhancing the heat transfer performance of the

PHPs. In addition, it is also pointed out that the present theoretical analyses of the PHP are restricted by further development of two-phase ow theories, and are concen-trated in the non-linear analyses. Numerical simulationsare expected to be another research focus, in particular of the combination of the nanouids and functional uids.

Keywords pulsating heat pipe (PHP), ow visualization,nanouids, nonlinear analysis

1 Introduction

Pulsating heat pipe (PHP), also known as oscillating heat pipe (OHP), is a novel passive heat transfer device, whichwas rst proposed by Akachi [1,2] in the early 1990s. It has

been widely investigated by scientists and engineers allover the world for its excellent features such as smallvolume, low fabricating cost, simple structure and high

heat transfer performance. The PHPs can be generallyclassied into closed end, closed loop, closed loop with

check valve, and PHP with open ends [3], as shown inFig. 1.

The operation principle of the PHP can be easilygeneralized as: the tube diameter is small enough (smaller than a critical value) to ensure the ow oscillations, i.e.

d


2/13

be categorized as either experimental measurements or theoretical analyses. Experimental studies are mainlyfocused on the enhanced heat transfer performances,effects of nanouids and other functional thermal uidsand visualization of different ow patterns of the PHPs,while theoretical investigations attempt to predict analy-tically and/or numerically the uid dynamics and two-

phase heat transfer mechanism associated with theoscillating ow in the PHPs.

2 Experimental investigations

Recent experimental studies are mainly focused on theenhanced heat transfer performance, the start-up procedureand the stable operating status of the PHPs. There are manyinteracting factors regarding to the heat transfer capabil-ities of a PHP, among which the charge ratio, heatingmethod, working uids properties, and scale effects of tubediameter have dominating impacts on the heat transfer

performance, compared to the number of turns, shapes of intersection, length of evaporation and condensationsection. The heat ux and overall thermal resistance are

two characteristic quantities for the measurement of thethermal performances for a PHP.

2.1 Enhanced heat transfer performance

The PHP is well recognized as one of the most promisingsolutions for the heat dissipation in terms of high ef cient and compact. Charoensawan and Terdtoon [5] investigatedthe thermal performance of a horizontal closed-looposcillating heat pipe (HCLOHP) made from copper capillary tubes with different inner diameters, evaporator lengths and number of turns. It was found that the start-up

procedure was dependent on the evaporator temperaturethat was related to the number of turns. It was also foundthat the critical number of turns, which was 26 for their experiment, mainly depended on the evaporator tempera-ture and inner diameter of the tube. The thermal

performance of the HCLOHP was improved by theincrease of the evaporator temperature and the decreaseof the evaporator/effective length. The proper workinguids were the mixture of water and ethanol for theHCLOHP with 1 mm inner diameter, but merely water for the HCLOHP with 2 mm inner diameter.

Yang et al. [6] conducted a great deal of experimentalstudies on the two at plate CLOHPs charged with ethanolin a thermal spreader conguration. Both of the two at

plates were made of aluminum with an overall size of 180mm 120 mm 3 mm. One of the structures had 40

parallel square channels with a cross-section of 2 mm 2mm, while the other had 66 parallel square channels with across-section of 1 mm 1 mm. The operations of the four different modes of the CLOHPs, as presented in Fig. 2,were obtained in their experiments. Considering their applications, the experimental study investigated the

possibility of embedding the PHP as an integrated structure

or a heat spreader, so as to provide a higher overall thermalconductivity to the host substrate.

Jiao et al. [7] fabricated an oscillating cryogenic heat pipe, as illustrated in Fig. 3, which consisted of a 418.5cm evaporator, a 418.5 cm condenser, and a 10 cm lengthof adiabatic section. Experimental results showed that themaximum heat transport capability of the OHP reached380 W with an average temperature difference of 49°C

between the evaporator and condenser when the cryogenicOHP was charged with liquid nitrogen at 48% (v/v) andoperated in a horizontal direction. The thermal resistancedecreased from 0.256 to 0.112 m2$K $W – 1 between the

Fig. 1 Four different types of PHPs: (a) Closed end; (b) closed loop; (c) closed loop with check valve; (d) PHP with open ends [3]

162 Front. Energy 2013, 7(2): 161 – 173


3/13

evaporator and the condenser. The results also showed that the amplitude of the temperature difference between theevaporator and the condenser decreased when the heat loadincreased due to the increase of the ow velocity.

Lin et al. [8] reported preliminary experimental results of using polydimethylsiloxane (PDMS) to manufacture avisual PHP with the length, width and internal diameter of 70, 65 and 2 mm, respectively, as demonstrated in Fig. 4.They also gave the details of the manufacturing process,and the vacuuming management for the lling and

packaging. Methanol and ethanol were used as workinguids at the lling ratio of 60%. The thermal performancewas tested at different heating power values (3 – 8 W) andthe ow visualization was conducted simultaneously.

The effective range, limitations and specic heat transfer states indicated the heat transfer capabilities of the PHPs,

which dened the application effectiveness in speciccases. Recent studies scrutinized these complicated issuesin order to attain insightful understandings.

