Experimental Thermal and Fluid Science 33 (2009) 1100–1105

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Experimental Thermal and Fluid Science

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The experimental study on flat plate heat pipe of magnetic working fluid

Zhang Ming, Liu Zhongliang *, Ma Guoyuan, Cheng ShuiyuanKey Laboratory of Enhanced Heat Transfer and Energy Conservation, Ministry of Education and Key Laboratory of Heat Transfer and Energy Conversion,Beijing Education Commission, College of Environmental and Energy Engineering, Beijing University of Technology, Beijing 100022, China

a r t i c l e i n f o a b s t r a c t

Article history:Received 30 May 2008Received in revised form 16 March 2009Accepted 19 June 2009

Keywords:Magnetic fluidFlat plate heat pipeElectronics cooling

0894-1777/$ - see front matter � 2009 Elsevier Inc. Adoi:10.1016/j.expthermflusci.2009.06.009

* Corresponding author. Tel.: +86 10 67391917; faxE-mail address: liuzhl@bjut.edu.cn (L. Zhongliang)

An effective thermal spreader can achieve uniform heat flux distribution and thus enhance heat dissipa-tion of heat sinks. Flat plate heat pipe is one of the highly effective thermal spreaders. Magnetic fluid isliquid and can be moved by the force of magnetic field. Therefore, the magnetic fluid is suitable to be usedas the working fluid of flat plate heat pipes which have a very small gap between evaporation and con-densation surfaces. We prepared a disk-shaped wickless flat plate heat pipe, and the distance betweenevaporation and condensation surfaces is only 1 mm. From experimental study, the effect of heat fluxand working fluid ratio on the performance of flat plate heat pipe is presented. Also we compared theexperimental results between the performance of water and magnetic fluid as working fluids.

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1. Introduction fluid can be increased much even in the absence of a magnetic

The magnetic fluid is a colloidal solution of many ultrafine mag-netic particles coated with surfactants and dispersed in a carrierliquid. The sizes of the particles are among 10–100 nm. The carrierliquid can be water, hydrocarbon, fluorocarbon etc. Therefore, themagnetic fluid possesses both magnetic and fluid properties. Mag-netic fluid consists of magnetic particles, carrier liquid and surfaceactive agent. Magnetic fluids have many practical engineeringapplications, such as sealing, bearing, magneto gravimetric separa-tion, damper and energy conversion. Water-based magnetic fluid ispreferred for heat transfer applications due to its higher latent heatover hydrocarbon liquids.

Since the appearance of magnetic fluid in 1960s, it had got wideapplications. Especially, the researchers pay much attention to thethermodynamic characteristics of magnetic fluid. Takahashi et al.[1] investigated the saturated pool boiling heat transfer of tolu-ene-solvent magnetic fluid on a horizontal surface in the verticallylinear profile of a magnetic field. The heat transfer is enhanced for adilute fluid or under a high field gradient, while being reduced for adense fluid at high heat flux under a low field gradient. Bashtovoiet al. [2] did the experimental study of heat transfer in magneticfluids under magnetic fields normal and tangential to the cylindri-cal steel specimen axis. It is shown that the orientation of the mag-netic field has a strong effect on the characteristics of boiling heattransfer under different boiling regimes. Liu et al. [3] made a poolboiling heat transfer comparison among water-based magnetic flu-ids in the absence and presence of a magnetic field. The experi-mental results show that the boiling heat transfer of magnetic

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field, and the presence of magnetic field can enhance the boilingheat transfer of magnetic fluid further.

