Thermal performance of a miniature loop heat pipe using water–copper nanofluid

Accepted Manuscript Thermal performance of a miniature loop heat pipe using water-copper nanofluid Zhenping Wan , Jun Deng , Bing Li , Yanxiao Xu , Xiaowu Wang , Yong Tang PII:

S1359-4311(14)00998-3

DOI:

10.1016/j.applthermaleng.2014.11.010

Reference:

ATE 6119

To appear in:

Applied Thermal Engineering

Received Date: 18 June 2014 Accepted Date: 7 November 2014

Please cite this article as: Z. Wan, J. Deng, B. Li, Y. Xu, X. Wang, Y. Tang, Thermal performance of a miniature loop heat pipe using water-copper nanofluid, Applied Thermal Engineering (2014), doi: 10.1016/j.applthermaleng.2014.11.010. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT

Thermal performance of a miniature loop heat pipe using water-copper nanofluid

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Zhenping Wan1, Jun Deng1, Bing Li1, Yanxiao Xu2, Xiaowu Wang3, Yong Tang1 Key Laboratory of Surface Functional Structure Manufacturing of Guangdong Higher Education Institutes, South China University of Technology, Guangzhou 510640, China 2

Department of Physics, School of Science, South China University of Technology, Guangzhou 510640,

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China

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3

Xiamen Institute of Technology Huaqiao University, Xiamen 361021, China

Corresponding author: Zhenping Wan

Tel.: +86-20-87110684 Fax: +86-20-87114634 E-mail address: [email protected]

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Abstract: The implementation of high power density coupled with limited space available in the cooling of electronics demands a highly efficient miniature loop heat pipe (mLHP). This study experimentally investigates the influence of a nanofluid on the thermal characteristics of a specially designed mLHP and explores the mechanism of heat transfer enhancement of the nanofluid in the mLHP. The nanofluid is

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composed of deionized water and Cu nanoparticles and has an average diameter of 50 nm. Reductions of 12.8% and 21.7% are achieved in the evaporator wall temperature and total thermal resistance, respectively,

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while the heat transfer coefficient (HTC) of the evaporator increases 19.5% when substituting the nanofluid with 1.0 wt% of deionized water at a heat load of 100 W. There is an optimal mass concentration for the nanofluids, which corresponds to the maximum heat transfer enhancement. The optimal mass concentration is 1.5 wt%. The thermal performance improvement of the mLHP using the nanofluid results from the reduction of the contact angle, the enhancement of boiling heat transfer, and a deposited nanoparticle coat on the boiling surface.

Key words: Miniature loop heat pipe; Nanofluid; Heat transfer enhancement; Electronics cooling

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ACCEPTED MANUSCRIPT Nomenclature Total thermal resistance (°C/W)

Tew

Average temperature of the evaporator wall (°C)

Tcw

Average temperature of the condenser wall (°C)

Tei

Inner wall temperature of the evaporator (°C)

Tw

Temperature of the heating wall (°C)

Tv

Temperature of the saturated vapor in the evaporator (°C)

Ts

Saturation temperature of the liquid (°C)

Q

Heat input power (W)

C

Constant

he

Heat transfer coefficient of the evaporator (W/m2·°C)

Ae

Heating area of the evaporator (m2)

Aei

Inner wall area of the evaporator corresponding to the heating area (m2)

t

Heating wall thickness of the evaporator (m)

kem

Thermal conductivity of the evaporator materials (W/m·°C)

kl

Thermal conductivity of the liquid (W/m·°C)

hfg

Latent heat of phase change (kJ/kg)

l

Side length of the measured rectangle (m)

ρl

Density of the liquid (kg/m3)

σ

Surface tension (N/m)

νl

Kinematic viscosity of the liquid (m2/s)

θ

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δ

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Rtot

Uncertainty

Contact angle (°)

1 Introduction Multicore central and graphic processing units of high end computing hardware, such as workstations, server computers, etc., are facing serious challenges in terms of thermal management due to the high integration and the miniaturization of electronic chips, which results in fluctuations in heat dissipation and limited space available for thermal control 2

ACCEPTED MANUSCRIPT system. Loop heat pipes (LHPs), which have exceptionally efficient heat-transfer features, are regarded as one of the promising solutions to this problem [1]. However, the main aim of the initial development of LHPs is to control the temperature of spacecraft. Therefore, it is necessary to miniaturize these devices to meet the challenging needs of electronics cooling.