Yang et al. [9] experimentally studied the operational

limitations of a CLPHP with the working uid of R123.The CLPHP consisted of 40 copper tubes with the inner diameters of 1 and 2 mm, respectively. The tested llingratios were 30%, 50% and 70%. Three operationalorientations, including the vertical bottom heated, hor-izontal heated and vertical top heated modes wereinvestigated. The CLPHPs with 2 mm inner diameter tubes had a lower thermal resistance in the verticalorientation with heating at the bottom, while the orienta-tion played almost no role for the CLPHP with 1 mm inner diameter tubes. Concerning the specic performance data,the CLPHP with the inner diameter of 1 mm tubes

Fig. 2 Different modes of PHP operations: (a) Model 1: thermosyphon mode; (b) Mode 2: combination of thermosyphon modesuperimposed by intermittent pulsations; (c) Mode 3: true self sustained pulsating action; (d) Mode 4: excess liquid condition withintermittent pulsations [6]

Fig. 3 Oscillating cryogenic heat pipe: (a) Schematic of experimental system; (b) PHP photograph and thermocouples location [7]

Xin TANG et al. Pulsating heat pipe review 163


4/13

achieved much higher dry-out heat uxes, approximately

1242 and 32 W/cm2 for axial and radial heat uxes,respectively. While the CLPHP with the inner diameter of 2 mm tubes achieved approximately 430 and 24 W/cm2 for axial and radial heat uxes, respectively. The best thermal

performance and maximum heat ux were obtained under the vertical bottom heated mode with the lling ratio of 50%.

Lin et al. [10] conducted experimental measurements onthe effective range of the miniature oscillating heat pipes(MOHPs) and the results indicated that the increase of theinner diameter or the decrease of the heat transfer lengthwas benecial to the start-up of the MOHPs. In their

experiments, the MOHP having three different heat transfer lengths ( L) of 100, 150 and 200 mm consisted of 4 meandering turns and inner diameters of 0.4, 0.8, 1.3 and1.8 mm. The effective range of the MOHPs was identied

by using pure water as the working uid. In order tomeasure the heat transport capability of the MOHPs, acorrelation, which was well agreed with the experimentalresults, was proposed in terms of several dominatingdimensionless parameters, including Di/L, Ja, Bo and Wa.

Khandekar et al. [11] found the multiple quasi-steady

states in a single loop PHP. The ow visualization

technique and continuous temperature measurement incrucial places, and the pressure at the inlet of theevaporator were applied in their experiments. Four distinct quasi-steady states were observed in these experimentalruns. Each quasi-steady state was characterized by aunique specic two-phase ow pattern and the correspond-ing effective device conductance, and revealed the strongthermo-hydrodynamic coupling guiding the thermal per-formance.

Lips et al. [12] also applied the ow visualizationtechnique to investigate the distinct heat transfer regimes inthe PHPs. The single tube experiments were performed to

highlight or reveal some phenomena usually mixed withothers when they occurred in real PHPs. The wallsuperheat temperatures of 15 and 35 K at the evaporator of the tube were displayed in Fig. 5. The wall superheat of 35 K at the evaporator led to a faster bubble expansion that deformed the receding meniscus and broke up the liquid

plug into several smaller plugs. While the wall superheat of 15 K, on the contrary, led to the nucleation of bubbles that were always lower than the tube diameter and did not affect the overall motion of the plug.

Fig. 4 PDMS visual PHP: (a) PDMS module; (b) dry out phenomenon of methanol/horizontal orientation [8]

Fig. 5 Visualization of the ow and the impact of bubble nucleation and expansion for two wall superheats [ 12]: (a) ΔT = 15 K; (b) ΔT =35 K

164 Front. Energy 2013, 7(2): 161 – 173


5/13

In order to enhance the heat transfer capability of thePHPs, novel structures were proposed to improve theoverall thermal performance.

Thompson et al. [13] investigated a three-dimensionalat-plate oscillating heat pipe (3D FP-OHP) with stag-gered micro-channels. Optimal thermal performance was

achieved when water was as working uid, the bottom-heating orientation and the heating width matched that of the heat pipe, as well as the thermal resistance on the order of 0.08°C/W. Only a slight increase in the thermalresistance occurred during the horizontal orientationwhen utilizing a larger heating width. The reason may be

partially attributed to the novel 3D design of the OHP.The heat transfer characteristics of a coupled PHP, as

depicted in Fig. 6(a), which consisted of the main PHPlled with distilled water and the synergistic oscillatingPHP lled with ethanol were investigated by Liu et al. [14].In the case of small temperature difference, the heat

transfer rate of the coupled PHP was better compared to thesingle PHP, as sketched in Fig. 6(b), which was only madeof the main PHP under the same condition. When thetemperature reached 50°C, the condenser section of thesynergistic oscillating PHP started oscillating, and anoptimal effect of enhanced heat transfer was obtained bythe two mutual incentive PHPs.

Thompson et al. [15] applied Tesla-type check valvesinto a FP-OHP in order to promote and sustain a desiredcirculatory ow for increasing the overall thermal

performance. The Tesla-type check valves FP-OHPconsistently possessed a lower thermal resistance than itscounterpart without check valves.

2.2 Effects of nanouids and other functional thermal uids

Owing to the advancement in electronic technology, the

demand of increasing heat loads and decreasing size of electronic devices for more effective cooling technology,nanouids, which are engineered by dispersing nano-sized

particles into base uids, have received great attentionduring the past decades [16] due to their enhanced heat transfer capabilities.

Ma et al. [17] have successfully actualized heat transfer enhancement of an OHP charged with nanouid consistedof high-performance liquid chromatography (HPLC) gradewater and 1.0% volume fraction of diamond nanoparticles.When the input power approached 100 W, the temperaturedifference between the evaporation and the condensationsections were reduced from 42°C to 25°C, whichsignicantly improved the heat transfer capability of theOHP.