The magnetic fluid possesses both magnetic and fluid proper-ties, so a magnetic fluid linear pump was invented. The magneticfluid can flow at the pumping force of the magnetic field. Parket al. [4–6] studied the pumping forces in the magnetic fluid linearpump. They derived the method to compute the pumping forces ofthe magnetic fluid linear pump and studied the new design to re-duce the discontinuities of the pumping forces of the magneticfluid linear pump. The boiling heat transfer of magnetic fluid canbe increased by magnetic field and the magnetic fluid also can beforced by magnetic field to flow. Therefore, the application of mag-netic fluid for heat transport device is studied. Shuchi et al. [7]introduced a new type of magnetic fluid which is a binary mixtureof a magnetic fluid and an organic solvent. By using the binarymagnetic fluid, a boiling two-phase flow of the fluid is obtainedwithout any deterioration of magnetic properties of the fluid. Aheat transport device is investigated and the effects of positionsof a magnetic field on heat transfer and driving force characteris-tics are particularly investigated experimentally. The applicationsof magnetic fluid to heat pipe working fluid are seldom studied.Nakatsuka et al. [8–9] studied the performance of heat pipe usingcitric ion-stabilized magnetic fluid (CMF) as working fluid. Theheat transfer was influenced by the application of magnetic fieldand was enhanced by a maximum of 30% compared to the field-free case. Furthermore, under the optimum magnetic field config-uration, the heat transfer by CMF heat pipe was 10% higher thanthat with water as working fluid. Irrespective of the presence or ab-sence of magnetic field water-based magnetic fluid degraded dur-ing boiling. However, the degradation of magnetic fluid wasavoided by heating the fluid in magnetic field.

Fig. 1. Experimental system. 1, Blower; 2, anemoscope; 3, flat plate heat pipe; 4, PC;5, data acquisition/switch unit; 6, transformer; 7, heating unit; and 8, heat sink.

Z. Ming et al. / Experimental Thermal and Fluid Science 33 (2009) 1100–1105 1101

The magnetic fluid had been used as the working fluid of con-ventional tubular heat pipe [8–9]. The magnetic field can increasethe boiling heat transfer of magnetic fluid, so the performance ofheat pipe is developed. We find that it is more suitable for applyingmagnetic fluid as the working fluid of flat plate heat pipe. In the flatplate heat pipe, the distance between evaporation surface and con-densation surface is only 1 mm, so it is much possible that lots ofmagnetic fluid is carried to the condensation surface under theforce of bubble growing and rupture. The magnet is able to drivethis part of liquid from condensation surface to evaporation sur-face. Especially, the magnet for wickless flat plate heat pipe canforce the magnetic fluid to accumulate around the heater on evap-oration surface. Without magnet, the working fluid is easy to de-tain in the marginal region on the evaporation surface.

The flat plate heat pipes can achieve more uniform heat flux dis-tribution than solid copper plate, that is, by replacing the solid cop-per plate with a flat heat pipe a more uniform heat flux distributioncan be obtained. These devices have a very high thermal conduc-tance. A flat plate heat pipe spreader is a kind of heat pipe withspecial arrangement. Compared to the conventional tubular heatpipe, the evaporator and condenser sections are the opposite sidesof it. The vapor channel is a very narrow space between the evap-oration and the condensation surfaces. The working fluid boils atthe evaporation surface and the generated vapor flows to and con-densates on the condensation surface. In this way the heat can betransferred from the evaporator to the condenser. It is called ‘‘va-por chamber” in some other papers. By using a flat plate heat pipea nearly isothermal cooling surface can be obtained. A uniformtemperature and thus a uniform heat flux distribution on the cool-ing surface also means a more effective heat dissipation from heatsource, according to heat transfer theory. The application of mag-netic fluid as working fluid of flat plate heat pipe can increasethe phase change heat transfer on evaporation surface and forcethe working fluid circulation when the flat plate heat pipe workson the present of magnetic field.

We did not study the negative effect of magnetic field to elec-tronic device, but the magnetic fluid can be a new choice as work-ing fluid of flat plate heat pipe. The magnetic fluid heat pipe can beapplied in the cooling devices of high power semiconductor diodelasers and light emitting diode which are not so sensitivity to mag-netic field. And also we did not study the deterioration of magneticproperties in boiling process. We prepared a disk-shape copper flatplate heat pipe without capillary wick. The distance of evaporationand condensation surfaces is only 1 mm. The experimental resultspresented the effect of heat flux, working fluid ratio and gravity tothe performance of magnetic fluid flat plate heat pipe.

