3-레벨 GTO 인버터를 위한 새로운 저손실 및 과전압 제한 스너버회로 설계

서재형 漢陽大學校 大學院 1995 국내석사

밀폐된 공간 내에서 자성유체의 자연대류 현상에 대한 수치적 연구

서재형 동아대학교 대학원 2011 국내석사

자성유체는 지름 10nm 정도의 마그네타이트, 페라이트, 철, 니켈 등과 같은 강자성체 미립자를 침전이나 응집이 생기지 않도록 오레인산 등의 계면활성제로 특수표면처리를 하여 에텔, 에스텔, 케로진 또는 순수한 물 등의 액체 용액 속에 안정하게 혼합시킨 콜로이드상태의 현탁액이며, 금속 특유의 강자성과 유체특유의 유동성을 겸비한 유체로 외견상 액상 자체가 자성을 갖는 것처럼 인공적으로 만들어진 일종의 고액혼상유체이다. 자성유체는 현재 그 미시적 구조가 복잡하기 때문에 자성유체에 관한 이론적 연구는 아직 충분한 성과를 얻지 못하고, 지배방정식계와 이러한 모델에 적용한 해석이 충분히 확립되어 있지 않다. 특히 외부자장의 영향 하에 있어서 자성유체의 흐름의 거동에 관해 상세한 연구가 필요하게 되었다. 따라서 본 연구에서는 작동유체를 자성유체로 한 밀폐된 정방형 용기를 모델로 하여 한쪽벽면을 가열벽면으로 하고 나머지 벽면은 냉각벽면으로 유지시키면서, 인가자장의 방향과 세기에 따른 자연대류 현상의 변화를 자성유체의 지배방정식계에 GSMAC법을 적용하여 수치 해석하였고 그 결과 아래와 같은 결론을 얻었다. 1. 자장을 가하지 않은 자성유체는 뉴턴 유체의 유동과 거의 같았다. 2. 해석모델의 우측벽면을 가열 면으로 했을 때, 수직으로 자장을 가한 경우 자장의 방향과 반대인 면의 근방에서 외관상 Grashof 수가 증가한 거동을 하였다. 수평으로 자장을 가한 경우 자장을 열 전달 방향과 같은 방향으로 가할 때 외관상 Grashof 수가 감소한 거동을 하였고, 반대방향으로 가한 경우 외관상 Grashof 수가 증가한 거동을 하였다. 3. 해석모델의 밑바닥을 가열 면으로 했을 때, 자성유체의 유동은 중력방향으로 일정한 자장을 가한 경우 외관상 Grashof 수가 증가한 거동을 하고, 반대방향으로 가한 경우 외관상 Grashof 수가 감소하는 거동을 하였다. 수평으로 자장을 가한 경우 자장의 방향과 반대인 면의 근방에서 외관상 Grashof 수가 증가한 거동을 하였다. 4. 해석모델의 우측벽면을 가열 면으로 했을 경우, 수직으로 자장을 가할 때 인가자장이 무 차원수 -4000 근방에서는 자기 체적력 과 부력이 균형을 이루는 무중력 상태가 되어 전도 열 전달 현상이 나타났다. 5. 자성유체에 가하는 자장의 세기와 방향에 대한 유동점의 개수와 위치의 변화가 나타났다. 6. 자성유체에 가하는 자장의 세기가 증가할수록 평균 Nusselt 수는 증가하였다. Magnetic fluid is the suspension of a colloidal state that 10nm sized ferromagnetic particles such as magnetite, ferrite, iron, nickel take special surface treatment using surfactant to avoid sedimentation and agglomeration and mix it and water or kerosene. Magnetic Fluid which has characteristics of the ferromagnetic metal and the fluidity of the liquid is artificially acts like it has the strong magnetism itself. In this study, temperature of a cubic cavity injected magnetic fluid is kept same at all wall except heated wall, By varying the magnetic field direction and intensity, numerically analyzed of the change in natural convection of a magnetic fluid in a cubic cavity. It is used GSMAC-FEM on the base of governing equations of magnetic fluids. The main results are showed as follows : 1. Without the applied magnetic field, the flow of magnetic fluid is similar to that of Newtonian fluid. 2. The case where the right side of the analysis model is the heating surface. when the magnetic field is applied vertically, It externally shows increased Grashof number in the opposite side of magnetic field direction. The case which horizontally applies the magnetic field. When the magnetic field is applied in the same direction as heat conduction, It externally shows decreased Grashof number. On the other hand, When the magnetic field is applied in the opposite direction as heat conduction, it externally shows increased Grashof number. 3. The case where the bottom of the analysis model is the heating surface. When the magnetic field is applied in the same direction as gravity, It externally shows increased Grashof number. On the other hand, When the magnetic field is applied in the opposite direction as gravity, it externally shows decreased Grashof number. The case which horizontally applies the magnetic field. It externally shows increased Grashof number in the opposite side of magnetic field direction. 4. The case where the right side of the analysis model is the heating surface. When the magnetic field is applied vertically, A gravity-free state shows when especially magnetic field near H=-4000 is applied, and it is a heat conduction phenomenon. 5. The number and position of the flowing point changed according to power and direction of magnetic field. 6. As the power of the impressed Magnetic Field increases in Magnetic Fluid, the Nusselt number grows.