Lin et al. [18] carried out a detailed experimental studyof the PHP with an inner diameter of 2.4 mm lled withsilver nanouid solution, and compared the result with that

of pure water. They tested the PHP with 20 nm silver nanouid at different concentrations (10010 – 6 and45010 – 6) at various lling ratios (20%, 40%, 60%, and80%, respectively) under different input heating power.When the heating power was 85 W, the average tempera-ture difference and the thermal resistance of the evaporator and condenser were decreased by 7.79 and 0.092°C/W,respectively.

Qu et al. [19] conducted an experimental investigationon the thermal performance of an OHP charged with basewater and spherical Al2O3 particles of 56 nm in diameter.The alumina nanouids improved the thermal performanceof the OHP signicantly, with an optimal mass fraction of

0.9% for the maximal heat transfer enhancement. Com- pared with that of pure water, the maximal thermalresistance was decreased by 0.14°C/W (or 32.5%) whenthe power input was 58.8 W at a lling ratio of 70% and at

Fig. 6 Comparison of coupled PHP and single PHP: (a) Setup one: the coupled PHP; (b) setup two: the single PHP (1 — box;2 — synergistic oscillating PHP; 3 — main PHP; 4 — water tank; 5 — water tank inlet; 6 — water tank outlet [14])



6/13

a mass fraction of 0.9%. The change of the surfacecondition on the evaporator due to the nanoparticlesettlement was found to be the major reason for theimprovement of thermal performance of the OHP. Ji et al.[20] performed another study on the particle size effect of Al2O3 on the OHP. Four different size particles with an

average diameter of 50 nm, 80 nm, 2.2μm and 20 μm wereexperimentally tested, respectively. The OHP achieved the

best heat transfer performance of a thermal resistance of 0.113°C/W when charged with 80 nm particles and water at an operation temperature of 25°C and a power input of 200 W.

Riehl and Santos [21] investigated an open loop PHPlled with water-copper nanouids. The addition of nanoparticles resulted in the increase of the thermalconductivity of the working uid, and the performanceanalysis indicated that the lm evaporation effect was more

predominant than the nucleate boiling at low heat loads.

When the higher heat loads were applied to the PHP, thenanoparticles acting as nucleation sites improved thenucleation boiling, resulting in the appearance of the

pulsating ow.Wang et al. [22] explored the heat transfer performance

of the PHPs charged with functional thermal uids(microcapsule uid FS-39E and nanouid Al2O3), andcompared it with that of pure water. Both of the functionalthermal uids signicantly enhanced the heat capability of the PHP. The results exhibited that microcapsule uid FS-39E was the best working uid when its best concentration(wt) was 1% in the bottom heating mode, and nanouidAl2O3 was the best working uid when its best concentra-

tion (wt) was 0.1% under the specic experimentalconditions.

Qiu et al. [23] conducted experiments on the heat transport performance of a PHP with water-based ferro-uids, which consisted of water and 1.39% Fe3O4nanoparticles with surfactant added. An external magneticeld of 0 – 60 kA/m was imposed to the PHP. The resultsindicated that the heat resistance of the PHP charged withferrouid was lower compared to the working uid of purewater and surfactant water solution. When the externalmagnetic elds were imposed, the heat transfer capabilitydecreased, contributing to the impacts of ferrouid volume

force. Investigation on the ferrouids PHP with imposedexternal magnetic elds was an interdisciplinary research,which revealed insightful potential application in spacetechnology.

2.3 Flow visualization

Different thermo-hydrodynamic status results in different ow patterns, which in turn, reveal specic characteristicworking conditions and heat transfer capabilities of thePHPs. Zhang and Faghri [4] briey summarized theinvestigation of ow pattern as follows:

The oscillatory slug ow driven by the pressuredifference between the heating and cooling sections wasthe dominant ow pattern in PHPs. For closed loop PHPs,the oscillatory slug ow might be combined with thecirculation of working uid. As the heat ux in a closedloop PHP increased, the circulation of working uid might

suppress the oscillating ow, and the ow pattern couldchange to the circulating slug ow. At even higher heat ux, the directional slug ow would change to directionalannular ow. Recent experimental investigations on theow visualization were listed in Table 1 [8,9,11,12,24 – 35].

3 Theoretical analyses

Although the PHPs have become a hot topic these yearsand many experimental studies have been conducted, themechanism of the uid ow and heat transfer behaviors of

the PHPs have not yet been well understood. Most of therecent theoretical studies have been focused on thenonlinear analyses of the pulsating or oscillating processesof the PHPs.

3.1 Nonlinear behavior analyses

In order to acquire more insightful understandings of thePHPs, especially their oscillating motions in regard to thenonlinear behavior, the nonlinear analytical methods have

been introduced to cooperate with the correspondingtheoretical studies of the PHPs.

Ma and Zhang [36] proposed a second-order model

using the central composite design method to express therelationship between the heat transfer performance of alooped copper-water OHP and the affecting factors of thecharging ratio, the inclination angle, and the heat input.The data analytical results indicated that the effect of theinclination angle on the heat transfer rate was the most signicant, followed by the effects of the heat input and thecharging ratio. However, many other issues, such as thenumber of turns, the inner diameters and the effectivelength of the evaporator and condenser etc., have not yet

been considered in their model.A series of mathematical models including the effects of

cavity size, capillary radius, heat ux level, working uidand bubble type on the start-up characteristics of theoscillating motion were investigated by Qu and Ma [33] to

better understand the heat transfer mechanism of a PHP. It was found that the cavity sizes on the capillary inner surface strongly affected the start-up performance, whichcan be improved by using a rougher surface, controllingthe vapor bubble type, and selecting the suitable workinguids.

