• 전기학회논문지
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• 제65권8호
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• pp.1340-1346
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• 2016

Utilization of Distributed Generations (DGs) using Renewable Energy Sources (RESs) has been constantly increasing as they provide a lot of environmental, economic merits. In spite of these merits, some problems with respect to voltage profile, protection and its coordination system due to reverse power flow could happen. In order to analyze and solve the problems, accurate modeling of DG systems should be preceded as a fundamental research task. In this paper, we present a PhotoVoltaic (PV) generation system which consists of practical PV cells with series and parallel resistor and an inverter for interconnection with a main distribution system. The inverter is based on controllable current source which is capable of controlling power factors, active and reactive powers within a certain limit related to amount of PV generation. To verify performance of the model, a distribution system based on actual data is modeled by using ElectroMagnetic Transient Program (EMTP) software. Computer simulations according to various conditions are also performed and it is shown from simulation results that the model presented is very effective to study DG-related researches.

• Journal of Advanced Marine Engineering and Technology
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• 제29권3호
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• pp.297-305
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• 2005

The evaporation heat transfer coefficient of $CO_2$ (R-744) in a horizontal tube was investigated experimentally. The experiments were conducted without oil in a closed refrigerant loop which was driven by a magnetic gear pump. The main components of the refrigerant loop are a receiver. a variable-speed pump. a mass flow meter. a pre-heater and evaporator (test section). The test section consists of a smooth. horizontal stainless steel tube of 7.75 mm inner diameter. The experiments were conducted at mass flux of 200 to $500\;kg/m^{2}s$. saturation temperature of $-5^{\circ}C\;to\;5^{\circ}C$. and heat flux of 10 to $40\;kW/m^2$. The test results showed the evaporation heat transfer of $CO_2$ has greatly effect on more nucleate boiling than convective boiling. The evaporation heat transfer coefficients of $CO_2$ are highly dependent on the vapor quality. heat flux and saturation temperature. The evaporation heat transfer coefficient of $CO_2$ is very larger than that of R-22 and R-134a. In making a comparison between test results and existing correlations. the present experimental data are the best fit for the correlation of Jung et al. But it was failed to predict the evaporation heat transfer coefficient of $CO_2$ using by the existing correlation. Therefore. it is necessary to develop reliable and accurate predictions determining the evaporation heat transfer coefficient of $CO_2$ in a horizontal tube.

• International Journal of Air-Conditioning and Refrigeration
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• 제13권4호
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• pp.167-174
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• 2005

The evaporation heat transfer coefficient of $CO_2$ (R-744) in a horizontal tube was investigated experimentally. The experiments were conducted without oil in a closed refrigerant loop which was driven by a magnetic gear pump. The main components of the refrigerant loop are a receiver, a variable-speed pump, a mass flow meter, a pre-heater and evaporator (test section). The test section consists of a smooth horizontal stainless steel tube of 7.75 mm inner diameter. The experiments were conducted at mass flux of 200 to $500kg/m^2s$, saturation temperature of $-5^{\circ}C\;to\;5^{\circ}C$, and heat flux of 10 to $40kW/m^2$. The test results showed the evaporation heat transfer of $CO_2$ has greater effect on nucleate boiling than convective boiling. The evaporation heat transfer coefficient of $CO_2$ is highly dependent on the vapor quality, heat flux and saturation temperature. The evaporation heat transfer coefficient of $CO_2$ is very larger than that of R-22 and R-134a. In comparison with test results and existing correlations, the best fit of the present experimental data is obtained with the correlation of Jung et al. But the existing correlations failed to predict the evaporation heat transfer coefficient of $CO_2$. Therefore, it is necessary to develop reliable and accurate predictions determining the evaporation heat transfer coefficient of $CO_2$ in a horizontal tube.

