Heat transfer scaling analysis of the single-phase natural circulation flow system

Yu, Xin-Guo,Choi, Ki-Yong Elsevier 2017 Applied thermal engineering Vol.126 No.-

<P><B>Abstract</B></P> <P>In this study, the heating power in the heat source or the core decay power is proposed as the reference parameter for the scaling analyses. From the scaling analyses assuming quasi-steady state flow, three system-level dimensionless numbers for the thermal heat transfer were obtained: the dimensionless power rate number, the dimensionless HX heat transfer number, and the dimensionless heat loss number. Based on the three local heat transfer processes, the dimensionless HX heat transfer number is further broken down into three dimensionless local heat transfer numbers: the dimensionless HX tube internal convective heat transfer number, the dimensionless HX tube conductive heat transfer number, and the dimensionless HX tube external convective heat transfer number, with each representing local heat transfer similarity. Analogically, three dimensionless local heat loss numbers were also derived. With the obtained dimensionless numbers, the scaling criteria for exact HX heat transfer similarity were derived. It was found the tube inner diameter should be scaled down in terms of the tube length scaling ratio for exact heat transfer similarity. In addition, three sets of scaling criteria for a full-pressure model, the new scaling criteria (general), the new scaling criteria (ideal), and the new scaling criteria (practical), were proposed.</P> <P><B>Highlights</B></P> <P> <UL> <LI> A set of system-level and local heat transfer similarity numbers were proposed. </LI> <LI> The proposed dimensionless numbers can be applied to analyze the heat transfer similarity. </LI> <LI> Three set of scaling criteria for the design of reduced-size thermal systems were proposed. </LI> <LI> The tube inner diameter was found to be related to the length scaling ratio by <SUB> d iR </SUB> = l ¯ R 0.75 . </LI> </UL> </P>

Numerical study on heat transfer characteristics in branch tube type ground heat exchanger

Choi, Hoon Ki,Yoo, Geun Jong,Pak, Jae Hun,Lee, Chang Hee Elsevier 2018 Renewable energy Vol.115 No.-

<P><B>Abstract</B></P> <P>A ground heat exchanger is an essential component of ground source heat pumps, which saves cooling and heating energy effectively. In this study, a branch tube type heat exchanger is suggested for a ground heat exchanger instead of commonly using the U-tube type for increasing heat transfer efficiency. The branch tube type heat exchanger has conjugate heat transfer including convective heat transfer between the walls of primary- and branch-tubes and internally circulating fluid, and conduction in the tube walls and grout. This conjugate heat transfer phenomenon is analyzed by steady 3-D numerical analysis using the finite volume method. In the analysis, temperature distribution, rate of heat transfer and pressure drop are compared for the branch tube type ground heat exchanger with 2, 4, 6, and 8 branch tubes together with a single U-tube type ground heat exchanger. Generally, the branch tube type shows better heat transfer performance compared to the U-tube type and the higher number of branch tubes give better heat transfer performance. Also, the branch tube type yields higher pressure drop than the U-tube type for the same mass flow rate of circulating fluid in the heat exchanger.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Branch tube type ground heat exchanger is proposed for enhanced heat transfer. </LI> <LI> Branch tube type ground heat exchanger has increased heat transfer area. </LI> <LI> The increased heat transfer rate is found in the out-flow region. </LI> <LI> Heat transfer rate is affected by number of branch tubes more than inlet velocity. </LI> </UL> </P>

공기조화, 냉동 분야의 최근 연구 동향

한화택(Hwataik Han),신동신(Dong Sin Shin),최창호(Chang-Ho Choi),이대영(Dae-Young Lee),김서영(Seo Young Kim),권용일(Yong-Il Kwon) 대한설비공학회 2008 설비공학 논문집 Vol.20 No.6

