• Journal of the Korean Society of Manufacturing Process Engineers
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• v.18 no.5
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• pp.60-65
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• 2019

Since nanofluids (NFs), which are a mixture of a small amount of nanoparticles and a bulk liquid solvent, were first proposed by Stephen Choi at the Argonne National Lab in 1995, they have been considered for use in many technical studies of power cooling systems and their practical application due to their high thermal conductivity and heat transfer coefficients compared to conventional coolants. Although nanofluids are a well-known form of engineering fluid that show great promise for use in future cooling systems, their underlying physics as demonstrated in experiments remain unclear. One proven method of determining the heat transfer performance of nanofluids is measuring the concentration of nanoparticles in a mixture. However, it is experimentally inefficient to build testbeds to systematically observe particle distributions on a nanoscale. In this paper, we demonstrate the distribution of nanoparticles under a temperature gradient in a solution using molecular dynamics simulations. First, temperature profiles based on substrate temperature are introduced. Following this, the radial pair distribution functions of pairs of nanoparticles, solvents, and substrates are calculated. Finally, the distribution of nanoparticles in different heating regions is determined.

• 한국태양에너지학회:학술대회논문집
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• 2012.03a
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• pp.194-199
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• 2012

In this study, for increasing the efficiency of solar collector, the thermal conductivities and viscosities of the pure water and ethanol oxidized multi-walled carbon nanofluids were measured. Nanofluids were manufactured by ultra-sonic dispersing oxidized multi-walled carbon nanotubes(OMWCNTs) in the pure-water and ethanol at the rates of 0.0005 ~ 0.1 vol%. the Thermal conductivities and viscosities of manufactured nanofluids were measured at the low temperature($10^{\circ}C$), the room temperature($25^{\circ}C$) and the high temperature($70^{\circ}C$). For measuring thermal conductivity and viscosity, we used Transient Hot-wire Method and Rotational Digital Viscometer, respectively. As a result, under given temperature conditions, thermal conductivity of the 0.1 vol% pure-water nanofluid improved 7.98% ($10^{\circ}C$), 8.34% ($25^{\circ}C$), and 9.14% ($70^{\circ}C$), and its viscosity increased by 37.08% ($10^{\circ}C$), 33.96% ($25^{\circ}C$) and 21.64% ($70^{\circ}C$) than the base fluids. Thermal conductivity of the 0.1 vol% ethanol nanofluids improved 33.72% ($10^{\circ}C$), 33.14% ($25^{\circ}C$), and 32.36% ($70^{\circ}C$), and its viscosity increased by 37.93% ($10^{\circ}C$), 31.92% ($25^{\circ}C$) and 29.42% ($70^{\circ}C$) than the base fluids.

• Journal of the Korean Solar Energy Society
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• v.29 no.5
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• pp.35-44
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• 2009

In this study, boiling heat transfer coefficients(HTCs) and critical heat flux(CHF) are measured on a smooth square flat copper heater in a pool of pure water with and without carbon nano tubes(CNTs) dispersed at $60^{\circ}C$. Tested aqueous nanofluids are prepared using multi-walled CNTs whose volume concentrations are 0.0001, 0.001, 0.01, and 0.05%. For dispersion of CNTs, polyvinyl pyrrolidone(PVP) is used in distilled water. Pool boiling HTCs are taken from $10kW/m^2$ to critical heat flux for all nanofluids. Test results show that the pool boiling HTCs of the nanofluids are lower than those of pure water in entire nucleate boiling regime. On the other hand, critical heat flux is enhanced greatly showing up to 200% increase at volume concentration of 0.001% CNTs as compared to that of pure water. This is related to the change of surface characteristics by the deposition of CNTs. This deposition makes a thin CNT layer on the surface and the active nucleation sites of heat transfer surface are decreased due to this layer. The thin layer acts as the thermal resistance and also decreases the bubble generation rate resulting in a decrease in pool boiling HTCs. The same layer, however, maintains the nucleate boiling even at very high heat fluxes and reduces the formation of large vapor canopy at near CHF resulting in a significant increase in CHF.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.33 no.11
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• pp.840-848
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• 2009

In the boiling heat transfer mechanism, CHF(critical heat flux) is the significantly important parameter of the system. So, many researchers have been struggling to enhance the CHF of the system in enormous methods. Recently, there were lots of researches about enormous CHF enhancement with the nanofluids. In that, the pool boiling CHF in nanofluids has the significantly increased value compared to that in pure water because of the deposition of the nanoparticle on the heater surface in the nanofluids. The aim of this study is the comparison of the effect of the nanoparticle deposited surface and the modified surface which has the similar morphology and made by MEMS fabrication. The nanoparticle deposited surface has the complex structures in nano-micro scale. Therefore, we fabricated the surfaces which has the similar wettability and coated with the micro size post and nano structure. The experiment is performed in 3 cases : the bare surface with 0.002% water-ZnO nanofluids, the nanoparticle deposited surface with pure water and the new fabricated surface with pure water. The contact angle, a representative parameter of the wettability, of the all 3 cases has the similar value about 0 and the SEM(scanning electron microscope) images of the surfaces show the complex nano-micro structure. From the pool boiling experiment of the each case, the nanoparticle deposited surface with pure water and the fabricated surface with pure water has the almost same CHF value. In other words, the CHF enhancement of the nanoparticle deposited surface is the surface effect. It also shows that the new fabricated surface follows the nanoparticle deposited surface well.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.37 no.9
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• pp.799-805
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• 2013