• Advances in nano research
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• 제10권1호
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• pp.77-90
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• 2021

In the present study, novel chitosan coated magnetic magnetite (Fe3O4) nanoparticles were successfully biosynthesized from mushroom, Agaricus campestris, extract. The obtained bio-nanocomposite material was used to investigate ultra-fast and highly efficient for removal of Ni2+ ions in a fixed-bed column. Chitosan was treated as polyelectrolyte complex with Fe3O4 nanoparticles and a Fungal Bio-Nanocomposite Material (FBNM) was derived. The FBNM was characterized by using X-Ray Diffractometer (XRD), Scanning Electron Microscopy-Energy Dispersive X-Ray Spectroscopy (SEM-EDS), Fourier Transform Infrared spectra (FTIR) and Thermogravimetric Analysis (TGA) techniques and under varied experimental conditions. The influence of some important operating conditions including pH, flow rate and initial Ni2+ concentration on the uptake of Ni2+ solution was also optimized using a synthetic water sample. A Central Composite Design (CCD) combined with Response Surface Modeling (RSM) was carried out to maximize Ni2+ removal using FBNM for adsorption process. A regression model was derived using CCD to predict the responses and analysis of variance (ANOVA) and lack of fit test was used to check model adequacy. It was observed that the quadratic model, which was controlled and proposed, was originated from experimental design data. The FBNM maximum adsorption capacity was determined as 59.8 mg g-1. Finally, developed method was applied to soft drinks to determine Ni2+ levels. Reusability of FBNM was tested, and the adsorption and desorption capacities were not affected after eight cycles. The paper suggests that the FBNM is a promising recyclable nanoadsorbent for the removal of Ni2+ from various soft drinks.

• Journal of Korean Biological Nursing Science
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• 제23권3호
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• pp.159-169
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• 2021

Purpose: The purpose of this study was to analyze research trends and find whether Post-Traumatic Stress Disorder (PTSD) of refugees could affect structural or functional changes of brains of those under MRI, focusing on volumes, functional connectivities, and metabolites. Methods: A literature search was done using PubMed, Embase, RISS, and KMBase to identify studies that matched our research purpose. A total of eight studies were identified using Prisma flow diagram by two reviewers independently. Results: Eight studies were identified. Three studies were on North Korean defectors as subjects. The number of studies that observed structural changes, functional changes, and metabolite changes in brains was 2, 5, and 2, respectively. Although each study observed various parts of the brain, anterior cingulate cortex (ACC) was observed commonly in three studies. The PTSD group showed reduction of ACC volume and N-acetyl-aspartate (NAA) metabolite in ACC compared to the non- PTSD group. When exposed to negative stimuli, the PTSD group showed higher neural activity than the non-PTSD group, but not vice versa. Conclusion: ACC showed significant difference in volume, neural activity, and NAA metabolite between the PTSD and the non-PTSD group, resulting in significant differences in structural changes, functional changes, metabolite changes, respectively. This study showed the need for conducting more research using various biomarkers to clarify the relationship between PTSD of refugees and their brain changes.

• Advances in concrete construction
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• 제14권2호
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• pp.103-113
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• 2022

On the basis of fabrication, the utilization of nano material in numerous industrial and technological system, obtained the utmost significance in current decade. Therefore, the current investigation presents a theoretical disposition regarding the flow of electric conducting Williamson nanoliquid over a stretchable surface in the presence of the motile microorganism. The impact of thermal radiation and magnetic parameter are incorporated in the energy equation. The concentration field is modified by adding the influence of chemical reaction. Moreover, the splendid features of nanofluid are displayed by utilizing the thermophoresis and Brownian motion aspects. Compatible similarity transformation is imposed on the equations governing the problem to derive the dimensionless ordinary differential equations. The Homotopy analysis method has been implemented to find the analytic solution of the obtained differential equations. The implications of specific parameters on profiles of velocity, temperature, concentration and motile microorganism density are investigated graphically. Moreover, coefficient of skin friction, Nusselt number, Sherwood number and density of motile number are clarified in tabular forms. It is revealed that thermal radiation, thermophoresis and Brownian motion parameters are very effective for improvement of heat transfer. The reported investigation can be used in improving the heat transfer appliances and systems of solar energy.

