• Title/Summary/Keyword: Thermal convection

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Numerical Analysis of Conjugate Heat Transfer for Various Ice-Ball Shapes (다양한 아이스 볼 형상에 대한 복합열전달의 수치해석)

  • Park, Seo Won;Kim, Myoung Soo;Jeon, Byoung Jin;Choi, Hyoung Gwon
    • Transactions of the Korean Society of Mechanical Engineers B
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    • v.40 no.9
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    • pp.605-612
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    • 2016
  • In this study, numerical simulations were conducted for conjugate heat transfer around ice balls in an encapsulated ice thermal storage system. Four shapes of ice balls were modeled; the default one was a sphere, and the other three shapes were designed to enhance convective heat transfer through the ball surface. The flow around the ball was laminar, for which the Reynolds number was 300, and both forced and natural convections inside and outside the balls were considered. The simulations revealed that the magnitude of convective heat transfer for the different shapes decreased in the following order: bone, dimple, hole, and sphere. For the entire simulation, the maximum difference in the average temperatures of water inside the capsules was found to be $0.9^{\circ}C$. Therefore, it can be said that the effect of ice-ball shape on the performance of the ice thermal storage system is significant, considering that more than 0.3 million balls are used in this system.

Heat Transfer Analysis around Transport Cask under Transport Hood (사용후핵연료 운반용기 덮개 내부 열전달 해석)

  • Lee, Dong-Gyu;Park, Jae-Ho;Jung, In-Su;Kim, Tae-Man;Yoon, Jeong-Hyun
    • Journal of Nuclear Fuel Cycle and Waste Technology(JNFCWT)
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    • v.9 no.3
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    • pp.161-167
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    • 2011
  • In case that the maximum temperature of any surface readily accessible during transport of a spent nuclear fuel (SNF) transport cask exceeds $85^{\circ}C$ in the absence of insolation under the ambient temperature of $38^{\circ}C$, personnel barriers or transport hood shall be used to prevent people from casual contact with the transport cask surface. Usually the air temperature within the hood and the hood surface temperature are calculated and further utilized as boundary conditions(free stream temperature and external radiation temperature) for thermal evaluation under normal conditions of transport. In this study, these temperatures are derived using the analytical method based on the heat transfer mechanism around the transport cask under transport hood assuming the thermal equilibrium. By comparing the analytical solutions with the results from the detailed calculations with CFD-computer-code FLUENT 12.1 it is verified that the analytical method is still efficient tool to estimate the temperatures and these temperatures can be further used as boundary conditions for thermal evaluation under normal conditions of transport.

Mass transfer study of double diffusive natural convection in a two-dimensional enclosure during the physical vapor transport of mercurous bromide (Hg2Br2): Part II. Mass transfer (브로민화 수은(I)(Hg2Br2) 물리적 증착공정의 2차원 밀폐공간에서 이중확산 자연 대류에서의 물질전달 연구: Part II. 물질전달)

  • Sung Ho Ha
    • Journal of the Korean Crystal Growth and Crystal Technology
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    • v.33 no.4
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    • pp.145-152
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    • 2023
  • The average Nusselt numbers in the source and crystal region for the variation of thermal Grashof number (Grt) in the range of 2.31 × 104 ≤ Grt ≤ 4.68 × 104 are obtained through numerical simulations. It is shown the average Nusselt number in the crystal region is more than twice as large as the average Nusselt number in the source region. The average Nusselt number in the source region shows an increasing tendency with increasing the thermal Grashof number, Grt, while the average Nusselt number in the crystal region shows a decreasing tendency with increasing thermal Grashof number, Grt. For the variation of the solutal Grashof number (Grs) in the ran ge of 3.28 × 105 ≤ Grs ≤ 4.43 × 105, the average Sherwood number in the source region and crystal region tends to decrease as the solutal Grashof number, Grs increases. The average Sherwood number in the crystal region is about four times greater than the average Sherwood number in the source region.

