• 대한기계학회논문집B
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• 제32권6호
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• pp.454-462
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• 2008

Since operating conditions are significantly different for heating and cooling mode operations in a $CO_2$ heat pump system, it is difficult to optimize the performance of the $CO_2$ cycle. In addition, the performance of a $CO_2$ heat pump is very sensitive to outdoor temperature and gascooler pressure. In this study, the cooling and heating performances of a variable speed $CO_2$ heat pump with a twin-rotary compressor were measured and analyzed with the variations of EEV opening and compressor frequency. As a result, the cooling and heating COPs were 2.3 and 3.0, respectively, when the EEV opening was 22%. When the optimal EEV openings for heating and cooling were 28% and 16%, the cooling and heating COPs increased by 3.3% and 3.9%, respectively, over the COPs at the EEV opening of 22%. Beside, the heating performance was more sensitive to EEV opening than the cooling performance. As the compressor speed decreased by 5 Hz, the cooling COP increased by 2%, while the heating COP decreased by 8%.

• 토지주택연구
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• 제9권4호
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• pp.25-32
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• 2018

Application of geothermal energy in buildings has been gaining popularity as it provides the benefits of both heating and cooling a building. Among the various types of geothermal energy systems, ground-coupled heat pump system is the most commonly applied one in South Korea. A ground heat exchanger plays an important role as a heat source in winter and a heat sink in summer. For the stable operation of a ground-coupled heat pump system, a ground heat exchanger should be sized so that it provides sufficient heating and cooling energy. Heating and cooling energies generated in ground heat exchangers mainly depend on the temperature difference between the heating medium in ground heat exchangers and the surrounding ground. In addition, the performance of ground heat exchangers influences the change in ground temperature. Therefore, it is necessary to consider this interrelation between the change in the ground temperature and the performance of ground heat exchanger for an accurate estimation of its performance. However, previous thermal analysis models for ground heat exchangers are not competent enough to allow a complete understanding of this interrelation. Therefore, this study proposes a three-dimensional equivalent, transient ground heat exchanger analysis model. First, a previous thermal analysis model for ground heat exchangers, including an analytical model, a g-function, and a numerical model are analyzed. Next, to overcome the limitations of the previous models, a three-dimensional equivalent, transient ground heat exchanger model is proposed. Finally, this study validated the proposed model with the measurement data of the thermal response test, sandbox test, and TRNSYS DST model. All validation results showed a good agreement. These findings helped us to investigate the thermal performance of ground heat exchangers more accurately than the analytical models, and faster than the numerical models. Furthermore, the proposed model contributes to the design of ground heat exchangers by considering the different operation conditions of buildings.

• 설비공학논문집
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• 제19권7호
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• pp.529-536
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• 2007

The perforated type element for a heat recovery ventilation system has been studied to improve the performance. Four holes of diameter of 6mm are punched out for each flow channel to break the boundary layer development and increase the turbulence. KS cooling and heating conditions and test procedures are applied for study. The efficiencies are compared to those of the typical element with smooth surface. For cooling operations, the temperature, latent and enthalpy efficiencies increase 2.5%, 18% and 8%, respectively. For heating operations, the temperature, latent and enthalpy efficiencies increase 3%, 5% and 3.2%, respectively.

• 한국수소및신에너지학회논문집
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• 제30권4호
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• pp.369-375
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• 2019

Air conditioners adopting high energy efficiency and low global warming potential (GWP) refrigerant are being developed globally. In case of commercial air conditioner, R32 with lower GWP than R410A is attracting industrial attentions. In this study, the performance of water-to-water geothermal heat pump adopting R32 is compared with that of the heat pump using R410A. By using R32 instead of R410A, heating capacity and COP in heating standard condition can be increased by 8.8% and 6.3% respectively. Also, cooling capacity and COP in cooling standard condition can be increased by 5.3% and 3.1% respectively by using R32. But in case of using R32, systemic improvements are required to decrease discharge temperature of compressor since discharge temperature rises abnormally in heating operations.

• 신재생에너지
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• 제16권4호
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• pp.41-48
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• 2020

This study performed a single hole operation method using a balancing well-cross-mixed underground heat exchanger, and conducted thermal performance studies of an SCW-type underground heat exchanger using a two-well. The study attempted to change the existing operating method of the two adjacent SCW underground heat exchangers with one ball each. The SCW-type geothermal heat exchanger is considered to enable up to 20% of bleed discharge at maximum load, which makes groundwater usage unequal. The efficiency factor of the geothermal system was improved by constructing the discharged water by cross-mixing two balancing wells to prevent the discharge of groundwater sources and keep the temperature of the underground heat exchanger constant. As a result of the cooling and heating operation with the existing SCW heat exchange system and the balancing well-cross-mixed heat exchange system, the measured performance coefficient improved by 23% and 12% in cooling and heating operations, respectively. In addition, when operating with a balanced cross-mixing heat exchange system, it has been confirmed that the initial basement temperature is constant with a standard deviation of 0.08 to 0.12℃.

