• Title, Summary, Keyword: thermal stratification

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Effect of an aspect ratio on thermal stratification in a solar seasonal thermal storage tank (태양열 계간 축열조 내부 열성층화에 대한 탱크 종횡비 영향 연구)

  • Kim, Seong Keun;Jung, Sung Yong
    • Journal of the Korean Society of Visualization
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    • v.18 no.2
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    • pp.28-34
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    • 2020
  • In this study, we numerically investigated the thermal stratification in solar seasonal thermal storage tanks. The vertical in/out flows were unsuitable for the thermal stratification in a large scale. The effect of an aspect ratio (AR) on the thermal stratification was investigated. When AR was less than 2, water adheres and flows along the upper wall due to buoyance and the surface effect. Thereafter, hot water flows down and a large scale vortex occurs in entire tank. For high AR, jet flows ejected from the inlet pipe impinges to the opposite wall and splits. The divided flows create two vortex flows in the upper and lower regions. These different flows strongly influence temperature and thermal stratification. The thermal stratification was evaluated in terms of the thermocline thickness and degree of stratification. Compared to ARs, the maximum degree of stratification was obtained with AR of 5 having the minimum thermocline thickness.

SIMULATION OF THERMAL STRATIFICATION IN INLET NOZZLE OF STEAM GENERATOR

  • Ji, Joon-Suk;Youn, Bum-Su;Jeong, Hyun-Chul;Kim, Sang-Nyung
    • Nuclear Engineering and Technology
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    • v.41 no.3
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    • pp.287-294
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    • 2009
  • Due to thermal hydraulics phenomena, such as thermal stratification, various events occur to the parts of a nuclear power plant during their lifetimes: e.g. cracked and dislocated pipes and thermally fatigued, bent, and damaged supports. Due to the operational characteristics of the parts of the steam generator feedwater inlet horizontal pipe, thermal stratification takes place particularly frequently. However, the thermal stress due to thermal stratification at the steam generator feedwater inlet horizontal pipe was not reflected in the design stage of old plants(Kori Unit No.1, 2, 3 and 4, Yeonggwang Unit No. 1 and 2, and Uljin Unit No. 1 and 2; referred to as old-style power plants hereinafter). Accordingly, a verification experiment was performed for thermal stratification in the horizontal inlet nozzle steam generator of old-style plants. If thermal stratification occurred in the horizontal pipe of an old-style power plant, numerical analysis of the temperature distribution of the pipes and fluids was conducted. The temperature distributions were compared at the curved part of the pipe and the horizontal pipe before and after the installation of the improved thermal sleeves designed to alleviate thermal stress due to thermal stratification. The thermal stress reduction measure was proven effective at the steam generator inlet horizontal pipe and the curved part of the pipe.

Status of Thermal Stratification Research on Piping System in Korea Nuclear Power Plant (국내원전 배관계통 열성층 연구개발 현황)

  • Lee, Sun Ki
    • Transactions of the Korean Society of Pressure Vessels and Piping
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    • v.12 no.2
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    • pp.25-33
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    • 2016
  • The thermal stratification phenomenon in the nuclear power plant can cause abnormal deformation of the piping, contact with the support, damage to the support system. Repetition of the thermal stratification phenomenon or variation of the thermal boundary layer can cause thermal fatigue. Thermal stratification phenomenon in nuclear power plants is still an ongoing issue and active research has been carried out. In this paper, the current situation in Korean nuclear power plants is described, followed by the status of research and the future problems on the thermal stratification phenomenon in Korea.

The Effect of Turbulence Penetration on the Thermal Stratification Phenomenon Caused by Leaking Flow in a T-Branch of Square Cross-Section (난류침투가 사각단면 T분기관 내 누설유동에 의해 발생한 열성층 현상에 미치는 영향)

