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      • Wind screen effect on water spray cooling performance in an air-cooled heat exchanger

        K. Han 대한기계학회 2019 대한기계학회 춘추학술대회 Vol.2019 No.11

        A numerical study was conducted to investigate the water spray cooling effect in an air-cooled heat exchanger equipped with a wind screen. Using a real scale operating condition, the effects of windscreen distance from fan, wind speed, wind screen length, and wind screen material were investigated. Dry bulb temperature drop and air flow rate into heat exchanger were used to describe the thermal performance of an air cooled heat exchanger. CFD results show inappropriate wind screen location is not helpful to minimizing fan performance deterioration. It was found that screen material and the vertical screen length should be determined carefully to maximize windscreen performance considering wind speed.

      • Heat exchanger capacity control by air recirculation in cold regions

        K. Han 대한기계학회 2019 대한기계학회 춘추학술대회 Vol.2019 No.11

        The purpose of this numerical simulation is to investigate the thermal capacity control method of a heat exchanger using air as cooling medium in a cold region. Various airflow restriction methods were modeled to estimate the air temperatures and velocities, which are critical parameters to air-cooled heat exchanger thermal performance. The impact of blocked air openings and air flow area restrictions were simulated with the help of computational fluid dynamics. It is found that by restricting airflow through heat exchanger inlets and outlets in different configurations, the average air velocity and air temperatures to the primary heat exchangers can be varied by to 0.4 ㎧ and 12 K, respectively.

      • Numerical optimization of microfin tube geometries in a superheated condition

        K. Han 대한설비공학회 2018 대한설비공학회 학술발표대회논문집 Vol.2018 No.6

        An optimized microfin tube geometry is derived with the assistance of computational fluid dynamics and the design of experiment to maximize heat transfer while minimizing pressure drop. Considered geometrical parameters are number of fins, spiral angle, and fin height. R410A is used as a working fluid, and simulations are conducted in a superheated condition for aluminum tubes with a 9.525 mm outside diameter. Non-dimensional parameters are introduced to evaluate heat transfer enhancement and pressure drop increase compared to a smooth tube having the identical outside diameter. Clear evidence of interaction between spiral angle and fin height is detected with statistical analysis.

      • NUMERICAL SIMULATION OF LUBRICANT PROPERTY IMPACT ON POOL BOILING

        K. Han 대한설비공학회 2018 대한설비공학회 학술발표대회논문집 Vol.2018 No.6

        Lubricant thermo-physical property impact on pool boiling was investigated with the assistance of computational fluid dynamics. The mixture of R123 refrigerants and lubricants (Hexane, mineral oil) was used as working fluid. The effect of density, thermal conductivity, surface tension, contact angle, viscosity, specific heat, and latent heat of lubricant on pool boiling was numerically analyzed. Thermal conductivity and density influence on heat transfer was relatively significant compared to other properties but they were not enough to explain the actual heat transfer enhancement level of pool boiling. Bubble dynamics responded to contact angle variation sensitively and nearby bubble behavior should be considered to explain high heat transfer improvement mechanism in actual measurement. The bubble departure diameter, the bubble departure time, and heat transfer coefficients at the bubble departure instant were presented with lubricant property change.

      • Computational Fluid Dynamics Modeling for Steam Condenser Corrosion

        K. Han 대한기계학회 2021 대한기계학회 춘추학술대회 Vol.2021 No.11

        Computational fluid dynamics (CFD) modeling was developed for uniform carbon dioxide corrosion, especially for steam condenser application. The electrochemical reactions of hydrogen ion reduction, carbonic acid reduction, and anodic dissolution of iron were taken into account. A single-phase model in aqueous carbon dioxide solutions was tuned with available experimental results and was expanded to homogeneous vapor-liquid mixture flow. The corrosion rate was analyzed while varying operating temperature, pH, fluid velocity, liquid volume fraction, and liquid droplet diameter. It was found that pH and temperature impact the corrosion rate significantly while the other variables showed relatively minor effects. The detailed electrochemical reaction modeling for computational fluid dynamics was described along with the limitations of two-phase flow application.

      • Numerical simulations on fault behaviors of heat pump systems with multi-indoor coils

        K. Han 대한기계학회 2021 대한기계학회 춘추학술대회 Vol.2021 No.11

        A series of heat pump system simulations are carried out to develop a fault detection method, especially for a system having multi-indoor coils. Five major faults like refrigerant overcharge, undercharge, reduced indoor coil air flow, reduced outdoor coil air flow, and superheat control are systematically varied in three distinctive levels. The system behavior is described with evaporation temperature, compressor discharge temperature, saturated condensing temperature, subcooling, indoor air side temperature rise, and outdoor air side temperature drop by imposing individual and simultaneous faults. The multi-fault behavior cannot be explained with individual fault characteristics. Simple fault prediction models are presented for both single and multi-indoor systems, respectively.

