Advances and Unsolved Issues in Pulsating Heat Pipes - CiteSeerX

Heat Transfer Engineering, 29(1):20–44, 2008 C Taylor and Francis Group, LLC Copyright  ISSN: 0145-7632 print / 1521-0537 online DOI: 10.1080/01457630701677114

Advances and Unsolved Issues in Pulsating Heat Pipes YUWEN ZHANG Department of Mechanical and Aerospace Engineering, University of Missouri-Columbia, Columbia, Missouri, USA

AMIR FAGHRI Department of Mechanical Engineering, University of Connecticut, Storrs, Connecticut, USA

Pulsating (or oscillating) heat pipes (PHP or OHP) are new two-phase heat transfer devices that rely on the oscillatory flow of liquid slug and vapor plug in a long miniature tube bent into many turns. The unique feature of PHPs, compared with conventional heat pipes, is that there is no wick structure to return the condensate to the heating section; thus, there is no countercurrent flow between the liquid and vapor. Significant experimental and theoretical efforts have been made related to PHPs in the last decade. While experimental studies have focused on either visualizing the flow pattern in PHPs or characterizing the heat transfer capability of PHPs, theoretical examinations attempt to analytically and numerically model the fluid dynamics and/or heat transfer associated with the oscillating two-phase flow. The existing experimental and theoretical research, including important features and parameters, is summarized in tabular form. Progresses in flow visualization, heat transfer characteristics, and theoretical modeling are thoroughly reviewed. Finally, unresolved issues on the mechanism of PHP operation, modeling, and application are discussed.

INTRODUCTION

vapor flows to overcome certain limitations inherent in conventional heat pipes. In the 1990s, Akachi et al. [2] invented a new type of heat pipe known as the pulsating or oscillating heat pipe (PHP or OHP). The most popular applications of PHP are found in electronics cooling because it may be capable of dissipating the high heat fluxes required by next generation electronics. Other proposed applications include using PHPs to preheat air or pump water. This review article will describe the operation of pulsating heat pipes, summarize the research and development over the past decade, and discuss the issues surrounding them that have yet to be resolved. Pulsating heat pipes, like conventional heat pipes, are closed, two-phase systems capable of transporting heat without any additional power input, but they differ from conventional heat pipes in several major ways. A typical PHP is a small meandering tube that is partially filled with a working fluid, as seen in Figure 1 [3]. The tube is bent back and forth parallel to itself, and the ends of the tube may be connected to one another in a closed loop, or pinched off and welded shut in an open loop (see Figure 1a and 1b). It is generally agreed by researchers that the closed-loop PHP has better heat transfer performance [4, 5]. For this reason, most experimental work is done with closedloop PHPs. In addition to the oscillatory flow, the working fluid can also be circulated in the closed-loop PHP, resulting in heat

Evolution in the design of the heat pipe—a type of passive two-phase thermal control device—has accelerated in the past decade due to continuous demands for faster and smaller microelectronic systems. As modern computer chips and power electronics become smaller and more densely packed, the need for more efficient cooling systems increases. The new design of a computer chip at Intel, for instance, will produce localized heat flux over 100 W/cm2 , with the total power exceeding 300 W. In addition to the limitations on maximum chip temperature, further constraints may be imposed on the level of temperature uniformity in electronic components. Heat pipes are a very promising technology for achieving high local heat-removal rates and uniform temperatures on computer chips. True development of conventional heat pipes (CHP) began in the 1960s; since then, various geometries, working fluids, and wick structures have been proposed [1]. In the last 20 years, new types of heat pipes—such as capillary pumped loops and loop heat pipes—were introduced, seeking to separate the liquid and Address correspondence to Professor Amir Faghri, Department of Mechanical Engineering, University of Connecticut, Storrs, CT 06269, USA. E-mail: [email protected]

20

Y. ZHANG and A. FAGHRI

21

lished, followed by the details of the modeling and/or experiment: theoretical approaches, major assumptions, the material used to manufacture the PHP, the geometry and configuration of the flow channel, number of parallel channels, inclination angles, channel diameters, the working fluids tested, the charge ratios that they were tested at, range of heat transferred by the PHP, a summary of the conclusions drawn by the investigator, and other significant comments. This article also presents the principles of operation, flow visualization, heat transfer, and modeling, as well as a discussion of the unresolved issues in PHP research. Figure 1 Different PHPs: (a) closed-end, (b) closed-loop, (c) closed-loop with check valve, and (d) PHP with open ends.

