International Journal of Research and Innovations in Science and Technology©SAINTGITS College of Engineering, INDIAwww.journals.saintgits.orgReview paper

A Review of Performance of Heat Pipe with NanofluidsJubin V Jose1*, A Ramesh2, Ebin Joshy3

1,3Government Engineering College, Thrissur, Kerala- 680009, India2Professor, Dept. of Mechanical Engineering Government Engineering College, Thrissur, Kerala, India

*Corresponding author E-mail: jubinvjose@gmail.com

Copyright © 2014 IJRIST. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use,distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

With the modern era of miniaturization of equipments, heat pipes have attracted major attention in the field of heattransfer. Nanofluids also have attracted a notable attention in recent days due to its superior heat transport properties.This review aim to compile the effect of nanofluid in heat pipes. Performance of different nanoparticles and differentbase-fluids are investigated. Most of the papers reviewed here reported an enhancement in performance of heat pipes.Existence of an optimum concentration of nanoparticles in base fluid was also reported. Paper also presents aperspective on possible research application.

Keywords: Heat pipe, heat transfer, nanofluid, nanoparticle, tilt angle, volume fraction.

1. Introduction

Nanotechnology is a field of emerging technology. From 1990’s, the researchers started study on applying nanoparticlesto enhance heat transfer. In 1995 Choi [1] proposed the concept of nanofluid. Nanofluids are stable colloidal suspensionof nano-meter sized particles in the base fluid. In the modern days, nanofluids have attracted the attention of researchgroups due to their enhanced heat transport properties.Heat pipe is a special type of heat exchanger that can transfer large amount of heat by phase change of working fluidand capillary action. It can transfer thermal energy about 1000 times than copper, the best known conductor. It is widelyused in electronic cooling systems, solar heaters etc. The application of nanofluid in heat pipe was first investigated in2003 H T Chient et al[2].The study was conducted on the improvement on thermal performance of disk shapedminiature heat pipe with nanofluid.Various mechanisms of heat transfer enhancement by nanofluids were proposed such as interface effect, Brownianmotion, thermophorisis, formation of porous layers etc.

2. Study of heat pipes with nanofluids

In 2007, Y.H Lin et al [3] investigated the performance of heat pipe with silver-water nanofluid. Experiment was donewith different concentrations, filling ratio and heating power. Higher value of filling ratio makes bubble formationdifficult. Also lower value of filling ratio lead to easy dry out. P Naphon et al[4] in 2008 performed experiment withwater, alcohol and nanofluid (alcohol+nanoparticle) (TiO2 21nm). Effect of tilt angle on performance was studied and itwas found that efficiency increases with increase in tilt angle due to the flow of liquid 600 (De-water) and 450(alcohol)was the best tilt angle with maximum heat transfer. With higher tilt angle thick liquid layer form over evaporator, whicheventually increase thermal resistance.

Effect of filling ratio was also studied. It was found that for higher filling ratio, there is only reduced space for vapour incondenser. Therefore evaporation of fluid decreases and optimum was found to be 66%. Z H Liu et al [5] conductedstudy with nanoparticle Cu, CuO and SiO with DI water as base fluid. Cu and CuO enhanced the performance whileSiO deteriorates the performance. Heat transfer coefficient at condenser and evaporator was found to increase. Heattransfer was found to increase with particle size. Reasons put forward by the author for getting the enhanced

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performance were: 1) increase in thermal conductivity of base fluid 2) decrease of solid-liquid contact angle betweennanofluid and heater surface 3) predominant Brownian motion 4) formation of coating layer on heater surface. Coatinglayer of Cu and CuO is compactly porous structure compactly aggregated nanoparticles while coating of SiO2 is slurrylick layer without porous structure and is not stable and washes away when cleaned.

Figure 1: TEM of the coating layer formed on the heat pipe surface compared to the surface without coating.

M. Shafahi et al. [6] studied thermal performance of rectangular and disc shaped heat pipe with nanofluid. Authorreports a substantial increase in thermal performance of flat shaped heat pipe. Heat load capacity of flat shaped heatpipe increases with particle loading. Sameer K et al. [7] in 2010 conducted a review on methodologies to predicthydrodynamic properties of uni-directional two-phase Taylor bubble flow.

