Design, construction, and performance testing of an isothermal naphthalene heat pipefurnaceJeehoon Choi, Yuan Yuan, Diana-Andrea Borca-Tasciuc, and Hwankook Kang

Citation: Review of Scientific Instruments 85, 095105 (2014); doi: 10.1063/1.4894526 View online: http://dx.doi.org/10.1063/1.4894526 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/85/9?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Back propagation neural network based control for the heating system of a polysilicon reduction furnace Rev. Sci. Instrum. 84, 125108 (2013); 10.1063/1.4847157 Ultra-high temperature isothermal furnace liners (IFLS) for copper freeze point cells AIP Conf. Proc. 1552, 851 (2013); 10.1063/1.4819655 Marangoni effect and heat pipe dry-out Phys. Fluids 18, 118103 (2006); 10.1063/1.2397586 StartUp Performance of A Loop Heat Pipe With Variable Heating Patterns and Periodic Cycles AIP Conf. Proc. 746, 106 (2005); 10.1063/1.1867124 Sorption Heat Pipe —A New Thermal Control Device for Space Applications AIP Conf. Proc. 654, 71 (2003); 10.1063/1.1541279

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REVIEW OF SCIENTIFIC INSTRUMENTS 85, 095105 (2014)

Design, construction, and performance testing of an isothermalnaphthalene heat pipe furnace

Jeehoon Choi,1,2,a) Yuan Yuan,2 Diana-Andrea Borca-Tasciuc,2,b) and Hwankook Kang3,a)

1Zalman Tech Co., Ltd., Anyang Gyunggi-do, Korea2Mechanical, Aerospace, and Nuclear Engineering Department, Rensselaer Polytechnic Institute,Troy, New York 12180-3590, USA3Dongeun Advanced Thermal System, Bucheon Gyunggi-do, Korea

(Received 25 December 2013; accepted 19 August 2014; published online 9 September 2014)

In this work, an isothermal naphthalene heat pipe furnace was developed to achieve uniformly radi-ant heating at temperature up to 300 ◦C. The startup, stability, and thermography tests were carriedout to evaluate the uniform temperature zone of the furnace. The temperature variance of the uniformtemperature zone was observed to be within ±1 ◦C along the axis of the furnace. To illustrate its capa-bilities, the furnace was used to successfully sinter thin layers of copper nanoparticle on microporouscopper wicks and form biporous wicks, materials of high interest to advanced heat pipe technologies.© 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4894526]

I. INTRODUCTION

Heat pipes, which are known as highly effective thermalmanagement devices, enable transfer of heat over consider-able distances. Since their development by Grover in the mid-1960s,1 heat pipes have been used in a wide variety of appli-cations from thermal control in air-space shuttles to cooling ofconsumer electronics such as laptops, cell phones, etc.2 Oneof the distinct characteristics of heat pipes is their ability toprovide a uniform temperature during operation.3 Therefore,isothermal devices such as ovens, chemical reactors, and fur-naces have adapted heat pipes for ensuring a uniform tem-perature distribution on otherwise unevenly heated surfaceareas.4–6

The objective of this paper is to investigate a furnace em-ploying tubular naphthalene heat pipes for achieving isother-mal wall temperature. This furnace is developed specificallyfor low temperatures (< 300 ◦C), being intended for nanopar-ticle sintering applications. The performance of the furnaceis experimentally investigated via infrared thermal imaging inparallel with thermocouple array monitoring. To demonstrateits functionality, the furnace was used to sinter thin layers ofcopper nanoparticles on microporous media to form biporouswicks, materials which are of high interest for advanced heatpipe technologies.

II. DESCRIPTION OF THE FURNACE AND THEPRINCIPLE OF OPERATION

Figure 1 shows the schematic of the furnace. The key partof the furnace is a tubular liner integrated with a concentricannular naphthalene heat pipe. The tube liner covers the innerwall made of steel and having an inner diameter of 159 mmand a length of 380 mm. The interior of the tubular liner was

a)Authors to whom correspondence should be addressed. Electronicaddresses: jhchoi@zalman.co.kr and choijeehoon@gmail.com, Tel.:82-70-4941-6199/1-518-423-0735 and hkkang67@hanmail.net, Tel.:82-32-675-9490.

b)E-mail: borcad@rpi.edu. Tel.: 1-518-276-2010.

designed large enough to accommodate a mold of maximum100 × 100 mm area and height of 100 mm. This size moldis necessary to be able to produce samples of sizes that aretraditionally employed for investigating transport propertiesof wicks for loop heat pipe applications,7–10 an applicationfor which present furnace is intended for. Therefore, the min-imum area that must have highly uniform temperature dis-tribution inside the furnace is larger than 130 mm in length(to accommodate for some less than perfect centering of themold).

