• 대한안전경영과학회지
• /
• 제20권4호
• /
• pp.7-13
• /
• 2018

This study investigates the enhancement of the oxygen diffusion rate in the cathode channel of a proton exchange membrane fuel cell (PEMFC) by pure oscillating flow, which is the same as the mechanism of human breathe. Three-dimensional numerical simulation, which has the full model of the fuel cell including electrochemical reaction, ion and electronic conduction, mass transfer and thermal variation and so on, is performed to show the phenomena in the channel at the case of a steady state. This model could analysis the oscillating flow as a moving mesh calculation coupled with electrochemical reaction on the catalyst layer, however, it needs a lot of calculation time for each case. The two dimensional numerical simulation has carried on for the study of oscillating flow effect in the cathode channel of PEMFC in order to reduce the calculation time. This study shows the diffusion rate of the oxygen increased and the emission rate of the water vapor increased in the channel by oscillating flow without any forced flow.

• 한국항공우주학회지
• /
• 제32권10호
• /
• pp.118-126
• /
• 2004

이산와류법(discrete vortex method)을 이용하여 진통하는 평판들로부터 추력발생을 연구하였다. 평판들과 후류들은 이산와류로 나타내었다. 진동하는 평판들 사이의 복잡한 공력상호작용을 정확히 계산하기 위하여 와핵모델(vortex core model)과 와핵첨가법(vortex core addition scheme)을 사용하였다. 단일 평판이 히빙진동을 할 경우 나타나는 후류를 계산하여 기존의 유동가시화 결과와 비교하였다. 피칭진동 시 평판에서 발생하는 후류형상이 평판들의 추진 특성에 미치는 영향을 조사하였다. 3가지 방식(1. 하나의 평판은 고정 다른 평판은 진동, 2. 두개의 평판이 같은 위상으로 진동, 3. 두개의 평판이 반대의 위상으로 진동)으로 진동하는 평판들의 추진특성을 계산하였다. 반대의 위상으로 진통하는 평판이 가장 큰 추진력을 보였다.

• 대한조선학회논문집
• /
• 제43권3호
• /
• pp.322-330
• /
• 2006

Numerical calculations are carried out for flow past a circular cylinder forced oscillating normal to the free-stream flow at a fixed Reynolds number equal to 185. The cylinder oscillation frequency ranged from 0.8 to 1.2 of the natural vortex-shedding frequency, and the oscillation amplitude extended up to 20% of the cylinder diameter. IBM (Immersed Boundary Method) with direct momentum forcing was adopted to handle both of a stationary and an oscillating cylinder Present results such as time histories of drag and lift coefficients for both stationary and oscillating cases are in good agreement with previous numerical and experimental results. The instantaneous wake patterns of oscillating cylinder with different oscillating frequency ratios showed the synchronized wakes pattern in the lock-in region and vortex switching phenomenon at higher frequency ratio than the critical frequency ratio.

• 대한기계학회논문집A
• /
• 제26권2호
• /
• pp.291-299
• /
• 2002

This paper characterizes hi-directionally oscillating flow in planar microdiffusers in order to evaluate the frequency-dependent flow rectification performance of the microdiffusers. In the theoretical study, we analyze a hi-directionally oscillating flow in the planar microdiffuser. In the experimental study, we fabricate two different microdiffuser prototypes, having different neck widths of 100 ㎛ (D100) and 300 ㎛(D300), respectively. The experimental net flow rates are measured as 116.6 $\mu$ι/min. and 344.4 $\mu$ι/min. for D100 and D300, respectively. The experimental flow rate of D300 decreases at the oscillating flow frequencies higher than 90Hz, at which the net boundary layer thickness is reduced to the microdiffuser neck width. It is experimentally verified that the flow rectification performance and the net flow rate of the microdiffusers tend to decrease when the boundary layer thickness is smaller than the diffuser neck width.

• 대한기계학회논문집
• /
• 제17권12호
• /
• pp.3208-3216
• /
• 1993

Heat transfer characteristics of the laminar oscillating flow in a circular pipe have been studied under the condition that the wall temperature of the pipe is distributed sinusoidally with the axial direction. The axial velocity was assumed to be uniform in radial direction and the temperature field was analyzed by means of the perturbation method. The results show that the difference between wall and section-time-averaged fluid temperature increases as the oscillating frequency increases and eventually converges to a constant value which is determined by the ratio of swept distance to the characteristic length of wall temperature distribution. Also it is shown that the dominant variable in the heat transfer process when swept distance ratio is greater than 1 is not thermal Womersley number(F) but thermal Womersley number multiplied by the square root of swept distance ratio. The variation of the time-averaged Nusselt number is obtained as a function of F. The results indicate that Nusselt number is proportional to $F_{\epsilon}^{1/2}$ when both of F and .epsilon. are much greater than 1.