transfer enhancement. Although an addition of a check valve (see Figure 1c) could improve the heat transfer performance of the PHPs by making the working fluid move in a specific direction, it is difficult and expensive to install these valves. Consequently, the closed-loop PHP without a check valve becomes the most favorable choice for the PHP structures. Recently, PHPs with a sintered metal wick have been prototyped by Zuo et al. [6, 7] and analyzed by Holley and Faghri [8]. The wick should aid in heat transfer and liquid distribution. There has also been some exploration into pulsating heat pipes in which one or both ends are left open without being sealed (see Figure 1d) [9–11]. Like a CHP, a PHP must be heated in at least one section and cooled in another. Often the evaporators and condensers are located at the bends of the capillary tube. The tube is evacuated and then partially filled with a working fluid. The liquid and its vapor will become distributed throughout the pipe as liquid slugs and vapor bubbles. As the evaporator section of the PHP is heated, the vapor pressure of the bubbles located in that section will increase. This forces the liquid slug toward the condenser section of the heat pipe. When the vapor bubbles reach the condenser, it will begin to condense. As the vapor changes phase, the vapor pressure decreases, and the liquid flows back toward the condenser end. In this way, a steady oscillating flow is set up in the PHP. Boiling the working fluid will also cause new vapor bubbles to form. The unique feature of PHPs, compared with conventional heat pipes, is that there is no wick structure to return the condensate to the heating section, and therefore there is no countercurrent flow between the liquid and the vapor. Due to the simplicity of the structure of a PHP, its weight is lower than that of conventional heat pipe, which makes PHP an ideal candidate for space application. Research on PHPs can be categorized as either experimental or theoretical. While experimental studies have focused on either visualizing the flow pattern in PHPs or characterizing the heat transfer capability of PHPs, theoretical examinations attempt to analytically and numerically model the fluid dynamics and/or heat transfer associated with oscillating two-phase flow. The existing experimental and theoretical research and their parameters are summarized in Table 1. The table lists the primary investigators, reference number, and the year the study was pubheat transfer engineering

PRINCIPLES OF OPERATION Although simple in their construction, PHPs become complicated devices when one tries to fully understand their operation: the thermodynamics driving PHP operation, the fluid dynamics governing the two-phase oscillating flow, heat transfer (both sensible and latent), and the physical design parameters of the PHP must all be considered. Thermodynamic Principles Heat addition and rejection and the growth and extinction of vapor bubbles drive the flow in a PHP. Even though the exact features of the thermodynamic cycle are still unknown, Groll and Khandekar [12] described it in general terms using a pressure/enthalpy diagram as seen in Figure 2. The temperature and vapor quality in the evaporator and condenser are known, or can be assumed, so the state at the outlets of the evaporator and condenser are known. Starting at the evaporator inlet, point A on the P-h diagram, the processes required to get to point B on the diagram can be simplified to heat input at a constant pressure combined with isentropic pressure increase due to bubble expansion. As one travels through the adiabatic section from the evaporator to the condenser, the pressure decreases isenthalpically. The thermodynamic process between the condenser’s inlet and outlet are complicated, but can be simplified to constant pressure condensation with negative isentropic work. An isenthalpic pressure drop in the adiabatic section completes the cycle. Because of the numerous assumptions made in this description, thermodynamic analysis is insufficient to study PHPs.

Fluid Dynamic Principles Fluid flow in a capillary tube consists of liquid slugs and vapor plugs moving in unison. The slugs and plugs initially distribute themselves in the partially filled tube. The liquid slugs 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/vapor interface. The slugs are separated by plugs vol. 29 no. 1 2008

N/A

Maezawa et al. None [27] (1996)

Copper

Copper

Materials

N/A

Teflon

N/A

None

Glass

N/A

Miyazaki and None Arikawa [49] (1999) Nishio [29] None (1999)

Gi et al. [4] (1999)

Copper/ polycarbonate

A continuous distribution of void fraction was assumed.