2. Experimental setup and procedures

The apparatus used in this study include a blower, a data acqui-sition/switch unit, T-type thermocouples, an anemoscope, a trans-former, a heating unit and a PC. Fig. 1 shows the scheme of theexperiment system. Air from a blower is forced to flow verticallyonto the heat sink to cool the flat plate heat pipe. The air velocityis measured by the anemoscope (testo 405-V1) with an uncertaintyof 5%. Heat flux is supplied by heating unit. Adjusting output volt-age of the transformer can change the input power of the heatingunit. The temperatures are measured by T-type thermocouplesand collected through a data acquisition system. The precision ofthe T-type thermocouples in our work is 0.1 �C.

Fig. 2 shows the configuration of the heating unit. The heatingunit is a copper rod with four electrical rod heaters. The rod diam-eter of its lower part is 60 mm and at its upper part diameter re-duces to 20 mm to obtain a high enough heating heat flux to theflat plate heat pipe. The copper rod is insulated and small insulat-

ing cushions are put between copper rod and supporting frame-work to reduce heat loss. In order to reduce contact resistancebetween the heating unit and the flat plate heat pipe, high conduc-tivity grease was applied to their contact surface. Tighteningscrews are also used to improve the contact between the flat plateheat pipe and the heating unit. One to four are the locations ofthermocouples that are used to measure the temperature distribu-tion of the copper rod. There are 12 thermocouples are buried in-side the copper rod to measure the temperature distribution ofthe copper rod. Every three thermocouples are used to measurethe temperature of one location. The locations of the thermocou-ples are shown in Fig. 2. So the temperatures of points 1 to 4 arethe mean value of temperature values from three thermocouples.The holes for burying the thermocouples are 10 mm in depth and1 mm in diameter. The four thermocouples in a row are locatedat 2 mm, 17 mm, 32 mm and 47 mm from the top surface of thecopper rod, respectively. The steady state temperature distributionalong the axial direction is thus obtained and is used to calculatethe temperature gradient. Then, the heat flux of the heating unitis deduced from Fourier’s law of heat conduction.

Comparing with the solid copper plate spreader, the flat plateheat pipe spreader should be applied when large base sections ofheat sinks and high heating power are involved. We know thatwe can reduce the spreading resistance of a solid copper platespreader by increasing plate thickness. So the flat plate heat pipespreader should be manufactured for small thicknesses, otherwise,it will present worse spreading performance than the solid copperplate in of equal dimensions.

In order to study the phase change heat transfer in the smallspace of flat plate heat pipe, we prepared a disk shape copper flatplate heat pipe without capillary wick. The distance of evaporationand condensation surfaces is only 1 mm which can be shown inFig. 3. The diameters of the top and bottom plates are both85 mm. In order to measure the temperature distribution of theevaporation and the condensation surface, 6 small holes weredrilled into the bottom and top plates to bury 6 thermocouples.Point 5 is at the center of evaporation surface. Points 6 to 10 are0 mm, 7.5 mm, 17.5 mm, 27.5 mm and 37.5 mm away from thecenter of condensation surface, respectively. To reduce the contactresistance and the temperature measurement error, high conduc-tivity grease is applied into these holes. We designed the thick-nesses of top and bottom plates to 3 mm, so that we can insertthe thermocouples to measure the temperatures of plates. In fact,it is unnecessary to design so big thicknesses for evaporation andcondensation surfaces. We manufactured an annular permanentmagnet to supply the magnetic field, so we can study the perfor-mance of magnetic fluid heat pipe on the present of magnetic field.The outer diameter and inner diameter are of 40 mm and 25 mm.The height of the annular permanent magnet is 10 mm. The surfacemagnetic field strength of the magnet is 0.4 T. The water-based

Fig. 2. Heating unit.

Fig. 3. Structure of flat plate heat pipe.

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magnetic fluid used in our work was provided by Jinke MagneticLiquid Co. Ltd. The magnetic particle is Fe3O4. The density of mag-netic fluid is 1.28 g/cm3 and specific magnetism is 0.032 T.

