• 대한기계학회논문집B
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• 제21권4호
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• pp.570-578
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• 1997

At design stage of new motor or when taking remedial action of old motor, a lot of information can be obtained from thermal parameters analysis. This study focused on the temperature rise of TEFC induction motor with respect to various thermal parameters. Frame heat transfer had the most important effect on coil temperature rise. But those of air gap and rotor fan had no effect. This fact shows fan action is more important than fin action in the case of rotor fan. Coil temperature can be more decreased by cooling near the heat sources than any other parts from the results of thermal conductivity and loss tests. Variation of cooling air flow rate and motor volume effects on coil temperature were also tested. These tests suggest that improvement of cooling fan performance is important in reducing the coil temperature rise. Thermal equivalent program was verified by comparison of some experimental results.

• 설비공학논문집
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• 제13권12호
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• pp.1266-1274
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• 2001

Heat transfer phenomena during melting process of the phase change material (ice) was studied by numerical analysis and experiments. In a horizontal ice storage tube, the natural convection caused an increase in melting rate. However, the reduction of the heating surface area caused a decrease in melting rate. Therefore, during the melting process of ice in a horizontal cylinder, the reduction of the heating surface area should be considered. Under the same heating wall and initial water temperature condition, the melting rate became higher for $V_s/V_tot/=0.545 \;than \;that\; for\; V_s/V_tot$/=1.00 due to the difference in the reduction of heating surface area. A modified melting model considering the equivalent thermal conductivity of liquid phase and volume reduction was proposed. The results of the model were compared with the measured values and found to be in good agreement.

• Nuclear Engineering and Technology
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• 제47권7호
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• pp.814-826
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• 2015

The most important advantage of nanoparticles is the increased thermal conductivity coefficient and convection heat transfer coefficient so that, as a result of using a 1.5% volume concentration of nanoparticles, the thermal conductivity coefficient would increase by about twice. In this paper, the effects of a nanofluid ($TiO_2$/water) on heat transfer characteristics such as the thermal conductivity coefficient, heat transfer coefficient, fuel clad, and fuel center temperatures in a VVER-1000 nuclear reactor are investigated. To this end, the cell equivalent of a fuel rod and its surrounding coolant fluid were obtained in the hexagonal fuel assembly of a VVER-1000 reactor. Then, a fuel rod was simulated in the hot channel using Computational Fluid Dynamics (CFD) simulation codes and thermohydraulic calculations (maximum fuel temperature, fluid outlet, Minimum Departure from Nucleate Boiling Ratio (MDNBR), etc.) were performed and compared with a VVER-1000 reactor without nanoparticles. One of the most important results of the analysis was that heat transfer and the thermal conductivity coefficient increased, and usage of the nanofluid reduced MDNBR.

• Nuclear Engineering and Technology
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• 제55권3호
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• pp.1140-1151
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• 2023

Fully Ceramic Microencapsulated (FCM) fuel is emerging advanced fuel material for the future nuclear reactors. The fuel pellet in the FCM fuel is composed of matrix and a large number of TRistructural-ISOtopic (TRISO) fuel particles which are randomly dispersed in the SiC matrix. The minimum layer thickness in a TRISO fuel particle is on the order of 10^-5 m, and the length of the FCM pellet is on the order of 10^-2 m. Hence, the heat transfer in the FCM pellet is a multi-scale phenomenon. In this study, three multi-scale heat conduction models including the Multi-region Layered (ML) model, Multi-region Non-layered (MN) model and Homogeneous model for FCM pellet were constructed. In the ML model, the random distributed TRISO fuel particles and coating layers are completely built. While the TRISO fuel particles with coating layers are homogenized in the MN model and the whole fuel pellet is taken as the homogenous material in the Homogeneous model. Taking the results by the ML model as the benchmark, the abilities of the MN model and Homogenous model to predict the maximum and average temperature were discussed. It was found that the MN model and the Homogenous model greatly underestimate the temperature of TRISO fuel particles. The reason is mainly that the conventional equivalent thermal conductivity (ETC) models do not take the internal heat source into account and are not suitable for the TRISO fuel particle. Then the improved ETCs considering internal heat source were derived. With the improved ETCs, the MN model is able to capture the peak temperature as well as the average temperature at a wide range of the linear powers (165 W/cm~ 415 W/cm) and the packing fractions (20%-50%). With the improved ETCs, the Homogenous model is better to predict the average temperature at different linear powers and packing fractions, and able to predict the peak temperature at high packing fractions (45%-50%).

• 한국지반공학회:학술대회논문집
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• 한국지반공학회 2010년도 춘계 학술발표회
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• pp.615-624
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• 2010

In this paper, the applicability of cement grout has been studied as an alternative to bentontite grout to backfill ground heat exchangers. To provide an optimal mixture design, the groutabilty and thermal conductivity of cement grouts with various mixture ratios were experimentally evaluated and compared. The unconfined compression strength of cement grout specimen was measured, which are exposed to cyclic temperature variation ranging from $50^{\circ}C$ to $-5^{\circ}C$. In addition, the integrity of the interface between circulating HDPE pipes and cement grout by performing equivalent hydraulic conductivity tests, in which a pipe locates at the center of the specimen.