Miyazaki and Wave equation of pressure was Akachi [48] derived. (1998)

N/A

Copper Pressure Miyazaki and Differential oscillation and relationship Akachi [28] oscillatory flow between (1996) reciprocally propagation excite each wave of p and other. α

N/A

None

Akachi et al. [2] (1996)

Assumptions

Theoretical approaches

Investigator (year)

Open/ closed

Closed

Closed

Open

Open

Open/ closed loop

Table 1 Summary of modeling and experiments on pulsating heat pipes (PHP)

Circular

Circular

Rectangular

Circular

Circular

Circular

Circular

Flow path geometry

10

4

50

60

80

254, 1000

Parallel channels

1

90, 0, −90

30–50

90

2

1.8, 2.4, 5.0

2

−90

Working fluid

42

25–70

50

50

20–180

50–1000

70–100, 450

Charge ratio q (W)

R142b

30–70

20–50, 60–100

20–100 70 Water, soapsuds, ethanol, R141b

R-142b

R142b

Water, R142b

0.7, 1.2 R142b

D (mm)

90, 0, −90

Inclination angle (◦ )

(Continued on next page)

Thermal resistance is independent of heat input and inclination angle if the number of turns is greater than 80. R142b performs better than water. Bottom heat mode performs better than top heat mode. Oscillation has no specific periodical feature. Optimized charge ratio for bottom and top heat modes are 70% and 35%, respectively. A symmetrical wave is obtained at proper charge ratio. The progressive wave for a closed-loop channel and the standing wave for a closed-end channel can be obtained from the wave equation. Measured wave velocities fairly agreed with Eq. (14). PHP performed best with charge ratio of 35%. PHP thermal conductivity is 500 times higher than copper. Heat transfer rate higher than the conventional heat pipe with the same diameter. Flow visualizations

Conclusions and comments

Circular

Open

Dobson [18] (2004)

2

2

Circular

Unlooped with open end

Dobson and Hams [9] (1999)

—

Rectangular

Closed

Mass, momentum, Liquid and vapor Copper phases are at and energy local equations of equilibrium. 1-D transient Convection two-phase flow dominate in are solved using axial direction. SIMPLEC scheme. Vapor is an ideal Copper Explicit finite gas. difference Incompressible scheme is used liquid. No heat to solve transfer in equations for liquid. motion and heat transfer. Mass, momentum, Vapor is ideal gas. Copper Momentum of and energy vapor bubble equations are and liquid film solved using are neglected. explicit scheme.

Zuo et al. [7] (2001)

—

8

Triangular

Rectangular

20

Parallel channels

Oscillatory flow Vapor is an ideal Sintered and plate Closed modeled similar gas. Laminar copper to mechanical liquid flow. vibration with Heat transfer is viscous neglected. damping.

Brass, acrylic

Circular

Flow path geometry

Zuo et al. [6] (1999)

N/A

Closed

Materials

Lee et al. [15] None (1999)

Assumptions

Open/ closed loop Closed

Theoretical approaches

Thin liquid film, Glass Hosoda et al. Numerical pressure loss at [19] (1999) solution of 1-D liquid and vapor bends, and viscous flow. dissipation are neglected.

Investigator (year)

Table 1 (Continued)

90–90

0

—

0, 90

30–90

90

Inclination angle (◦ ) Water

Working fluid

3.34

3.34

—

—

Water

Water

Water

Water

1.5 × Ethanol 1.5

1.2

D (mm)

—

40–80

40–80

20–80

30–90

—

10–250

5–250

—

80–220

Charge ratio q (W)

The dominate forces for liquid plug motion are vapor pressure difference, friction and gravity.

Thrust produced by PHP is 0.0027N. Open-ended PHP mounted on a float in water.

PHP performed best at charge ratio of 60%. Numerical results for pressure are higher than experimental results but oscillation is simulated. Oscillation caused by formation or extinction of bubbles. No fluid circulation. Most active oscillation is observed in bottom heating with charge ratio of 40–60%. The wick structure distributes liquid evenly, and reduces local temperature fluctuation. Thermal resistance is as low as 0.16◦ C/W at an optimum charge ratio of 70%. Experiment shows that performance of PHP is sensitive to charge ratio. Numerical results were not reported in the paper.

