XXVIII Congresso UIT sulla Trasmissione del CaloreBrescia, 21-23 Giugno 2010

1. INTRODUCTION

As recent technology has made it possible to producenano-sized particles, researchers tried to disperse this size(determined to be < 100 nm) of particles, instead of millimeteror micrometer-sized ones, inside conventional heat transferfluids such as water, oil, and ethylene glycol. These new classof fluids are called nanofluids. A nanofluid can be defined as afluid in which solid particles with sizes below 100 nm aresuspended stably and dispersed uniformly. The base fluid usedis usually a traditional heat transfer fluid, e.g., water, oil, andethylene glycol.

A lot of researchers observed the phenomenon of higherthermal conductivity of various nanofluids compared to that ofthe base fluids. However, there is a main difference betweenthe results, i.e., some results showed that the increase ofthermal conductivity of nanofluids is an anomaly that cannotbe predicted by the existing conventional model for calculatingthe effective thermal conductivity of liquids containingsuspended solid particles [1 - 4] while some others showed thatthe increase is not an anomaly and can be predicted by usingthe existing model [5, 6]. Thus, the research regarding thermalconductivity increase of nanofluids needs to be extended untilthe physical phenomena responsible for it are known.

It is true that increase in thermal conductivity will improvethe heat transfer behavior of fluid. However, a number of othervariables also play key roles such as in convection heat transfercases. Convection heat transfer, beside the thermophysicalproperties of the fluid, is also influenced by the velocity of the

flow and by the geometry of the system through which the fluidis flowing. Hence, the heat transfer performance/behavior ofnanofluids must be tested directly under flow conditions.

Xuan and Li [7] reported that in turbulent forced convection,the heat transfer coefficient of Cu-water nanofluids flowinginside a uniformly heated tube remarkably increased. The heattransfer coefficient increased by around 39% for 2 vol.%nanoparticle concentration compared to that of water.Furthermore, it was observed that the increase of nanoparticleconcentration would also increase the heat transfer coefficient.For the same Reynolds number, the heat transfer coefficientincrease ranged from 6% to 39% with the increase of particleconcentration from 0.5 vol.% to 2 vol.% respectively.Interestingly, the experimental results showed that, fornanoparticle volume concentration of 1%, 1.2%, 1.5%, and2%, no significant increase in pressure drop compared to thatof water. Thus, it is no need to be worried about the drawbackof pumping power increase.

Maiga et al. [8] investigated, numerically, laminar andturbulent forced convection of water- 2O3 and ethyleneglycol- 2O3 nanofluids inside a uniformly heated circulartube. It was found that heat transfer at the tube wall wasenhanced for both laminar and turbulent flow compared to thatof base fluids. The enhancement increased with the increase ofparticle loading. However, this also resulted in the increase ofwall shear stress which causes the undesirable increase ofpumping power and contradicts the results of Xuan and Li [7].The particle volume concentrations used in this numericalstudy were 1%, 2.5%, 5%, 7.5%, and 10%.

ESTIMATION OF EFFECTIVENESS OF USING NANOFLUIDS FLOWINGINSIDE A CIRCULAR TUBE FOR HEAT TRANSFER APPLICATIONS

Eldwin Djajadiwinataa*, Hany al-Ansaryb, Khalid al-Dakkanc, Abdulaziz al-Jariwic

a Graduate Study of Department of Mechanical Engineering, King Saud University, P.O. Box 800, Riyadh 11421,Kingdom of Saudi Arabia

b Department of Mechanical Engineering, King Saud University, P.O. Box 800, Riyadh 11421,Kingdom of Saudi Arabia

c National Nanotechnology Center (NNC), King Abdulaziz City for Science and Technology (KACST),P. O. Box 6086, Riyadh 11442, Kingdom of Saudi Arabia

*Corresponding author, email address: eldwin_dj@yahoo.com

ABSTRACTThis study assesses the heat transfer application of nanofluids, particularly Al2O3-water, CuO-water, Al2O3-EG/water mixture(60:40wt), and CuO-EG/water mixture (60:40wt), flowing turbulently (Re around 5000 to 15000) inside a circular tube. The

h/P)n/(h/P)b h Pconvection heat transfer coefficient and the pumping power, respectivelbase fluids, respectively. It is found that for mass flow rate of 0.1 kg/s and mean bulk temperature of 25ºC, CuO-water nanofluidsreached maximum and minimum effectiveness of 1.106 at 1.5 vol.% nanoparticles and 1.093 at 3 vol.% nanoparticles,respectively, while Al2O3-water nanofluids reached maximum and minimum effectiveness of 1.087 at 2.5 vol.% nanoparticlesand 1.067 at 1% vol. nanoparticles, respectively.outperforms that of Al2O3 nanofluids. On the other hand, for EG/water mixture based nanofluids, with mass flow rate of 0.3 kg/sand various mean bulk temperature and particle volume concentration, generally the Al2O3outperforms that of This paper also shows that heat transfer application of nanofluids mainly depends on theirthermophysical properties and the assessment must be done by observing the ratio between h and P.Keywords: Nanofluids, copper oxide, alumina, pressure drop, pumping power, convection coefficient