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Maydanik et al. [2] developed two LHPs that employed a nickel-based cylindrical evaporator with a wick sintered from nickel powder. The cylindrical evaporators were soldered to copper heat spreaders, which ensured a reliable thermal contact with the flat heat spreaders of the CPUs that were being cooled. Nagano and Nishigawara [3] fabricated a small

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LHP with an outer diameter of 12 mm and a length of 77 mm from an evaporator made of stainless steel. A small LHP with a cylindrical evaporator requires a saddle between the

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evaporator and the flat-plate parts that are cooled. This configuration results in greater thermal contact resistance in the contact region. Therefore, mLHPs with flat evaporators have been presented and fabricated. Li and Wang [4] carried out a thorough experimental investigation on a compact copper-water LHP with a flat, square evaporator with dimensions of 30 mm × 30 mm × 15 mm. Singh et al. [5] designed two prototypes of miniature LHPs. One LHP had a

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10-mm thick disk-shaped evaporator with a 30-mm diameter. The other LHP had a rectangular-shaped evaporator that had a 47×37 mm2 planar area and was 5 mm thick; this LHP was used for the thermal control of laptop computers. Choi et al. [6] investigated the

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thermal performance of an mLHP in which the evaporator was designed to fit a 30×30 mm2 CPU chip. Zhang et al. [7] conducted a series of experiments to investigate the effect of temperature oscillation on the startup behavior and the operating performance in a miniature

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LHP with an 8-mm thick flat evaporator. Choi et al. [8] designed a cylindrical evaporator with a flat base plate, and vapor removal channels were machined on the evaporator base plate. In this case, the thermal contact conductance increases significantly because a portion of the wick is inserted into the vapor removal channels. To promote condensation on the mLHP, a number of arrayed pins were machined out on the upper wall of the condenser, and the condenser structure was optimized [9]. All of the work mentioned above focused on design miniaturization or structure optimization of the evaporator and condenser and the enhancement of the heat transfer. In fact, the working fluid is essential to achieve enhanced mLHP thermal performance because LHPs 3

ACCEPTED MANUSCRIPT utilize the phase change of the working fluid to efficiently transfer the heat from the evaporator to the condenser. Nanofluids are a new type of fluids consisting of uniformly dispersed and suspended nanometer-sized particles in a host fluid and have unprecedented thermal characteristics, such as high thermal conductivity [10,11] and considerable

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enhancement of forced convective [12-14] and boiling heat transfer [15-17]. Inspired by this, some studies on the use of nanofluids in heat pipes have been performed recently [18-21]. However, few studies have presented nanofluids for use in LHPs. Li et al. [22] investigated the effects of nanofluids on the transient and steady operation characteristics of a miniature

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capillary pump loop. Gunnasegaran et al. [23] experimentally studied the heat transfer characteristics of a nanofluid in a LHP. Riehl [24] investigated the heat transport behavior of

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an mLHP using a water-nickel nanofluid, and this study increased the pressure drop, and greater operation temperatures were observed when the LHP was changed to make use of a nanofluid.

The goal of the present work was to investigate the thermal performance of a specially designed mLHP cooling system that used a water-based nanofluid with copper nanoparticles.

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The contact angle and the evaporation rate of the Cu-water nanofluid and with deionized water are measured, and the deposited layer of Cu nanoparticles on the evaporator wall is observed to explore the possible mechanisms that account for the effects of nanofluids on the

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thermal performance of the mLHP. The thermal performance comparison of the mLHP operating with the Cu-water nanofluid and with deionized water is presented and discussed,

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which demonstrates the benefits of using nanofluids in this application.