M G Mousa [8] conducted experiment with pure water and Al2O3-water based nanofluid in heat pipe. Surfacetemperature was found to decrease with increase in nanoparticle concentration. Thermal resistance reduces withincrease in filling ratio and reach minimum at 0.4 filling ratio. Porous layer formation was reported to be thepredominant reason for improvement in performance as it increases the wettability. Author reported optimumconcentration of 0.6 to 0.8 volume fraction of nanoparticle.

In 2012, Y H Hung et al. [9] investigated performance of heat pipe with Al2O3-water nanofluid (0.5, 1, 3 wt %).Chitosan 0.2% was used as dispersant in this experiment. The optimum filling ratio was at 20-40% range. Tilt angle formaximum performance was 40-700. For lower concentration, higher thermal performance was obtained at lower fillingratio. For higher concentration, higher thermal performance was obtained at high filling ratio. At higher concentrationof nanoparticle author reported a drop in thermal efficiency due to drop in convective heat transfer. Author also reportsthat at evaporator with vaporization nanoparticle remain at the evaporator surface which leads to 1) rapid increase innanofluid concentration at evaporator 2) lead to premature dry out.

N Putra et al. [10] investigated performance with 5 different nanofluids. Thermal conductivity was found to increasewith particle concentration. Temperature difference between the ends decreases with particle concentration. Henceevaporator temperature gets reduced. Author reports the formation of coating layer on wire mesh which 1) improvedcapiliary structure 2) improved surface wettability 3) reduces contact angle 4) increases the surface roughness. Meshheat pipe with best performance was Al2O3-water nanofluid with 5% volume concentration.

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Figure3: SEM image screen mesh wick (1)pure water(2) Al2O3–water, (3) Al2O3 ethylene glycol, (4) TiO2–water,(5) TiO2–ethylene glycol, (6) ZnO–ethylene glycol

In 2012 M. Keshavarz, M Oravej and S Razavarz [11] conducted investigation with pure water and aluminum oxidenanoparticle. Heat pipe with nanofluid found to operate at higher heat loads. Thermal resistance of heat pipe withnanofluid was found to be less. This is because, for pure water large bubble nucleation size at evaporator creates highthermal resistance. With nanofluid suspended particle bombard with vapour bubble during bubble formation. Hencenucleation size of vapour bubble will be much smaller for nanofluid.

L.G Asirvatham et al.[12] investigated the performance of heat pipe with silver nanofluid in D.I water. In theinvestigation vapour and wall temperature was measured at three points of each region which show the more accuratevariation compared to measuring only at each region. Vapour temperature get reduced due to high conductivity ofnanofluid. Reduction in evaporator surface temperature with increase in concentration of nanoparticle is due toaugmented heat transfer and increase in evaporator surface area. Reasons leading to drop in evaporator and condensertemperature are 1) formation of porous coating layer 2) increase of surface roughness 3) enhancement in bubbleformation growth and collapse 4) decrease in contact angle which increase surface wettability and capiliary pumpingability.

Figure2: SEM image of the screen mesh wick surface after the experiment with nanofluid as working fluid.

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3. Conclusion

By the review on the various researches done on the performance of heat pipe with nanofluid, most of the authorsreported enhancement in performance. This shows the applicability of nanofluid as working fluid. From the journalsreviewed here, it was found that there was an optimum particle loading concentration above which it affects theperformance. Operating range increase, hence dry out occur at high heat flux and thermal resistance found to decreasewhile for certain nanoparticles, performance deteriorates. Hence the structure of porous layer formed at evaporatorsurface is an important parameter. There are still challenges in the study of phase change of nanofluids. It is one of theareas which are still to be explored. The effect of surfactants to stabilize nanofluids also has to be taken intoconsideration while accounting the effect of nanofluids.

Acknowledgements

The author wishes to thank Mr. Sreejith A, M.Tech student of GEC Thrissur for his help and support to complete thiswork.

References

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