Earticle

To mitigate power transmission losses between power plants and consumers, Medium-Voltage Direct Current (MVDC) systems, which transmit power as DC and convert it back to AC at the destination, are increasingly being developed. However, these MVDC conversion systems, which are rich in semiconductor devices, generate significantly more heat than conventional transformers. Consequently, the thermal management capabilities of traditional air-cooling methods are proving insufficient, necessitating the adoption of liquid cooling systems. This study aims to optimize the energy efficiency of MVDC liquid cooling systems. Finally, we demonstrated a optimized system that minimizes the pump's power consumption by dynamically adjusting the coolant flow rate to each thermal module based on the automatic control valve system.

통신 시스템에서 제한된 총 전력으로 여러개의 부채널로 이루어진 채널의 입력과 출력 사이의 상호정보를 최 대화하는 문제의 해는 Waterfilling 구조를 가진다. 채널 상태 정보(CSI)를 알고 있을 때 OFDM이나 MIMO는 병렬의 독립된 부채널들로 분해 될 수 있다. 제한된 전력 하에 채널용량에 접근하는 전송속도를 위한 최적의 부채널 전력할당 문제의 해는 Waterfilling 으로 구할 수 있다. Waterfilling은 상태가 좋은(SNR이 높은) 부채 널에 더 많은 전력을 할당 하고 상태가 나쁜 채널들은 적은 전력이나 전력을 할당하지 않음으로서 상태가 좋은 부 채널들의 전송속도를 높이고 결 과적으로 전체 전송속도를 채널용량에 접근하게 한다. Waterfilling은 총 전력 제한을 만 족하는 정확한 수면 높이를 찾 는데 일반적으로 수면 높이를 추정하고 갱신해 나가는 반복적 알고리즘이 사용된다. 이 과정에서 부채널들에 대한 채 널이득 제곱의 역수들의 부분합(             ) 계산이 반복적으로 필요하다. 본 논문에서는 이런 부분합들을 초기화 단계 에서 미리 계산하여 배열을 만들고 임의의 부분합 계산을 배열 참조로 대치함으로서 Waterfilling 알고리즘의 계산 시 간을 줄였다.

In communication systems the solution of the problem of maximizing the mutual information between the input and output of a channel composed of several subchannels under total power constraint has a waterfilling structure. OFDM and MIMO can be decomposed into parallel subchannels with CSI. Waterfilling solves the problem of optimal power allocation to these subchannels to achieve the rate approaching the channel capacity under total power constraint. In waterfilling, more power is alloted to good channels (high SNR) and less or no power to bad channels to increase the rate of good channels, resulting in channel capacity. Waterfilling finds the exact water level satisfying the power constraint employing an iterative algorithm to estimate and update the water level. In this process computation of partial sums of inverse of square of subchannel gain is repeatedly required. In this paper we reduced the computation time of waterfilling algorithm by replacing the partial sum computation with reference to an array which contains the precomputed partial sums in initialization phase.

The helical-coiled once-through steam generator (H-OTSG) has the advantages of compact structure and strong heat transfer ability, and is appropriate for lead-cooled fast reactor. The two-phase flow instability may cause mechanical vibration and thermal fatigue of heat transfer tube bundles, posing a serious threat to the safe operation of the H-OTSG. In this work, the secondary side of the H-OTSG characterized as multi-parallel channels is modeled by RELAP5/MOD3.4 code, and the oscillation behavior during start-up and the influence of structural and operating parameters on system stability are studied based on time-domain method. The results indicate that the pressure, flow rate, and temperature of the secondary fluid exhibit density wave oscillations at the heating section in a (n-2,2) pattern. In addition, increasing inlet throttling, reducing outlet throttling, controlling subcooling within an appropriate range and avoiding operation under low loads are all beneficial for improving the system stability of the helical-coiled once-through steam generator.

The flow pulsation of two parallel channels is investigated using RANS and URANS approaches. The parallel channels are connected with a small gap and have different cross section areas. The ratio of a right side area and a left side area ($A_R$ / $A_L$) is 0.5. Computations are conducted using a CFX code. Turbulence models adopted for RANS are Reynolds stress model and Shear Stress Transport (SST) model. The bulk Reynolds number is 60,000. Predicted results are compared with the experimental result of Lee et al. and show the flow pulsation with the frequency of about 100 Hz at the center of the gap.

Flow pulsation in the gap connecting with two parallel channels is investigated by RANS and URANS approaches. The two parallel channels are connected by a small channel called for a gap. The parallel channels are designed to have different cross section area with its ratio of 0.5. Computations are conducted using a CFX 11.0 code. The bulk Reynolds number is 60,000. Predicted results are compared with the previous experimental result. Mean velocity profile at the center of gap region are compared with experiments for its validation. Spectral analysis on the lateral velocity in the center of the gap is presented. Auto and cross correlation for the axial-flow velocity pattern are presented. The unsteady structure of the flow pulsation was visualized in the region of the gap in the parallel channel.

