• Transactions of the Korean Society of Mechanical Engineers B
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• v.37 no.3
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• pp.205-212
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• 2013

The objective of this study was to investigate the feasibility of replacing a conventional plate fin-tube evaporator with a spiral fin-tube evaporator by comparing the performance of domestic refrigerator-freezers adopting either the plate fin-tube evaporator or spiral fin-tube evaporator. Experiments were conducted for the domestic refrigerator-freezers using either a 2-column and 15-row plate fin-tube evaporator or three spiral fin-tube evaporators with 11, 13, and 15 tube rows (N). The optimum refrigerant charge decreased with a decrease in the number of tube rows. The power consumptions of the domestic refrigerator-freezers using the spiral fin-tube evaporators with N = 11 and 13 were 2.8% and 1.5% lower than those using the plate fin-tube evaporator, respectively. In addition, the cooling capacity of the spiral fin-tube evaporator with N = 13 was 3%-7% higher than that of the plate fin-tube evaporator under the frosting condition. In a cooling speed test, all of the evaporators showed similar performances.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.13 no.12
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• pp.5655-5660
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• 2012

The objective of this study is theoretically to suggest the effective heat exchanger design method using boundary layer analysis. The boundary layer formation and interruption on rectangular plate and round plate fins are explained and the heat transfer coefficients showed with the variation of the velocity and temperature boundary layer. In addition, the flow pattern on one plate fin surface considered as external flow and flow pattern between fins considered as internal flow. As a result, theoretical method for the boundary layer interruption avoidance is suggested and the heat transfer coefficient of the round plate fin was higher than that of the rectangular plate fin because of the less thermal and velocity boundary layer thickness except the centerline.

• Journal of Industrial Technology
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• v.20 no.B
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• pp.21-26
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• 2000

Heat loss, fin effectiveness and efficiency of a plate fin are investigated as a function of non-dimensional fin length and Biot number using a two-dimensional separation of variables method. The value of temperature of the left side is set to be different from that of the right side for this plate fin to satisfy the real physical condition. Also temperature distribution within this plate fin is listed. One of the results shows that the fin can be considered to be useful in view of fin effectiveness on the given range of Biot number when non-dimensional fin length is larger than 3.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.34 no.5
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• pp.515-522
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• 2010

The objective of this study is to investigate the air-side heat transfer characteristics of a spirally-coiled circular fin-tube heat exchanger for various geometric parameters under non-frosting conditions. The heat transfer characteristics of the heat exchanger were analyzed with respect to heat exchanger geometries, and then, the characteristics were compared with those of rectangular-plate fin-tube heat exchangers with discrete fins. The heat transfer coefficient increased with a decrease in the number of tube rows and an increase in the fin pitch. The optimum length of the L-foot was 2.7 mm. In addition, the heat transfer rate increased with a decrease in the tube pitch and the tube thickness. The heat transfer coefficient of the spirally-coiled circular fin-tube heat exchanger was 24.3% higher than that of the rectangular-plate fin-tube heat exchanger.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.17 no.10
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• pp.930-937
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• 2005

The objective of this study is to investigate the heat transfer performance of flat plate finned-tube heat exchangers with large fin pitch. In this study, twenty-two heat exchangers were tested with a variation of fin pitch, number of tube row, and tube alignment. The heat transfer coefficient decreased with a reduction of the fin pitch and an increase of the number of tube row. The staggered tube alignment improved heat transfer performance more than $10\%$ compared to the inline tube alignment. A heat transfer correlation was developed from the measured data for flat plate finned-tubes with large fin pitch. The correlation yielded good predictions of the measured data with mean deviations of $3.6\%\;and\;6.4\%$ for the inline and staggered tube alignment, respectively.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.9 no.3
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• pp.292-299
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• 1997

Ammonia has been used as refrigerant for more than 100 years in absorption as well as in compression systems. Due to its poisonous and inflammable properties, however, its use has been mainly on heavy industrial plants in which regular maintenance are available. For these systems, condensers are generally water-cooled. This is suitable for large systems over 20 RT but is not suitable for small systems. In order to apply ammonia for a small system, it is important to adopt an air-cooled condenser. In this study, simple numerical analysis of an air-cooled condenser for an ammonia refrigeration system has been carried out. The condenser is designed as horizontal tubes with plate fins attached at the outer surface to enhance the air-side heat transfer rate. Effects of fin shape and arrangement are studied in detail. Since the local heat transfer coefficient is highest at the leading edge, heat flux is highest at the edge and decreases along the distance. Conditions of inlet air are also varied in the study and condenser length that is required for full condensation is calculated. The results show that it is important to enhance both the air-side and internal heat transfer coefficients.

• Proceedings of the Korean Vacuum Society Conference
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• 2013.08a
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• pp.278.2-278.2
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• 2013

