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Performance analysis of grooved heat pipe using nano fluids.

INTRODUCTION

A heat pipe or heat pin is a heat-transfer device that combines the principles of both thermal conductivity and phase transition to efficiently manage the transfer of heat between two solid interfaces. At the hot interface within a heat pipe, which is typically at a very low pressure, a liquid in contact with a thermally conductive solid surface turns into a vapour by absorbing heat from that surface. The vapour condenses back into a liquid at the cold interface, releasing the latent heat. The liquid then returns to the hot interface through either capillary action or gravity action where it evaporates once more and repeats the cycle. In addition, the internal pressure of the heat pipe can be set or adjusted to facilitate the phase change depending on the demands of the working conditions of the thermally managed system. A typical heat pipe consists of a sealed pipe or tube made of a material with high thermal conductivity such as copper or aluminium at both hot and cold ends. A vacuum pump is used to remove all air from the empty heat pipe, and then the pipe is filled with a fraction of a percent by volume of working fluid(or coolant) chosen to match the operating temperature. Examples of such fluids include water, ethanol, acetone, sodium, or mercury. Due to the partial vacuum that is near or below the vapour pressure of the fluid, some of the fluid will be in the liquid phase and some will be in the gas phase. The use of a vacuum eliminates the need for the working gas to diffuse through any other gas and so the bulk transfer of the vapour to the cold end of the heat pipe is at the speed of the moving molecules. In this sense, the only practical limit to the rate of heat transfer is the speed with which the gas can be condensed to a liquid at the cold end.

Inside the pipe's walls, an optional wick structure exerts a capillary pressure on the liquid phase of the working fluid. This is typically a sintered metal powder or a series of grooves parallel to the pipe axis, but it may be any material capable of exerting capillary pressure on the condensed liquid to wick it back to the heated end. The heat pipe may not need a wick structure if gravity or some other source of acceleration is sufficient to overcome surface tension and cause the condensed liquid to flow back to the heated end.A heat pipe is not a thermosiphon, because there is no siphon. Thermosiphons transfer heat by single-phase convection. Heat pipes contain no mechanical moving parts and typically require no maintenance, though non-condensing gases (that diffuse through the pipe's walls, result from breakdown of the working fluid, or exist as impurities in the materials) may eventually reduce the pipe's effectiveness at transferring heat. This is significant when the working fluid's vapour pressure is low. Maryam Shafahi et al [1], conducted experiments using cylindrical pipe utilizing nano fluids. They considered three of the most common nanoparticles, namely Al2O3, CuO, and TiO2 as the working fluid. A substantial change in the heat pipe thermal resistance, temperature distribution, and maximum capillary heat transfer of the heat pipe is observed when using a nanofluid. The nanoparticles within the liquid enhance the thermal performance of the heat pipe by reducing the thermal resistance while enhancing the maximum heat load it can carry. They also investigated the effect of particle size on the thermal performance of the heat pipe. They found that smaller particles have a more pronounced effect on the temperature gradient along the heat pipe. Xue Fei Yang et al [2],perfomed experiments on the heat transfer performance of a horizontal micro-grooved heat pipe using CuO nanofluid as the working fluid. CuO nanofluid was a uniform suspension of CuO nanoparticles and deionized water. The average diameter of CuO nanoparticles was 50 nm. Mass concentration of CuO nanoparticles varied from 0.5 wt% to 2.0 wt%. Experimental results showed that CuO nanofluid can improve the thermal performance of the heat pipe and there is an optimal mass concentration which is estimated to be 1.0 wt% to achieve the maximum heat transfer enhancement. Operating pressure has apparent influences on both the heat transfer coefficients and the CHF of nanofluids. The minimum pressure corresponds to the maximum heat transfer enhancement. Under an operating pressure of 7.45 kPa, the heat transfer coefficients of the evaporator can be averagely enhanced by 46% and the CHF can be maximally enhanced by 30% when substituting CuO nanofluids for water. Guo-Shan Wang et al [3],have conducted experiments to investigate the operation characteristics of a cylindrical miniature grooved heat pipe using aqueous CuO nanofluid as the working fluid at some steady cooling conditions. The experiments were carried out under both the steady operation process and the unsteady startup process. Their experiment results showed that substituting the nanofluid for water as the working fluid can apparently improve the thermal performance of the heat pipe for steady operation. The total heat resistance and the maximum heat removal capacity of the heat pipe using nanofluids can maximally reduce by 50% and increase by 40% compared with that of the heat pipe using water, respectively. For unsteady startup process, substituting the nanofluid for water as the working fluid, not only improve the thermal performance, but also reduce significantly the startup time.