An analytical study is conducted to assess the efficiency of a flat-plate solar collector using nanofluids. The nondimensionalized 2D heat diffusion equation is solved by assuming a wavelength-independent extinction coefficient and intensity to obtain the analytical solution of the temperature distribution in the flat-plate solar collector. The dimensionless temperature distribution is investigated as functions of the volume fraction of the nanofluids, magnitude of heat loss, and collector's depth based on the analytical solution when using water-based single-walled carbon nanohorn (SWCNH) nanofluids as a working fluid. Finally, the efficiency of the flat-plate solar collector using the nanofluids is predicted and compared with that of the conventional solar collector. The results indicate that the efficiency of the nanofluid solar collector is better than that of the conventional solar collector under specific geometrical conditions.

• Journal of the Korean Solar Energy Society
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• v.32 no.4
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• pp.9-16
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• 2012

Nanofluids using Carbon Nanotubes have a excellent thermal characteristic. In this study, for increasing the efficiency of solar collector, the thermal conductivity and viscosity of Ethanol-Oxidized Multi-walled Carbon Nanofluids were measured. Nanofluids were manufactured by ultra-sonic dispersing Oxidized Multi-walled Carbon Nanotubes(OMWCNTs) in ethanol at the rates of 0.0005 ~ 0.1 vol%. The thermal conductivity and viscosity of manufactured nanofluids were measured at the low temperature($10^{\circ}C$), the room temperature($25^{\circ}C$) and the high temperature($70^{\circ}C$). For measuring thermal conductivity and viscosity, we used transient hot-wire method and rotational digital viscometer, respectively. As a result, under given temperature conditions, thermal conductivity of the 0.1 vol% nanofluids improved 33.74% ($10^{\circ}C$), 33.14% ($25^{\circ}C$) and 32.36% ($70^{\circ}C$), and its viscosity increased by 37.93% ($10^{\circ}C$), 31.92% ($25^{\circ}C$) and 29.42% ($70^{\circ}C$) than the base fluids.

• Proceedings of the SAREK Conference
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• 2008.06a
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• pp.130-135
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• 2008

The objectives of this study are to investigate the combined heat and mass transfer enhancement using binary nanofluids as the working fluids in a $H_2O$/LiBr absorber. The result of heat and mass transfer experiment with the additives(Arabicgum, 2E1H) showed that the heat and mass transfer performance of binary nanofluid with 2E1H enhanced significantly in comparison with that without additive. In the case of 0.01wt% $Al_2O_3$ binary nanofluids with 2E1H, the vapor absorption rate increased up to 77% in comparison with that without additive. The heat transfer rate of 0.01wt% $Al_2O_3$ binary nanofluids with 2E1H increased up to 19%. Based on the experimental results, it is recommended that the $Al_2O_3$ binary nanofluid be good with 2E1H to improve the heat and mass transfer performance.

• Nuclear Engineering and Technology
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• v.44 no.4
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• pp.429-436
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• 2012

Critical heat flux (CHF) is the thermal limit of a phenomenon in which a phase change occurs during heating (such as bubbles forming on a metal surface used to heat water), which suddenly decreases the heat transfer efficiency, thus causing localized overheating of the heating surface. The enhancement of CHF can increase the safety margins and allow operation at higher heat fluxes; thus, it can increase the economy. A very interesting characteristic of nanofluids is their ability to significantly enhance the CHF. Nanofluids are nanotechnology-based colloidal dispersions engineered through the stable suspension of nanoparticles. All experiments were performed in round tubes with an inner diameter of 0.01041 m and a length of 0.5 m under low pressure and low flow (LPLF) conditions at a fixed inlet temperature using water, 0.01 vol.% $Al_2O_3$/water nanofluid, and SiC/water nanofluid. It was found that the CHF of the nanofluids was enhanced and the CHF of the SiC/water nanofluid was more enhanced than that of the $Al_2O_3$/water nanofluid.

• Journal of computational fluids engineering
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• v.19 no.3
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• pp.37-43
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• 2014

In this paper, hydraulic & thermal developing and fully developed laminar forced convection flow of a water-$Al_2O_3$ nanofluid in a circular horizontal tube with uniform heat flux at the wall, are investigated numerically. A single phase model employed with temperature independent properties. The thermal entrance length is presented in this paper. The variations of the convective heat transfer coefficient and shear stress are shown in the entrance region and fully developed region along different nanoparticles concentration and Reynolds numbers. Convective heat transfer coefficient for nanofluids is larger than that of the base fluid. It is shown that heat transfer is enhanced and shear stress is increased as the particle volume concentration increases. The heat transfer improves, as Reynolds number increases.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.30 no.9 s.252
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• pp.898-904
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• 2006

A new method measuring thermal conductivity of fluids is proposed in this research. It is based on the steady state heat transfer from a hot central cylinder to a cold outer cylinder located concentrically. This method guarantees more stable measurement than conventional THM(transient hot-wire method) due to its simplicity of theoretical principle. Measurements was made for the three nanofluid samples with different particle concentration of pure, 2% and 4%. Nanofluids are made by mixing the pure transformer oil with AlN nano particles. Design of the sensor module and experimental procedures are explained and comparison of the measuring data between present method and THM was made in detail.