그래핀은 우수한 전기적, 기계적, 광학적 특성들로 인하여 전자소자, 센서, 에너지 재료 등으로의 응용이 가능하다고 알려진 단 원자층의 탄소나노재료이다. 특히 그래핀을 전자소자로 응용하기 위해서는 캐리어 농도, 전하 이동도, 밴드갭 등의 전기적 특성을 향상시키거나 제어하는 것이 요구되며, 에너지 소재로의 응용을 위해서는 높은 전기전도도와 함께 기능화를 통한 촉매작용을 부여하여 효율을 향상시키는 것이 요구된다. 일반적으로 화학적 도핑은 그래핀의 전기적 특성을 제어하는 효율적인 방법으로 알려져 있다. 화학적 도핑의 방법으로 질소, 수소, 산소 등 다양한 이종원소를 열처리 또는 플라즈마 처리함으로써 그래핀을 구성하는 탄소원자를 이종원자로 치환하거나 흡착시켜 기능화 처리된 그래핀을 얻는 방법들이 제시되었다. 이중 플라즈마를 이용한 도핑방법은 저온에서 처리가 가능하고, 처리시간, 공정압력, 인가전압 등 플라즈마 변수를 변경하여 도핑정도를 비교적 수월하게 제어할 수 있다는 장점을 가지고 있다. 본 연구에서는 열화학기상증착법으로 합성된 그래핀을 직류 플라즈마로 처리함으로써 효율적인질소도핑 조건을 도출하고자 하였다. 그래핀의 합성은 200 nm 두께의 니켈 박막이 증착된 몰리브덴 호일을 사용하였으며, 원료가스로는 메탄을 사용하였다. 그래핀의 질소 도핑은 평행 평판형 직류 플라즈마 장치를 이용하여 암모니아($NH_3$) 플라즈마로 처리하였으며, 플라즈마 파워와 처리시간을 변수로 최적의 도핑조건 도출 및 도핑 정도를 제어하였다. 그래핀의 질소 도핑 정도는 라만 스펙트럼의 G밴드의 위치와 반치폭(Full width at half maximum; FWHM)의 변화를 통해 확인하였다. NH3 플라즈마 처리 후 G밴드의 위치가 장파장 방향으로 이동하며, 반치폭은 감소하는 것을 통해 그래핀의 질소도핑을 확인하였다.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.22 no.3
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• pp.171-179
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• 2010

Thermal performance of louver fin and plate fin in a thermoelectric cooling system with a duct-flow type fan arrangement is analytically evaluated. The thermoelectric cooling system consists of a thermoelectric module and two heat exchanger fins. The analytic results show that the optimized louver fin has lower thermal resistance than plate fin. The COP and heat absorbed rate of the thermoelectric cooling system with optimized louver fins are 10.3% and 5.8% higher than optimized plate fins, respectively.

• Proceedings of the Korean Vacuum Society Conference
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• 2015.08a
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• pp.238.1-238.1
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• 2015

2차원 탄소나노재료인 그래핀은 본연의 우수한 물성으로 인하여 전자소자, 에너지 저장매체, 유연성 전도막 등 다양한 분야로의 응용가능성이 제기되었다. 그러나 실제적인 응용을 위해서는 그래핀의 구조적인 결함을 최소화하며, 특성을 자유로이 제어하거나 향상시키는 공정의 개발이 필요하다. 일반적으로 화학적 도핑은 그래핀의 전기적 특성을 제어하는 효율적인 방법으로 알려져 있다. 화학적 도핑의 방법으로는 그래핀을 특정 가스 분위기에서 고온 열처리하거나 활성종들이 존재하는 플라즈마에 노출시킴으로써, 그래핀을 구성하는 탄소원자를 이종원자로 치환하거나 표면에 흡착시켜 기능화 된 그래핀을 얻는 방법 등이 제시되었다. 특히 플라즈마를 이용한 도핑방법은 저온에서 단시간의 처리로 효율적인 도핑이 가능하고, 인가전력, 처리시간 등의 플라즈마 변수를 변경하여 도핑정도를 수월하게 제어할 수 있다는 장점을 가지고 있다. 그러나 플라즈마 내에 존재하는 극성을 띄는 다양한 활성종들로 인하여 그래핀에 구조적인 결함을 형성하여 오히려 특성이 저하될 수 있어 이를 고려한 플라즈마 공정조건의 설정이 필수적이다. 따라서 본 연구에서는 플라즈마에 노출된 그래핀의 Raman 특성을 고찰함으로써 화학적 도핑과 구조적인 결함의 경계를 확립하고 구조결함의 형성을 최소화한 효율적인 도핑조건을 도출하였다. 고품질 그래핀은 물리적 박리법을 이용하여 300 nm 두께의 실리콘 산화막이 존재하는 실리콘 웨이퍼 위에 제작하였으며, 평행 평판형 직류 플라즈마 장치를 이용하여 전극의 위치, 인가전력, 처리시간을 변수로 암모니아(NH3) 플라즈마를 방전하여 그래핀의 Raman 특성변화를 관찰하였다. 그래핀의 구조적 결함 및 도핑은 라만 스펙트럼의 D, G, D', 2D밴드의 강도비와 G밴드의 위치와 반치폭(Full width at half maximum; FWHM)의 변화를 통해 확인하였다. 그 결과, 인가전력과 처리시간이 증가함에 따라 그래핀의 도핑레벨이 증가되고, 이후에는 도핑효과가 없어지고 결함의 정도가 상승하는 천이구역이 존재하며, 이를 넘어서는 너무 높은 인가적력의 처리는 그래핀에 결함을 형성하여 구조적인 붕괴를 야기함을 확인하였다.

• Transactions of the Korean Society of Mechanical Engineers
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• v.13 no.2
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• pp.299-306
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• 1989

The combined convective heat transfer from vertical parallel plates with constant temperature has been studied by numerical method. The governing equations for the system are solved by the finite difference method and successive over relaxation scheme for Re$_{L}$ = 50 - 500, Gr = 10$^{4}$, Pr = 0.7. Results for various plate spacings and plate lengths are as follows ; For various plate spacings, the mean Nusselt number increases and then decreases as the dimensionless plate spacing increases. The optimum plate spacing for maximum mean Nusselt number decreases with increasing Reynolds number and can be expressed as a function of Reynolds number. For various plate lengths, the mean plate Nudest number increases as the dimensionless plate length decreases and Reynolds number increases.