• 한국정보통신학회논문지
• /
• 제7권3호
• /
• pp.492-498
• /
• 2003

본 논문은 캐스코드결합에 의한 마이크로파 자기발진믹서를 설계하기 위하여 믹서가 최적 변환이득을 나타내는 바이어스 조건에 대해 분석하였다. 마이크로파 자기발진믹서는 두 개의 GaAs MESFET를 케스코드 결합시켰으며 상위 MESFET는 비교적 높은 Q값을 가지는 유전체공진기에 의해서 발진기로 동작시키고 아래쪽 FET는 저잡음 특성과 최적의 변환이득을 나타내는 믹서로서 동작시켰다. 분석결과 드레인 전압은 $V_{ds}=2.5V$이고 게이트바이어스 전압은 $V_{gs1}=-0.2V와 \;V_{g2}=0V$로 선정하였을 때 설계된 5.15Ghz의 발진기 출력은 5.92dBm, 위상잡음은 -132.0dBc@100KHz, 믹서의 변환손실은 약 -3dB를 나타내어 이론에 의한 자기발진믹서를 설계할 수 있음을 보였다.

• 대한기계학회논문집B
• /
• 제25권9호
• /
• pp.1272-1280
• /
• 2001

An experimental study has been conducted to elucidate the resonance of natural convection in a side-heated square enclosure having a mechanically oscillating bottom wall. Under consideration is the impact of the imposed oscillating frequency, amplitude and the system Rayleigh number on the fluctuation of air temperatures. The experimental results show that the magnitude of the fluctuation of air temperature is substantially augmented at a specific forcing frequency of the oscillating bottom wall. The resonant frequency is increased with the increase of the Rayleigh number and it is little affected by the amplitude of the oscillating wall. It is also found that the resonant frequency is relevant to the Brunt- V$\"{a}$iS$\"{a}$l$\"{a}$ frequency which represents the stratification degree of the system.

• 전력전자학회논문지
• /
• 제15권1호
• /
• pp.1-8
• /
• 2010

본 논문은 별도의 PWM-IC없이 저가 구현이 가능한 자려 발진 플라이백 컨버터의 대기전력 저감을 위한 회로를 제안한다. 제안된 자려발진 플라이백 컨버터는 기존 자려발진 플라이백 컨버터의 스위치 초기 동작을 위한 경로에서 발생하는 지속적인 전력손실 문제를 개선하기 위해 DC-Blocking 캐패시터를 삽입한 형태로써 약 1W의 대기전력 저감효과 및 시스템의 전력 변환 효율 향상이 가능하다. 본 논문에서는 제안된 회로의 동작원리를 설명하고, 실제 35W급 전원시스템을 구현하여 기존 회로와의 비교를 통해 제안된 회로의 타당성을 검증한다.

• International Journal of Air-Conditioning and Refrigeration
• /
• 제11권1호
• /
• pp.1-9
• /
• 2003

The examinations of the operating mechanism of an oscillating capillary tube heat pipe (OCHP) using the visualization method revealed that the working fluid in the OCHP oscillated to the axial direction by the contraction and expansion of vapor plugs. The contraction and expansion were due to the formation and extinction of bubbles in the evaporating and condensing part, respectively The actual physical mechanism, whereby the heat which was transferred in such an OCHP was complex and not well understood. In this study, a theoretical model of the OCHP was developed to model the oscillating motion of working fluid in the OCHP. The differential equations of two-phase flow were applied and simultaneous non-linear partial differential equations were solved. From the analysis of the numerical results, it was found that the oscillating motion Of working fluid in the OCHP was affected by the operation and design conditions such as the heat flux, the charging ratio of working fluid and the hydraulic diameter of flow channel. The simulation results showed that the proposed model and solution could be used for estimating the operating mechanism in the OCHP.

• 한국해양공학회지
• /
• 제11권2호
• /
• pp.77-90
• /
• 1997

Wave deformation due to Oscillating water column (OWC) plant was studied. To solve this problem, three dimensional numerical method based on Improved Green integral equation was applied. Method condition was considered as well as fixed condition and freely floating condition. From the calculation results, main characteriatic of wave deformation due to OWC plant were discussed. Also, some calculations for the floating barge were performed to confirm the validity of numerical solution of the method.