A review on the papers published in the Korean Journal of Air-Conditioning and Refrigeration Engineering in 2006 has been accomplished. Focus has been put on current status of research in the aspect of heating, cooling, ventilation, sanitation and building environments. The conclusions are as follows. (1) The research trends of fluid engineering have been surveyed as groups of general fluid flow, fluid machinery and piping, etc. New research topics include micro heat exchanger and siphon cooling device using nano-fluid. Traditional CFD and flow visualization methods were still popular and widely used in research and development. Studies about diffusers and com￢pressors were performed in fluid machinery. Characteristics of flow and heat transfer and pip￢ing optimization were studied in piping systems. (2) The papers on heat transfer have been categorized into heat transfer characteristics, heat exchangers, heat pipes, and two-phase heat transfer. The topics on heat transfer characteristics in general include thermal transport in a cryo-charnber, a LCD panel, a dryer, and heat generating electronics. Heat exchangers investigated include pin-tube type, plate type, ventilation air-to-air type, and heat transfer enhancing tubes. The research on a reversible loop heat pipe, the influence of NCG charging mass on heat transport capacity, and the chilling start-up characteristics in a heat pipe were reported. In two-phase heat transfer area, the studies on frost growth, ice slurry formation and liquid spray cooling were presented. The studies on the boiling of R-290 and the application of carbon nanotubes to enhance boiling were noticeable in this research area. (3) Many studies on refrigeration and air conditioning systems were presented on the practical issues of the performance and reliability enhancement. The air conditioning system with multi indoor units caught attention in several research works. The issues on the refrigerant charge and the control algorithm were treated. The systems with alternative refrigerants were also studied. Carbon dioxide, hydrocarbons and their mixtures were considered and the heat transfer correlations were proposed. (4) Due to high oil prices, energy consumption have been attentioned in mechanical building systems. Research works have been reviewed in this field by grouping into the research on heat and cold sources, air conditioning and cleaning research, ventilation and fire research including tunnel ventilation, and piping system research. The papers involve the promotion of efficient or effective use of energy, which helps to save energy and results in reduced environmental pollution and operating cost. (5) Studies on indoor air quality took a great portion in the field of building environments. Various other subjects such as indoor thermal comfort were also investigated through computer simulation, case study, and field experiment. Studies on energy include not only optimization study and economic analysis of building equipments but also usability of renewable energy in geothermal and solar systems.

주기적인 채널형상을 갖는 고밀도 열교환기의 열유동 성능해석

김민환(M.H. Kim),이원재(W.J. Lee) 한국전산유체공학회 2007 한국전산유체공학회지 Vol.12 No.3

A periodic CFD approach for the performance analysis of compact high temperature heat exchangers is introduced and applied to selected benchmark problems, which are a fully developed 2D laminar heat transfer, a conjugate heat transfer between parallel plates which have exact solutions, and a heat transfer in a real high temperature heat exchanger module. The results for the 2D laminar heat transfer and the 2D conjugate heat transfer showed a very good agreement with the exact solutions. For the high temperature heat exchanger module, the pressure drops were predicted well but some difference was observed in the temperature parameters when compared to the full channel CFD analysis due to assumptions introduced into the periodic approach. Considering its assumptions and simplicities, however, the results showed that the periodic approach provides physically reasonable results and it is sufficient to predict the performance of a heat exchanger within an engineering margin and with much less CPU time than the case of a full channel analysis.

태양광발전 전력변환장치용 GaN-FET의 방열효율 향상을 위한 PCB 방열구조 열해석

이천규(Lee Cheonkyu),정효재(Jeong Hyo Jae) 한국태양에너지학회 2022 한국태양에너지학회 논문집 Vol.42 No.6

In 2021, the solar power generation capacity of South Korea was 14.6 GW, accounting for 71% of the total renewable energy generation capacity. The market for photovoltaic power conversion devices with high efficiency and power density is expanding rapidly in the country. Gallium Nitride (GaN), a compound semiconductor with a wide band gap, can be used under high voltage and current compared to silicon materials, resulting in its wide use in photovoltaic power conversion devices. Excellent heat dissipation performance of GaN-based devices should be achieved due to their significantly high power and efficiency. In this study, the heat dissipation characteristics of a PCB with FR4 (Flame Retardant 4) material composed of various heat dissipation structures for the effective heat dissipation of GaN-FETs for power conversion systems applicable to a 3 kW solar power generation system were investigated using thermal analysis. Part of the heat generated from the GaN-FET was directly dissipated on its surfaces to the external air, and the other part was transferred to the PCB through conduction and then discharged to the external air through convection on the surfaces of the PCB package. Via holes were considered to improve the heat transfer rate of the PCB in the thickness direction, and a heat sink was applied to expand the heat transfer area. In this study, the heat dissipation characteristics were investigated based on four types of heat dissipation structures of the PCB using the ANSYS transient thermal analysis program. The effect of via holes exposed to external air were analyzed using two types of heat dissipation structures. In addition, two other types of heat dissipation structures were analyzed to compare the top and top/bottom cooling of the PCB package. It was observed that the efficiencies of heat dissipation of the via holes that were unexposed to external air and in both the top and bottom cooling cases were more advantageous than that of the exposed holes and the top cooling case, respectively.