• Advances in nano research
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• 제16권4호
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• pp.341-352
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• 2024

This study investigates the heat and mass transfer characteristics of a MoS₂ nanoparticle suspension in ethylene glycol over a porous stretching sheet. MoS₂ nanoparticles are known for their exceptional thermal and chemical stability which makes it convenient for enhancing the energy and mass transport properties of base fluids. Ethylene glycol, a common coolant in various industrial applications is utilized as the suspending medium due to its superior heat transfer properties. The effects of variable thermal conductivity, variable mass diffusivity, thermal radiation and thermophoresis which are crucial parameters in affecting the transport phenomena of nanofluids are taken into consideration. The governing partial differential equations representing the conservation of momentum, energy, and concentration are reduced to a set of nonlinear ordinary differential equations using appropriate similarity transformations. R software and MATLAB-bvp5c are used to compute the solutions. The impact of key parameters, including the nanoparticle volume fraction, magnetic field, Prandtl number, and thermophoresis parameter on the flow, heat and mass transfer rates is systematically examined. The study reveals that the presence of MoS₂ nanoparticles curbs the friction between the fluid and the solid boundary. Moreover, the variable thermal conductivity controls the rate of heat transfer and variable mass diffusivity regulates the rate of mass transfer. The numerical and statistical results computed are mutually justified via tables. The results obtained from this investigation provide valuable insights into the design and optimization of systems involving nanofluid-based heat and mass transfer processes, such as solar collectors, chemical reactors, and heat exchangers. Furthermore, the findings contribute to a deeper understanding of stretching sheet systems, such as in manufacturing processes involving continuous casting or polymer film production. The incorporation of MoS₂-C₂H₆O₂ nanofluids can potentially optimize temperature distribution and fluid dynamics.

• Korean Journal of Radiology
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• 제22권9호
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• pp.1537-1546
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• 2021

Objective: To assess the role of arterial spin-labeling (ASL) perfusion MRI in identifying cerebral perfusion changes after indirect revascularization in children with moyamoya disease. Materials and Methods: We included pre- and postoperative perfusion MRI data of 30 children with moyamoya disease (13 boys and 17 girls; mean age ± standard deviation, 6.3± 3.0 years) who underwent indirect revascularization between June 2016 and August 2017. Relative cerebral blood flow (rCBF) and qualitative perfusion scores for arterial transit time (ATT) effects were evaluated in the middle cerebral artery (MCA) territory on ASL perfusion MRI. The rCBF and relative time-to-peak (rTTP) values were also measured using dynamic susceptibility contrast (DSC) perfusion MRI. Each perfusion change on ASL and DSC perfusion MRI was analyzed using the paired t test. We analyzed the correlation between perfusion changes on ASL and DSC images using Spearman's correlation coefficient. Results: The ASL rCBF values improved at both the ganglionic and supraganglionic levels of the MCA territory after surgery (p = 0.040 and p = 0.003, respectively). The ATT perfusion scores also improved at both levels (p < 0.001 and p < 0.001, respectively). The rCBF and rTTP values on DSC MRI showed significant improvement at both levels of the MCA territory of the operated side (all p < 0.05). There was no significant correlation between the improvements in rCBF values on the two perfusion images (r = 0.195, p = 0.303); however, there was a correlation between the change in perfusion scores on ASL and rTTP on DSC MRI (r = 0.701, p < 0.001). Conclusion: Recognizing the effects of ATT on ASL perfusion MRI may help monitor cerebral perfusion changes and complement quantitative rCBF assessment using ASL perfusion MRI in patients with moyamoya disease after indirect revascularization.

• Investigative Magnetic Resonance Imaging
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• 제4권1호
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• pp.42-51
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• 2000