• Steel and Composite Structures
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• 제31권5호
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• pp.517-527
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• 2019

To move forward in large steps rather than in small increments, the community would benefit from a systematic and comprehensive database of multi-scale composites and measured properties, driven by comprehensive studies with a full range of types of fiber-reinforced polymers. The multi-scale hierarchy is a promising chemical approach that provides superior performance in synergistically integrated microstructured fibers and nanostructured materials in composite applications. Achieving high-efficiency thermal conductivity and mechanical properties with a simple surface treatment on single-walled carbon nanotubes (SWCNTs) is important for multi-scale composites. The main purpose of the project is to introduce ozone-treated SWCNTs between an epoxy matrix and basalt fibers to improve mechanical properties and thermal conductivity by enhancing dispersion and interfacial adhesion. The obvious advantage of this approach is that it is much more effective than the conventional approach at improving the thermal conductivity and mechanical properties of materials under an equivalent load, and shows particularly significant improvement for high loads. Such an effort could accelerate the conversion of multi-scale composites into high performance materials and provide more rational guidance and fundamental understanding towards realizing the theoretical limits of thermal and mechanical properties.

• 한국지반공학회논문집
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• 제30권4호
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• pp.93-99
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• 2014

최근 들어 경제적인 지열에너지 활용을 위하여 에너지 파일의 적용이 확대되고 있다. 특히 더 높은 열 교환 효율을 확보하고자 에너지 파일의 경우 통상적인 U자형 또는 W자형과 같은 라인형 지중 열교환기가 아닌 코일형 지중 열교환기를 매입하는 경우가 늘어나고 있다. 본 연구에서는 매립지 부지에 PHC 코일형 열교환기 형태의 에너지 파일을 설치하고 240시간 동안 현장 열응답 시험(thermal response test)을 실시하였다. 또한 현장에서 지층별로 시료를 채취하여 실내에서 현장 지층 물성으로 시료를 재조성한 후 비정상 탐침법(non-steady state probe method)을 이용하여 지층별 열전도도와 열확산계수를 측정하였고 등가의 열물성으로 환산하였다. 실험 결과 현장 열응답 시험에 의한 지반의 열전도도와 실내 탐침법으로 측정된 지반의 열전도도는 5%내에서 일치하였으며 아울러 지반의 또 하나의 중요한 설계인자인 등가 열확산계수의 측정방법도 제시하였다.

• 한국지반공학회논문집
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• 제29권10호
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• pp.49-56
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• 2013

최근 들어 경제적이고 친환경적인 에너지 활용을 위하여 지열에너지 필요성이 증대되고 있다. 지반의 열전도도(ground thermal conductivity)와 보어홀 열저항(borehole thermal resistance)은 지열 히트펌프 시스템(geothermal heat pump system)의 설계 과정에서 매우 중요한 변수이다. 본 논문에서는 일반 수직밀폐형에서의 U, W 타입의 지중 열교환기(ground heat exchanger)를 매립지 지반에 설치한 후 100시간 연속 운전 조건으로 현장 열성능 실험(thermal performance test)을 수행하였다. 또한 보어홀 열저항 산정 모델들을 이용하여 열효율을 산정한 후 이를 실험값과 비교하였다. 실험 결과 기존에 주로 적용되고 있는 shape factor(SF) 모델보다 multi-pole과 equivalent diameter(EQD) 모델이 계측값과 잘 일치하였다.

• 한국지반공학회논문집
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• 제29권1호
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• pp.93-107
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• 2013

입상체의 열전도도 산정에 관한 연구는 다공질 매질이나 지반공학에서 다양하게 사용될 수 있다. 입상체의 열전도도 산정은 입자들 사이의 에너지 관계에 대한 모사를 통해 "유효 열전도도"를 획득하는 것으로 발전하였다. 본 연구는 불포화토의 유효 열전도도를 산정하기 위해 3차원 개별 요소법을 이용하여 입자를 생성하고 기존 네트워크 모델을 수정하여 적용하였다. 수정된 네트워크 모델을 검증하기 위해 3가지 다른 크기의 글라스 비즈와 주문진사를 이용하여 실내시험을 통해 흙-수분 특성 곡선과 포화도에 따른 시료의 열전도도를 산정하였다. 수정된 네트워크 모델에서는 흙-수분 특성 곡선을 사용하여 입자 사이의 평균 유효 열전달 실린더 반경을 조정하고 모델에 적용하였다. 일련의 실내시험과 수정된 네트워크 모델을 사용하여 결과를 비교한 결과, 흙-수분 특성 곡선을 적용한 네트워크 모델은 주어진 불포화 조건에서 입상체 시료의 유효 열전도도를 합리적으로 모사할 수 있는 것으로 나타났다. 또한, 포화상태의 유효 열전달 실린더 반경 계수로 정규화한 예측식을 제안하였다. 제안한 예측식을 통해 기존 네트워크 모델을 사용하여 불포화 상태에서 입상체의 유효 열전도도를 산정할 수 있다.

• 대한기계학회논문집A
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• 제22권3호
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• pp.588-594
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• 1998

Reliable three-dimensional models of woven fabric composites had scarcely been proposed for their geometric complexity. Simplified models, mostly one- or two-dimensional, currently used are not considered effective enough because of their oversimplifications. In this paper, the equivalent thermal conductivities and elasticity properties of woven fabric composites are computed using homogenization technique. The computational results show that the strength and thermal conductivity linearly increase with fiber volume fraction and that the variations of undulation of fibers has little effect on equivalent material properties. Homogenization technique is proved useful in the study of woven fabric composites and may find a lot more applications in the area.