Turbulent flow characteristics on the gap of two parallel channels are investigated using LES(large eddy simulation) approach. Two parallel channels have the same cross-section area and are connected by the narrow channel named the gap. Turbulent flow near the gap makes the flow pulsation along the streamwise direction of two channels. The flow condition is the Reynolds number of $2.5{\times}10^{-5}$. We compared the predicted results with the previous experimental results and presented the axial mean velocity, turbulent intensities, Reynolds shear stresses and turbulent kinetic energy.

Compared to the solid-desiccant system using rotors, the liquid-desiccant system has many advantages. including low regeneration temperature and removal of VOC, bacteria, and particulate matter. One deficiency of the liquid system, however, is the carryover of liquid desiccant into the system. In this study, a parallel-channel type of dehumidifying element made of cellulose/PET was developed; it minimized the liquid carryover even at relatively high velocities. Dehumidification tests were conducted using LiCl to change the frontal velocity from 2.0 to 4.0 m/s, and the results were compared with those obtained from commercial Celdek. Virtually no carryover was observed for the new sample up to 4.0 m/s, whereas the carryover was observed above 3.0 m/s for Celdek. The amount of dehumidification increased as frontal velocity increased. At 2.0 m/s, the amounts of dehumidification between the new sample and Celdek were approximately equal. At 4.0 m/s, however, Celdek yielded 16% more dehumidification. Celdek also yielded 41 to 48 percent larger pressure drops. Comparison of data from both the new sample and the Celdek with theoretical predictions or those from correlations showed significant disagreement, apparently because of the poor wetting of the liquid over sample surfaces, or deficiency of the correlations that resulted from ignoring the existence of liquid film.

본 연구에서는 병렬 마이크로 채널에서 FC-72의 비등 열전달 특성을 파악하기 위한 실험적 연구를 수행하였다. 병렬 마이크로 채널은 깊이 0.2 mm, 폭 0.45 mm, 길이 60 mm의 15개의 마이크로 채널로 구성되었으며, 실험은 열유속 $16.4kW/m^2$와 $25.6kW/m^2$의 조건에서 수행되었으며, 이때 질량유속 300, 400 그리고 $500kg/m^2s$의 범위에서 이루어졌다. 실험을 통해 측정된 열전달 계수는 낮은 건도에서는 건도가 증가함에 따라 급격하게 감소하며, 일정 건도 이상에서는 일정하게 유지되었다. 본 연구를 통해 마이크로 채널에서 FC-72의 비등 열전달 메커니즘을 확인하였으며, 실험에서 얻어진 열전달 계수는 열전달 계수를 예측하기 위한 기존의 상관식들과 비교 분석하였다.

In this study, an experimental study was performed to understand the boiling heat transfer characteristics of FC-72 in parallel micro-channels. The parallel micro-channels contained channels having a $0.2mm{\times}0.45mm$ [$H{\times}W$] cross section and length of 60 mm. And heat flux was varied from 16.4 to $25.6kW/m^2$ and mass fluxes from 300 to $500kg/m^2s$. The measured heat transfer coefficient was sharply decreased at lower vapor quality and then it was kept approximately constant as the vapor quality is increased. From the experimental results, the boiling heat transfer mechanism of FC-72 was confirmed and the measured heat transfer coefficient was compared and analyzed with the existing correlations to predict the heat transfer coefficient.

본 연구에서는 FC-72를 작동유체로 사용하여 병렬 마이크로 채널에서의 2상 유동 마찰 압력 강하를 예측하기 위한 실험적 연구를 수행하였다. 병렬 마이크로 채널은 깊이 0.2 mm, 폭 0.45 mm, 길이 60 mm의 15개의 마이크로 채널로 구성되었으며, 실험은 질량유속 $152.2{\sim}584.2kg/m^2s$, 열유속 $7.5{\sim}28.3kW/m^2$ 범위에서 이루어졌다. 실험에서 얻어진 자료는 기존의 마찰 압력 강하를 예측하기 위한 상관식들과 비교 분석하였다. 기존의 상관식은 일반적으로 균질 모델과 분리류 모델을 사용한다. 본 연구에서는 분리류 모델을 사용한 기존의 상관식을 수정하여 새로운 상관식을 제안하였으며, 그 결과 Mean Absolute Error 9.6%내에서 실험 결과를 잘 예측하였다.

In this study, an experimental study was performed to predict the two-phase frictional pressure drop of FC-72 in parallel micro-channels. The parallel micro-channels consist of 15 channels with depth 0.2 mm, width 0.45 mm and length 60 mm. And tests were performed in the ranges of mass fluxes from 152.2 to $584.2kg/m^2s$ and heat fluxes from 7.5 to $28.3kW/m^2$. The experimental data was compared and analyzed with existing correlations to predict the pressure drop. The existing methods to predict the pressure drop used the homogeneous model and the separated model. In this study, the new correlation was proposed by modified existing correlation using the separated model, and the new correlation predicted consequently with the experimental data within MAE of 9.6%.