  • 홍석우;최영돈;박민수
    • Korean Journal of Air-Conditioning and Refrigeration Engineering
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    • v.15 no.3
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    • pp.239-245
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    • 2003
  • In the nuclear power plant, emergency core coolant system (ECCS) is furnished at reactor coolant system (RCS) in order to cool down high temperature water in case of emergency. However, in this coolant system, thermal stratification phenomenon can occur due to coolant leaking in the check valve. The thermal stratification produces excessive thermal stresses at the pipe wall so as to yield thermal fatigue crack (TFC) accident. In the present study, effects of turbulence penetration on the thermal stratification into T-branches with square cross-section in the modeled ECCS are analysed numerically. $textsc{k}$-$\varepsilon$ model is employed to calculate the Reynolds stresses in momentum equations. Results show that the length and strength of thermal stratification are primarily affected by the leak flow rate of coolant and the Reynolds number of the main flow in the duct. Turbulence penetration into the T-branch of ECCS shows two counteracting effects on the thermal stratification. Heat transport by turbulence penetration from the main duct to leaking flow region may enhance thermal stratification while the turbulent diffusion may weaken it.

The Effect of Turbulence Penetration on the Thermal Stratification Phenomenon Caused by Coolant Leaking in a T-Branch of Square Cross-Section

  • Choi, Young-Don;Hong, Seok-Woo;Park, Min-Soo
    • International Journal of Air-Conditioning and Refrigeration
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    • v.11 no.2
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    • pp.51-60
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    • 2003
  • In the nuclear power plant, emergency core coolant system (ECCS) is furnished at reactor coolant system (RCS) in order to cool down high temperature water in case of emergency. However, in this coolant system, thermal stratification phenomenon can occur due to coolant leaking in the check valve. The thermal stratification produces excessive thermal stresses at the pipe wall so as to yield thermal fatigue crack (TFC) accident. In the present study, effects of turbulence penetration on the thermal stratification into T-branches with square cross-section in the modeled ECCS are analysed numerically. Standard k-$\varepsilon$ model is employed to calculate the Reynolds stresses in momentum equations. Results show that the length and strength of thermal stratification are primarily affected by the leak flow rate of coolant and the Reynolds number of duct. Turbulence penetration into the T-branch of ECCS shows two counteracting effects on the thermal stratification. Heat transport by turbulence penetration from main duct to leaking flow region may enhance thermal stratification while the turbulent diffusion may weaken it.

A Numerical Analysis on Thermal Stratification Phenomenon by In-Leakage in a Branch Piping

  • Park Jong-Il
    • Journal of Mechanical Science and Technology
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    • v.19 no.12
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    • pp.2245-2252
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    • 2005
  • Thermal stratification in the branch piping of power plants can be generated by turbulent penetration or by valve leakage. In this study, a numerical analysis was performed to estimate the thermal stratification phenomenon by in-leakage in the SIS branch piping of nuclear power plant. Leakage rate, leakage area and leakage location were selected as evaluation factors to investigate the thermal stratification effect. As a result of the thermal stratification effect according to leakage rate, the maximum temperature difference between top and bottom of the horizontal piping was evaluated to be about 185K when the valve leakage rate was about 10 times as much as the allowed leakage rate. For leakage rate more than 10 times the allowed leakage rate, the temperature difference was rapidly decreased due to the increased mixing effect. In the result according to leakage area, the magnitude of temperature difference was shown in order of $3\%,\;1\%\;and\;5\%$ leakage area of the total disk area. In the thermal stratification effect, according to the leakage location, temperature difference when leakage occurred in the lower disk was considerably higher than that of when leakage occurred in the upper disk.

Comparison of DME HCCI Operating Ranges for the Thermal Stratification and Fuel Stratification based on a Multi-zone Modeling (Multi-zone 모델링을 통한 온도성층화와 농도성층화가 존재하는 DME HCCI 엔진의 운전영역에 관한 수치해석연구)

  • Jeong, Dong-Won;Lim, Ock-Taeck
    • Transactions of the Korean Society of Automotive Engineers
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    • v.19 no.2
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    • pp.35-41
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    • 2011
  • This work investigates the potential of in-cylinder thermal stratification and fuel stratification for extending the operating ranges in HCCI engines, and the coupling between thermal stratification and fuel stratification. Computational results areemployed. The computations were conducted using both a custom multi-zone version and the standard single-zone version of the Senkin application of the CHEMKINII kinetics rate code, and kinetic mechanism for di-methyl ether (DME). This study shows that the potential of thermal stratification and fuels stratification for extending the high-load operating limit by a staged combustion event with reduced pressure-rise rates is very large. It was also found that those stratification offers good potential to extend low-load limit by a same mechanism in high-load. However, a combination of thermal stratification and fuel stratification is not more effective than above stratification techniques for extending the operating ranges showing similar results of fuel stratification. Sufficient condition for combustion (enough temperature for) turns misfire in low-load limit to operate engines, which also leads to knock in high-load limit abruptly due to the too high temperature with high. DME shows a potential for maximizing effect of stratification to lower pressure-rise rate due to the characteristics of low-temperature heat release.