Based on the experiments of the ow visualization,Sakulchangsatjatai et al. [34] introduced one of the most exhaustive models for the CEPHP based on the previous

166 Front. Energy 2013, 7(2): 161 – 173


7/13

Table 1 A brief summary of ow visualization experiments in recent years

Researchers Materials Scales structure Working uids Conclusions and comments

Gi et al. [24] Teon Open/Closed loop, ow pathgeometry is circular, 20 parallel

channels, ID 2 mm

R142b One of the earliest ow visuali-zation experiments on PHPs.

Hosoda et al. [25] Glass Closed loop, circular, 20 parallelchannels, ID 1.2 mm

water The propagation procession of vapor bubbles and a numerical

simulation of oscillating werereported, and the PHP performed beat at a charge ratio of 60%.

Lee et al. [26] Brass, Acrylic Closed loop, rectangular channelgeometry, 8 parallel channels, ID

1.5 mm

Ethanol The generation and extinction of bubbles resulted in the oscilla-

tion movement. Most activeoscillation is observed in bottomheating mode with a charge ratio

of 40 – 60%.

Tong et al. [27] Pyrex glass Closed loop, circular,14 parallelnumbers, ID 1.8 mm

Methanol Circulation was observed, andincreasing heat input led to the

circulation velocity. The circula-tion directions were random,which could be clockwise or

counter-clockwise.Cai et al. [28] Quartz, Copper Closed, open loop, circular, 12

and 50 parallel channels, ID 2.2,2.4 mm

Ethanol, water, acetone,ethanol, ammonia

Evaporation and boiling in thinliquid lm on the evaporator walland generation and collapse of tiny bubbles suspended in the

liquid slug were observed. Fluidswith low latent heats are recom-mended to promote oscillatory

motion.

Khandekar et al. [29] Glass/copper Closed loop, circular, 10 parallelchannels, ID 2 mm

Water, ethanol Effect of gravity is negligible.Bubble formation and collapse

are discussed.

Khandekar, et al. [30] Pyrex glass Closed loop, circular, 10 parallelchannels, ID 2 mm

R-123 An empirical correlation was proposed. Flow oscillates with

low amplitude/ high frequency at horizontal mode.

Khandekar and Groll [31] Glass/copper Closed loop, circular, 2 parallelchannels, ID 2 mm

Ethanol PHP did not operate in thehorizontal mode. Different

ranges of heat input resulted indifferent ow patterns.

Xu et al. [32] Glass/copper Closed loop, circular, 8 parallelchannels, ID 2 mm

Water, methanol High-speed ow visualizationresults were presented. There

existed the bulk circulation ow,which lasts longer and the local

ow direction switch ow.

Qu and Ma [33] Glass Closed loop, circular, 4 parallelchannels, ID 3 mm

Water A theoretical analysis was con-ducted to determine the primary

factors affecting the startupcharacteristics of a PHP. Thestartup visualization was applied

to aid the investigation.

Sakulchangsatjatai et al.[34]

Pyrex glass Closed end, circular, number of turns 2, ID 2 mm

R123 With the help of a visualizationstudy, a mathematical model that

could ef ciently represent the behavior of the working uid in a

CEPHP in an inclined positionwas proposed.

Chen et al. [35] Glass Closed loop, ow path geometryis circular, ID 2 mm, the number

of parallel channels is 10

Deionized water Flow visualization was carriedout for better understanding of the mechanism of PHP alongwith the mathematical model

proposed.



8/13

work of Faghri and Zhang [37,38], by adding empiricalassumptions to the nucleate boiling frequency, the bubblelength and the liquid lm thickness.

Arslan and Özdemir [39] conducted a series of experiments on an OHP, consisting of three interconnectedcolumns of which the dimensions were large enough toneglect the effect of the capillary forces. A correlationfunction of the dimensionless numbers such as Kinetic

Reynolds C pΔT

hfg

(where C p is the heat capacity, ΔT is the

temperature difference, and hfg is the heat transfer coef cient) and the geometric parameters were proposedto express the overall heat transfer coef cient.

Based on neural network, an approach of nonlinear autoregressive moving average model with exogenousinputs (NARMAX) was applied to the thermal instabilityof the CLPHP by Chen et al. [40]. Their workapproximated the nonlinear behavior of the CLPHP witha linear approximation method that can establish therelationship among the response temperature differences

between the evaporator, adiabatic, and condenser sections.Lee and Chang [41] introduced a methodology of a

nonlinear autoregressive with exogenous (NARX) neuralnetwork to analyze the thermal dynamics of a PHP in bothtime and frequency domains. The derived models yielded asatisfactory consistence between the measured and pre-dicted results.

The articial neural network (ANN), usually calledneural network (NN), is a mathematical model and isinspired by the structure and functional aspects of

biological neural networks. Modern neural networks arenonlinear statistical data modeling tools and are usuallyused to model complex relationships between inputs andoutputs or to nd patterns in data. The main disadvantage

of using ANN is that it requires a large diversity of trainingfor real-world operation. Another drawback of the ANN isthat the algorithms are not linked to the physical

phenomena heading the dynamics of the system and canonly predict the PHP behavior in the range of the present experimental data.

Song and Xu [42] carried out a comprehensive study onthe chaotic behaviors of the PHPs. The nonlinear analysisthey used was based on the recorded time series of temperatures on the evaporation, adiabatic, and condensa-tion sections. They conrmed that the PHPs weredeterministic chaotic systems, not periodic or randomsystems. Three typical attractor patterns were identied.Hurst exponents and Kolmogorov entropies were appliedto present the thermal performance and chaotic behavior of the PHPs.