PC에 이어 스마트폰, 태블릿 등 모바일 기기 보급이 활성화되면서 온라인 쇼핑 채널이 다양해졌다. 국경을 넘어서 온라인으로 상품 주문이 가능해지면서, 수입상품도 좀 더 저렴한 가격으로 구매하려는 욕구가 생기기 시작했다. 해외직구와 더불어 병행수입은 기존 유통방식에 추가되었다. 국내에서 리바이스 청바지 사태가 일어난 이후 정부는 병행수입을 인정하며 권장해왔다. 병행수입시장은 관세청의 ‘병행수입물품 통관인증제’ 도입에 힘입어 소비자들에게 신뢰성을 부각하였고 해외직구와 더불어 성장하고 있다. 본 논문은 이러한 배경하에서 병행수입의 개념과 발생원인, 우리나라의 법령으로 제시된 허용·금지기준, 문제점을 분석했다. 특히 문제점과 개선점을 통해 향후 병행수입에 대한 이해를 돕고자 하였다.

As smartphone, tablet and other mobile devices become new distribution channels through online and mobile communication, ordering goods online across borders gets the urge to sales and consumption in a more reasonable price. Parallel import has also been added to the existing distribution channels with foreign imports. The government agreed a parallel import market after Levis Jeans situation. In this paper, the concept and causes of the parallel import under this background were analyzed. In particular, the analysis of the problem from the perspectives of companies, consumers and national level will help the understanding of future parallel import market.

연구목적: 본 연구의 목적은 사례분석을 통해 병행수입과 관련된 정식수입업체의 보증정책전략을 확인하고 그 효과를 분석하는데 있다. 또한 병행수입과 정식수입간의 갈등을 조율할 수 있는 정부정책의 필요성을 확인한다. 논문구성/논리: 본고의 구성은 다음과 같다. 먼저 제 2장에서 병행수입의 개요와 병행수입에 대한 정식수입업체의 보증정책에 대해 개관하며, 제 3장에서는 우리나라 오토바이 시장과 병행수입에 대한 Honda Korea의 보증정책 사례를 분석한다. 제 4장에서는 병행수입에 대한 정책적인 측면을 진단하고 특히 금융지원과 보증정책에 대해 구체적으로 논의한다. 마지막으로 5장에서는 본 연구의 내용을 요약 정리한다. 결과: 본 연구결과, 병행수입업체에 비해 가격경쟁력이 열위인 정식수입업체가 구매를 유인하기 위한 전략은 차별적인 보증정책임을 확인하였다. 이와 관련하여 병행수입품에 대한 정식수입업체의 보증정책을 무차별보증, 유상보증, 보증거부의 세 가지로 분류하고 Honda Korea의 사례를 통해 유상보증정책에서 보증거부정책으로의 변화가 제품전략변화와 맞물려 높은 시장점유율을 확보할 수 있었음을 확인하였다. 또한 정식수입과 병행수입, 두 가지 유통채널의 갈등을 조율할 수 있는 명확한 정부정책의 필요성을 확인하였다. 독창성/가치: 본 연구는 통계자료를 기반으로 한 특정기업에 대한 사례분석을 통해 표면적으로 나타나지 않은 병행수입과 보증정책의 영향에 대해 확인한 논문으로 의의가 있으며, 향후 병행수입에 대한 정부 정책수립에 있어 시사점을 제공할 수 있다.

Purpose: This paper addresses the warranty policies of authorized distribution channels in order to cope with unauthorized channels. In addition, the role of the government, in order to tone down the channel conflicts and improve consumers’ rights, is discussed. The motorcycle market, focusing on Honda Korea’s warranty policy, is demonstrated for the understanding of readers. Composition/Logic: The paper is organized as follows. First, the present condition of parallel imports and warranty policies of authorized channels are described in Section 2. Section 3 deals with the motorcycle market in Korea overall and analyzes a case for warranty policy under parallel imports focusing on Honda Korea. Section 4 presents the government’s acts for parallel imports and discusses two aspects regarding financial support and warranty policies. Finally, the conclusion is summarized in the last section. Findings: The results show that a discriminative warranty strategy acted upon by authorized channels which are less competitive than a parallel importer is the effective way to attract buyers. In this regard, this study categorizes the authorized channels' warranty policies for parallel imports as a universal warranty, after sales service at a cost, and denial of service. In the case of Honda Korea, changing warranty policy combined with product strategy was able to secure a high market share. We also identified the necessity of clear government policies that can coordinate both, the authorized channels and parallel imports. Originality/Value: This study is meaningful as it confirms the effects of parallel import and warranty policies that have not been seen through the case analysis of a specific company based on statistical data. In addition, this study explains implications for the establishment of government policies on parallel imports in the future.