Subchannel Analysis of PWR Single Fuel Assembly Using COBRA-SFS

Ju-Chan Lee,Doyun Kim,Kyung-Sik Bang,Seunghwan Yu 한국방사성폐기물학회 2022 한국방사성폐기물학회 학술논문요약집 Vol.20 No.1

Detailed temperature distributions of the spent fuel are required to evaluate the long-term integrity of the dry storage system. In this study, a subchannel analysis method was established to obtain the detailed temperatures of a spent fuel using the COBRA-SFS code. The SAHTT (Single Assembly Heat Transfer Test) model was selected as the subchannel analysis. It was developed at the PNL to investigate heat transfer characteristics of spent PWR fuel under dry storage conditions. The SAHTT has a 15×15 rod array with simulated rods 0.42 in. (10.7 mm) in diameter. Control rod thimbles were modeled with unheated rods. The COBRA-SFS input consists a detailed subchannel model with 256 subchannels, 225 rods, and 8 slab nodes. The heat generation rate was axially uniform with total power of 1.0 kW. Subchannel analyses were performed for the vertical orientation under three different backfills of air, helium, and vacuum. For the vacuum backfill, the peak temperature was the highest and temperature gradients the sharpest only due to the radiation heat transfer effect. For the helium backfill, peak temperature was lowest and the axial profiles flattest due to the higher conductivity and lower density of helium. Subchannel analyses were also performed to evaluate the effect of thermal parameters such as surface emissivity, convective heat transfer coefficients, and flow resistance coefficients on the PCT (Peak Cladding Temperature). The PCT was affected by the emissivity of the fuel rod and the basket, and in particular, the basket emissivity had a greater effect. The PCT was affected by the Nusselt number, but the range of the Nusselt number is around 3.66. Therefore, the effect of the Nusselt number on the PCT will not be significant. As a result of the analysis according to the flow resistance coefficients, the PCT was affected by the wall friction factor, but the loss coefficients from the space grid had little effect. Subchannel technique obtained from this work can be used to predict the detailed temperature distributions of spent fuel assembly.

Transient 해석에 의한 열응력 및 변형 해석 기법에 관한 연구

이상훈(Sang-Hoon Lee),정일중(Il-Jung Jeong),김동은(Dong-Eun Kim),한국남(Goog-Nam Han),이석순(Seok-Soon Lee),황영진(Young-Jin Hwang) 대한기계학회 2007 대한기계학회 춘추학술대회 Vol.2007 No.10

This study is a deformation analysis by transferring heat imposed on a duct as example especially using transient analysis method. The heat transfer analysis is performed in order to obtain the temperature distribution data which were used in thermal analysis. The thermal stress analysis is performed to verify stability of the excavator rotation part body On the other hand the thermal stress analysis is performed to look into the failure and the deformation behavior of structure by thermal expansion. The finite element analysis for 3D model is performed by ABAQUS/Standard and heat transfer analysis is performed by STAR-CD V3.22.