Conclusions and comments

Stainless steel

N/A

N/A Adiabatic, heat input was modeled as a sudden pressure rise. No liquid film. N/A Copper

N/A

N/A

None Kiseev and Zolkin [43] (1999)

Wong et al. Mass and [50] (1999) momentum balances in a Lagrangian frame.

None

None

Lin et al. [30] (2000)

Lin et al. [31] (2001)

Tong et al. None [20] (2001)

Shafii et al. Mass, momentum, Vapor is an ideal N/A gas. [13] (2001) and energy Incompressible equations for liquid. No each liquid slug pressure loss in and vapor plug bends. are solved.

Pyrex glass

Copper

—

Dobson [11] (2005)

Open/ closed

Closed

Open

Open

Open

Open

—

Circular

Circular

Circular

Circular

Circular

Circular

—

4

14

40

40

4

46

—

1.5, 3.0 Water

−90

Methanol

FC-72, FC-75

Acetone

—

Acetone

Water

1.8

1.75

1.75

—

1.1

3.34

0, 90

0, 90

0, 90

0

0

0

61.4, 89.47

60

30–50

25–50

50

60

—

0–80

50

140– 2040

140– 2040

—

15–300

—

(Continued on next page)

Optimum charge ratio is 38%. Operation is better in horizontal. No operation at 25% charge. Optimum charge ratio is 50%. FC-72 performed better than FC-75. Operation is better in horizontal. No operation at 25% charge. Performance is independent of orientation. Circulation was observed, and circulation velocity increases with increasing heat input. Circulation can be either clockwise or counter-clockwise. Majority (95%) of heat is transferred by sensible heat. Latent heat serves only to drive oscillating flow. Effect of gravity is negligible.

Use of an open PHP to pump water. Mass flow rate of the pump is 0.2 mg/s for 100 mm height. Evaporator temperature is increased by 30% by increasing acceleration from −6g to 12g. The pressure pulse induces oscillation but is damped out by friction between the liquid and pipe wall.

Closed, open

N/A

Copper Heat input and charge ratio from 52 data sets are inputted to ANN.

—

None

Artificial Neural Network (ANN) is used to predict PHP performance.

N/A

Khandekar et al. [16] (2002)

Khandekar et al. [56] (2002)

Khandekar et al. [24] (2002)

Glass/ copper

Closed

Closed

Aluminum/ glass, Closed copper/ glass

Quartz, copper

N/A

None

Materials

Cai et al. [17] (2002)

Assumptions

Open/ closed loop Open/ closed

Theoretical approaches

Radial conduction N/A Shafii et al. Thin film only in thin [14] (2002) evaporation and film. Neglecting condensation shear stress at were solved to liquid-vapor get latent heat interface. transfer coefficient.

Investigator (year)

Table 1 (Continued)

12, 50

4

Parallel channels

Circular

Circular

10

10

Rectangular, 12, 12, 10 rectangular, circular

Circular

Circular

Flow path geometry

0, 45, 90

90

0–90

45, 0

—

Inclination angle (◦ )

Working fluid 64.21– 0–119 89.5

Charge ratio q (W)

2

2

Water, ethanol

Ethanol

2.2×2, Water, ethanol 1.5×1, 2.0

0–100

0–100

10–70

5–15

—

25–70

50, 40– 100–600 2.4, 2.2 Ethanol, 60 water, acetone, ethanol, ammonia

1.5, 3.0 Water

D (mm)

Heat transfer is due mainly to the exchange of sensible heat. Higher surface tension results in a slight increase in total heat transfer. No operation for high charge ratio. Propagation and extinction of bubbles are observed. Fluids with low latent heats are recommended to promote oscillatory motion. The metal PHP did not operate in horizontal orientation but operated vertically as thermosyphon. Performance depends on orientation, charge ratio, and cross-section geometry. ANNI is trained by experiments. Effects of diameter, number of turns, length, inclination angle, and fluid properties are not in the model. Effect of gravity is negligible. Bubble formation and collapse are discussed.