Recent Progress in Air-Conditioning and Refrigeration Research : A Review of Papers Published in the Korean Journal of Air-Conditioning and Refrigeration Engineering in 2013 (설비공학 분야의 최근 연구 동향 : 2013년 학회지 논문에 대한 종합적 고찰)

  • Lee, Dae-Young;Kim, Sa Ryang;Kim, Hyun-Jung;Kim, Dong-Seon;Park, Jun-Seok;Ihm, Pyeong Chan
    • Korean Journal of Air-Conditioning and Refrigeration Engineering
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    • v.26 no.12
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    • pp.605-619
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    • 2014
  • This article reviews the papers published in the Korean Journal of Air-Conditioning and Refrigeration Engineering during 2013. It is intended to understand the status of current research in the areas of heating, cooling, ventilation, sanitation, and indoor environments of buildings and plant facilities. Conclusions are as follows. (1) The research works on the thermal and fluid engineering have been reviewed as groups of fluid machinery, pipes and relative parts including orifices, dampers and ducts, fuel cells and power plants, cooling and air-conditioning, heat and mass transfer, two phase flow, and the flow around buildings and structures. Research issues dealing with home appliances, flows around buildings, nuclear power plant, and manufacturing processes are newly added in thermal and fluid engineering research area. (2) Research works on heat transfer area have been reviewed in the categories of heat transfer characteristics, pool boiling and condensing heat transfer and industrial heat exchangers. Researches on heat transfer characteristics included the results for general analytical model for desiccant wheels, the effects of water absorption on the thermal conductivity of insulation materials, thermal properties of Octadecane/xGnP shape-stabilized phase change materials and $CO_2$ and $CO_2$-Hydrate mixture, effect of ground source heat pump system, the heat flux meter location for the performance test of a refrigerator vacuum insulation panel, a parallel flow evaporator for a heat pump dryer, the condensation risk assessment of vacuum multi-layer glass and triple glass, optimization of a forced convection type PCM refrigeration module, surface temperature sensor using fluorescent nanoporous thin film. In the area of pool boiling and condensing heat transfer, researches on ammonia inside horizontal smooth small tube, R1234yf on various enhanced surfaces, HFC32/HFC152a on a plain surface, spray cooling up to critical heat flux on a low-fin enhanced surface were actively carried out. In the area of industrial heat exchangers, researches on a fin tube type adsorber, the mass-transfer kinetics of a fin-tube-type adsorption bed, fin-and-tube heat exchangers having sine wave fins and oval tubes, louvered fin heat exchanger were performed. (3) In the field of refrigeration, studies are categorized into three groups namely refrigeration cycle, refrigerant and modeling and control. In the category of refrigeration cycle, studies were focused on the enhancement or optimization of experimental or commercial systems including a R410a VRF(Various Refrigerant Flow) heat pump, a R134a 2-stage screw heat pump and a R134a double-heat source automotive air-conditioner system. In the category of refrigerant, studies were carried out for the application of alternative refrigerants or refrigeration technologies including $CO_2$ water heaters, a R1234yf automotive air-conditioner, a R436b water cooler and a thermoelectric refrigerator. In the category of modeling and control, theoretical and experimental studies were carried out to predict the performance of various thermal and control systems including the long-term energy analysis of a geo-thermal heat pump system coupled to cast-in-place energy piles, the dynamic simulation of a water heater-coupled hybrid heat pump and the numerical simulation of an integral optimum regulating controller for a system heat pump. (4) In building mechanical system research fields, twenty one studies were conducted to achieve effective design of the mechanical systems, and also to maximize the energy efficiency of buildings. The topics of the studies included heating and cooling, HVAC system, ventilation, and renewable energies in the buildings. Proposed designs, performance tests using numerical methods and experiments provide useful information and key data which can improve the energy efficiency of the buildings. (5) The field of architectural environment is mostly focused on indoor environment and building energy. The main researches of indoor environment are related to infiltration, ventilation, leak flow and airtightness performance in residential building. The subjects of building energy are worked on energy saving, operation method and optimum operation of building energy systems. The remained studies are related to the special facility such as cleanroom, internet data center and biosafety laboratory. water supply and drain system, defining standard input variables of BIM (Building Information Modeling) for facility management system, estimating capability and providing operation guidelines of subway station as shelter for refuge and evaluation of pollutant emissions from furniture-like products.