Yuan et al. [43] proposed a model for the uid ow andheat transfer characteristics on the liquid slug andneighboring vapor plugs in a PHP. A new energy equationfor the liquid slug was proposed by the aid of Lagrangemethod and the latent heat was used as the outer heat input in the vapor energy equation. The gravity effect could be

reasonably demonstrated by a forced vibration of the singledegree of the freedom system with viscous damping.

3.2 Numerical simulation

Previous theoretical models for the PHPs were principallyfocused on the lumped, one dimensional, or quasi-one-dimensional, and many unrealistic assumptions wereintroduced [4]. Owing to the development of newtheoretical models, recent numerical simulations havegained different approaches.

Hemadri et al. [44] proposed a three dimensional

(Continued )Researchers Materials Scales structure Working uids Conclusions and comments

Lin et al. [8] PDMS Closed loop, ow path geometryis circular, ID 2 mm, 12 parallel

channels

Methanol, ethanol, water The ow visualization of PDMSPHP with three different working

uids was conducted.

Yang et al. [9] Aluminum, transparent

polycarbonate plate

Closed loop, ow path geometry

is rectangle, the number of parallel channels 40/60

Ethanol Four typical ow modes were

observed.

Khandekar et al. [11] Copper, glass Closed loop, ow path geometryis circular, ID 2mm, number of

turns is 2

Ethanol Four quasi-steady states, eachcharacterized by a unique speci-c tow-phase ow pattern andcorresponding effective deviceconductance, were observed.

Lips et al. [12] Glass Closed loop, ow path geometryis circular, ID 2.4mm, single tube

Pentane The visualization of a single branch of a PHP, of which the

test section was adiabatic or non-adiabatic, was conducted. Themain heat transfer mechanism

was discussed.

168 Front. Energy 2013, 7(2): 161 – 173


9/13

computational heat transfer simulation to complement their experimental study with a high resolution infrared camerato obtain spatial temperature proles of the radiators,which were embedded with the PHP in order to conjugateheat transfer conditions on their surface. The simulationindicated that the advantage of any enhanced thermal

conductivity device embedded in a radiator plate (such asthe PHP) would decrease, as the absolute value of thethermal conductivity of the enhancement device/structureincreased beyond a particular value. This phenomenon wasin accordance with the extended surface heat transfer analogy. One of the experimental results and simulationconsequence were exhibited in Fig. 7.

Liu and Hao [45] developed a mathematical model based on the volume of uid (VOF) method to investigateheat and mass transfer behaviors in a CLOHP, byconsidering the effects of the vapor-liquid interface andsurface tension. Under various working conditions, the

proposed model could successfully simulate the initialdistribution of the working uids in the CLOHP, thesophisticatedow pattern including bubbly ow, slugow,semi-annular/annular ow, back ow, and the ow patterntransition during the start-up processes of the CLOHP. Thesemi-annular/annular ow and slug ow at the initial stageof the working uid circulation were shown in Fig. 8.

Recently, a novel numerical model was developed byMameli et al. [46], taking into account the effects of thelocal pressure losses due to the presence of turns, whichwere neglected by previous models. The CLPHPs weresimulated under different working conditions, such as

different working uids of ethanol, R123 and FC-72,different number of turns, different inclination angles aswell as different inputs heat uxes at the evaporator.Although the simulation results of the liquid momentum,the maximum tube temperature and the equivalent thermalresistances were in good qualitative and quantitative

accordance with the experimental data given in literature,further direct experimental validations are still in demandto test the practical application of the numerical models.

Xu et al. [47] proposed a 3D unsteady model of vapor-liquid two phase ow and heat transfer in a at-plate PHP(FP-PHP). A numerical simulation was conducted to studythe thermal-hydrodynamic characteristics in two different type of the FP-PHP, of the traditional FP-PHP havinguniform channels and the FP-PHP with micro groovesincorporated into the evaporator section. The one withmicro grooves was effective for the heat transfer enhance-ment and possessed a smaller thermal resistance at high

heat loads.In order to better understand the enhanced heat transfer behaviors of the PHP/OHP by the magnetic uids, Tang et al. [48] developed a 2D numerical model based onCOMSOL Multiphysics software combined with user dened function. Preliminary numerical simulations werecarried out in coupling with three elds of thermal, uidow and magnetic. The additional volumetric force caused

by the external static magnetic elds was imposed to themagnetic uids of the numerical model. The inuences of the single-phase magnetic uid on the enhanced heat transfer characteristics for two parallel plates were

Fig. 7 Comparison of experimental and simulated thermal proles with working embedded PHP at different heat inputs [44]: (a) 120 Winput power (i) simulated with K eff = 1800 W/(m$K) (ii) corresponding experimental prole; (b) 150 W input power (i) simulated with K eff = 2300 W/(m$K) (ii) corresponding experimental prole



10/13

simulated under non-uniform static magnetic eld [48].The typical distributions of the magnetic ux density andthe magnetic vector potential were displayed in Fig. 9. The

pressure proles inside the two parallel plates were presented in Fig. 10, at the inlet velocity of 3 m/s and at the heating ux of 20 W/cm2 on both top and bottom

plates.The numerical results showed the heat transfer coef -

cient of the magnetic uids between the two parallel plates

can be enhanced by the external static magnetic

elds, asgiven in Fig. 11. The uid ow was in the range of laminar ( Re< 105) at various inlet velocities from 1 to 3 m/s. It showed that the heat transfer enhancement by the externalmagnetic eld was obvious at lower inlet velocity and

decreased as the inlet velocity increased because of incomplete heat exchanging.

