• Journal of the Korean Solar Energy Society
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• v.36 no.3
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• pp.63-74
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• 2016

The nanofluids are the fluids with excellent thermal property, it is expected as a working fluid of the next generation. The nanofluids are well known that if it is used in the boiling heat transfer system, the critical heat flux is enhanced up to 200%, and the thermal conductivity is increased up to from 10 to 160%. However, the fouling phenomenon can be occurred that nanoparticles of nanofluids are deposited on the heat transfer surface. Therefore, to investigate relation between nanofluid and fouling, this study is carried out using oxidized graphene nanofluid. Also it compared and analyzed the critical heat flux and the boiling heat transfer coefficient. As the result, in case of oxidized graphene deposition for fouling, the critical heat flux is increased up to 20% more than oxdized graphene nanofluid. However, the boiling heat transfer coefficient is decreased down to about $6kW/m^2K$ at $1,000kW/m^2$ more than pure water.

• Nuclear Engineering and Technology
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• v.34 no.2
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• pp.165-175
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• 2002

Effects of gap sizes (3.9-44.3 mm) of vertical annuli on nucleate pool boiling heat transfer of water at atmospheric pressure have been obtained experimentally. Through the study, tubes of the closed bottom have been investigated and results are compared with those of a single unconfined tube. According to the results, the annular condition gives much increase in heat transfer coefficient at moderate heat fluxes. The increase is much enhanced 3s the gap size decreases. At the same tube wall superheat (about 3.1 K) the heat transfer coefficient for the least gap size (i.e., 3.9 mm) is more than three times greater than that of the unconfined tube. However, deterioration of heat transfer occurs at high heat flux for confined boiling.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.25 no.2
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• pp.101-108
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• 2013

This paper is presented an experimental study of flow boiling heat transfer characteristics of ammonia, and is focused on pressure gradient and heat transfer coefficient of the refrigerant flow inside horizontal small tube with inner diameter of 3.0 mm and length of 2000 mm. The direct heating method is applied for supplying heat to the refrigerant, where the test tube is uniformly heated by electric current. The local heat transfer coefficients were obtained over a heat flux range of 20 to $80kW/m^2$, a mass flux range of 50 to $500kg/m^2s$, a saturation temperature range of 0 to $10^{\circ}C$, and quality up to 1.0. The pressure drops increase with increasing mass flux and heat flux, and with decreasing saturation temperature. The heat transfer coefficients increase with increasing mass flux and saturation temperature in middle and high quality region. And the local heat transfer coefficient increase with increasing heat flux in low quality region. The heat transfer coefficient of the experimental result was compared with six existing heat transfer coefficient correlation. A new boiling heat transfer coefficient correlation based on the superposition model for ammonia in small tubes is developed average deviation of -0.17% and mean deviation of 10.85%.

• The Magazine of the Society of Air-Conditioning and Refrigerating Engineers of Korea
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• v.10 no.3
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• pp.220-227
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• 1981

The purpose of this paper is to investigate the potential ability of sintered metal tube to promote heat transfer. In the experiment for Freon - 11, the boiling heat transfer on the sintered metal tube of bronze element is investigated and compared with that of the bronze tube (bare tube) atmospheric pressure. The experimental results are obtained as follows : 1) For sintered metal tubes of bronze element with particle diameters which ranges from $79({\mu})\;to\;461({\mu})$ and bare tube, boiling characteristic curves are expressed as : a) Sintered metal tube $$q{\propto}{\Delta}T^{1.05\~1.373}$$ b) Brae tube $$q{\propto}{\Delta}T^{3.096}$$ 2) Compared with that of the bare tube at low temperature difference$({\Delta}T_{sat})$, boiling heat transfer coefficient of the sintered bronze tube are relatively high. 3) There is tendency that curves of boiling heat transfer coefficients of sintered ·bronze tube and bare tube approach each other at rather high temperature difference. It is due to the increasing rate of the former heat transfer coefficient along with temperature difference is smaller than that of the latter. 4) Referring to particle diameter, optimum condition, i. e. , maximum heat transfer coefficient is found to be at approximately 2 mm thickness of sintered layer with $D_p=150({\mu})$.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.21 no.2
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• pp.294-302
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• 1997

Falling-film evaporation experiments for aqueous solution of lithium bromide (LiBr) were performed on a horizontal smooth 19.05-mm-dia copper tube. Average heat transfer coefficients were obtained with varied film Reynolds numbers, system pressures, LiBr concentrations and degrees of wall superheat. Heat transfer coefficients increase with increasing system pressure and decreasing concentration. For degrees of wall superheat, the heat transfer coefficient did not't show the distinct trend. For this experimental ranges, heat transfer coefficients showed maximum values at an optimal film Reynolds number. The results of this work were compared with pool boiling data reported previously, and it was shown that the heat transfer performance is superior to the pool boiling.

