• International Journal of Aeronautical and Space Sciences
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• v.18 no.3
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• pp.395-402
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• 2017

The local heat transfer and total pressure drops of developed turbulent flows in the ribbed rectangular convergent/divergent channels with ${\Lambda}/V-shaped$ ribs have been investigated experimentally. The channels have the exit hydraulic diameter ($D_{ho}$) to inlet hydraulic diameter ($D_{hi}$) ratios of 0.67 for convergence and 1.49 for divergence, respectively. The ${\Lambda}/V-shaped$ ribs with three different flow attack angles of $30^{\circ}$, $45^{\circ}$, and $60^{\circ}$ are manufactured with a fixed rib height (e) of 10 mm and the ratio of rib spacing (S) to height (e) of 10 on the walls. Thermal performances of the ribbed rectangular convergent/divergent channels are compared with the smooth straight tube under identical pumping power. The results show that the flow attack angle of $45^{\circ}$ with ${\Lambda}-shaped$ rib has the greatest thermal performance at all the Reynolds numbers studied in the convergent channel; whereas, the flow attack angle of $60^{\circ}$ with V-shaped rib has the greatest thermal performance over Reynolds number of 30,000 in the divergent channel.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.17 no.12
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• pp.1139-1144
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• 2005

This study has been performed to investigate the thermal performance of variable conductance heat pipe (VCHP) with meshed wick. The length of condenser portion in a VCHP is varied by the expansion of inert gas with the operation temperature, and the heat transport capacity is thus varied with the operating temperature. In this study, numerical evaluation of the VCHP is made for the thermal performance of VCHP, based on the diffusion model of inert gas. Water is used as a working fluid and nitrogen as a control inert gas in the copper tube. As a result, the thermal performance of VCHP has been compared with that of constant conductance heat pipe (CCHP) according to the variation of operation temperature. Maximum heat transport capacity of VCHP is mainly presented for operation temperature and the variation of operation temperature is also presented for heat transfer rate of VCHP.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.17 no.9
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• pp.849-854
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• 2005

In this paper, the cooling performance of a microchannel heat sink with nano-particle-fluid suspensions ('nanofluids') is numerically investigated. By using theoretical models of thermal conductivity and viscosity of nanofluids that account for the fundamental role of Brownian motion respectively, we investigate the temperature contours and thermal resistance of a microchannel heat sink with nanofluids such as 6nm copper-in-water and 2nm diamond-in-water. The results show that a microchannel heat sink with nanofluids has high cooling performance compared with the cooling performance of that with water, the classical coolant. Nanofluids reduce both the thermal resistance and the temperature difference between the heated microchannel wall and the coolant.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.26 no.6
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• pp.765-771
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• 2002

This Paper presents numerical solutions of two-dimensional Euler equations for supersonic steady and unsteady flows with heat addition in a convergent-divergent duct, The Van Leer FVS (flux vector splitting) method in generalized coordinates is employed in order to calculate the inviscid strong shock waves caused by thermal choking. We discuss on transient characteristics, start and unstart phenomena caused by thermal choking, limit of equivalence ratio to avoid thermal choking and fluctuation of specific thrust caused by thermal choking. We prove that thermal choking is a serious problem in view of engine performance.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.29 no.11 s.242
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• pp.1503-1508
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• 2005

Scanning Thermal Microscope (STU) has been known for its superior resolution for local temperature and thermal property measurement. However, commercially available STU probe which is the key component of SThM does not provide resolution enough to explore nanoscale thermal phenomena. Here, we developed a SThM probe fabrication process that can achieve spatial resolution around 50 m. The batch-fabricated probe has a thermocouple junction located at the end of the tip. The size of the thermocouple junction is around 200 m and the distance of the junction from the very end of the tip is 150 m. The probe is currently being used for nanoscale thermal probing of nano-material and nano device.