도시 대중주택의 실외공간특성에 관한 연구 : 청주지역을 중심으로

서재형 청주대학교 산업경영대학원 1993 국내석사

The aim of this study is to find the characteristics of the outdoor space in the popular detatched houses(PDH) built in Chongju city, from 1986 to 1991. In this paper several elements of the outdoor space in the PDH was studied vertically and horizontally. This study consists of five chapters as follows; Chapter 1, Introduction - The purpose and background of this study. Chapter 2, Defining concept of the PDH - Defining several terms used in this study. - Review of the meaning of the outdoor space in the PDH already studied. - Investigation of the building code related to the PDH. Chapter 3, Survey and analysis of the characteristics of the outdoor space on the ground level, the 2nd floor level and the flat roof. Chapter 4, Suggestions for a good outdoor space quality in the PDH. Chapter 5, Conclusion. Conclusion of this study are as follows: 1. The outdoor space on the ground level is not enough to function as a main garden, as building ratio comes to 60 percent(legal limitations). 2. The outdoor space of the 2nd floor is left to a pathway function. 3. The space of the flat roof becomes half of the site area, as compare with the space of ground level come relatively to small. So, it must be considered that roof garden could be function like a main garden. 4. Legal proceedings, raising the ratio of planting area and lowering the builing ratio, is desirable for a good outdoor space. In is the hope of this study to that these suggestions could be used as the basic information for the outdoor space planning & design of the outdoor space in the PDH.

身體像 改善 프로그램을 通한 身體滿足度 및 自我尊重感 向上 效果

서재형 한서대학교 교육대학원 교육학과 상담심리교육전공 2003 국내석사

The purpose of this study is to define how Body Image Improvement Program affect middle schoolgirls' Body Cathexis and Self-esteem. For the matter defined above, Body Image Improvement Program was carried out to middle schoolgirls to understand their physical condition objectively and make their body image better. For the study, 24 middle schoolgirls were selected after testing all of second and third grade middle schoolgirls at G middle school located in Taean-Gun. Half of them selected for the experimental group and half of them were selected for the control group. Both groups were tested with Body Cathexis and Self-Esteem test before real experiment. Body Image Improvement Program had eight steps and each step was allocated for 60 minutes. It was also carried out once a week from 4:30 p.m. to 5:30 p.m. for eight weeks. After Body Image Improvement Program was carried out to both groups, body cathexis and self-esteem also carried out to them. Pre-test condition and post-test result were compared to each other. The data were analyzed by t-test to verify the difference of the average between pre-test condition and post-test result in both the experimental group and the control group. The results of the study are as follows. 1. Middle schoolgirls who attended Body Image Improvement Program improved physical self-esteem more than those who didn't. 2. The average value of the experimental group, which attended Body Image Improvement Program had significant difference in scholastic achievement and general self-esteem especially including self-esteem related with friends, physical self-esteem and physical capacity self-esteem. The average value of the group had no significant difference in domestic self-esteem and characteristic self-esteem. The average value of the control group, which didn't attend Body Image Improvement Program had no significant difference in all of the testing sector such as scholastic achievement and general self-esteem especially including self-esteem related with friends, domestic self-esteem, physical self-esteem, characteristic self-esteem and physical capacity self-esteem. 3. According to physical post-test result and the assessment from the participant who attended Body Image Improvement Program, they feel that they are happier than before and their quality of life is improving more than that, and they have optimistic attitude about their life at the other aspects of their life as well as at the aspect of body cathexis and self-Esteem after attending Body Image Improvement Program.

