• Journal of Advanced Marine Engineering and Technology
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• 제34권2호
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• pp.259-266
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• 2010

다중관식 헬리컬 코일형 가스냉각기내 $CO_2$의 열유량과 압력강하는 LMTD 방식을 이용하여 예측하였고 그 결과를 실험값과 비교하였다. $CO_2$와 냉각수의 유량은 각각 0.06~0.075 kg/s이고, 가스냉각기의 냉각압력은 8~10 MPa이다. 초임계 $CO_2$의 냉각시의 열유량과 압력강하는 LMTD 방식을 이용하여 예측하였고, 이때 냉매측 열전달과 압력강하식은 각각 Gnielinski와 Dittus-Boelter 식을 사용하였다. LMTD법으로 예측한 값과 실험값을 비교한 결과, $CO_2$의 열유량과 압력강하는 상대적으로 좋은 일치를 보였다.

• 설비공학논문집
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• 제13권11호
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• pp.1049-1058
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• 2001

The performance and design analysis for a NWD cooling tower using a combined wet and dry type fill are numerically investigated and compared with the experimental results. The Stoecker's method is applied to the wet section and LMTD or NTU-Effectiveness method to the wet and dry sections. The efficiency ratio of the NWD cooling tower to a wet type crossflow cooling tower is 59.34%. The predicted result shows a good agreement with the experimental data within 1.4% error. Plume abatement is far better with a NWD cooling tower than a counterflow cooling tower. It costs less than a conventional wet/dry tower because the finned exchanger is eliminated. This method also leaves out complexity in structure and Intricacy in operation.

• 설비공학논문집
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• 제8권2호
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• pp.299-306
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• 1996

This work presents an analytical model, so called modified LMTD method, to predict the thermal performance of finned-tube heat exchanger under frosting conditions. In this model, the total heat transfer coefficient and effective thermal conductivity of the frost layer were defined as a function of frost surface temperature. The surface temperature of the frost layer formed on the heat exchanger was calculated through the analysis of the heat and mass transfer process in the air and frost layer. To examine the validity of this analytical model, the computed results from the present model, such as heat transfer rate, frost mass and thickness of frost, were compared with the ones of the expermental work and LMED method.

• 한국산학기술학회논문지
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• 제14권2호
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• pp.517-522
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• 2013

아이스크림제조기는 원액통과 냉각통의 증발온도가 $4^{\circ}C$와 $-8^{\circ}C$로 각각 다르므로 다수의 모세관을 이용하는데 이 경우 서로 냉매 유량에 영향을 주므로 냉동사이클 설계가 복잡하다. 본 연구에서는 증발온도가 서로 다른 두개의 증발기(냉장 및 냉동)가 장착된 소프트 아이스크림 제조기의 성능을 해석하였다. 압축기 해석에는 효율모델을 적용하였고, 모세관은 1차원 유동으로 간주하여 해석하였으며, 응축기와 증발기는 UA-LMTD법을 적용하여 해석하였다. 요소부품에서의 엔탈피, 압력 및 질량의 평형을 적용하여 냉동사이클 시뮬레이션 프로그램을 개발하였고 R404A를 사용하는 아이스크림 제조기에 적용한 결과, 냉장과 냉동 증발기의 온도를 $3^{\circ}C$ 이내로 예측하여 측정치와 대체로 일치하였다.

• 설비공학논문집
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• 제26권9호
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• pp.401-408
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• 2014

This paper proposes an optimum operation method for open type ground heat exchangers. A series of TRTs and artificial heating/cooling operations were carried out while monitoring temperature in the hole of SCW. The ground temperature naturally increases with depth, but a switch between the cooling/heating mode results in a change in the distribution of ground temperature. The effect of the mode change was evaluated by performing LMTD and COMSOL multiphysics analysis for a reduced model with the depth of 150 m. As a result, in the cooling mode, the upstream operation is more efficient than the downstream operation and reduces EWT by $2.26^{\circ}C$. On the other hand, in the heating mode, the downstream operation is advantageous over the upstream operation and increases EWT by $3.19^{\circ}C$. The merit of the optimum operation will be enhanced for the typical dimension of SCW with a depth of 400~500 m. In the future, an open type ground heat exchanger system adopting the optimum operation with variation in the ground temperature will be used in practice.