Experiments on the Thermal Stratification in the Branch of NPP

  • Kim Sang Nyung;Hwang Seon Hong;Yoon Ki Hoon
    • Journal of Mechanical Science and Technology
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    • v.19 no.5
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    • pp.1206-1215
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    • 2005
  • The thermal stratification phenomena, frequently occurring in the component of nuclear power plant system such as pressurizer surge line, steam generator inlet nozzle, safety injection system (SIS), and chemical and volume control system (CVCS), can cause through-wall cracks, thermal fatigue, unexpected piping displacement and dislocation, and pipe support damage. The phenomenon is one of the unaccounted load in the design stage. However, the load have been found to be serious as nuclear power plant operation experience accumulates. In particular, the thermal stratification by the turbulent penetration or valve leak in the SIS and SCS pipe line can lead these safety systems to failure by the thermal fatigue. Therefore in this study an 1/10 scaledowned experimental rig had been designed and installed. And a series of experimental works had been executed to measure the temperature distribution (thermal stratification) in these systems by the turbulent penetration, valve leak, and heat transfer through valve. The results provide very valuable informations such as turbulent penetration depth, the possibility of thermal stratification by the heat transfer through valve, etc. Also the results are expected to be useful to understand the thermal stratification in these systems, establish the thermal strati­fication criteria and validate the calculation results by CFD Codes such as Fluent, Phenix, CFX.

A study on Characteristics of Heat Flow of Low Temperature Latent Thermal Storage System (저온 잠열 축열조내의 열유동 특성에 관한 연구)

  • Lee, W.S.;Park, J.W.
    • Solar Energy
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    • v.19 no.4
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    • pp.33-43
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    • 1999
  • The study on ice thermal storage system is to improve total system performance and increase the economical efficiency in actual all-conditioning facilities. To obtain the high charging and discharging efficiencies in ice thermal storage system, the improvement of thermal stratification is essential, therefore the process flow must be piston flow in the cylindrical type. With the relation of the aspect ratio(H/D) in the storage tank, the stratification is formed better as inlet flow rate is smaller. If the inlet and the outlet port are settled at the upside and downside of the storage tank, higher storage rate could be obtainable. In case that the flow directions inside the thermal storage tank are the upward flow in charging and the downward in discharging, thermal stratification is improved because the thermocline thickness is maitained thin and the degree of stratification increases respectively. In the charging process, in case of inlet flow rate the thermal stratification has a tendency to be improved with the lower flow rate and smaller temperature gradient in case of inlet temperature, the large temperature difference between inflowing water and storage water are influenced from the thermal conduction. The effect of the reference temperature difference is seen differently in comparison with the former study for chilled and hot water. In the discharging process, the thermal stratification is improved by the effect of the thermal stratification of the charging process.

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Methods to Characterize the Thermal Stratification in Thermal Energy Storages (열에너지 저장소 내 열성층화를 평가하기 위한 기법)

  • Park, Dohyun;Ryu, Dong-Woo;Choi, Byung-Hee;SunWoo, Choon;Han, Kong-Chang
    • Tunnel and Underground Space
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    • v.23 no.1
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    • pp.78-85
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    • 2013
  • A primary objective in creating a stratified thermal storage is to maintain the thermodynamic quality of energy, so thermally stratified energy can be extracted at temperatures required for target activities. The separation of the thermal energy in heat stores to layers with different temperatures, i.e., the thermal stratification is a key factor in achieving this objective. This paper introduces different methods that have been proposed to characterize the thermal stratification in heat stores. Specifically, this paper focuses on the methods that can be used to determine the ability of heat stores to promote and maintain stratification during the process of charging, storing and discharging. In addition, based on methods using thermal stratification indices, the degrees of stratification of stored energy in Lyckebo rock cavern in Sweden were compared and the applicability of the methods was investigated.