4 Applications of PHPs

The PHPs/OHPs are now mainly applied in the cooling of electronic devices and superconducting magnets, in energysaving systems for heat recovery [49 – 51] and other eldsdemanding highly ef cient heat transfer rates. However,the commercial availabilities are still quite limited [4].According to previous reviews and reports, few manufac-tured PHPs, regarded as standard items, have beencommercialized up to the present.

Maidanik et al. [52,53] investigated a compact CLPHP

Fig. 8 Vapor-liquid two-phase ow in adiabatic sections of vertical tubes at the initial stage of working uid circulation (t = 9 s) [45]

Fig. 9 Distribution of magnetic ux density and magnetic vector potential

Fig. 10 Pressure prole of magnetic uids under external staticmagnetic eld

170 Front. Energy 2013, 7(2): 161 – 173


11/13

cooler made of a copper tube with an inner diameter of 1.2 mm for the application of electronic cooling. TheCLPHP, charged with R141b, water and methanolrespectively, was equipped with a light copper radiator,as shown in Fig. 12.

The experiments conducted by Nuntaphan et al. [54]indicated that the heat transfer rate of the wire-on-tube heat exchanger can be enhanced by using an OHP. Mito et al.[55] proposed a highly effective cooling technique for asuperconducting magnet incorporating the cryogenic OHPas the cooling panels in the coil windings.

As reported by Srimuang and Amatachaya [51], the PHPhad already been applied in energy saving systems.Recently, Supirattanakul et al. [56] applied a CLOHPwith check valves (CLOHP/CV), as illustrated in Fig. 13,in order to reduce the energy consumption in the split typeair conditioning system. The CLOHP/CV was fabricatedwith the copper tube having an inner diameter of 2.03 mmand charged with the working uids of R134a, R22 and

R502. Compared to the conventional air conditioningsystem without CLOHP/CV, the system coupled withCLOHP/CV achieved the highest value of 14.9% and17.6% for COP and EER, respectively.

Considering the increasing practical demands and huge potential markets for highly ef cient heat transfer rates, it can be concluded that the PHPs/OHPs will denitelyattract more interest and applications in the cooling of electronic devices and superconducting magnets, in energysaving systems for heat recovery.

5 Conclusions

Owing to their outstanding heat transfer capabilities and

Fig. 11 inuences of the inlet velocity and heat transfer coef cient for various inlet velocities with/without magnetic elds [48]: (a)Inuences of inlet velocity; (b) heat transfer coef cient

Fig. 12 External view of a cooler on the basis of CLPHP [51]

Fig. 13 Position of check-valves on OHP [54]



12/13

simple structure, the PHPs/OHPs have received worldwidefocus in academic and engineering elds. Meanwhile, thecomplicated operational pulsating behavior and two-phaseow mechanism in the channels of the PHPs have alsogained much attention.

As mentioned above, a great deal of experimental

studies indicated that the visualization technique combin-ing the scrutiny investigated images of the ow patternsand the experimental data of heat transfer performance,contributed a lot to further understanding of the thermo-hydrodynamic mechanism within the PHPs. The char-acteristics of the start-up procedure and dry-out phenom-ena are presently the main research topics. With theapplications of nanouids and other functional thermaluids to the heat transfer enhancement of the PHPs/OHPs,the outstanding heat transfer capabilities of the PHPs willgain more attentions in various elds.

Many practical and sophisticated mathematical models

of the PHPs are expected to be proposed for theoreticalanalyses, in particular of the nonlinear behavior analyticalmethod. Owing to the limitations of the two-phase owtheories, the mundane of the pulsating or oscillating ow

behaviors and heat transfer mechanisms are requiredfurther development. Moreover, numerical simulationswill denitely attract extensive attention with the fast development of supercomputers.

It is worth mentioning that the PHPs will be graduallyexpanded from high-tech elds, like aerospace andelectronics cooling to commercial and civilian elds.

Acknowledgements This project is nancially supported by the National

Natural Science Foundation of China (Grant No. 51006069).

References

1. Akachi H. Structure of micro-heat pipe. US Patent 5219020, 1993-

06-15

2. Akachi H. L-type heat pipe. US Patent 5490558, 1996-02-13

3. Khandekar S. Thermo-hydrodynamics of closed loop pulsating heat

pipe. Dissertation for the Doctoral Degree. Germany: University of

Stuttgart, 2004

4. Zhang Y W, Faghri A. Advances and unsolved issues in pulsating

heat pipes. Heat Transfer Engineering, 2008, 29(1): 20 –

445. Charoensawan P, Terdtoon P. Thermal performance of horizontal

closed-loop oscillating heat pipes. Applied Thermal Engineering,

2008, 28(5,6): 460 – 466

6. Yang H H, Khandekar S, Groll M. Performance characteristics of

pulsating heat pipes as integral thermal spreaders. International

Journal of Thermal Sciences, 2009, 48(4): 815 – 824

7. Jiao A J, Ma H B, Critser J K. Experimental investigation of

cryogenic oscillating heat pipes. International Journal of Heat and

Mass Transfer, 2009, 52(15,16): 3504 – 3509

8. Lin Y H, Kang S W, Wu T Y. Fabrication of polydimethylsiloxane

(PDMS) pulsating heat pipe. Applied Thermal Engineering, 2009,

29(2,3): 573 – 580

9. Yang H H, Khandekar S, Groll M. Operational limit of closed loop

pulsating heat pipes. Applied Thermal Engineering, 2008, 28(1):