• 한국식품과학회지
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• 제11권2호
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• pp.86-92
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• 1979

시판(市販) 식용대두유(食用大豆油)에 항산화제(抗酸化劑) BHA, BHT, PG를 200 ppm씩 첨가(添加)하여 $150{\pm}2^{\circ}C$에서 30분간(分間) 가열(加熱)한 후 23.5시간(時間) 방냉(放冷)하는 조작(操作)을 6회(回) 반복(反復)하였다. 그리고 이 반복과정(反復過程)에 따르는 항산화제(抗酸化劑)의 잔존율(殘存率)의 변화(變化)와 기질(基質)의 일부(一部) 물리적(物理的), 화학적성질(化學的性質)의 변화(變化)를 조사(調査)하였다. 그 결과(結果)는 아래와 같다. 1. 반복가열조작(反復加熱操作)의 회수(回數)에 따르는 항산화제(抗酸化劑)의 잔존율(殘存率)은 2회(回 )의 경우 BHA 60.5%, BHT 55.7%, PG 40.5%로써 PG는 초기농도(初期濃度)의 1/2이상(以上)이 손실(損失)되었다. 한편, 6회(回)의 경우, BHA 5.7%, BHT 3.0%, PG 1.8%로써 첨가(添加)된 항산화제(抗酸化劑)의 거의 전부(全部)가 손실(損失)되었다. 2. 반복가열조작(反復加熱操作)에 따르는 control을 포함하는 모든 기질(基質)의 굴절율(屈折率), 점도(粘度)의 변화(變化)는 초기(初期)에는 그 증가(增加)가 원만했으나 4회이후(回以後)에는 모두 급격(急激)히 증가(增加)했다. 한편 color index는 초기(初期)에는 급격(急激)히 증가(增加)했으나 2회이후(回以後)에 는 그 증가속도(增加速度)는 상당(相當)히 감소(減少)되었다. 3. 또한 항산화제(抗酸化劑)의 잔존율(殘存率)이 감소(減少)함에 따라 기질(基質)에 대한 각항산화제(各抗酸化劑)의 산패억제작용(酸敗抑制作用)도 급격(急激)히 저하(低下)되었다. 즉, 6회(回) 반복가열(反復加熱)후에는 각항산화제(各抗酸化劑)가 첨가(添加)되었든 기질(基質)들의 과산화물가(過酸化物價), TBA가(價), 산가(酸價)는 control의 그것과 별차(差)가 없었다.

• 한국군사과학기술학회지
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• 제17권4호
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• pp.413-421
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• 2014

Since the structural temperature of a flight vehicle flying at high speed rises rapidly due to aerodynamic heating, it is necessary for optimum structural design to obtain proper material properties at high temperature by taking into account of its operational environment. For a special alloy, analysis data on strength change due to exposure time to high temperature are very limited, and most of them are for an exposure time longer than 30 minutes for long term operations. In this study, base and weld metal samples of Ti-6Al-4V alloy had been prepared and high temperature tensile tests with induction heating were performed, and then high temperature strength characteristics and strength recovery characteristics through cooling have been analyzed. Pre-tests to determine maximum heating rate were performed, and response characteristics for temperature control were confirmed. As a result, high temperature tensile strength appeared to be lower than that of room temperature, but it was higher than that of high temperature of 30 minite exposure listed in MMPDS. In strength recovery through cooling Ti-6Al-4V alloy has shown higher recovery rate compared with other alloys.

• 대한기계학회논문집B
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• 제21권10호
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• pp.1262-1272
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• 1997

An experimental study on the frequency control of an inverter heat pump to get the desired indoor room temperature has been conducted for the performance characteristics during the steady, 4, 8, and 16 step frequency operations. The heat pump model used in this study was operated to meet the experimental conditions of ASHRAE standard. The performance of the system was tested by measuring the temperature and pressure of the refrigerant, and cooling capacity, power consumption, etc. of the system. As the controlling frequency steps increased, the running time of the compressor increased as well as the electric consumption of the system and the cooling energy due to the wall heating load. However, the average cooling COP was improved.

• Tribology and Lubricants
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• 제15권2호
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• pp.199-205
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• 1999

Using a coupled thermal-mechanical analysis, the thermal distortion of the ventilated disk brakes has been investigated based on the air cooling effects during 15 braking operations. The FEM results show that the bendings and distortions of the disk toward the left side are decreased, but the sinusoidal distortion of the disk rubbing surface along the arc length of the vent hole is highly increased by increasing the convective air cooling effects, which is heavily related to the squeal, wear and micro-thermal crackings at the rubbing surfaces due to uneven dissipation rates of friction heatings.

• 대한설비공학회지:설비저널
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• 제17권6호
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• pp.696-707
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• 1988

This study simulates a typical solar system using the transient simulation program TRNSYS, and calculates the maximum cooling load of the model room of $50m^2$. In this study, energy rate control method is used in calculating a maximum cooling load. On the ground of the maximum cooling load of the model room, the variables that have an effect on the solar collection performance of the solar system are made a selection. Also in this study the trend of the solar collection performance is shown as the variables change. The results show that the variables which have an effect on the collection performance are collector area, collector mass flow rate, collector slope and the volume of storage tank, and the optimal value of Ac/Vt is not constant but varies as the collector area and the collector mass flow rate. Also the results show that for cooling system the optimal value of the collector slope is latitude minus $15^{\circ}$ during the seasonal operations, and twenty percent of the maximum cooling load is saved with the aid of the solar energy.