• Journal of the Korean Solar Energy Society
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• v.26 no.1
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• pp.7-12
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• 2006

The dish type solar thermal concentrating system can collect the solar energy above $800^{\circ}C$. It has a concentration ratio of 800 and total reflector area of $49m^2$. To operate solar receivers at high temperature, the optimum aperture size is obtained from a comparison between maximizing absorbed energy and minimizing thermal losses. The system efficiency is defined as the absorbed energy by working fluid in receiver divided by the energy coming from the concentrator. We find that system efficiency is stable in case of flow rate of above 6lpm. The system efficiency are 64.9% and 65.7% in flow rate of 6lpm and 8lpm, respectively. The thermal performance showed that the maximum efficiency and the factor of thermal loss in flow rate of 8lpm are 68% and 0.0508.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.35 no.11
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• pp.1119-1126
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• 2011

In conventional microchannel heat exchangers, only one kind of fluid (hot or cold) flows in each plate. The channels contain different kinds of fluid depending on the vertical position, but they have the same kind of fluid at all horizontal positions. Therefore, there is a slower heat transfer rate in the horizontal direction than in the vertical direction. We propose a microchannel heat exchanger in which hot and cold fluid flows alternately in each plate to improve the thermal performance. This novel channel array requires a special design for the inlet and outlet. The proposed channel array has a faster heat transfer rate than a conventional channel array. The thermal performance of the novel channel array increases with increasing Reynolds number and Prandtl number, but it decreases as the ratio of solid to fluid thermal conductivity increases.

• Journal of Mechanical Science and Technology
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• v.15 no.3
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• pp.357-365
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• 2001

The aim of this paper is to perform the experiment and the numerical simulation for investigating the heat transfer in a regenerator system with ceramic honeycomb and to suggest a useful correlation for optimization of the regenerator system. For achieving this, the effects of some parameters were investigated, e. g., switching time, cell size and length of honeycomb on the mean temperature efficiency. The measured temperatures by R-type thermocouples were compared with the predictions by means of the commercial package, STAR-CD. A useful correlation for thermal efficiency was newly proposed as a function of the normalized switching time, defined in terms of switching time, cell size and length of honeycomb. The results showed that the thermal efficiency is above 90% and the normalized heat exchange rate is higher than 80% when the normalized switching time is less than 1000.

• Journal of Mechanical Science and Technology
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• v.17 no.8
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• pp.1196-1202
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• 2003

The concept of hydrogen enriched LPG fuelled engine can be essentially characterized as low emissions and reduction of backfire for hydrogen engine. The purpose of study is obtaining low-emission and high-efficiency in LPG engine with hydrogen enrichment. In order to determine the ideal compression ratio, a variable compression ratio single cylinder engine was developed. The objective of this paper is to clarify the effects of hydrogen enriched LPG fuelled engine on exhaust emission, thermal efficiency and performance. The compression ratio of 8 was selected to minimize abnormal combustion. To maintain equal heating value, the amount of LPG was decreased, and hydrogen was gradually added. In a similar manner, the relative air-fuel ratio was increased from 0.8 to 1.3 in increment of 0.1, and the ignition timing was controlled to be at MBT each case.

• Journal of the Korean Solar Energy Society
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• v.28 no.2
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• pp.28-33
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• 2008

The purpose of this study was experimentally to investigate thermal performance of heat pipe for evacuated tubular solar collector. Two sets of evacuated tubular solar collector with different condenser shape of heat pipe were prepared. The experiments were performed under the same operating condition with an indoor testing apparatus. Also, the experiments were carried out various testing conditions including inclination, flow rate, and incident heat flux. The results of thermal performance of collector with enlarged condenser showed that $F_R({\tau}{\alpha})$ was 0.6572 and $F_RU_L$ was -2.0086 at $40^{\circ}$. And the results of thermal performance of collector with straight condenser showed that $F_R({\tau}{\alpha})$ was 0.6233 and $F_RU_L$ was -1.4996 at the same inclined angle.