      • 북한지역의 인구지리적 고찰

        이간용 서울대학교 지리교육과 2000 地理敎育論集 Vol.44 No.-

        This paper is intended to explain the spatial aspects of population in North Korea. The findings include: a population concentration in the Midwest (42.9%) as well as a more sparse population in the north (8.7%). The west and east distribution ratio has changed from 59.41 in 1945 to 67:33 in 1993. The population density is higher in the south 175.5 than in the north 81.3 per ㎢. The density in the west (220.9 per ㎢) is higher than that in the east (111.6 per ㎢). Because the north has an urban settlement structure, the degree of urbanization is higher (average 70%) than in the south (average 41%). These spatial patterns of population have been affected by the topographical differences as well as a government policy that encourages regional equalization. This policy has been developed intensely in the Midwest.

      • KCI등재

        사회과 지리 학습에서 우리나라 초·중등 학생들의 사고 특성과 문제점 : SOLO 분류적 접근

        이간용 한국사회과교육연구학회 2004 사회과교육 Vol.43 No.2

        사고 학습을 강조하는 기존의 연구들은 우리나라 학생들이 사고력이 빈약하다는 점을 공통의 출발점으로 삼고 있지만, 사고의 전개 과정 중 어느 단계나 맥락에서 문제가 있는지는 구체적으로 밝히지 않은 채 논의를 전개해 온 것 같다. 이에 본 논문은 사회과 지리 학습에서 우리나라 초·중·고 학생들이 지리적 사고 과정에서 보여주는 특성과 문제점을 구체적, 실증적으로 밝히고자 SOLO식 순차적 평가 과제를 개발하여 학생들에게 제시한 다음, 그 반응을 분석한 사례 연구이다. 분석 결과, 학생들은 단일 및 다중 정보의 파악 면에서는 탁월한 능력을 보였지만, 지식의 질적 전환이 이루어지는 '관계화 단계'에서는 정답률이 현저하게 저하되고 있었다. 즉, 정도의 차이는 있으나 거의 모든 학령대에서 지리적 정보간 '관계 짓기'나, 혹은 공간 관계의 파악 과정에서 심각한 어려움을 겪는 특성과 문제점을 확인할 수 있었다. This paper intends to elucidate the problem in thinking process of Korean students through SOLO model-based testlets. They were asked to respond to these testlets. The results are as follows; Most of the Korean students have difficulty for a 'relational stage' at SOLO levels or contexts regardless of school grade. Thus, the researchers who would study these tasks should reinforce the instructions and assessment methods to fill up and deal with the problem. Also geography teaching-learning activities and studies should bring focus into interrelating a geographical concepts, knowledges, and informations; that is, looking for the spatial relationships.

      • KCI등재

        Spatially-offset Fluorescence Spectroscopy (SOFS) Using Ring Illumination and Point Collection for Sub-surface Measurements in Layered Tissue

        Khan Mohammad Khan,Ragesh Kumar,Hemant Krishna,Nirmalya Ghosh,Shovan Kumar Majumder 대한의용생체공학회 2016 Biomedical Engineering Letters (BMEL) Vol.6 No.4

        Purpose To report development of a depth-sensitivefluorescence spectroscopy system based on the configurationof point collection fixed at the centre of an illumination ringon the surface of a target sample. Methods The system makes use of an axicon for convertingthe collimated laser beam into ring shaped illumination andachieves the ability of subsurface interrogation by varyingthe radii of the illumination rings thereby introducing spatialoffsets between collection and illumination. The system wasvalidated on a non-biological phantom comprising a thin tissuepaper overlaying a thick dye card and also on a biologicaltissue sample that was a chicken leg tissue consisting of athin epithelial membrane on the top of a much thicker muscletissue. Results It was found that while the fluorescence spectracorresponding to zero spatial offset are dominated by thefluorescence emission bands associated with the top layer ofthe layered samples, the fluorescence spectra measured withspatial offsets contain higher contribution of fluorescencesignatures characteristics of the bottom layer as compared tothe spectrum measured with the zero offset. Conclusions The SOFS based system is able to interrogatesubsurface depths beyond the reach of the conventionalconfocal fluorescence by simply varying the position ofthe axicon in the excitation arm. Due to the simplicity inits instrumentation, the approach has the potential to bedeveloped into a portable system for in situ measurement ina tissue.

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