Before filling the heat pipe, it must be weighed. Then a vacuumpump is used to bring the heat pipe to vacuum condition and theworking fluid is filled into it. The filling amount is a little bit overthe expected value. After that, the heat pipe is vacuumized again.But this time, it is heated by hot water during the vacuumizingprocess in order to extract as much residual air as possible and alsoto assure the heat pipe is full of vapor of the working fluid. At thismoment the pressure transducer indicates that the pressure valuein the heat pipe is 30 Pa. When the weight of the heat pipe reachesthe expected value, the vacuum valve is closed to keep the vacuumcondition. To test its tightness, the heat pipe is left in room condi-tions for 24 h, and if the pressure of the heat pipe as measured bythe pressure transducer then is equal to the saturated pressure ofthe room temperature then we say that the sealing can maintainthe vacuum condition for the experimental time. Otherwise theheat pipe is brought to vacuum condition again.

In order to simulate the state of magnetic fluid on the present ofannular magnetic field, we put a quartz glass plate on the annular

Fig. 4. The state of the magnetic fluid

magnet and then put some magnetic fluid on the quartz glass plate.From Fig. 4, we can see that the magnetic fluid distributes annu-larly under the effect of magnetic field. When we turned the quartzglass plate over, the magnetic fluid did not drop down and kept theannular distribution on quartz glass plate. Therefore, if we put theannular magnet near the heat source, most of the liquid innerthe flat plate heat pipe will stay near the heat source. The liquid boilsat the evaporation surface and the vapor flows to and condenses onthe condensation surface. In this way the heat can be transferredfrom the evaporator to the condenser. Because of the small distancebetween evaporation and condensation surface (1 mm), the liquidwhich condenses on condensation surface is easy to return theevaporation surface on the present magnetic field. It is possible forthe wickless flat plate heat pipe to work versus gravity.

3. Results and discussion

Fig. 5 shows the temperature responses of magnetic fluid flatplate heat pipe at different heat flux. The working fluid of the flatplate heat pipe is magnetic fluid and the working fluid ratio is53.5%. We use the volume ratio of working fluid and cavity of heatpipe to define the working fluid ratio. The room air temperature is28 �C and the air velocity that impinges on the heat sink is 9.7 m/s.At the beginning of experiment, we set the heat flux as 20.9 W/cm2, and then we increased the heat flux at a certain time to seethe performance at different heat flux. We can see that the temper-ature difference between evaporation and condensation surfacesincreased with time before the startup of the flat plate heat pipe.Then the flat plate heat pipe starts up suddenly at a certain tem-perature difference. It is can be understood that this temperaturedifference is essential for the start of the boiling process on the

on the present of magnetic field.

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Fig. 5. Temperature responses of magnetic fluid flat plate heat pipe at different heatflux.

Z. Ming et al. / Experimental Thermal and Fluid Science 33 (2009) 1100–1105 1103

evaporation surface. The startup of the flat plate heat pipe showsthe beginning of the boiling inner the vapor channel. After thestartup of heat pipe, the temperature difference between evapora-tion and condensation surfaces decreases suddenly and the tem-perature values fluctuate at a certain range. The bubbles formedon the smooth surface are bigger in size and fewer in number thanthose on the capillary structure surface. Therefore, the fluctuationof temperature is caused by the discontinuous departure of bub-bles from smooth evaporation surface. And also the big size bubblecan carry hot liquid to impinge on the condensation surface whichis only 1 mm away from evaporation surface, so the temperaturefluctuations happen on both evaporation and condensation sur-faces. The temperature difference between evaporation and con-densation surfaces also increases with the heat flux.

Fig. 6 shows the startup of magnetic fluid flat plate heat pipe atdifferent heat flux. The working fluid is magnetic fluid and theworking fluid ratio is 53.5%. The room air temperature is 28 �Cand the air velocity that impinges on the heat sink is 9.7 m/s.The results show the temperature responses of evaporation andcondensation surface when heat fluxes are 20.9 W/cm2, 25.2 W/cm2 and 34.9 W/cm2. The flat plate heat pipe starts up suddenlyand the temperatures fluctuate in a certain range. With the in-

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Fig. 6. The startup of magnetic fluid flat plate heat pipe at different heat flux.

crease of heat flux, the flat plate heat pipe will spend less time tostart up, but the temperature difference between evaporationand condensation surfaces will be much bigger. That is becausethe bigger temperature difference is needed to enable boiling andcondensation inside the flat plate heat pipe at higher heat flux.