ANSYS를 이용한 화재 후 온도에 따른 합성보 성능에 관한 해석적 연구

곽성신(Sung-Shin Kwak),최병정(Byong-Jeong Choi) 한국산학기술학회 2019 한국산학기술학회논문지 Vol.20 No.1

화재발생 시 구조물의 부재는 온도상승에 의하여 본래의 강성을 잃게 되어 하중 지지력이 감소하게 된다. 구조 부재에 급격한 내력 상실은 구조물의 붕괴 및 인명 피해로 이어질 수 있다. 구조물의 화재 시간이 길어지게 되면 구조 부재에 대한 내력 상실은 더욱 증가하게 된다. 건축물의 붕괴를 방지하기 위하여 부재 손상 여부의 파악은 매우 중요한 과제이나 국내에서 부재의 잔존내력을 진단하고 평가하는 지침이 될 만한 자료는 미비한 실정이다. 따라서 본 연구는 구조 부재 중합성구조인 합성보를 유한요소해석을 통하여 내화성능을 분석하고자 한다. 합성보 모델링은 한국전력기술의 협조를 받아 원자력 발전소 보조건물(Electrical Penetration Room, EPR)에 사용되는 도면을 바탕으로 수행하였다. 유한요소해석 프로그램인 ANSYS를 이용하여 정해석 단계의 열전달 해석과 구조해석을 수행하였다. 열전달 해석 결과로 얻어진 온도분포를 데이터로 확보하여 구조해석에 열전달해석 결과를 반영시켰다. 잔존성능을 분석하기 위하여 합성보의 온도분포와 열 영향을 받은 구조해석의 최대변위 결과를 도출하여 실험 결과 데이터와 구조해석 결과 데이터를 비교분석하였다. In the case of fire, a structure loses its original stiffness due to the temperature rise, and the load bearing capacity decreases. The loss of structural strength increases with increasing fire time of the structure. To prevent the collapse of buildings, it is very important to understand whether or not the members are damaged. On the other hand, there is insufficient data to be a guideline for diagnosing and evaluating the residual strength of the members in Korea. Therefore, this study examined the resistance performance by Finite-Element-Analysis of composite beams, which are composite structures among structural members. Composite beam modeling was carried out based on the model used in the Electrical Penetration Room (EPR) in cooperation with KEPCO. The heat transfer analysis and structural analysis of the critical phase were performed using ANSYS, a finite element analysis program. ANSYS was used to perform heat transfer analysis and structural analysis at the static analysis. To analyze the residual performance, the temperature distribution of the composite beam and the maximum displacement result of the heat-affected structure analysis were derived and the experimental data and the structural analysis result data were compared and analyzed.

Performance of a Shell-and-Tube Heat Exchanger with Spiral Baffle Plates

손영석,신지영,Son, Yeong-Seok,Sin, Ji-Yeong 대한기계학회 2001 KSME International Journal Vol.15 No.11

In a conventional shell-and-tube heat exchanger, fluid contacts with tubes flowing up and down in a shell, therefore there is a defect in the heat transfer with tubes due to the stagnation portions . Fins are attached to the tubes in order to increase heat transfer efficiency, but there exists a limit. Therefore, it is necessary to improve heat exchanger performance by changing the fluid flow in the shell. In this study, a highly efficient shell-and-tube heat exchanger with spiral baffle plates is simulated three-dimensionally using a commercial thermal-fluid analysis code, CFX4.2. In this type of heat exchanger, fluid contacts with tubes flowing rotationally in the shell. It could improve heat exchanger performance considerably because stagnation portions in the shell could be removed. It is proved that the shell-and-tube heat exchanger with spiral baffle plates is superior to the conventional heat exchanger in terms of heat transfer.

냉각탑 공기가열기용 전열판의 수치해석적 성능 비교

이을종(Lee, Eul-Jong),김정식(Kim, Jeong-Sig),김내현(Kim, Nae-Hyun) 한국산학기술학회 2012 한국산학기술학회논문지 Vol.13 No.12

본 연구에서는 백연방지용 냉각탑의 공기가열기 적용을 목적으로 현재 냉각탑의 충진재로 적용되고 있는 쉐브론형, 웨이브형 그리고 딤플형 전열판 형상에 대한 수치해석을 수행하였다. 해석 결과 동일 소비동력 대비 전열 량은 평판보다 고성능 전열판에서 높게 나타났다. 열전달계수는 웨이브 형상에서 가장 크고 쉐브론 형상, 딤플 형상 순으로 나타났다. 웨이브 형상의 경우 교차하는 절곡형상 사이의 왕성한 혼합유동이 열전달 계수를 증진시킨 것으로 판단된다. 마찰계수는 열전달계수와 유사한 경향을 보였다. 하지만 동일 크기 쉬트에서의 전열량 및 압력손실은 쉐브 론형에서 가장 크게 나타났다. 이는 쉐브론 형상의 전열면적이 다른 형상에 비하여 월등히 크기 때문이다. In this study, numerical analysis was performed on three shapes of heat transfer plates (chevron, wave and dimple type), which are currently used as fillers of cooling towers. Results show that heat transfer rates per consumed power were larger for enhanced plates as compared with that of plain plate. Highest heat transfer coefficient was obtained for wave shape followed by chevron and dimple shape. For wave shape, cross corrugations induced significant mixing of fluids, which enhanced the heat transfer. Friction factor yielded a similar trend with the heat transfer coefficient. However, heat transfer rate and pressure drop per sheet was the largest for chevron shape, due to the largest heat transfer area per sheet.