Forced Convection Modelling of a Solar Central Receiver using Nonisothermal Cylinders in Crossflow (비등온 실린더 모델을 이용한 태양로의 강제 대류에 의한 열 손실 분석)

  • Chun, Won-Gee;Jeon, Myung-Seok;Jeon, Hong-Seok;Auh, P. Chung-Moo;Boehn, Robert F.
    • Solar Energy
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    • v.10 no.3
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    • pp.13-18
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    • 1990
  • When nonuniform thermal boundary conditions are imposed on the surface of a circular cylinder in crossflow, the heat transfer characteristics can be quite different compared to what is found for isothermal or constant heat flux boundary conditions. In the present analysis, two kinds of nonuniform boundary conditions along the circumference of the cylinder are considered in a uniform stream of air: step changes and linear profiles. Step changes in temperature can arise on the surface of an external, cylindrical, solar central receiver. As the working fluid(water) flows through the vertical tubes that ring the circumference of Solar One(a solar central receiver in Barstow, California), the solar flux on the receiver heats the water from a liquid to a superheated state. In this process, portions of the receiver panels, and thus portions of the circumference of the cylinder, function as a preheater, boiler, or superheater. Hence the surface temperature can vary significantly around the cylinder. Common engineering practice has been to use an average wall temperature with an isothermal cylinder heat transfer coefficient when estimating the convective loss in these kinds of situations.

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Experimental Study on the Determination of Heat Transfer Coefficient for the KURT (KURT 내 열전달계수 결정에 관한 실험적 연구)

  • Yoon, Chan-Hoon;Kwon, Sang-Ki;Kim, Jin
    • Tunnel and Underground Space
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    • v.19 no.6
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    • pp.507-516
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    • 2009
  • In cases of high-level radioactive waste repositories, heat load is apparent by radioactive waste decay. The safety of a waste repository would be influenced by changing circumstances caused by heat transfer through rock. Thus, a ventilation system is necessary to secure the waste repository. The first priority for building an appropriate ventilation system is completing a computer simulation research with thermal rock properties and a heat transfer coefficient. In this study, the heat transfer coefficient in KURT was calculated using the measurement of inner circumstance factors that include dry bulb and wet bulb temperature, rock surface temperature, and barometric pressure. The heater that is 2 m in length and 5 kw in capacity heats the inside of rock in the research module by $90^{\circ}C$. As a result of determining the heat transfer coefficient in the heating section, the changes of heat transfer coefficient were found to be a maximum of 7.9%. The average heat transfer coefficient is approximately 4.533 w/$m^2{\cdot}K$.

Numerical Investigation on Experiment for Passive Containment Cooling System (피동 원자로건물 냉각계통 실험에 관한 수치적 연구)

  • Ha, Hui Un;Suh, Jung Soo
    • Journal of the Korean Society of Safety
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    • v.35 no.3
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    • pp.96-104
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    • 2020
  • The numerical simulations were conducted to investigate the thermal-fluid phenomena occurred inside the experimental apparatus during a PCCS, used to remove heat released in accidents from a containment of light water nuclear power plant, operation. Numerical simulations of the flow and heat transfer caused by wall condensation inside the containment simulation vessel (CSV), which equipped with 18 vertical heat exchanger tubes, were conducted using the commercial computational fluid dynamics (CFD) software ANSYS-CFX. Shear stress transport (SST) and the wall condensation model were used for turbulence closure and wall condensation, respectively. The simulation using the actual size of the apparatus. However, rather than simulating the whole experimental apparatus in consideration of the experimental cases, calculation resources, and calculation time, the simulation model was prepared only in CSV. Selective simulation was conducted to verify the effects of non-condensable gas(NC gas) concentration, CSV internal pressure, and wall sub-cooling conditions. First, as a result of the internal flow of CSV, it was observed that downward flow due to condensation occurred surface of the vertical tube and upward flow occurred in the distant place. Natural convection occurred actively around the heat exchanger tube. Due to this rising and falling internal flow, natural circulation occurred actively around the heat exchanger tubes. Next, in order to check the performance of built-in condensation model using according to the non-condensable gas concentration, CSV internal flow and wall sub-cooling, the heat flux values were compared with the experimental results. On average, the results were underestimated with and error of about 25%. In addition, the influence of CSV internal pressure and wall sub-cooling was small, but when the condensate was highly generated due to the low non-condensable gas concentration, the error was large compared to the experimental values. This is considered to be due to the nature of the condensation model of the CFX code. However, in spite of the limitations of CFD, it is valid to use the built-in condensation model of CFD for PCCS performance prediction from a conservative perspective.