Fig. 7 shows the temperature responses of magnetic fluid flatplate heat pipe at different working fluid ratios. The working fluidis magnetic fluid and heat flux is 25.2 W/cm2. The room air temper-ature is 28 �C and the air velocity that impinges on the heat sink is9.7 m/s. When the working fluid ratio is 100%, it means the flatplate heat pipe is full of magnetic fluid. When the working fluid ra-tio is 0%, it means the flat plate heat pipe is empty. We can see thatthe temperature difference between evaporation and condensationsurfaces is more than 30 �C when the working fluid ratio is 0%.When the heat pipe is full of working fluid, it also can not startup. When the working fluid ratio is too low, such as 40.7%, the tem-peratures of evaporation and condensation surfaces fluctuate tran-sitorily at some time, but it can not start up at last. Our experimentresults show that the optimal working fluid ratio of this magneticfluid flat plate heat pipe is 53.5%. We also found that the heat pipecan work normally at the proper working fluid ratio range. The crit-ical values of the ratio range depend on the heat flux and the qual-ity of the flat plate heat pipe, so it is not easy to be determined inexperiment.

Fig. 8 shows the dry out characteristics of magnetic fluid flatplate heat pipe which is obtained accidentally. The working fluidis magnetic fluid and working fluid ratio is 41.2%. The room airtemperature is 27 �C and the air velocity that impinges on the heatsink is 9.7 m/s. At the beginning, we set the heat flux as 30.5 W/cm2. When the temperatures increase to more than 40 �C, the tem-peratures of evaporation and condensation surfaces start to fluctu-ate suddenly at the same time. Some time later, the temperaturedifference between evaporation and condensation surfaces in-creases suddenly, instead of decreasing. That shows the flat plateheat pipe dry out and the temperatures increase quickly. The tem-perature of point 10 is even bigger than the temperature of point 6which shows the heat pipe is disabled at all and the heat is trans-ferred by conduction. After we changed the heat flux to 25.2 W/cm2, the temperatures of evaporation and condensation surfacesdecrease simultaneously and then the heat pipe starts up suddenlyagain. The temperature difference between evaporation and con-densation surfaces decrease and the temperatures start to fluctuateat the same time. From this experimental result, we can see thatthe flat plate heat pipe will dry out at 30.5 W/cm2 when the work-ing fluid ratio is 41.2%, but it can work normally at 25.2 W/cm2.Therefore, 41.2% is the minimal working fluid ratio which canmaintain the normal work of flat plate heat pipe when the heat fluxis 25.2 W/cm2.

Fig. 9 shows the performance of magnetic fluid and water flatplate heat pipe working versus gravity. The air velocity that im-pinges on the heat sink is 9.7 m/s and the heat flux is 25.2 W/m2.In this experiment, we turned over the experiment rig, so the evap-oration surface is on top and the condensation surface is at bottom.We can see that the magnetic fluid flat plate heat pipe can start upwhen the working fluid ratio is 70.5%, but can not start up whenthe working fluid ratio is 61.8%. From Fig. 4, we can understandthat the magnetic fluid can be attracted by the magnetic fieldand reverse gravity. The pumping force mainly depends on the spe-cific magnetism of the magnet. The gravity and friction both candamp the flow of magnetic fluid, so it needs more working fluidto form circular loop in heat pipe. The magnetic fluid flat plate heatpipe should work at high working fluid ratio. We also studied theperformance of water flat plate heat pipe working versus gravity.The results show that the water heat pipe can not start up at allkinds of working fluid ratio, because there are not any capillarystructures in this flat plate heat pipe.

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Fig. 7. Temperature responses of magnetic fluid flat plate heat pipe at different working fluid ratio.

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Fig. 8. Dry out characteristics of magnetic fluid flat plate heat pipe.