Conclusions and comments

Copper

Charoensawan None et al. [34] (2003)

N/A

N/A

Vapor is an ideal Liquid–vapor Zhang and gas. pulsating flow Faghri [53] in PHP with (2003) arbitrary number of turns is investigated.

Liquid slug Heat transfer in Copper oscillation is evaporator is described by modeled as the balance of convective thermally boiling in a driven, tube. capillary, frictional, and elastic restoring forces. Evaporation and Vapor is saturated N/A Zhang and and isothermal. condensation Faghri [10] Neglecting on thin film left (2002) behind by liquid inertia, shear stress, and slug is solved. interfacial thermal resistance effects. Vapor is an ideal N/A Zhang et al. Liquid–vapor gas. [52] (2002) pulsating flow in a U-shaped miniature tube is investigated.

Ma et al. [32] (2002)

Circular

Open

Closed

Circular

Circular

Circular

Open

Open

Circular

Open

10–46

Any

2

1

4

—

−90

—

—

—

Acetone

—

—

—

—

50 1.0, 2.0 Water, ethanol, R-123

—

−90

0, 90

—

1.67

0

0

Overall heat transfer is dominated by sensible heat transfer. Frequency and amplitude are not affected by surface tension.

Minimum onset temperature difference is 15◦ C. Range of operational temperature difference is studied. Model underpredicts temperature drops.

(Continued on next page)

The amplitude and frequency of oscillation were correlated to the heat transfer coefficients and temperature difference. — Amplitude and circular frequency decrease by decreasing the lengths of the heating and cooling sections. Increasing the charge ratio resulted in a decrease of amplitudes and an increase of circular frequency. 200–1100 Gravity has a significant effect on PHP performance. Minimum number of turns is needed for a horizontal PHP to operate. Performance improves by increasing the diameter and the number of turns.

—

—

5–20

Assumptions

N/A

N/A

N/A

N/A

N/A

Theoretical approaches

None

None

Investigator (year)

Khandekar et al. [23] (2003)

Khandekar et al. [33] (2003)

None Khandekar and Groll [21] (2004)

Rittidech et al. None [3] (2003)

Rittidech et al. None [35] (2005)

Table 1 (Continued)

Copper

Copper

Glass/ copper

Copper

Pyrex, glass

Materials

Open

Open

Closed

Closed

Closed

Open/ closed loop

Circular

Circular

Circular

Circular

Circular

Flow path geometry

0

0, 90

0, 90

0, 90

Inclination angle (◦ )

— 16 per PHP, 32 PHPs

38–84

2

10

20–58

Parallel channels R-123

Working fluid 50

5,000– 70,000 W/m2

Charge ratio q (W)

Conclusions and comments

Flow oscillates with low amplitude/high frequency at horizontal mode. Capillary slug and semi-annular/ annular flow depend on heat input and inclination angle. Experimental results are correlated using empirical model. Optimum charge ratios 0–100 5–65, 2 Water, for three fluids are 30, 5–60, ethanol, 20, and 35%, 5–25 R-123 respectively. Orientation affects performance. Horizontal mode did not work. 2 Ethanol 0–100 14.8–74.4 PHP did not operate in horizontal mode. Capillary slug flow and annular flow depends on heat input. 50 2,000– Ethanol, 0.55, For R-123, heat flux 12,000 1.05, Water, increases with W/m2 R123 2.03 increasing diameter, but the trend is the opposite for ethanol. Correlation for heat transfer was proposed based on experiments. 2 Water, 50 1460–3504 PHPs were used as an air R-123 (Total) preheater for energy thrift in a dryer. Performance improves with increasing evaporator temperature. PHP with R-123 performs better than PHP with water.

2

D (mm)

N/A

None

None

None

Gu et al. [44] (2004)

Riehl [36] (2004)

Zhang et al. [5] (2004)

—

Circular

Open and closed

Open

Sakulchangsatjatai et al. [51] (2004)

Circular

Circular

Rectangular

Circular

Mass, momentum, Vapor is an ideal — gas. and energy Incompressible equations for liquid. No each liquid slug pressure loss in and vapor plug bends. are solved. Katpradit et al. None N/A Copper [37] (2005)

Open

Closed

—

Open and closed

Copper

Aluminum

N/A

Copper

N/A

N/A

Vapor bubbles are uniformly distributed.