목적: Projection-type Fast Spin Echo (PFSE) 영상 기법은 일반적인 Fast Spin Echo (FSE) 기법과 비교하여 환자의 움직임과 혈류에 강한 장점이 있는 반면. $T_2$ 대조도(contrast)를 조절하기가 어려운 단점이 있다. 본 연구에서는 PFSE의 대조도를 이론적으로 분석하였고 컴퓨터 모의실험을 통하여 다양한 effective echo time (TE) 을 갖는 일반적인 FSE와 비교, 분석하였다. 또한 인체 실험을 통하여 제안한 PFSE 영상기법으로 움직 임과 혈류에 강인한 $T_2$ 강조 영상을 얻을 수 있음을 보였다. 대상 및 방법: 본 연구에서는 1.OT 전신 MRI 시스템에서 새로운 k-space의 배치를 갖는 PFSE 펠스 시권스를 구현하여, PFSE와 FSE 방식의 $T_2$ 대조도를 컴퓨터 모의설험과 인체 실험을 통하여 비교, 분석하였다. 컴퓨터 모의실험에서는 서로 다른 $T_2$ 값을 갖는 팬텀을 구현하여 다양한 effective TE에 대한 FSE 영상과 PFSE 영상을 재구성하여 대조도를 비교하였다. 인체 설험에서는 multi-slice $T_2$ 강조 두부 영상을 PFSE와 FSE로 얻어 영상기법간의 $T_2$ 대조도를 비교하였다. 결과: 이론적인 분석에서 PFSE의 $T_2$ 대조도는 effective TE가 80-l00ms 정도의 FSE 영상과 등가하게 나타나 $T_2$ 강조 영상을 얻을 수 있을 것으로 판단되었다. 컴퓨터 모의실험에서 PFSE 재구성 영상은 effective TE가 96ms인 FSE 영상과 대조도가 비슷하게 나타났다. 인체 실험에 서도 PFSE 영상은 effective TE가 96ms인 FSE 영상과 비슷하게 나타났으며. PFSE 방법이 FSE 방법에 비하여 움직 엄과 혈류와 관련한 artifact에 강인함을 확인 할 수 었었다. 결론: PFSE 기법은 k-space의 극좌표계에서 서로 다른 각도를 갖는 여러 line틀을 다중 스핀 에코 기법으로 측정하는 방식이다. PFSE기법은 FSE와 비교하여 환자의 움직임과 혈류에 강한 장점이 있는 반면, $T_2$ 대조도를 조절하기가 어려운 단점이 있다. 본 연구에서는 PFSE 방식으로 FSE와 대등한 $T_2$ 대조도 ($T_2$ 강조 영상)를 얻을 수 있음을 이론과 컴퓨터 모의실험 밝히고, 인체 실험을 통하여 확인하였다.

• 한국자기학회지
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• 제25권3호
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• pp.67-73
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• 2015

본 연구는 철기(Fe based) $Fe_{73.5}Si_{13.5}B_9Nb_3Cu_1$ 나노결정 합금의 분말코어(powder core)의 연자기적 특성 향상을 위한 기초연구로서, 절연 코팅제의 첨가량 및 분말입도에 따른 투자율, 코어손실 및 DC 바이어스 특성을 주로 조사하였다. 우선 합금조성을 PFC 장치를 이용하여 비정질 합금리본을 제조한 후, 열처리, 미분쇄 및 분급하여 얻어진 합금분말에 절연 코팅제(PEI)의 첨가량을 0.5, 1.0, 2.0, 2.5 wt%로 변화시켜 $16ton/cm^2$으로 압축성형 및 결정화 열처리하여 제조한 토로이달 나노결정 분말코어($OD12.7mm{\times}ID7.62mm{\times}H4.75mm$)는 절연 코팅제 함량이 증가할수록 투자율은 감소하였지만, 코어손실 및 DC 바이어스 특성은 향상됨을 확인하였다. 이러한 이유는 합금분말 절연 코팅제 첨가량이 증가할수록 비정질 합금분말 입자가 적어져 분말코어의 성형밀도가 낮아지기 때문으로 추정되었으며, 절연 코팅제의 함량은 1 wt%가 가장 적합한 것으로 판단되었다. 또한 절연 코팅제 함량을 1 wt%로 고정하고, 합금분말의 입도에 따라 제조한 분말코어의 경우, 실효투자율 및 코어손실은 입도가 클수록 우수하였지만, DC 바이어스 특성은 인가자장이 증가함에 따라 더욱 나빠짐을 확인하였다. 그 이유는 합금분말 표면의 코팅층 두께 차이에 의한 절연효과, 잔류기공 혹은 분말코어의 성형밀도 차이 등에 기인하는 것으로 추정되었다.