1104 Z. Ming et al. / Experimental Thermal and Fluid Science 33 (2009) 1100–1105

Fig. 10 shows the temperature responses of magnetic fluid flatplate heat pipe. The working fluid is magnetic fluid and the work-ing fluid ratio is 46.8%. The room air temperature is 25 �C and the

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Fig. 9. The performance of magnetic fluid and wate

air velocity that impinges on the heat sink is 9.7 m/s. At the begin-ning of experiment, we set the heat flux as 25.2 W/cm2, and thenwe increased the heat flux at every certain time to see the perfor-mance of it at different heat flux. We can see that the suddendecreasing of the temperature difference between evaporationand condensation surfaces does not happen when the workingfluid ratio is 46.8%. The liquid film of evaporation surface is thinnerwhen the working fluid ratio is lower, so the heat pipe does notneed big temperature difference to start up. At different heat flux,the magnetic fluid flat plate heat pipe shows very good perfor-mance regarding temperature leveling in the condensation surface.Temperatures of the five points are almost same at all kinds ofheat flux, except the result at high heat flux. In order to dissipateheat efficiently, the heat sink is generally much larger than theheat source. So a flat plate heat pipe spreader is usually placedbetween the heat source and the heat sink to level the temperaturedistribution. The magnetic fluid flat plate heat pipe can achievemore uniform heat flux distribution on the condensation surface.

Fig. 11 shows the temperature responses of water flat plate heatpipe. The working fluid of the flat plate heat pipe is water and theworking fluid ratio is 51.2%. The room air temperature is 18 �C andthe air velocity that impinges on the heat sink is 9.7 m/s. The heatflux is 25.2 W/m2. We can see that temperature difference on thecondensation surface of water flat plate heat pipe is much bigger

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r flat plate heat pipe working reverse gravity.

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Fig. 10. Temperature responses of magnetic fluid flat plate heat pipe.

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Fig. 11. Temperature responses of water flat plate heat pipe.

Z. Ming et al. / Experimental Thermal and Fluid Science 33 (2009) 1100–1105 1105

than magnetic fluid flat plate heat pipe. The edge temperatures ofcondensation surface also do not fluctuate as those on magneticfluid flat plate heat pipe. That because the wickless flat plate heatpipe has a very small space in vapor channel, the liquid is easy tobe pressed by the vapor to the edge of the vapor channel. The li-quid will block the vapor to flow and condense on the condensa-tion surface. If we use the magnetic fluid as the working fluidand put magnet near the evaporation surface. The liquid will be at-tracted by magnetic field and the circulation of working fluid canbe improved.

4. Conclusions

We used magnetic fluid as the working fluid of the flat plateheat pipe. With the presence of magnetic field, the effect of heatflux and working fluid ratio on the performance of wickless flatplate heat pipe is studied. From the experimental results and theabove discussions, it is possible to draw the following conclusions:The wickless flat plate heat pipe starts up suddenly at a certaintemperature difference when the working fluid ratio is at highlevel. After the startup of heat pipe, the temperature difference be-tween evaporation and condensation surfaces decreases suddenlyand the temperature values fluctuate at a certain range. The opti-mal working fluid ratio of this magnetic fluid flat plate heat pipe is53.5% and the flat plate heat pipe only can work normally at theproper working fluid ratio range. The water flat plate heat pipecan not start up versus gravity at all kinds of working fluid ratio,because there are not any capillary structures in this flat plate heatpipe. On the present of magnetic field, the magnetic fluid flatplate heat pipe could work versus gravity at high working fluid ra-tio. The liquid magnetic fluid can be attracted by magnetic fieldand the circulation of working fluid can be improved, so the mag-netic fluid flat plate heat pipe can achieve more uniform heat fluxdistribution on the condensation surface than water flat plate heatpipe.

Acknowledgement

This work is supported by Beijing Education Committee ProjectNo. KM200510005002, Key Lab Project of Beijing University ofTechnology, Beijing Natural Science Foundation Project No.3092007 and Beijing Outstanding Scholar Program (2006).

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