Gyudong Lee

Among the various types of fuel cells, the polymer electrolyte fuel cell (PEFC) is one of the prospective power sources for automotive applications, stationary cogeneration systems, and mobile electronic devices. The PEFC is very sensitive to the high temperature environment inside the fuel cell, and non-uniform temperature distribution reduces its performance. In this study, the performance of cooling plates for the PEFC was investigated by using three-dimensional computational fluid dynamics w

Viscous dissipation effects in circular micro-tubes and rectangular micro-channels were investigated theoretically in a fully developed laminar flow region. From the energy conservation law, simple theoretical equations were derived for the evaluation of the viscous dissipation effect. The proposed equations were verified by comparing to other researchers’ experimental measurement and numerical results. Wall heat flux, fluid velocity, operating temperature, tube diameter, and working fluids’ eff

이 연구는 남북관계에서 국내정치적 제약요인이 갖는 영향력의 정도를 밝히는 것을 목적으로 한다. 남북관계에 관한 기존 연구는 국제정치 현실이라는 구조적 제약조건에 주목하여 국내정치 행위자들의 역할과 영향력을 간과했다. 그러나 남북한의 정치 발전과 변화를 고려하면, 남북관계의 동학에서 국내정치적 제약요인은 매우 강화되었다. 이 연구는 양면게임이론을 이론적 자원으로 2007년 남북정상회담의 협상과정을 실증적으로 분석했다. 2007년 남북정상회담의 쟁점은 경제협력 강화와 군사적 긴장 완화를 추구한 남한의 입장과 서해 해상경계선을 재설정하려는 북한의 입장 차이에서 비롯되었다. 남한의 경우 안보문제를 중시하는 보수세력의 격렬한 반대로 협상의 자율성이 상당히 위축되었으며, 북한의 경우는 유일지배방식에 의한 반대 의견의 부재 또는 통제 가능성으로 인해 자율성이 상대적으로 크게 나타났다. 즉 국내정치적 제약요인이 강하게 작용한 남한의 윈셋은 축소되었고, 국내정치적 제약요인에서 자유로운 북한의 윈셋은

The PEM(polymer electrolyte membrane) fuel cell is one of the promising fuel cell systems as a new small power generating device for automobiles and buildings. The optimal design of cooling plates installed between MEA(membrane electrode assembly) is very important to achieve high performance and reliability of the PEMFC because it is very sensitive to temperature variations. In this study, six types of cooling plate models for the PEMFC including basic serpentine and parallel shapes were design

The performance experiments for a microchannel printed circuit heat exchanger (PCHE) of high-performance and high-efficiency on the two technologies of micro photo-etching and diffusion bonding were performed in this study. The microchannel PCHE were experimentally investigated for Reynolds number in ranges of 100 ~ 700 under various flow conditions in the hot side and the cold side. The inlet temperatures of the hot side were conducted in range of 40oC ~ 50oC while that of the cold-side were fi

A flow analysis for the erosion-corrosion damaged automotive water pump which causes vehicle fire is numerically performed using the CFX program, computational fluid dynamics (CFD) code. The blade bending deformation and the blade clearance enlargement are considered in the analysis of performance reduction. For the cavitation analysis, the homogeneous multiphase model is adopted using the Ralyleigh-Plesset model for the rate equation controlling vapor generation and condensation.

Experiments to measure the pool boiling heat transfer on the micro-fin surfaces were performed with PF5060. The effects of various orientation and subcooling of heat surface on pool boiling performance were investigated under various heat-flux conditions for plain and micro-fin surfaces. The comparison between the results of this study and those of previous work shows a similar trend at the same conditions. From the results, it is proved that nucleate boiling performance is strongly dependent on

The present research is an experimental study on characteristics of cooling behavior for the adjacent copper blocks with surface roughness or micro-porous coated surface. The experiments were carried out at saturation state or within subcooled states of PF5060. The effects of heater orientation and the intervals between heating surfaces or substrates were investigated under various heat flux conditions. The boiling performance of copper block with micro-porous coated surface was better than that

Microscale heat transfer and microfluidics have become increasingly important to overcome some very complex engineering challenges. The use of very small passages to gain heat transfer enhancement is a well documented method for achieving high heat flux dissipation. In this study, the performance evaluation of micro-channel plated heat exchangers with straight, V-shaped and Y-shaped channels has been experimentally carried out under the counterflow condition. It is found that the mixing effect i

This paper presented the heat transfer and pressure drop characteristics for micro plate heat exchanger with straight channel. The metal sheets for straight channel are manufactured by chemical etching and fabricated micro plate heat exchangers by using the vacuum brazing of bonding technology. The performance experiments are performed within the Reynolds numbers range of 15∼250 under the same flow rate conditions for hot and cold sides. The inlet temperature of hot and cold water are conducted

Condensation heat transfer coefficients in a 7.92 mm inside diameter copper smooth tube were obtained experimentally for R22, R134a, and R410A. Working conditions were in the range of 30-40oC condensation temperature, 95-410 kg/m2s mass flux, and 0.15-0.85 vapor quality. The experimental data were compared with the eight existing correlations for an annular flow regime. Based on the heat-momentum analogy, a condensation heat transfer coefficients correlation for the annular flow regime was devel

기존 마이크로채널 열교환기는 한 플레이트에 고온 또는 저온, 한 종류의 유체만이 흐르고 있다. 채널 내부를 흐르는 유체의 수직 방향으로는 다른 종류의 유체가 존재하지만, 수평 방향으로는 같은 종류의 유체가 존재한다. 그로 인해 수평 방향의 열전달률은 수직 방향에 비하여 낮게 나타나게 된다. 열교환기 성능 향상을 위하여 한 플레이트에서 고온, 저온 유동이 번갈아가며 존재하는 새로운 채널 배열을 제안하였다. 새로운 채널 배열을 위해서는 특별한 입구 및 출구 설계가 필요하다. 제안된 채널 배열을 통하여 기존 열교환기보다 높은 열전달률을 얻을 수 있다. Reynolds 수와 Prandtl수가 증가할수록 새로운 채널 배열로 인한 열 성능 향상이 증가하고, 고체와 유체의 열전도계수 비가 증가할수록 열 성능향상은 감소한다.

나타내었다.본 연구의 압력강하 실험조건이 층류영역에 해당하므로 층류에서의 마찰계수상관식과 실험값을 비교하였다. 본 실험에 사용된 시험부와 같이 길이가 짧은 경우, 시험부 전체 구간이 속도경계층이 완전히 발달하지 않은 입구유동에 해당하고 이와 같은 입구유동 영역에서는 완전 발달된 유동에 비해 압력강하가 크게 나타나는 것으로 알려져 있다. 본 실험에서 채널 직경 4mm의 경우 Re＝446, 3mm의 경우 Re＝595, 2mm의 경우 Re＝893 이상일 때 시험부 전 구간이 입구영역에 해당하게 된다.냉매가 단상인 경우 히터로부터 냉매로 전달된 열량, 은 식(1)과 같이 계산하였다.

Experiments on evaporative spray cooling on the square plate heaters with plain or micro-porous coated surfaces were performed in this study. Micro-porous coated surfaces were made by using DOM[Diamond particle, Omegabond 101, Methyl-Ethyl-Keton] method. In case of purely air-jet cooling, the micro-porous coating doesn't affect the cooling capacity. In spray cooling three different flow patterns(complete wetting, evaporative wetting, dryout) are observed on both plain and micro-porous coated sur