• 태양에너지
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• 제17권4호
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• pp.23-33
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• 1997

본 연구에서는 Rankine 사이클을 이용한 해양 온도차 발전 시스템을 컴퓨터로 모사했고 작동유체를 변화시켜 가면서 그 성능을 비교하였다. 증발기와 응축기는 UA(총괄열전달계수$\times$전열면적)와 LMTD(대수평균온도차) 방법으로 모사하였으며 터빈과 펌프는 등엔트로피 효율로 모사하였다. 작동유체로는 R22, R290, R1270, R134a, R125, R143a, R32, R410A 그리고 암모니아 등을 사용하였다. 모사 결과 OTEC 발전 시스템의 효율은 작동유체에 상관없이 거의 일정한 것으로 드러났다. 한편 증발기 출구에서의 과열도와 응축기 출구에서의 과냉도는 시스템의 성능에 거의 영향을 미치지 않는 것으로 나타났지만 터빈의 효율과 열교환기의 크기는 시스템 성능에 큰 영향을 미치는 것으로 나타났다. 마지막으로 따뜻한 표층수와 차가운 심해수 사이의 온도차가 $20^{\circ}C$이상일 때 실제로 전기를 생산할 수 있다는 사실을 확인했다.

• 한국신재생에너지학회:학술대회논문집
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• 한국신재생에너지학회 2008년도 춘계학술대회 논문집
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• pp.626-629
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• 2008

In this study, a geothermal power generation system using the Kalina cycle was investigated by the simulation method. The Kalina cycle system can be used for the utilization of a low-temperature heat sources such as geothermal and industrial waste heat that are not hot enough to produce steam. The sea/river water can be considered as a cooling media. A steady-state simulation model was developed to analyze and optimize its performance. The model contains a turbine, a pump, an expansion valve and heat exchangers. The turbine and pump were modelled by an isentropic efficiency, while a condenser, an evaporator and a regenerative heat exchanger were modeled by UA-LMTD method with a counter-flow assumption. The effect of the ammonia fraction at the separator inlet on the cycle performance is investigated in detail.

• 한국수소및신에너지학회논문집
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• 제32권1호
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• pp.63-67
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• 2021

Accurate measurements of the heat transfer coefficients of concentric annular space for the test section is important to measure the tube-side heat transfer coefficients of working fluids. This paper presents the annular side heat transfer coefficients of concentric annuli with variation of tube diameter ratios using Wilson plot method. The test facility has a straight, horizontal test section with an active length of 3.0 m. Inner/outer diameters of test tubes are 7.0/7.5 and 8.0/8.56 mm, respectively. An outer diameter of annulus side is 16.0 mm. The test results show that convective heat transfer coefficients in annuli increase with annular diameter ratio. The correlations for convective heat transfer coefficients in annuli are also developed.

• 대한설비공학회지:설비저널
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• 제10권4호
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• pp.297-303
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• 1981

Optimal design of shell and tube heat exchanger system with the working fluids which may condense outside the tubes has been carried out under specified inlet and outlet conditions. Independent variables such as number of parallel series, tube diameter, distribution pitch, tube side pressure loss, baffle cut and shell side pressure loss as well as dependent variables such as shell diameter, number of tubes, number of serial series and number of baffles were all characterized according to the standard. Exhaustive search method was used to construct a computer program together with the calculation of heat transfer rate by LMTD method. stress analysis of maj or parts was made to examine their dimensions satisfying heat transfer and pressure loss requirements. Cost estimation based on the installation, operation and maintenance was also made, A few representative variables, heat transfer area, shell diameter and pressure loss, were used to express cost function, finally giving the optimal selection of all tentative solutions.

• 대한설비공학회:학술대회논문집
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• 대한설비공학회 2008년도 하계학술발표대회 논문집
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• pp.782-787
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• 2008

The Kalina cycle simulation study was carried out for a preliminary design of a geothermal power generation system. The Kalina cycle system can be used for the utilization of a low-temperature heat sources such as geothermal and industrial waste heat that are not hot enough to produce steam. The sea/river water can be considered as a cooling media. A steady-state simulation model was developed to analyze and optimize its performance. The model contains a turbine, a pump, an expansion valve and heat exchangers. The turbine and pump were modelled by an isentropic efficiency, while a condenser, an evaporator and a regenerative heat exchanger were modeled by UA-LMTD method with a counter-flow assumption. The simulation results show that the power generation efficiency over 10% is expected when a heat source and sink inlet temperatures are $100^{\circ}C$ and $10^{\circ}C$ respectively.