2 Experimental apparatus and procedure 2.1 Design of the mLHP

Maydanik’s loop heat pipe is a typical prototype that consists of a cylindrical evaporator, a water-cooled condenser, a vapor line and a liquid line. The evaporator absorbs heat, and the working fluid in the evaporator vaporizes. The vapor enters the condenser through the vapor line and is condensed into a liquid. The condensed liquid reflows into the evaporator by means of capillary force. To intensify the evaporation and to facilitate condensed liquid

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ACCEPTED MANUSCRIPT reflowing, a special mLHP was constructed. The schematic configurations of the evaporator are shown in Fig. 1 in which a flat evaporator made of red copper was partitioned partially into a boiling chamber and a suction chamber using a separator. In the suction chamber, a sintered red copper powder sheet with a porosity of 53% was filled to be used as a porous

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wick. Unlike the wick in a conventional loop heat pipe, the wick in the suction chamber of the mLHP only provides a capillary force that drives the condensed liquid back. In the boiling chamber, arrayed copper sheets with grooves and caves were welded to the substrate to improve the evaporation efficiency of the evaporator. The arrayed copper sheets were placed

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in parallel to provide regular passages for the vapor flow and extend the heat transfer area. The surface morphologies of the arrayed copper sheets are shown in Fig. 2. The grooves and

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caves can act as boiling nucleation points that can intensify the boiling, and the capillary force induced by the grooves and caves can propel the working fluid to flow on the surface of the copper sheets. The working fluid circulation is indicated in Fig. 1(a) by arrows. Vapor outlet Liquid inlet

Wick

Suction chamber

Separator

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Boiling chamber

Separator

Cover plate

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Thermocouple location

Heat input

Evaporator house

Arrayed copper sheets

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(a)

(b)

Fig. 1. Configuration of the evaporator: (a) Schematic drawing, (b) Close-up view Grooves Caves

Fig. 2. Surface morphologies of the arrayed copper sheets 5

ACCEPTED MANUSCRIPT The schematic configurations of the condenser are shown in Fig. 3. The condenser was integrated into an air-cooled radiator with a large fined surface that was made of aluminum. To intensify condensation, staggered diamond-shaped pins were machined in the inner wall of the condenser house. Table 1 lists the structure parameters of the mLHP. Plate fins

Diamond-shape pin Vacuum orifice

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Condenser house Liquid outlet

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Condenser cover

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Thermocouple loation

(a)

Vapor inlet

(b)

Fig. 3. Configuration of the condenser: (a) Condenser embedded in an air-cooled radiator, (b) Close-up view of the condenser house

Table 1 Structural parameters of the mLHP (Unit: mm) copper sheet: height x thickness x gap

30 × 40 × 10

30 × 40 × 6

10 × 2.5 × 2.1

outline size

inner chamber

diamond-shape pin

pin numbers

4 × 4× 15

25

60 × 60 × 15

vapor line: external diameter ×

liquid line: external diameter × internal

internal diameter × length

diameter × length

10 × 8 × 350

8 × 6 × 350

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suction chamber

55 × 50 × 18

76 × 80 × 95

line

boiling chamber

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condenser

outline size

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evaporator

thickness

porosity

average pore size

permeability

6

53%

65 µm

6.13×10-12 m2

2.2 Preparation of nanofluid In the present investigation, nanofluid samples were prepared by dispersing Cu nanoparticles with mean diameter of 50 nm in deionized water. The dispersion was carried out using magnetic stirring for 30 minutes followed by ultrasonic vibration for 4 hours to ensure proper mixing of nanoparticles into the base fluid. For a better dispersion, some amount of 6

ACCEPTED MANUSCRIPT sodium dodecyl benzene sulfonate (SDBS) was added to the nanofluid samples. 2.3 Experimental setup for measurement The experimental system consisted of an mLHP, a power supply and a data acquisition system. A copper block with an embedded heater was used as the heat-load simulator. The

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area of the copper block was the same as the bottom area of the evaporator. The heating power to the copper block was controlled by a voltage regulator. Employing heat-conducting paste made it possible to achieve good thermal contact between the copper block and the evaporator. The evaporator and the copper block were enclosed with a thermal insulating material. The

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heat loss was estimated to be approximately 3% or less based on the thermal conductivity and the thickness of the insulation material. The evaporator and the condenser were placed

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vertically and horizontally, respectively. The condenser was approximately 10 mm above the evaporator. Eight standard K-type thermocouples with a measurement accuracy of ±0.5ºC were mounted to monitor the temperature of the system, and the solid dots in Fig. 1 and Fig. 3 indicate the locations of the thermocouples. All of the thermocouples were embedded on the outer surface of the mLHP system. The data acquisition system Model 4718 produced by

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National Instruments was used to monitor the mLHP and to record the test data at a time interval of 1 s. The whole LHP system was vacuumed to a pressure of 2000 Pa and was then charged with the working liquid. The working fluid charge was 20 ml. The vapor in the