An Experimental Study on Braking Thermal Damage of Brake Disk Cover (브레이크 디스크 커버의 제동 열손상에 대한 실험적 연구)

  • Ko, Kwang-Ho;Moon, Byung-Koo
    • Journal of Digital Convergence
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    • v.13 no.11
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    • pp.171-178
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    • 2015
  • The disk cover is installed to protect brake disk and calliper and it's removed right before delivering to customers. The temperature of disk cover was measured driving test vehicles(2000cc, diesel) in this study. The highest temperature measured for the driving test(120km/h-braking(0.3G)-stop-120km/h-braking(0.5G)-stop) was $260{\sim}270^{\circ}C$ in the upper part of the disk cover and the temperature varied considerably around the disk cover. It can be inferred from this temperature distribution around the cover that the major heat transfer from hot disk to cover was through convection. In other words, the hot air generated by braking friction moved up to the upper part of the disk cover. And only the upper area of the disk cover was melted down during this driving test. The thickness of disk cover was increased to 1.0mm from 0.7mm and 1 paper of masking tape was pasted in the upper region of the disk cover. Then the cover endured the heated air formed by braking friction during the driving test.

Particle deposition on a rotating disk in application to vapor deposition process (VAD) (VAD공정 관련 회전하는 원판으로의 입자 부착)

  • Song, Chang-Geol;Hwang, Jeong-Ho
    • Transactions of the Korean Society of Mechanical Engineers B
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    • v.22 no.1
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    • pp.61-69
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    • 1998
  • Vapor Axial Deposition (VAD), one of optical fiber preform fabrication processes, is performed by deposition of submicron-size silica particles that are synthesized by combustion of raw chemical materials. In this study, flow field is assumed to be a forced uniform flow perpendicularly impinging on a rotating disk. Similarity solutions obtained in our previous study are utilized to solve the particle transport equation. The particles are approximated to be in a polydisperse state that satisfies a lognormal size distribution. A moment model is used in order to predict distributions of particle number density and size simultaneously. Deposition of the particles on the disk is examined considering convection, Brownian diffusion, thermophoresis, and coagulation with variations of the forced flow velocity and the disk rotating velocity. The deposition rate and the efficiency directly increase as the flow velocity increases, resulting from that the increase of the forced flow velocity causes thinner thermal and diffusion boundary layer thicknesses and thus causes the increase of thermophoretic drift and Brownian diffusion of the particles toward the disk. However, the increase of the disk rotating speed does not result in the direct increase of the deposition rate and the deposition efficiency. Slower flow velocity causes extension of the time scale for coagulation and thus yields larger mean particle size and its geometric standard deviation at the deposition surface. In the case of coagulation starting farther from the deposition surface, coagulation effects increases, resulting in the increase of the particle size and the decrease of the deposition rate at the surface.

ANALYSIS OF HEAT TRANSFER ON SPENT FUEL DRY CASK DURING SHORT-TERM OPERATIONS (사용후핵연료 건식 용기의 단기운영공정 열전달 평가)

  • Kim, H.;Lee, D.G.;Kang, G.U.;Cho, C.H.;Kwon, O.J.
    • Journal of computational fluids engineering
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    • v.21 no.2
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    • pp.54-61
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    • 2016
  • When spent fuel assemblies from the reactor of nuclear power plants(NPPs) are transported, the assemblies are exposed to short-term operations that can affect the peak cladding temperature of spent fuel assemblies. Therefore, it needs to perform the analysis of heat transfer on spent fuel dry cask during the operation. For 3 dimensional computational fluid dynamnics(CFD) simulation, it is proposed that the short-term operation is divided into three processes: Wet, dry, and vacuum drying condition. The three processes have different heat transfer mode and medium. Metal transportation cask, which is Korea Radioactive Waste Agency(KORAD)'s developing cask, is evaluated by the methods proposed in this work. During working hours, the boiling at wet process does not occur in the cask and the peak cladding temperatures of all processes remain below $400^{\circ}C$. The maximum peak cladding temperature is $173.8^{\circ}C$ at vacuum drying process and the temperature rise of dry, and vacuum drying process occurs steeply.