Study on heat transfer characteristics of a high power LED cooling system with ferrofluid

서재형 동아대학교 대학원 2018 국내박사

The objectives of this study are to investigate the thermal and flow behavior characteristics of the ferrofluid in different cavities with various the thermal properties of the ferrofluid, the external magnetic fields and the boundary conditions for an application of the thermal management system. In addition, heat transfer phenomena of a high power LED cooling system and its influence on illuminance characteristics of the high power LED were investigated experimentally. The numerical model for prediction of a Nusselt number of the ferrofluid was developed and the predictions of the present model showed a good agreement within 2.70% of the data of other researchers. The heat and flow characteristics of the ferrofluid within the tested cavity were found to depend on both magnetic intensity and magnetic volume fractions of magnetite. The mean Nusselt number and the expansion speed of the isotherm contours of the ferrofluid increased with the larger temperature difference between the top and bottom walls of the tested cavity. In the cases that the magnet was located near the local heated surface and located at the opposite side of the cooling area of the cavity, the cooling effect of ferrofluid in the cavity was enhanced. Therefore, in the present study, the location of the magnet was considered near the local heated surface and opposite to cooling area of the cavity for better heat transfer performances. As the application research, the radial fin heat sink filled with the ferrofluid was designed with 24 multi fins of 1.0 mm thickness for the effective cooling of the generated heat during the high power LED operation. The thermal resistance between the surface and the junction points of the high power LED cooling device with the ferrofluid decreased by 15.5% and 18.2%, respectively, compared to those with the air and the water. The illuminance efficacy of the high power LED cooling device with the ferrofluid increased by 45.4% and 51.0% compared to those of the air and the water, respectively. The junction temperature and the illuminance efficacy characteristics of the high power LED cooling device with the magnet at all working voltages decreased by 9.7% and increased by 6.9%, respectively, compared to those without the magnet. As a result, the heat transfer and illuminance characteristics of the high power LED cooling system with the radial heat sink filled with the ferrofluid suggested that it could be suitable as the vehicle headlamps. Additionally, based on the results and discussion obtained in this study, the followings could be suggested as fundamental research for both the cooling performance improvement and cooling system compactness for the electronics with high heat flux. 전자 제품에 사용되는 반도체 칩은 열에 대하여 높은 민감도를 가지고 있어 작동 온도의 증가에 따라 칩의 고장 가능성이 크게 증가한다. 따라서 전자 장치의 지속적인 성능과 안전성을 보장하기 위하여 적절한 열관리 기술의 적용은 필수적이며, 효율적인 열관리를 위한 방법 중 하나로 나노유체를 이용한 연구가 활발히 진행되고 있다. 자성유체는 나노유체의 일종으로 외부 자기장에 반응하여 높은 열전달 특성을 나타내지만 복잡한 열유동 특성으로 인하여 실제 열 시스템에 적용하는 것은 어렵다. 본 연구의 목적은 다양한 공간에서 자성유체의 열유체 거동 및 열전달 특성에 관한 연구를 수행하여 자성유체를 적용한 고출력 LED 냉각 시스템을 제안하였고 시스템의 성능 특성을 분석하였다. 자성유체의 열유체 거동 및 열전달 특성 분석을 위하여 다양한 강자성체 입자 함량 및 외부 자기장 조건이 고려되었다. 밀폐 공간 내 자성유체의 Nusselt 수를 예측하기 위하여 수치해석 모델을 제안하였고 기존 연구자의 결과와 비교하여 2.70% 오차 이내로 예측되었다. 자성유체의 열유동 특성은 자기장 세기와 자성유체의 자성입자 함량에 의존적으로 나타났고 자성유체의 평균 Nusselt 수와 열팽창 속도는 해석공간의 고온 및 저온부간 온도차가 클수록 증가하였다. 또한 자기장 방향이 국소 가열 영역 근방이고 냉각 영역의 반대측에 배치되는 경우 자성유체의 냉각 효과가 향상되었다. 또한 응용 연구로서 자성유체를 적용한 고출력 LED 냉각 시스템의 열전달 현상과 이에 의한 조도 특성의 영향을 실험적으로 연구하였다. 고출력 LED에서 발생되는 열을 효과적으로 방열시키기 위하여 1 mm 두께의 24 핀이 적용된 히트싱크를 포함하는 방열 장치를 설계하였고 자성유체, 물, 공기를 작동유체로 적용하였다. 고출력 LED 방열 시스템에 자성유체를 적용할 경우 LED 접합 온도는 공기 및 물을 작동유체로 적용한 경우와 비교하여 각각 15.5% 및 18.2% 감소되었고, 조도 효율은 각각 45.4% 및 51.0% 향상되었다. 자성유체를 적용한 고출력 LED 방열 시스템에 자석을 적용할 경우 LED 접합 온도는 자석을 적용하지 않은 경우와 비교하여 9.7% 감소되었고, 조도 효율은 6.9% 향상되었다. 결론적으로 제안된 자성유체를 적용한 고출력 LED 시스템의 열전달 특성 및 조도 성능 특성은 자동차용 헤드램프 적용 조건에 적합한 것으로 검토되었다.