The concentric annular heat pipe has an inner diameterof 159 mm, an outer diameter of 260 mm, and a length of200 mm, much larger than the required size for the furnaceregion with highly uniform temperature. Mantelli et al. re-ported that naphthalene may react chemically with the con-tainer material, leading to considerable gas generation anda subsequent reduction in the heat transfer capacity of theheat pipe.11 Based on the study of Anderson et al. who re-viewed compatibility of naphthalene with materials used inheat pipe applications, 316 l stainless steel was used as con-tainer material.12 Same material was also used for the porouswick consisting of 50 mesh screen determined as optimizedpore size on the basis of a pressure balance arising from thedifference in capillary pressure across liquid-vapor interfacesin the evaporation and condensation regions. The mesh porouswick was coupled to the container wall by diffusion bonding.The lower part of the tubular heat pipes were mounted on asemi-cylindrical ceramic band heater (Fig. 1(a)) which con-sists of a helically wound resistance coil made from nickel-chrome wire. This wire is laid on a bed of ceramic insula-tion (NCH-1W, 2400 W from Omega) with the help of twostainless steel band clamps. The heat pipe and the heater wereinsulated with ceramic wools to minimize heat losses to thesurrounding. Finally, the outer shell of the furnace enclosingall the components discussed was also made of 316 l stainlesssteel. A picture of the built furnace is shown in Fig. 2.

The furnace uses naphthalene as working fluid becauseit covers the desired temperature range (Fig. 3) and is more

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095105-2 Choi et al. Rev. Sci. Instrum. 85, 095105 (2014)

FIG. 1. Schematic of heat pipe furnace: (a) tubular liner integrated with con-centric annular naphthalene heat pipe; (b) furnace assembly.

environmental friendly than mercury. Moreover, its versatilityis well documented for heat pipe applications.14, 15

The operational principle of the heat pipe furnace is il-lustrated in Fig. 4 and briefly explained next. Once poweris turned on and the semi-cylindrical ceramic band heaterheats up, naphthalene starts to sublimate inside the porouswick near the interface with the heater (Fig. 4(a)). The heatis then gradually transferred to the upper part of heat pipe(Fig. 4(b)), while the temperature of the heater increases dueto continuous heat addition and the transient regime. The heatpipe temperature also increases until the saturated vapor pres-sure becomes equal in all vapor regions inside the heat pipe.At this time point, the heat pipe becomes nearly isothermal(Fig. 4(c)). As depicted in Fig. 5, the vapor phase of naphtha-lene is collected inside the vapor collection spaces built intothe porous wick and moved toward the wick surface near tothe interior wall where heat is rejected to the furnace cavityby radiation (Fig. 4(d)) and the vapor changes to the liquidphase. The liquid returns via capillary pumping effects withinthe porous wick that provides the driving forces for flow cir-culation inside the heat pipe and the loop is closed.

The furnace was designed to perform in a vacuum ofmaximum 1 mTorr to minimize oxidization during sintering.The door of the furnace mouth was sealed by silicon O-rings.Vacuum gauge, vacuum valve, and exhaust gas port were allconnected to the end of the furnace. The O-ring and vacuum

FIG. 2. Furnace with cover: (a) closed and (b) open.

FIG. 3. The useful operating temperature range of the heat pipe work-ing fluid. This figure is drawn on the basis of data on working fluidproperties.3, 4, 12, 13

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095105-3 Choi et al. Rev. Sci. Instrum. 85, 095105 (2014)

FIG. 4. Heat transfer paths of the heat pipe furnace: (a) Ceramic band heater heats up, and then Naphthalene starts to sublimate near the interface with heater;(b) heat is gradually transferred to the upper heat pipe; (c) heat spreads everywhere throughout heat pipe. The heat pipe becomes nearly isothermal; (d) heat isrejected to the furnace cavity by radiation. Heat pipe operates in steady state.

parts are cooled down by a water chiller during operation. Inaddition, three disk-shaped radiation shields made of the bur-nished stainless steel of 1 mm thickness were placed on bothends of the interior of the furnace to minimize the heat lossescaused by the water cooling of valves and ports placed at thetwo ends.

