• Journal of the Korean Electrochemical Society
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• v.9 no.3
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• pp.118-123
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• 2006

Degradation of polymer electrolyte membrane fuel cell (PEMFC) that is facilitated by on/off cycles is one of the most important issues for commercialization of fuel cell vehicles. When a PEMFC stack is shut down, residual hydrogen and induce high voltage equivalent to open circuit voltage to the cathode side that might cause sintering of Pt catalyst and facilitate formation of hydrogen peroxide at the anode side that might decompose $Nafionc\'{A}$ membrane. In this study, degradation of PEMFC exposed to repetitive on/off cycles was investigated by measuring i-V characteristics, ac impedance, cyclic voltammograms, gas leak, cross-sectional SEM images, and TEM images. To prevent degradation of PEMFC caused by the residual gases, hydrogen was removed from anode gas channel by gas-purging and by using a dummy resistance, that were found to be a very effective method.

• Journal of Aerospace System Engineering
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• v.17 no.4
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• pp.28-33
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• 2023

In this study, computational fluid dynamics was used to analyze the flow bias generated as air supplied by a fan passes through ducts, piping, and a coal separation screen. The flow bias of the air flow is mostly caused by the spatial characteristics of the fan volute and duct, and the internal baffle and the coal separation screen at the outlet cause strong pressure losses that dampen the flow bias. ANSYS CFX was used for computational fluid dynamics, and since the baffle and the coal separation screen are shaped like perforated plates with many small holes uniformly distributed, actual modeling for analysis was not possible. Therefore, the Porous Loss Model was applied. The evaluation of the flow bias was analyzed based on the velocity distribution of the Porous Loss Model at the outlet surface of the coal separation screen obtained from the computational fluid dynamics results.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.36 no.6
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• pp.625-632
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• 2012

The objective of this paper is to demonstrate the cathode channel flooding effects at different stoichiometries in proton exchange membrane (PEM) fuel cells by using visualization techniques. The phenomena of liquid water formation and removal caused by current variations were also examined experimentally. Tests were conducted at cathode stoichiometries of 1.5 and 2.0, and the anode stoichiometry was fixed at 1.5. It is found that at an air-side stoichiometry of 2.0, liquid water begins to form and the flooding occurs faster than at an air-side stoichiometry of 1.5. Also, when the air-side stoichiometry of 1.5 is maintained, the dry-out phenomena is observed in the dry-out area 7.8 A following the field of flooding. Thus, a stoichiometry of 1.5 produced better performance in terms of membrane electrode assembly (MEA) durability and hydrogen ion conductivity than did a stoichiometry of 2.0, in which dry-out occurs beyond 8A.

• 유체기계공업학회:학술대회논문집
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• 2002.12a
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• pp.147-151
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• 2002

Cross flow fan system is widely used for various applications, especially for the air-moving device of heaters, air-conditioners, and air-curtains. Although there are efforts for the optimization of cross-flow fan flow path with different methods of approach, it is still being investigated by many researchers through experimentally and/or theoretically, because the flow pattern of the cross flow fan is not stereotyped. This paper presents some results from numerical experiments for the optimization of the flow path through a cross-flow fan to be applied to indoor wall-mounted room heater. Two dimensional analysis has been applied to a specific fan system including inlet and diffuser outlet. Flow characteristics art presented and discussed for two different flow path at three different operating conditions represented by rotational speed(800, 1,000, 1,200 rpm) of the In. According to the simulated results for the specific fan system under consideration, it could be found that the flow pattern resembles each other at different rotational speed (to say from 800 rpm to 1,200 rpm) for a fixed flow path, while the secondary flows mostly absorbs the speed effects. By changing the flow path significant increase in volume flow rate is estimated upto 2.65 at the same rotational speed. According to the present experience, fan flow path design can be performed more efficiently by incorporating this type of numerical experiments combined with the model tests.

• Transactions of the Korean Society of Mechanical Engineers
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• v.16 no.10
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• pp.1990-1997
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• 1992

The characteristic of supersonic flow with condensation along a wavy wall of small amplitude in channel is investigated through the direct marching method of characteristics. The very complex problem that may appear where the overlapping of the same family characteristics occurs, can be satisfactorily solved by means of the modified method suggested by Zucrow. In the present study for the case of supersonic moist air flow, the dependency of location of formation and reflection of oblique shock wave generated by the wavy wall, and the distributions of flow properties, on the relative humidity and temperature at the entrance of wavy wall is clarified by plots of streamline, ios-Mach umber and ios-flow properties. Also, it is confirmed that the wavy wall plays an important key role in the formation of oblique shock wave, and that the effect of condensation on the flow field appears apparently.