船舶用 파워팩 制御器用 FlexRay 通信의 硏究

서재형 목포해양대학교 2013 국내석사

NMEA0183, 2000 communication method is applied to previous maritime electronic devices. The portion of ethernet communication has been used to Voyage Data Recoder(DVR), the reliability is not guaranteed since communication reliability is SIL(Safety Integrity Level) of a single step. In this paper, therefore, a high reliability FlexRay communication method which has 10Mbps at SIL level 4 is researched to apply the ship's steering device. The TDMA structure with time trigger method is planned through FlexRay communication protocol research and plan. That has, however, communication delay according to NMEA2000 bus status, the exact communication cycle is planned in FlexRay communication. This research is performed to include development and estimation of FlexRay communication for reasonable certification, the reliability is secured since error occurs only 1 frame among 4,215,182 frames. The FlexRay communication with duplication structure is planned to add error detection function, research which the error itself is prevented is performed through error restoring research in the future. The result product of this research will be applied to powerpack, also research for level up will be magnified.

후판 압연시 판 평탄도 불량 분석 및 저감 방법 연구

서재형 부산대학교 대학원 2019 국내박사

국방정보체계의 OTP를 이용한 사용자 인증 연구

서재형 숭실대학교 정보과학대학원 2014 국내석사

都市大衆住宅의 層外空間 特性에 관한 硏究 : 淸州地域을中心으로

서재형 청주대학교 1994 국내석사

SUS 304의 정면밀링 가공시 발생하는 공구파손 검출에 관한 실험적인 연구

서재형 全北大學校 大學院 1994 국내석사

This experimental study is mainly investigated on the detection of cutting tool failure in the machining of SUS 304 by face milling. In this study, the Acoustic Emission signal and cutting force signal from the metal cutting process are monitored in order to identify the cutting tool state. The catastrophic cutting tool failure are suddenly occurred by several mechanisms. The abrupt cutting tool failures deteriorate the surface roughness of workpiece and impulse the spindle of machine tools. Excessive cutting forces which are generated in tool failure deform the structure of machine tools. Therefore, confidential monitoring methods are adopted and the abrupt cutting tool failure and damage are detected effectively. In-process detection of the cutting tool state is possible by monitoring the level of Acoustic Emission and cutting force. In the case of cutting tool failure, damaged cutting tool changes the original depth of cut for oneself and it is called by the name of non-cutting action. The level of cutting force signal is varied with the occurrence of non-cutting action and the level of Acoustic Emission signal is varied too. This study adopts the mean cutting forces and AE parameters in order to detect the cutting tool failure and estimate the total behavior of tool failure From the result of cutting experiments, cutting tool failure in face milling can be detected by mean cutting forces and AE parameters. AE parameters are AE RMS, AE energy, AE count, AE duration. The past observations of tool failure detection have been focused on the tool failure detection itself, but this study suggests that the detection and estimation methods of cutting tool failure are based on the cutting conditions and machinability factors. Only the overall estimation of tool failure which are related cutting conditions and machinability factors can explain the mechanisms of tool failure and damage. Finally, these data from the metal cutting process will be used in the basic field of precision and economic cutting process.