To protect against heating beyond the allowable tempera-ture ranges for naphthalene, the temperature of the heater was

FIG. 5. Flow paths in the tubular heat pipe.

controlled with the help of a proportional integral differential(PID) controller (TZN4W from Autonix, Inc.) and an arrayof K-type sheath thermocouples (thickness of 0.125 mm fromOmega Engineering) with an uncertainty of 0.4%. One wasinserted into the hollow interior of the furnace (Ti) and theother was also attached on the cover of the heat pipe (Te) onthe side, close to the heater. At steady-state these thermocou-ples were moved to probe the temperature along the axis ofthe furnace in the region required to have high temperatureuniformity. A MV-1000 (from Yokogawa Co., Ltd.) data ac-quisition unit was used to monitor and record the temperatureat each position.

III. FURNACE TESTING AND DISCUSSION

A. Temperature distribution

Naphthalene is solid under vacuum condition at roomtemperature. During startup, the naphthalene evaporates at thehot side. The vapor then moves towards the interior wall of thecontainer which is cold. Because the cooling rate could be toohigh during startup, the vapor may solidify at the inner wall,stopping the flow of liquid naphthalene back to the hot in-terface. This may cause instable furnace behavior with large

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095105-4 Choi et al. Rev. Sci. Instrum. 85, 095105 (2014)

FIG. 6. Operational characteristics of the furnace heated up to achieve tem-perature set points of (a) Ti = 200 ◦C, and (b) Ti = 270 ◦C. Ti is the tempera-ture point right in the middle of the hollow interior of the furnace and Te theexterior covering temperature point of the heat pipe close to the heater. Twotemperature points are collinear on the x-axis and the y-axis, as shown in theinset of Fig. 6(a).

temperature fluctuations.11 To understand if this is an issuein current setup configuration, the furnace was investigatedat two different operating temperatures of 200 ◦C and 270 ◦C,respectively.

The operational characteristics are shown in Figs. 6(a)and 6(b), which show the temperature in the furnace as func-tion of time for each operating temperature. Thermocouplepositions were as shown in the inset of Fig. 6(a). As can beseen from Fig. 6, the furnace achieved stable startup (no largetemperature oscillations) and steady state operation for bothtemperature settings.

Further understanding of the isothermal characteristic ofthe furnace can be obtained from Fig. 7, which show Ti and Teat steady state as function of distance x along the axis of thefurnace. Figure 7 shows that the temperature is uniform over

FIG. 7. Temperature profiles obtained from moving thermocouples along thetransverse direction while the furnace operates at steady state: operation at200 ◦C; operation at 270 ◦C; Ti denotes temperature points of the hollow in-terior as function of distance x along the axis of the furnace. Te denotes theexterior covering temperature points of the heat pipe along the x-axis of thefurnace.

a large portion of the furnace with a temperature difference ofless than 3 ◦C over for the entire length of furnace, despite thewater cooling applied at the two ends.

B. Tests using infrared thermography technique

As shown in Fig. 2(b), the furnace was designed to allowfor partial removal of the most outer cover in order to ob-serve the temperature distribution when the furnace operatesat steady state. Typically, characterization of furnace temper-ature is extremely limited when carried out with thermocou-ples, which measure spot temperature; problem areas can beeasily missed. For an overall evaluation of the temperatureuniformity, the temperature distribution on the cover of theheat pipe was visualized using an infrared thermography tech-nique. The IR camera employed, FLIR ThermaCAM P660,has a thermal sensitivity of 30 mK at room temperature andthe uncertainty of 1%. The camera was mounted on a tripodand focus on the exposed exterior of the heat pipe throughopened furnace cover. To reduce the emissivity-induced er-rors on temperature readings, the initial calibration of the IRcamera was carried out at 19 ± 1 ◦C by measuring the temper-ature of a particular point on the exterior surface of the heatpipe using both a thermocouple and the IR camera. The emis-sivity value input to the camera was modified until the twotemperature readings matched. Through this calibration theemissivity of the exterior heat pipe surface was determined as0.33.