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condenser was cooled by forced air convection from an axial DC fan operating at 2000 RPM that was mounted onto the radiator. The ambient temperature was 25±2 ºC. The total thermal resistance, the average wall temperature and the heat transfer

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coefficient of the evaporator are three important parameters to describe the thermal performance of LHP. The total thermal resistance of mLHP, Rtot, is defined as the following [23]:

Rtot =

Tew − Tcw Q

(1)

The average heat transfer coefficient of the evaporator, he, is calculated by the following equation: he =

Q (Tei − Tv ) Ae 7

(2)

ACCEPTED MANUSCRIPT where Tei is calculated by the following formula: Tei = Tew −

Qt . k em Aei

(3)

2.4 Experimental uncertainty analysis

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The experimental uncertainty results from the experimental conditions and measurement the accuracy of instruments. According to Eq. (1), the uncertainty of the total thermal resistance can be calculated by

Rtot

2

2

 δQ   δTew   δTcw   +   =   +  Q T T    ew   cw 

2

(4)

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δRtot

According to Eqs. (2) and (3), the uncertainty of the heat transfer coefficient of the

2

δhe

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evaporator can be estimated as follows: 2

2

2

 δQ   δTew   δAei   δAe   δt   +   +   +   = 2  +  he  Q   Tew   Aei   Ae   t 

2

(5)

The heat loss was estimated to be approximately 3% or less based on the thermal conductivity and the thickness of the insulation layer, and the measurement accuracy of the

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power meter.

δQ Q

= 0.03

(6)

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As mentioned in section 2.3, the measurement accuracy of the thermocouples is 0.5°C; therefore, the maximum uncertainty in the measurement of temperature δT is ±0.5ºC. Because

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the heating area of the evaporator Ae and the inner wall area corresponding to the heating area of the evaporator Aei are both rectangles, the uncertainty of the metered area is

δA = δl1 × δl2

(7)

where δl1 and δl2 are the lengths of two side of the metered area. Owing to the accuracy of vernier caliper used, δl is assumed to be ±0.02 mm, so δA can be calculated as 0.0004 mm2. Similarly, the uncertainty of the heating wall thickness of evaporator, δt, can be assumed to be ±0.02 mm. Equations. (1) and (2) are based on the uncertainties of the heat power, the measurement of the temperature, the heating area and the heating wall thickness of evaporator. According to

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ACCEPTED MANUSCRIPT Eqs. (4) and (5), these calculations are performed by taking the aforementioned uncertainties of the parameters to eliminate the experimental errors. The maximum uncertainties in the heat transfer coefficients of evaporation and the total thermal resistance are found to be 4.57% and

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3.75%, respectively.

3 Contact angle and boiling characteristics of water-copper nanofluid 3.1 Contact angle

The addition of nanoparticles to the working fluid can alter its solid-liquid contact angle,

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θ, which influences the capillary force and the wettability of surface. In this subsection, the contact angle of the Cu-water nanofluid with different mass concentration on a clean red

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copper surface is measured using an optical contact angle measuring device (SL200B, Kino Industry). Figure 4 shows that the surface contact angle decreases from 66.53º for deionized water to 26.29º, 11.05º and 8.14º for Cu-water nanofluids with 1.0 wt%, 1.5 wt% and 2.0 wt% copper nanoparticles, respectively. The drastic decrease in the solid-liquid contact angle of the nanofluid can increase the capillary force and improve surface wettability. As a result, more

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liquid can be attached to the surface of the wall and a dryout phenomenon can be avoided.

Fig. 4. Static contact angles of different working fluids on clean red copper: (a) deionized water, (b) Cu-water nanofluid 1.0 wt%, (c) Cu-water nanofluid 1.5 wt%, (d) Cu-water nanofluid 2.0 wt%

3.2 Boiling characteristics 9

ACCEPTED MANUSCRIPT The Cu-water nanofluid boiling test rig used in the current study is shown in Fig. 5. The boiling test rig consisted of a liquid filling funnel, a double glazing reaction kettle, a condenser-west tube and a condensed liquid recycling vessel. The system pressure was vacuumed to 2000 Pa prior to each experiment. The nanofluid or deionized water was then

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allowed to fill the inner layer of the reaction kettle. The water was heated using a constant temperature bath that flowed through the outside layer of double glazing reaction kettle. The temperature of the heating water was maintained at 50ºC. The nanofluid or deionized water in the inner layer of the reaction kettle was heated and vaporized, and then the vapor entered the

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condenser-west tube. The vapor flowing in condenser-west tube was cooled by room temperature water, and the condensed water was recycled into the condensed liquid recycling

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vessel.

