• 설비공학논문집
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• 제27권2호
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• pp.63-68
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• 2015

The fin and tube type heat exchangers widely used in air conditioners have been developed to improve on the heat transfer performance and compactness. This study presents the new type of tube for the heat exchanger to improve the heat transfer performance by increasing the heat transfer area per unit volume in the air-conditioner heat exchanger. The new type tube can be used for mechanical expansion facility, due to the two-port copper tube. Numerical calculation shows that the heat exchanger using the two-port copper tube outperforms the conventional heat-exchanger using a circular copper tube, in terms of the increased heat transfer coefficient and higher pressure drop. The calculation results were experimentally validated and are in agreement with the experimental results. Compared to the heat exchanger using a conventional circular tube, the heat exchanger with a two-port tube increased the heat transfer coefficient up to 21%, and the pressure dropped up to 16%.

• 동력기계공학회지
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• 제9권1호
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• pp.30-35
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• 2005

Repeated ribs are used on heat exchanger surfaces to promote turbulence and to enhance convective heat transfer. Applications include fuel rods of gas-cooled nuclear reactors, inside cavities of turbine blades, and internal surfaces pipes used in heat exchangers. Despite the great number of literature papers, only few experimental data concerns detailed distributions of friction factors and heat transfer coefficients in square channels varying the number of rough walls. This issue was tackled by investigating effects of different number of ribbed walls on heat transfer and friction characteristics in square channel. The rough wall had a $45^{\circ}$ inclined square rib. Uniform heat flux was maintained on the whole inner heat transfer channel area. The heat transfer coefficient and friction factor values increased with increasing the number of rough walls.

• 설비공학논문집
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• 제15권5호
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• pp.422-431
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• 2003

Theory of the building heat transfer is generally limited to the heat flux to the surfaces of windows and walls, which influences the indoor climate of a building, in the field of architectural environmental engineering. While the heat flux from the buildings to their environment has been considered in the viewpoint of urban climate, its conventional theory have been rarely examined. The purpose of this study is to propose a building-urban heat transfer model for defining the relation between the building and the urban climate by extending the building heat transfer model. In this study, the extended building heat transfer model, where response factor method is used, is established on the urban space and the indoor space by the boundary of building envelopes. Computer simulation (HASP/ACLD) is conducted on the subjected urban area by the established building-urban heat transfer model. As a result it is logically proved that the short waves of solar radiation, which interact with long Waves of radiation from the buildings and the earth, increase the urban air temperature ana buildings largely influence on the urban climate.

• 수산해양기술연구
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• 제30권1호
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• pp.33-44
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• 1994

This work studies for boiling and condensation heat transfer performance of trapezoidally shaped integral-fin tubes having fin densities from 748fpm to 1654fpm. For comparison, tests are made using a plain tube having the same inside and outside diameter as that of the root of fins of finned tubes. Hahne's theoretical model and Webb's theoretical model are used to predict the R-11 boiling heat transfer coefficient and condensing heat transfer coefficient respectively for plain tube and all integral-fin tubes. Experiments are carried out using R-11 as working fluid. This work is limited to film-wise condensation and pool boiling on the outside surface of plain tube and 4 low integral-fin tubes. In case of condensation, the refrigerant condenses at saturation state of 32$^{\circ}C$ on the outside tube surface cooled by coolant and in case of boiling. the refrigerant evaporates at saturation state of 1bar on the outside tube surface. The amount of non-con-densable gases in the test loop is reduced to a negligible value by repeated purging. The actual boiling and condensing processes occur on the outside tube surfaces. Hence the nature of this surface geometry affects the heat transfer performances of condenser and evaporator in refrigerating system. The condensation heat transfer coefficient of integral-fin tube is enhanced by both extended tube surface area and surface tension. The ratio of the condensation heat transfer coefficients of finned to plain tubes is greater than that of surface area of finned to plain tubes, while ratio of the boiling heat transfer coefficient of finned to plain tubes shows reverse result. As a result, low integral-fin tube can be used in condenser more effectively than used in evaporator.

• 동력기계공학회지
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• 제10권3호
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• pp.32-37
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• 2006

In this paper, we present the flow and heat transfer characteristics with the array of impinging jet nozzles by using the numerical computation and experiment. Numerical solutions were obtained for dimensionless gap H=6, dimensionless outlet length L=10 and Reynolds number Re=1500 by using the commercial CFD code, CFX-5. Experimental and numerical results were agreed well with each other. It was found that the impinging jet with circular array nozzles generated the uniform heat transfer area and the maximum heat transfer is higher than rectangular array nozzles for certain parameter sets. It is one of the most important utilities providing steam to turbine in order to supply mechanical energy in thermal power plant. It is composed of thousands of tubes for high efficient heat transfer.