Figure 8 shows the infrared thermal images taken on therunning furnace without cover at steady state; the correspond-ing steady-state thermocouple temperatures are shown inFig. 6(b). The temperature distribution depicted by Fig. 8is almost uniform throughout the entirely exposed heat pipe

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095105-5 Choi et al. Rev. Sci. Instrum. 85, 095105 (2014)

FIG. 8. Infrared thermal image of the cover of the heat pipes. Red area rep-resents the heat pipe surface and blue area is the ceramic heater band clampmade of stainless steel. Temperature set point is Ti = 270 ◦C.

surface except for the stainless steel band clamps. The temper-ature readings at thermocouple monitoring points (in the caseof Ti = 270 ◦C) shown in Fig. 7 were found to be very closeto IR camera readings despite the natural convection effectson the exposed surfaces. As seen from thermocouple data, thetemperature difference of the uniform temperature zone wasless than 1 ◦C along the furnace axis.

C. Sintering layers of copper nanoparticles ontomicroporous wicks

To demonstrate furnace functionality, thin layers ofcopper nanoparticles were sintered onto microporous copperwicks. Biporous wicks have recently emerged as promisingmaterials for heat transfer technologies such as heat pipes16

or miniature loop heat pipes (mLHP).17 The role of thenanoparticles texturing on the microporous samples is toimprove capillary and heat transfer performance.18–22 Whilethe process for fabricating biporous wicks is more com-plex, increasing manufacturing cost and affecting potentialtechnology transfer, providing a low temperature sinteringsolution is economically appealing and feasible since copper(a key material used in heat pipes and mLHPs) nanoparticlescan be sintered at temperature below 300 ◦C.23–25 A mainchallenge during sintering is providing a highly uniformtemperature within the furnace. Non-uniform temperaturedistribution may result in large temperature gradients insidethe sample and post-sintering deformation due to shrinkageand thermal expansion.

The microporous wicks were sintered using high tem-perature furnace, as described in detail in reference of Choiet al.10 Here, only a thin layer of copper nanoparticles wasadded to the top surface of the existing microporous wicks.The copper nanoparticle powder (Cu 300 nm from SkySpringNanomaterials, Inc.) was spread over the surface to form alayer which was approximately 0.3 mm in thickness. After-ward, the sample was placed in a graphite mold and left insidethe center portion of the furnace. The furnace was then evacu-ated and the temperature of the heater was set to reach approx-imately 280 ◦C. The steady state temperature was reached in

FIG. 9. Interior furnace temperature (Ti) as function of time.

about 30 min (Fig. 9), and the sample was left to stay at thistemperature for 6 h, after which the oven cooled by naturalconvection assisted by water cooling on the two ends.

After sintering, the nanoparticle coated porous wick wascleaned with acetone in an ultrasonic cleaner. The surface ofthe wick covered with a porous layer is smooth in appear-ance as compared to the original wick surface. Porous mediabefore and after sintering nanoparticle thin film is shown inFigs. 10(a)–10(c). A scanning electron micrograph of the re-gion close to the interface of the sintered nanoparticle layer is

FIG. 10. Microporous wick porous media (a) before coating nanoparticles;(b) after nanoparticle sintering; (c) after ultrasonic cleaning; (d) scanningelectron micrograph on the interface of the sintered nanoparticle layer.

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095105-6 Choi et al. Rev. Sci. Instrum. 85, 095105 (2014)

shown in Fig. 10(d), which clearly show the presence of thenanoporous layer on top of the microporous substrate.

IV. CONCLUSION

This work presents an isothermal, low-temperature heatpipe furnace for metallic nanoparticles sintering. The furnaceincorporates a concentric annular heat pipe to achieve uniformheat distribution. Naphthalene was selected as a working fluidbecause it operates well on the temperature range of inter-est (200 ◦C–300 ◦C). The furnace tests showed stable startupand steady state operation. The temperature distribution mea-surement of the furnace demonstrated a uniform temperaturezone, where the temperature variance was within ±1 ◦C. Todemonstrate the functionality of the furnace, nanoparticle sin-tering was carried out onto the surface of microporous wicks.The composite materials, known as biporous wicks, havepotential applications in emerging two-phase heat transferdevices.

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