III

3

4

1

IV

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II

Fig. 5. Photograph of test rig for measuring the evaporation rate of the nanofluid and water

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I–liquid filling funnel, II–double glazing reaction kettle, III–condenser-west tube, IV–condensed liquid recycling vessel. 1–heating water inlet, 2–heating water outlet, 3–cooling water inlet, 4–cooling water outlet

Table 2 lists the volume of recycled liquid collected after 180 ml Cu-water nanofluid and deionized water boiled for 20 min in the double glazing reaction kettle. From Tab. 2, the volume of the recycled liquid condensed from the boiling of Cu-water nanofluid increases by 7 ml compared with the amount collect when using the deionized water. Therefore, the addition of the Cu nanoparticles to the deionized water can increase water recirculation rate during boiling and, consequently, enhance the boiling heat transfer and the evaporation 10

ACCEPTED MANUSCRIPT capacity of the base water. Table 2 Volume of the recycled liquid Working fluid

Cu-water nanofluid (1.5 wt%)

deionized water

Volume of recycled liquid (ml)

30

23

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According to experimental results from Xuan and Li [25], the presence of nanoparticles in water increases the thermal conductivity of the working fluid. The heat flux of a one-phase liquid that is boiling can be expressed by

2 k h ρ (T − Ts ) 2 k l (Tw − Ts ) 3 + C 2 l fg l w σTs vl σTs

(8)

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q = C1

From Eq. 8, the heat flux increases with the thermal conductivity. According to diffusion

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theory, the nanofluid can be treated as a one-phase fluid integrating micro movement and thermal diffusion of nanoparticles [25]. Therefore, the relationship that the heat flux increases with the thermal conductivity of the fluid still holds for a nanofluid. Therefore, the fact that the nanofluid can increase the thermal conductivity of the base fluid results in a better heat transfer performance.

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3.3 Cu nanoparticle-deposited coat on the arrayed copper sheets Boiling at the evaporator is greatly influenced by the surface condition of the arrayed copper sheets. Nanoparticle deposition on the surface of the arrayed copper sheets is examined by

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scanning electronic microscope (SEM) after being operated with the nanofluid. Figure 6 shows an SEM image of the Cu nanoparticles-deposited coat on the arrayed copper sheets.

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From Fig. 5, the size of Cu nanoparticle agglomerates deposited on the boiling surface (arrayed copper sheets) is several tens to one hundred nanometers.

Fig. 6. SEM image of the Cu nanoparticle-deposited coat on the arrayed copper sheets

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4 Experimental results and discussion In the following sections, the thermal performances of the mLHP charged with Cu-water nanofluid with various mass concentrations and deionized water alone will be investigated experimentally.

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4.1 Startup and operation characteristics of the mLHP To understand the influence of the nanofluid on the thermal performance of the mLHP, the experiments at startup and at steady operation are carried out under the same cooling conditions. Figures 7 (a) and (b) show the startup and the operation characteristics of the

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mLHP charged with Cu-water nanofluid with 1.0 wt% and deionized water alone, respectively, with a heating power of 100 W. The temperatures of the evaporator wall, the vapor outlet, the

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condenser wall and the working liquid at the outlet of the condenser are presented in Fig. 7. The evaporator wall temperature is 66.1ºC for the mLHP when using the nanofluid and is 75.8ºC when using the deionized water when the mLHP reaches steady operation. This result suggests that 12.8% reduction in evaporator wall temperature is achieved by substituting the nanofluid for deionized water as the working fluid. The condenser of the mLHP when using

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the nanofluid begins to function when the evaporator wall temperature reaches 35 ºC, while the condenser of the mLHP when using deionized water begins to function when the evaporator wall temperature reaches 50 ºC. Approximately 12 min later, the mLHP reaches

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steady operation when using the nanofluid, yet it takes 17 min for the mLHP to reach steady operation when using deionized water. Once the mLHP filled with the nanofluid reaches

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steady operation, the temperatures of the evaporator wall and condenser wall progress stably with time. However, the temperatures of evaporator wall and the condenser wall undergo temperature fluctuations for 20 minutes after the mLHP with deionized water reaches steady operation. It is observed that the mLHP with the nanofluid possesses quicker startup characteristics, greater heat removal capacity and more stable operation.