• International Journal of Air-Conditioning and Refrigeration
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• v.14 no.4
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• pp.156-165
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• 2006

Convective boiling heat transfer coefficients were measured in a horizontal minichannel with R-l34a. The test section was made of stainless steel tube with an inner diameter of 3.0 mm and a length of 2m. It was uniformly heated by applying electric current directly to the tube. Local heat transfer coefficients were obtained for heat fluxes from 10 to $40kW/m^2$, mass fluxes from 200 to $600kgT/m^2s$, qualities up to 1.0, and the inlet saturation temperature of $10^{\circ}C$. The experimental results were mapped on Wojtan et $al.'s^(7)$ and Wang et $al.'s^(8)$ flow pattern maps. The nucleate boiling was predominant at low vapor quality whereas the convective boiling was predominant at high vapor quality. Laminar flow appeared in the flow with minichannel. The experimental results were compared with six existing two-phase heat transfer coefficient correlations. A new boiling heat transfer coefficient correlation based on the superposition model for refrigerants was developed with mean and average deviations of 10.39% and -3.66%, respectively.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.12 no.5
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• pp.502-510
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• 2000

Evaporation heat transfer characteristics were studied in a horizontal tube using R22/R114 non-azotropic refrigerant mixture. the heat transfer coefficient was high in the upper part for pure refrigerants, and heat transfer coefficient was low in the lower part for refrigerant mixtures. In the low quality region where nucleate boiling was dominant, the average heat transfer coefficient was low. In the region where forced convection was dominant, heat transfer coefficient was high. Results show that the heat transfer coefficient for pure refrigerants obtained by experiments were lower than those of Yoshida et al. but agreed well with Jung et al., and Chen et al. data. But the heat transfer coefficients for refrigerant mixtures were lower about 20% than those predicted by the equation for pure refrigerant.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.20 no.7
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• pp.2298-2314
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• 1996

Electric field effect on boiling of refrigerants R11, R113, and FC72 has been investigated experimentally. One purpose of the experimental investigation is to determine the effects of the electrode arrangements on electrohydrodynamic boiling of the above mentioned liquids. The test equipment employed in the experiment consists of a shell and tube heat exchanger with six or six and twelve rows of electrode wires around the tube. It has been found that the applied voltage promotes the boiling heat transfer coefficient except FC72. Boiling heat transfer enhancement obtained is about 230% for R11, 280% for R113. It has also been observed that bubbles detached from the tube aggregate at the place where the electrical gradient force balances with the buoyancy one. These aggregated bubbles force to decrease the boiling heat transfer coefficient as well as to reduce the voltage needed to the dielectric breakdown.

• Proceedings of the SAREK Conference
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• 2006.06a
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• pp.68-73
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• 2006

The present paper deals with an experimental study of boiling heat transfer characteristics of R-290, and is focused on pressure gradient and heat transfer coefficient of the refrigerant flow inside horizontal smooth minichannel with inner diameter of 3.0 mm and length of 2000 mm. The direct heating method applied for supplying heat to the refrigerant where the test tube was uniformly heated by electric current which was applied to the tube wall. The experiments were conducted with R-290 with purity of 99.99% at saturation temperature of 0 to $10^{\circ}C$. The range of mass flux is $50{\sim}250kg/m^2s$ and heat flux is $5{\sim}20kW/m^2$. The heat transfer coefficients of R-290 increases with increasing mass flux and saturation temperature, wherein the effect of mass flux is higher than that of the saturation temperature, whereas the heat flux has a low effect on increasing heat transfer coefficient. The significant effect of mass flux on heat transfer coefficient is shown at high quality, the effect of heat flux on heat transfer coefficient at low quality shows a domination of nucleate boiling contribution. The heat transfer coefficient of the experimental result was compared with six existing heat transfer coefficient correlation. Zang et al.'s correlation(2004) gave the best prediction of heat transfer coefficient.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.22 no.10
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• pp.704-709
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• 2010

The effect of oil on flow boiling of R-123 in a plain tube bundles was experimentally investigated for a range of quality and heat flux. It is shown that the heat transfer coefficient decreased as the oil concentration increased. Comparison with the previous pool boiling data reveals that the reduction of heat transfer coefficient by oil is more pronounced in pool boiling, and the difference increased with the increase of oil concentration and heat flux. Within the experimental range, the variation of mass flux or quality has negligible effect on the heat transfer coefficient.