Zhen-Hua Liu et al [4], have conducted experiments to investigate the thermal performance of an inclined miniature grooved heat pipe using water-based CuO nanofluid as the working fluid. Their study focused mainly on the effects of the inclination angle and the operating pressure on the heat transfer of the heat pipe using the nanofluid with the mass concentration of CuO nanoparticles of 1.0wt%. The experiment was performed at three steady sub-atmospheric pressures. Experimental results showed that the inclination angle has a strong effect on the heat transfer performance of heat pipes using both water and the nanofluid. The inclination angle of 45 corresponded to the best thermal performance for heat pipes using both water and the nanofluid. The investigation indicated that the thermal performance of an inclined miniature grooved heat pipe can be strengthened by using CuO nanofluid. CK Loh et al [5] studied about heat pipe performance with different wick structures subjected to different orientations. Wick structures with low capillary limit work best under gravity assisted conditions, where the evaporator is located below the condenser. The experimental heat pipes orientation test produced the following

1. Heat source orientation and gravity have less effect on sintered powder metal heat pipes due to the fact that the sintered powder metal wick has the strongest capillary action

* It is not desirable to use groove or mesh heat pipes when the orientation of the evaporator (heat source) is on top of the condenser (heat sink).

For 6mm OD, the groove heat pipe has better thermal performance than mesh and sintered powder metal in the +90[degrees] to 0[degrees] range.

II. Experimental setup:

The heat pipe is fabricated in copper tube of internal grooves. The pipe is divided into three sections, evaporator section, adiabatic section & condenser section. The evaporator or heater section is heated using a sheet type heater wound around it. The adiabatic section is completely insulated. The condenser section is surrounded by a shell of 20mm diameter, with water inlet and water outlet tubes. The heat pipe is first charged with CuO--nanofluids, prepared using Ultra--sonic Homogenizer, is charged into Vacuumised heat pipe, which is maintained at 36cm of Hg. Eight thermocouples of K- type is installed at various sections of the heat pipe. The specifications of the heat pipe are as follows.

* Heat pipe--100W capacity

* Wattmeter--0-1000W

* Auto-transformer--230 V,5 A

* Rotameter--max. 330 ml/min.

* Data Logger and Laptop

* Thermocouples--9 nos.

* Total length of pipe, [L.sub.t] =600 mm

* Evaporator length, [L.sub.e] =180mm

* Adiabatic section length, [L.sub.a] =220mm

* Condenser length, [L.sub.c] = 200 mm

* Outer & Inner Diameters = 10mm & 9.3 mm respectively

III.Experimental procedure:

The experiment, after optimising using DOE, is carried out with varying the samples/fluids. This is the beginning of the experimental process. The heat pipe is first filled with the Nano fluids and the thermocouples are connected in the Data-logger and configuration is done in it. The Wattmeter and the Auto-transformer connections are checked. First set of reading is taken based on the combination obtained on applying DOE. The same procedure is repeated for the processes involving the varying of Heat Input from 40W to 80W, varied in steps of 10W, the varying of Orientation of Heat Pipe from 0[degrees] to 90[degrees] varied in steps of 22.5[degrees] increment, the varying of Flow Rate of cooling water from 80ml/min to 120ml/min, varied in steps of 20ml/min. The corresponding temperatures i.e., T1 to T9 are noted and simultaneously the vacuum pressure reading is noted from the Pressure Gauge. The readings are taken till obtaining the steady-state. Next step is the changing of working fluid to DI water. After changing the working fluid the same procedure is followed, as followed for the Nano fluids. The readings are tabulated in the tabular column and calculations are done for finding out the Thermal resistance and Efficiency for each and every step. These values are plotted in the graphs and comparison is done between Nanofluids and DI water outputs.

RESULT AND DISCUSSION

From fig:2 it thermal resistance increased from [0.sup.0] to [45.sup.0] and then steady after all other orientation. This is due to Brownian motion of CuO nanoparticles inside the pipe. The Brownian motion its cost grooved heat pipe. Surface tension of the DI water is less, it affect both condensation and evaporation inside the pipe. So its low as compared to CuO--nanofluids.