12

80

80

70

70

Temperature [ºC]

60 50 Evaporator wall Vapor outlet Condenser wall Liuid outlet

40 30

5

10

15

20

25

30

50 40

Evaporator wall Vapor outlet Condenser wall Liquid outlet

30 20

20 0

60

0

35

5

Time [min]

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Temperature [ºC]

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10

15

20

25

30

35

Time [min]

(a)

(b)

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Fig. 7. Startup and operation characteristics of the mLHP at heat load of 100 W using (a) Cu-water nanofluid with 1.0 wt%, (b) deionized water

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Figure 8 compares the temperature difference between the evaporator wall and the condenser wall when using the nanofluid and deionized water alone as the working fluid. It is apparent from Fig. 8 that the maximum temperature difference of the mLHP when using the nanofluid is considerably lower than the temperature difference within the mLHP when using deionized water alone during the startup process. The temperature difference of the mLHP

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when using the nanofluid is still less than the temperature difference within the mLHP when

30 25

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Temperature difference [ºC]]

using deionized water alone when the mLHP reaches steady operation.

20

Cu-water nanofluid 1.0% wt Deionized water

15 10

5 0 0

5

10

15

20

25

30

Time [min]

Fig. 8. Temperature difference between the evaporator and condenser

4.2 Heat transfer coefficient of the evaporator and the total thermal resistance of mLHP Figure 9 shows the evaporation heat transfer coefficients (HTCs) of the mLHP when using the nanofluid and when using deionized water alone. It is obvious from Fig. 9 that the evaporation HTCs increase with an increase in the heat flux. The evaporation HTCs of the 13

ACCEPTED MANUSCRIPT nanofluid are considerably greater than those of the deionized water, and the HTCs can be increased by 19.5% when substituting the nanofluid for deionized water at a heat load of 100 W. Figure 10 presents the total thermal resistance (TTR) of the mLHP when using the nanofluids and deionized water alone. From Fig. 10, the TTRs decrease with an increase in

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the heat flux and are very small no matter which working fluid was used. At heat input power of 100 W, the TTRs are 0.065 ºC/W and 0.083 ºC/W when using the nanofluid and deionized water, respectively, are used as the working fluid, and the TTR decreased by 21.7% when substituting the nanofluid for deionized water.

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1300

900 700

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2

h e [W/m ·ºC]

1100

Cu-water nanofluid 1.0% wt Deionized water

500 300 0

25

50

75

100

125

Heat input power [W]

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Fig. 9. Heat transfer coefficient of the evaporator at different heating powers 0.3

Cu-water nanofluid 1.0% wt Deionized

0.2

0.15

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R tot [ºC/W]

0.25

0.1

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0.05

0 0

25

50

75

100

125

Heat input power [W]

Fig. 10. Total thermal resistance of the mLHP at different heating powers

From Figs. 7–10, it can be observed that both the evaporator wall temperature and the total thermal resistance of the mLHP when using the Cu-water nanofluid decrease, and the HTC of the evaporator increases compared with the HTC of the mLHP when using deionized water. The nanofluid can carry away more heat from the boiling chamber of the evaporator due to greater evaporation rate during boiling. Conversely, the enhanced capillary force of the 14

ACCEPTED MANUSCRIPT nanofluid drives more working fluid to flow through the arrayed copper sheets, and the better surface wettability of the nanofluid causes the working fluid to be well distributed on the surface of arrayed copper sheets. Additionally, the nanoparticle-deposited layer can create additional active nucleation sites by splitting a single nucleation site into multiple sites, and

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the irregular nanopores formed on the nanoparticle-deposited layer affect the bubble release frequency and diameter [18]; therefore, the boiling heat transfer is further enhanced. Consequently, a lower evaporator wall temperature, a lower temperature difference between the evaporator and condenser, a faster startup process, greater heat transfer coefficients and a

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smaller total thermal resistance have been obtained as compared with the parameters as obtained from the LHP operating with deionized water alone. Therefore, the mLHP filled with

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the nanofluid has a greater heat transfer performance.