• New & Renewable Energy
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• v.3 no.4
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• pp.8-15
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• 2007

Liquid water in flow channel is an important factor that limits the steady and transient performance of PEM fuel cells. A computational fluid dynamics study based on the volume-of-fluid [VOF] multi-phase model was conducted to understand the two-phase flow behavior of liquid water in cathode gas channels. The liquid water transport in $180^{\circ}{\Delta}$ bends was investigated, where the effects of surface characteristics (hydrophilic and hydrophobic surfaces], channel geometries (rectangular and chamfered corners], and air velocity in channel were discussed. The two-phase flow behavior of liquid water with hydrophilic channel surface and that with hydrophobic surface was found very different; liquid water preferentially flows along the corners of flow channel in hydrophilic channels while it flows in rather spherical shape in hydrophobic channels. The results showed that liquid water transport was generally enhanced when hydrophobic channel with rounded corners was used. However, the surface characteristics and channel geometries became less important when air velocity was increased over 10m/s. This study is believed to provide a useful guideline for design optimization of flow patterns or channel configurations of PEM fuel cells.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.19 no.6
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• pp.441-446
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• 2007

An experimental study on the countercurrent two-phase flow in narrow rectangular channels has been peformed. Countercurrent flow limitation (CCFL) was investigated using air and water in 760mm long, 100mm wide, vertical test sections with 1 and 3mm channel gaps. Tests were systematically performed with downward liquid superficial velocities and upward gas velocities covering 0 to 0.125 and 0 to 3.5m/s ranges, respectively. As the gap width of rectangular channel increased the CCFL water superficial velocity decreased for the given air superficial velocity. Slight increase of the air superficial velocity resulted in the abrupt decrease of water velocity when $j_g=2{\sim}4m/s$. The critical superficial velocity of air, at which the downward flow of water was no longer allowed, also decreased with the increase of gap width. The experimental results were compared with the previous correlations, which were mainly for round tubes, and the qualitative trends were found to be partially acceptable. However the quantitative discrepancies were hardly neglected. New correlation of CCFL was developed and showed good agreement with the experimental data.

• Proceedings of the Korean Nuclear Society Conference
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• 1996.11a
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• pp.334-339
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• 1996

Midloop 운전중 RHR 기능 상실사고를 모의 실험한 Bethsy 6.9d에 대해 CATHARE2 코드를 이용하여 해석하였다. 이 실험의 초기조건은 계통수위를 고온관 중간까지 낮추고, 그 윗부분은 비응축 가스로 차 있는 midloop 상태를 유지하는 것이다. 잔열은 원자로 정지 2일 후를 가정한 노심출력을 사용하였으며, 계통내 방출유로는 상부의 Upper head vent와 가압기 vent 및 고온관 1에 연결된 Letdown line과 수위지시계 방출유로가 열려 있다고 가정하였다. 또한 세 개의 loop중 증기발생기 한대만 이유 가능하고, 나머지 두 대는 이차측이 공기로 가득 차 있는 상태를 유지하였다. 이 연구의 주된 목적은 midloop 운전중 RHR 기능 상실사고에 대한 위와같은 상태에서 계통의 열수력적 현상을 실험을 통해 이해하고 코드 예측능력을 평가하는 것이다. CATHARE2 코드 계산결과 대체적으로 실험의 현상을 잘 모의하고 있으나 다음 사항에 대해서는 차이를 보이고 있다. 첫째 노심내 물의 혼합을 적절히 모의하지 못하여, 노심내 국부적 증기 발생 시점이 실험에 비해 약 250초 빨리 나타났다. 둘째 노심에서 고온관으로의 물의 유입이 많아 고온관에서 기포율이 실험에 비해 낮게 나타났다. 마지막으로 밀림관(surge line)에서 물의 유입에 의한 압력차가 실험보다 높게 나타났다.

• Transactions of the Korean hydrogen and new energy society
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• v.23 no.5
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• pp.476-483
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• 2012

Proper water management is crucial for the efficient operation of polymer electrolyte membrane (PEM) fuel cell. Especially, for automotive applications, A novel water management that can avoid both membrane dry-out and flooding is a very important task to achieve good performance and efficiency of PEM fuel cells. The aim of this study is to investigate the liquid water behavior on the gas diffusion layer (GDL) surface and in the cathode flow channel of a PEM unit fuel cell under flooding conditions. For this purpose, a transparent unit fuel cell is devised and fabricated by modifying the conventional PEM fuel cell design. The results of water droplet behavior under flooding conditions are mainly presented. The water distributions in the cathode flow channels with cell operating voltage are also compared and analyzed. Through this work, it is expected that the data obtained from this fundamental study can be effectively used to establish the basic water management strategy in terms of water removal from the flow channels in a PEM fuel cell stack.

• Journal of the Korean Electrochemical Society
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• v.9 no.4
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• pp.170-177
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• 2006

Optimum design of flow channel in the separation plate of Proton Exchange Membrane Fuel Cell is very prerequisite to reduce concentration over potential at high current region and remove the water generated in cathode effectively. In this paper, fully 3 dimensional computational model which solves anode and cathode flow fields simultaneously is developed in order to compare the performance of fuel cell with parallel and interdigitated flow channels. Oxygen and water concentration and pressure drop are calculated and i-V performance characteristics are compared between flows with two flow channels. Results show that performance of fuel cell with interdigitated flow channel is hi민or than that with parallel flow channel at high current region because hydrogen and oxygen in interdigitated flow channel are transported to catalyst layer effectively due to strong convective transport through gas diffusion layer but pressure drop is larger than that in parallel flow channel. Therefore Trade-off between power gain and pressure loss should be considered in design of fuel cell with interdigitated flow channel.