At flow rate of 100ml/min proper condensation takes place which leads proper transfer occurs in both CuO -nanofluids and DI water. When comparing the graphs achieving the maximum efficiency of 97% in grooved heat pipe filled with CuO--nanofluid. This is because of the orientation, proper heat input and flow rate of grooved heat pipe which paves the exact condensation cycling of working fluid

EFFICIENCY CURVES FOR CuO NANOFLUID

EFFICIENCY CURVES FOR DI WATER

Conclusion:

The overall analysis of the results obtained gives a clear idea about the parameters which play an important role in the heat transfer in the heat pipe. The heat pipe charged with CuO --nanofluids, as working fluid, shows better performance with maximum efficiency of heat transfer being 97.6%. This can be attributed to the nanoparticles, which absorb some heat during the heat transfer.

It is found the percentage of increase in heat transfer about 11.3% more at CuO --nanofluid heat pipe than DI water heat pipe at the 450 orientation and 100ml/min flow rate. This experiment variable combination (60W heat input, 450 orientation and 100ml/min flow rate) is the most influencing scientific combination, which is also obtained from design of experiment method. Thus, we concluded that using nanofluids, we obtained heat transfer almost equal to 100%, and the most efficient heat transfer leads to energy conservation and energy saving.

REFERENCES

[1.] Maryam Shafahi, Vincenzo Bianco, Kambiz Vafai, Oronzio Manca, 2010. "An investigation of the thermal performance of cylindrical heat pipes using nanofluids", International Journal of Heat and Mass Transfer, 53: 1-3: 376-383 ISSN: 00179310.

[2.] Xue Fei Yang, Zhen-Hua Liu, Jie Zhao, 2008. "Heat transfer performance of a horizontal micro-grooved heat pipe using CuO nanofluid", Journal of Micromechanics and Microengineering, 18(3): 035-03 ISSN: 09601317

[3.] Guo-Shan Wang, Bin Song, Zhen-Hua Liu, 2010. "Operation characteristics of cylindrical miniature grooved heat pipe using aqueous CuO nanofluids", Experimental Thermal and Fluid Science, 34(8): 14151421 ISSN: 08941777

[4.] Zhen-Hua Liu, Yuan-Yang Li, Ran Bao, 2010. "Thermal performance of inclined grooved heat pipes using nanofluids", International Journal of Thermal Sciences, 49(9): 1680-1687 ISSN: 12900729.

[5.] Dong-ryun Shin, Seok-ho Rhi, Taek-kyu Lim, Ju-chan Jang, 2011. " Comparative study on heat transfer characteristics of nanofluidic thermosyphon and grooved heat pipe", Journal Of Mechanical Science And Technology, 25(6): 1391-1398

[6.] Sameer Khandekar, Yogesh M. Joshi, Balkrishna Mehta, 2008. Thermal performance of closed two- phase thermosyphon using nanofluids, International Journal of Thermal Science, 47: 659-667.

[7.] Zhang, 2001. Innovative heat pipe systems using a new working fluid, International Communications of Heat Mass Transfer, 28(8): 1025-1033.

[8.] Chi, S.W., 1976. Heat pipe theory and practice, McGraw-Hill, Washington.

[9.] Amir Faghri, 1995. Heat pipe science and technology, Taylor & Francis, Washington.

[10.] Peterson, 1994. An introduction to heat pipes, Wiley & Sons.

(1) V. Aruna and (2) N. Alagappan

(1) Assistant Professor In Mechanical Engineering, SMR East Coast College Of Engineering & Technology, Tamilnadu, India.

(2) Assistant Professor In Mechanical Engineering, Annamaiai University, Tamiinadu, India.

Received 28 February 2017; Accepted 22 May 2017; Available online 6 June 2017

Address For Correspondence: V. Aruna, Assistant Professor In Mechanical Engineering, SMR East Coast College Of Engineering & Technology, Tamilnadu, India. E-mail: arunaveer@gmail.com

Caption: Fig. 1: Experimental setup for heat pipe

Caption: Fig. 2: Efficiency of heat pipe for CuO-nanofluid and DI water
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Author:Aruna, V.; Alagappan, N.
Publication:Advances in Natural and Applied Sciences
Article Type:Report
Date:Jun 1, 2017
Words:2199
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