4.3 Influence of mass concentration of the nanofluid on the thermal performance of the mLHP

Figures 11 (a) and (b) shows the influence of the heat input power on the average evaporator wall temperature and the HTC of the evaporator at different concentrations of

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nanoparticles. Figure 11 suggests that that evaporator wall temperature and the HTC of evaporator increase with an increase in the heat input power. Furthermore, it can be observed from Fig. 11 (a) that at steady operation the evaporator wall temperatures are 66.1 ºC, 60 ºC

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and 62.7 ºC at the particle concentration of 1.0%, 1.5% and 2.0%, respectively, when the heat input power is 100 W. When the heat input power increases to 150 W, the evaporator wall

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temperatures are 86.8 ºC, 76.7 ºC and 80.4 ºC for particle concentration of 1.0%, 1.5% and 2.0%, respectively. As shown in Fig. 10 (b), the HTC of the evaporator at a nanoparticle concentration of 1.5% is greatest among the three nanoparticle concentrations of 1.0%, 1.5% and 2.0% when heat input power is equal to or greater than 50 W. Therefore, there is an optimal particle concentration, which is approximately 1.5 wt%, for the Cu-water nanofluids in the present experiments. Figure 12 indicates the changing trends of the TTR when the heating power was increased at nanoparticle concentrations of 1.0%, 1.5% and 2.0%. Figure 12 also demonstrates the point because the TTR at the nanoparticle concentration of 1.5% is always smallest. For instance, the TTRs are 0.053 ºC/W, 0.039 ºC/W and 0.077 ºC/W for the particle concentration of 1.0%, 1.5% and 2.0%, respectively, when the heat input power is 150 15

ACCEPTED MANUSCRIPT W. 90

1600 Nanofluid 1.0 wt% Nanofluid 1.5 wt% Nanofluid 2.0 wt%

1200

2

70

1400 h e [W/m ·ºC]

60 50

1000 Nanofluid 1.0 wt% 800

40

600

30

400 0

25

50

75

100

125

150

Nanofluid 1.5 wt% Nanofluid 2.0 wt%

0

175

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Temperature [ºC]

80

25

50

75

100 125 150 175

Heat input power [W]

Heat input power [W]

(b)

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(a)

Fig. 11. Influence of the heating power on: (a) the temperature of evaporator wall, (b) the heat transfer

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coefficient of the evaporator at different mass concentration of nanoparticles

0.4 0.35

Nanofluid 1.0 wt% Nanofluid 1.5 wt% Nanofluid 2.0 wt%

0.25 0.2 0.15

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R tot [ºC/W]

0.3

0.1

0.05

0

0

25

50

75

100 125 150 175

EP

Heat input power [W]

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Fig. 12. Influence of the heating power on the total thermal resistance of the mLHP at different mass concentration of nanoparticles

5 Conclusion

(1) The startup and transient operation of the mLHP indicates that both the startup time and the transient stage are reduced, and the evaporator wall temperature and the total thermal resistance decrease by 12.8% and 21.7%, respectively, when substituting the nanofluid with 1.0 wt% nanoparticles in place of deionized water alone at a heat input power of 100 W. The evaporation heat transfer coefficient of the mLHP when using the nanofluid increases by 19.5% for the same operating condition. 16

ACCEPTED MANUSCRIPT (2) There is an optimal mass concentration, which is 1.5 wt% of nanoparticles, that corresponds to the maximum heat transfer enhancement. (3) The heat transfer enhancement from the use of nanofluids in the evaporator of the mLHP may result from the following three aspects: (i) the addition of Cu nanoparticles in the

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base water can increase the water recirculation rate and enhance boiling heat transfer; (ii) the decrease of the solid-liquid contact angle of the nanofluid can increase the capillary force and surface wettability; (iii) the nanoparticle-deposited coat on the arrayed copper sheets in the boiling chamber of the evaporator can create more new active nucleation sites to enhance the

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boiling heat transfer.

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Acknowledgements

This paper was supported by the National Natural Science Foundation of China (51375176, U0934005) and supported by the Fundamental Research Funds for the Central Universities of China (2013ZZ017)

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Research Highlights

> Contact angle of Cu-water nanofluid is studied.

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> A miniature loop heat pipe (mLHP) is designed for electronics cooling system.

> Water recirculation rate of nanofluid is more than that of pure water during boiling.

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> The effects of nanofluid on thermal performance of mLHP are investigated.