• 한국소성가공학회:학술대회논문집
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• 한국소성가공학회 2004년도 추계학술대회논문집
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• pp.107-110
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• 2004

Application of aluminum alloy has been increasing for most of industry area because aluminum has a good mechanical properties and castability, especially automotive field for weight reduction. But, Furnace industry is sluggish. The purpose of this study is numerical analysis of aluminum holding furnace for reasonableness estimation when we design for new model of furnace. The numerical simulation involving fluid flow of inside air and heat transfer to fireproof material is presented in order to improve the understanding of aluminum furnace. First of all, we are carried out numerically for the two dimensional inside convection and surface radiation heat transfer in a square enclosure. Subsequently, we are established the analysis method of aluminum furnace considering natural convective heat transfer

• Nuclear Engineering and Technology
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• 제41권10호
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• pp.1285-1292
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• 2009

The effects of the narrowed upside gap on nucleate pool boiling heat transfer in a vertical annulus were investigated experimentally. For the study, a stainless steel tube with a diameter of 25.4 mm and saturated water that kept an atmospheric condition were used. The ratio between the gaps measured at the upper and the lower regions of the annulus ranged from 0.18 to 1. Two different lengths of the modified gap also were investigated. The change in heat transfer due to the modified gap became evident as the gap ratio decreased and the length of the gap increased. As the gap ratio became less than 0.51, a significant decrease in heat transfer was observed compared to the plain annulus. The longer gap size resulted in an additional decrease in heat transfer. The major cause for the tendency was attributed to the formation of lumped bubbles around the upper region of the annulus followed by the increased flow friction between the fluid and the surface around the modified gap.

• 대한안전경영과학회지
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• 제19권4호
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• pp.317-322
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• 2017

When the heat flux on the heating surface following changing heat condition in the boiling heat transfer system exceeds critical heat flux, the critical heat flux phenomenon is going over to immediately the film boiling area and then it is occurred the physical destruction phenomenon of various heat transfer systems. In order to maximize the safe operation and performance of the heat transfer system, it is essential to improve the CHF(Critical Heat Flux) of the system. Therefore, we have analysis the effect of improving CHF and characteristics of heat transfer following the nanoparticle coating thickness. As the results, copper nanocoating time are increased to CHF, and in case of nano-coatings are increased spray-deposited coating times more than in the fure water; copper nanopowder is increased up to 6.40%. The boiling heat transfer coefficients of the pure water are increased up to 5.79% respectively. Also, the contact angle is decreased and surface roughness is increased when nano-coating time is increasingly going up.

• 대한기계학회논문집B
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• 제40권9호
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• pp.557-567
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• 2016

태양열 가열을 도입한 해양온도차발전용 열교환기(증발기와 응축기)설계 최적화가 수행되었다. 출력은 100kW이고 작동유체는 R134a이며 고성능 상용튜브를 사용하였다. 열전달면적과 압력강하는 관수의 증가와 관통로수의 감소에 따라 서로 상반되는 경향이 존재하므로 이를 해결하기 위하여, 설비투자비에 관련되는 열전달면적과 압력강하에 관련되는 운전비용 최소화를 고려한 두 목적함수를 갖는 유전자 알고리즘(GA)을 이용하여 다목적설계최적화를 수행하였다. 설계최적화 결과, 구현 가능한 최적의 열전달면적 및 압력강하의 조합들이 적정한 관수 및 관통로 수에 대하여 존재하였다. 도출된 증발기와 응축기의 Pareto 선들은 설계자들에게 재정적인 면을 고려하여 선택할 수 있도록 넓은 범위의 최적해를 제공하였다. 또한, 총열전달면적 중 응축기의 열전달면적이 증발기 쪽보다 크게 나타났다.

• Journal of Mechanical Science and Technology
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• 제18권8호
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• pp.1435-1450
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• 2004

Experimental data are presented which describe the effects of a combustor-level high free-stream turbulence on the near-wall flow structure and heat/mass transfer on the endwall of a linear high-turning turbine rotor cascade. The end wall flow structure is visualized by employing the partial- and total-coverage oil-film technique, and heat/mass transfer rate is measured by the naphthalene sublimation method. A turbulence generator is designed to provide a highly-turbulent flow which has free-stream turbulence intensity and integral length scale of 14.7% and 80mm, respectively, at the cascade entrance. The surface flow visualizations show that the high free-stream turbulence has little effect on the attachment line, but alters the separation line noticeably. Under high free-stream turbulence, the incoming near-wall flow upstream of the adjacent separation lines collides more obliquely with the suction surface. A weaker lift-up force arising from this more oblique collision results in the narrower suction-side corner vortex area in the high turbulence case. The high free-stream turbulence enhances the heat/mass transfer in the central area of the turbine passage, but only a slight augmentation is found in the end wall regions adjacent to the leading and trailing edges. Therefore, the high free-stream turbulence makes the end wall heat load more uniform. It is also observed that the heat/mass transfers along the locus of the pressure-side leg of the leading-edge horseshoe vortex and along the suction-side corner are influenced most strongly by the high free-stream turbulence. In this study, the end wall surface is classified into seven different regions based on the local heat/mass transfer distribution, and the effects of the high free-stream turbulence on the local heat/mass transfer in each region are discussed in detail.