49 – 59

10. Lin Z R, Wang S F, Chen J J, Huo J P, Hu Y X, Zhang W.

Experimental study on effective range of miniature oscillating heat

pipes. Applied Thermal Engineering, 2011, 31(5): 880 – 886

11. Khandekar S, Gautam A P, Sharma P K. Multiple quasi-steady statesin a closed loop pulsating heat pipe. International Journal of Thermal

Sciences, 2009, 48(3): 535 – 546

12. Lips S, Bensalem A, Bertin Y, Ayel V, Romestant C, Bonjour J.

Experimental evidences of distinct heat transfer regimes in pulsating

heat pipes (PHP). Applied Thermal Engineering, 2010, 30(8,9):

900 – 907

13. Thompson S M, Cheng P, Ma H B. An experimental investigation of

a three-dimensional at-plate oscillating heat pipe with staggered

microchannels. International Journal of Heat and Mass Transfer,

2011, 54(17,18): 3951 – 3959

14. Liu J H, Shang F M, Liu D Y. Experimental study on enhanced heat

transfer characteristics of synergistic coupling between the pulsatingheat pipes. Energy Procedia, 2012, 16: 1510 – 1516

15. Thompson S M, Ma H B, Wilson C. Investigation of a at-plate

oscillating heat pipe with Tesla-type check valves. Experimental

Thermal and Fluid Science, 2011, 35(7): 1265 – 1273

16. Das S K, Choi S U S, Patel H. Heat transfer in nanouids – a review.

Heat Transfer Engineering, 2006, 27(10): 3 – 19

17. Ma H B, Wilson C, Borgmeyer B, Park K, Yu Q, Choi S U S,

Tirumala M. Effect of nanouid on the heat transport capability in an

oscillating heat pipe. Applied Physics Letters, 2006, 88(14): 143116

(3 pages)

18. Lin Y H, Kang S W, Chen H L. Effect of silver nano-uid on

pulsating heat pipe ther mal performance. Applied Thermal

Engineering, 2008, 28(11,12): 1312 –

1317

19. Qu J, Wu H, Chen P. Thermal performance of an oscillating heat

pipe with Al2O3 – water nanouids. International Communications in

Heat and Mass Transfer, 2010, 37(2): 111 – 115

20. Ji Y L, Ma H B, Su F M, Wang G Y. Particle size effect on heat

transfer performance in an oscillating heat pipe. Experimental

Thermal and Fluid Science, 2011, 35(4): 724 – 727

21. Riehl R R, Santos N. Water-copper nanouid application in an open

loop pulsating heat pipe. Applied Thermal Engineering, 2012, 42:

6 – 10

22. Wang S F, Lin Z R, Zhang W B. Experimental study on pulsating

heat pipe with functional thermal uids. International Journal of

Heat and Mass Transfer, 2009, 52(21,22): 5276 –

527923. Qiu S H, He Y Q, Bi Q C. Experimental investigation on pulsating

heat pipes lled with magnetic uid. Journal of Xi’an Jiaotong

University, 2010, 7(44): 6 – 8

24. Gi K, Sato F, Maezawa S. Flow visualization experiment on

oscillating heat pipe. In: Proceedings of 11th International Heat Pipe

Conference. Tokyo, Japan, 1999, 149 – 153

25. Hosoda M, Nishio S, Shirakashi R. Meandering closed loop heat

transport tube (propagation phenomena of vapor plug). In:

Proceedings of 5th ASME/JSME Joint Thermal Engineering

Conference. San Diego, USA, 1999, 1 – 6

26. Lee W H, Jung H S, Kim J H, Kim J S. Flow visualization of

oscillating capillary tube heat pipe. In: Proceedings of 11th

172 Front. Energy 2013, 7(2): 161 – 173


13/13

International Heat Pipe Conference. Tokyo, Japan, 1999, 131 – 136

27. Tong B Y, Wong T N, Ooi K T. Closed-looped pulsating heat pipe.

Applied Thermal Engineering, 2001, 21(18): 1845 – 1862

28. Cai Q, Chen R, Chen C L. An investigation of evaporation, boiling

and heat transport performance in pulsating heat pipe. In:

Proceedings of ASME International Mechanical Engineering

Congress & Exposition. New Orleans, USA, 2002, 99 –

10429. Khandekar S, Groll M, Charoensawan P, Terdtoon P. Pulsating heat

pipes: Thermo-uidic characteristics and comparative study with

single phase thermosyphon. In: Proceedings of 12th International

Heat Transfer Conference. Grenoble, France, 2002, 459 – 464

30. Khandekar S, Charoensawan P, Groll M, Terdtoon P. Closed loop

pulsating heat pipes, Part B: visualization and semi-empirical

modeling. Applied Thermal Engineering, 2003, 23(16): 2021 – 2033

31. Khandekar S, Groll M. An insight into thermo-hydrodynamic

coupling in closed loop pulsating heat pipes. International Journal of

Thermal Sciences, 2004, 43(1): 13 – 20

32. Xu J L, Li Y X, Wong T N. High speedow visualization of a closed

loop pulsating heat pipe. International Journal of Heat and Mass

Transfer, 2005, 48(16): 3338 – 3351

33. Qu W, Ma H B. Theoretical analysis of startup of a pulsating heat

pipe. International Journal of Heat and Mass Transfer, 2007, 50

(11,12): 2309 – 2316

34. Sakulchangsatjatai P, Chareonsawan P, Waowaew T, Terdtoon P,

Murakami M. Mathematical modeling of closed-end pulsating heat

pipes operating with a bottom heat mode. Heat Transfer Engineer-

ing, 2008, 29(3): 239 – 254

35. Chen P H, Lee Y W, Chang T L. Predicting thermal instability in a

closed loop pulsating heat pipe system. Applied Thermal Engineer-

ing, 2009, 29(8,9): 1566 – 1576

36. Ma Y X, Zhang H. Analysis of heat transfer performance of

oscillating heat pipes based on a central composite design. ChineseJournal of Chemical Engineering, 2006, 14(2): 223 – 228