Liang and Ma Vapor bubble is [54] (2004) considered as gas spring.

10, 20, 30

—

6

13

96

—

0.66, 1.06, 2.03

—

−90

0, 90

1.18

1.5

50–60

1×1

5–60

60–90 FC-72, ethanol, water

—

—

—

50 R-123, ethanol, water

—

10–50

1.4–5.9

—

Acetone, 50 ethanol, isopropyl, alcohol, methanol, water

R114

—

1, 2, 5 Water

90

0, 90

—

0

(Continued on next page)

Isentropic bulk modulus generates stronger oscillations than the isothermal bulk modulus. PHP performed better in microgravity than normal or hyper gravity. New equation of critical diameter in microgravity is developed. Performance is better when operating in a horizontal orientation. Better performances were obtained when acetone was used in vertical orientation and methanol was used on horizontal orientation. Open loop PHP did not work. A minimum heat input is necessary to initiate pulsating flow. Closed loop PHP. Optimum charge ratio is 70% for all three fluids. Model is same as Shafii et al. (2001). The predicted heat transfer rate is compared to experimental results in literature. Heat flux increases with decreasing evaporator length, and increasing latent heat and number of turns. Correlation to predict heat transfer characteristics was proposed.

N/A

Copper

Stainless steel, copper

Closed

—

Circular

Circular

24

24

90

None

N/A

Ma et al. [45] (2006)

90

0

8

90

None

Circular

8

Inclination angle (◦ )

Cai et al. [40] (2006)

Closed

Circular

Parallel channels

−90, −45, 90

Copper

Closed

Flow path geometry

Mass, momentum Liquid is incompressible. and energy Neglecting equations are losses at bends. solved for PHP Saturated vapor with sintered with negligible copper wick flow friction. and varying channel diameter.

N/A

Xu and Zhang None [41] (2005)

Glass/ copper

Materials

Open/ closed loop

Holley and Faghri [8] (2005)

N/A

None

Xu et al. [25] (2005)

Assumptions

Theoretical approaches

Investigator (year)

Table 1 (Continued)

Water

FC-72

1.65

20–60

10–25.6

Nanofluid 50 (water with diamond nanoparticles)

5–336

40, 55, 100–400 70

70

10, 30

Charge ratio q (W)

Water, 70 methanol

Working fluid

1.397, Water 1.568

2

2

D (mm)

Flow circulation was observed. Flows in some channels are in the opposite direction of bulk circulation Both startup and steady thermal oscillations were studied. Oscillation flow at low heating power displays random behavior and becomes quasi-periodic at high heat power. Varying diameter between parallel channels induces flow circulation and may increase heat transfer capacity. Bottom heat mode performed better than top heat mode. Sensitivity to gravity decreases when increasing the number of channels. Minimal temperature difference and fluctuation appear at operating temperature between 120◦ C and 160◦ C. At 100 W, the temperature difference can be reduced from 42◦ C to 25◦ C for the nanofluid OHP as opposed to the pure water OHP.

Conclusions and comments

N/A

Chiang et al. None [47] (2007)

Heat conduction The contribution Khandekar of PHP is and Gupta in the radiator modeled using plate is solved [42] (2007) an effective using FLUENT. thermal conductivity.

N/A

None

Qu et al. [39] (2007)

Laplace Pressure transformation difference was used to between solve the ODE evaporator and that accounts condenser is for the balance related to of thermally temperature driven, difference by frictional, and Clapeyronelastic restoring Clausis forces. equation. Charoensawan Nondimensional Four dimensionless empirical and numbers are correlation for Terdtoon identified. heat transfer of [38] (2007) PHP is proposed.

Ma et al. [55] (2006)

Open

Closed

Aluminum

Aluminum

Closed

Closed

Copper

Copper

—

N/A

Circular

Square, triangle

0

22

26, 36

90, 0

90, 0

90, −90

10, 22, 32, 0 52

—

Square, trian- 16 gle

Circular

Circular

Water, acetone

2

1, 1.5

Water

Ethanol, acetone, nanofluid

Water

1, 1.5, Water, 2 ethanol

1.65

7.3–33.3 W/cm2

—

50

20–62.5

20–80