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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.
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.
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.
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.
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.
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.
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.
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.
Vehicular Ad-hoc Networks (VANETs) are comprised of wireless mobile nodes characterized by a randomly changing topology, high mobility, availability of geographic position, and fewer power constraints. Orthogonal Frequency Division Multiplexing (OFDM) is a promising candidate for the physical layer of VANET because of the inherent characteristics of the spectral efficiency and robustness to channel impairments. The susceptibility of OFDM to Inter-Carrier Interference (ICI) is a challenging issue. The high mobility of nodes in VANET causes higher Doppler shifts, which results in ICI in the OFDM system. In this paper, a frequency domain com-btype channel estimation was used to cancel out ICI. The channel frequency response at the pilot tones was estimated using a Least Square (LS) estimator. An efficient interpolation technique is required to estimate the channel at the data tones with low interpolation error. This paper proposes a robust interpolation technique to estimate the channel frequency response at the data subcarriers. The channel induced noise tended to degrade the Bit Error Rate (BER) performance of the system. Parallel concatenated Convolutional codes were used for error correction. At the decoding end, different decoding algorithms were considered for the component decoders of the iterative Turbo decoder. A performance and complexity comparison among the various decoding algorithms was also carried out.