37. Zhang Y, Faghri A, Shai M B. Analysis of liquid-vapor Pulsating

ow in a U-shaped miniature tube. International Journal of Heat and

Mass Transfer, 2002, 45(12): 2501 – 2508

38. Zhang Y, Faghri A. Heat transfer in a pulsating heat pipe with open

end. International Journal of Heat and Mass Transfer, 2002, 45(4):

755 – 764

39. Arslan G, Özdemir M. Correlation to predict heat transfer of an

oscillating loop heat pipe consisting of three interconnected

columns. Energy Conversion and Management, 2008, 49(8):

2337 – 2344

40. Chen P H, Lee Y W, Chang T L. Predicting thermal instability in aclosed loop pulsating heat pipe system. Applied Thermal Engineer-

ing, 2009, 29(8,9): 1566 – 1576

41. Lee Y W, Chang T L. Application of NARX neural networks in

thermal dynamics identication of a pulsating heat pipe. Energy

Conversion and Management, 2009, 50(4): 1069 – 1078

42. Song Y X, Xu J L. Chaotic behavior of pulsating heat pipes.

International Journal of Heat and Mass Transfer, 2009, 52(13,14):

2932 – 2941

43. Yuan D Z, Qu W, Ma T Z. Flow and heat transfer of liquid plug and

neighboring vapor slugs in a pulsating heat pipe. International

Journal of Heat and Mass Transfer, 2010, 53(7,8): 1260 – 1268

44. Hemadri V A, Gupta A, Khandekar S. Thermal radiators with

embedded pulsating heat pipes: Infra-red thermography and

simulations. Applied Thermal Engineering, 2011, 31(6,7): 1332 –

1346

45. Liu X D, Hao Y L. Numerical simulation of vapor-liquid two-phaseow in a closed loop oscillating heat pipe. In: Proceedings of the

ASME 2009 International Mechanical Engineering Congress and

Exposition, Lake Buena Vista, USA, 2009, 609 – 617

46. Mameli M, Marengo M, Zinna S. Numerical model of a multi-turn

closed loop pulsating heat pipe: effects of the local pressure losses

due to meanderings. International Journal of Heat and Mass

Transfer, 2012, 55(4): 1036 – 1047

47. Xu D H, Chen T F, Xuan Y M. Thermo-hydrodynamics analysis of

vapor-liquid two-phase ow in the at-plate pulsating heat pipe.

International Communications in Heat and Mass Transfer, 2012, 39

(4): 504 – 508

48. Tang X, Zhang H, Sha L L, Ju Y L. The ow and heat transfer

simulation of ferrouid in non-uniform static magnetic eld. In:

Proceeding of Multi-phase Flow Conference of Chinese Society of

Engineering Thermodynamics. Xi’an, China, 2012, 1206235 (in

Chinese)

49. Rittidech S, Dangeton W, Soponronnarit S. Closed-ended oscillat-

ing heat pipe (CEOHP) air-preheater for energy thrift in a dryer.

Applied Energy, 2005, 81(2): 198 – 208

50. Meena P, Rittidech S, Poomsa-ad N. Closed-loop oscillating heat

pipe with check valves (CLOHP/CVs) air-preheater for reducing

relative humidity in drying systems. Applied Energy, 2007, 84(4):

363 – 373

51. Srimuang W, Amatachaya P. A review of the applications of heat

pipe heat exchangers for heat recovery. Renewable & SustainableEnergy Reviews, 2012, 16(6): 4303 – 4315

52. Maidanik Y F, Dmitrin V I, Pastukhov V G. Compact cooler for

electronics on the basis of a pulsating heat pipe. Applied Thermal

Engineering, 2009, 29(17,18): 3511 – 3517

53. Dmitrin V I, Maidanik Y F, Pastukhov V G. Development and

investigation of compact cooler using a pulsating heat pipe. High

Temperature, 2010, 48(4): 565 – 571

54. Nuntaphan A, Vithayasai S, Vorayos N, Vorayos N, Kiatsiriroat T.

Use of oscillating heat pipe technique as extended surface in wire-

on-tube heat exchanger for heat transfer enhancement. International

Communications in Heat and Mass Transfer, 2010, 37(3): 287 –

29255. Mito T, Natsume K, Yanagi N, Tamura H, Tamada T, Shikimachi K,

Hirano N, Nagaya S. Development of highly effective cooling

technology for a superconducting magnet using cryogenic OHP.

IEEE Transactions on Applied Superconductivity, 2010, 20(3):

2023 – 2026

56. Supirattanakul P, Rittidech S, Bubphachot B. Application of a

closed-loop oscillating heat pipe with check valves (CLOHP/CV) on

performance enhancement in air conditioning system. Energy and

Building, 2011, 43(7): 1531 – 1535


Documents

study pipes