http://chineseinput.net/에서 pinyin(병음)방식으로 중국어를 변환할 수 있습니다.
변환된 중국어를 복사하여 사용하시면 됩니다.
Review : Thermal contact problems at cryogenic temperature
Jeong, Sangkwon,Park, Changgi The Korea Institute of Applied Superconductivity a 2015 한국초전도저온공학회논문지 Vol.17 No.4
This paper addresses technical problems of thermal contact conductance or resistance which inevitably occurs in most cryogenic engineering systems. The main focus of this paper is to examine what kind of physical factors primarily influences the thermal contact resistance and to suggest how it can be minimized. It is a good practical rule that the contact surface must have sub-micron roughness level with no oxide layer and be thinly covered by indium, gold, or Apiezon-N grease for securing sufficient direct contact area. The higher contact pressure, the lower the thermal contact resistance. The general description of this technique has been widely perceived and reasonable engineering results have been achieved in most applications. However, the detailed view of employing these techniques and their relative efficacies to reduce thermal contact resistances need to be thoroughly reviewed. We should consider specific thermal contact conditions, examine the engineering requirements, and execute each method with precautions to fulfil their maximum potentials.
Park, Jiho,Jeong, Sangkwon,Cha, Jeongmin IPC Science and Technology Press 2017 International journal of refrigeration Vol.75 No.-
<P><B>Abstract</B></P> <P>This research paper focuses on the experimental investigation of the Stirling-type pulse tube refrigerator with cold compression concept. Due to this innovative feature, the pulse tube refrigerator can reach lower temperature effectively other typical single-stage Stirling-type pulse tube refrigerators. The experiment as a proof of concept is carried out to demonstrate the capability of the pulse tube refrigerator operating between 80 K and 20 K. The cold linear compressor, which is submerged in a liquid nitrogen bath, produces cold mass flow with the efficiency of 85% for all the frequencies. At the lowest temperature part of the pulse tube refrigerator, the no-load temperature of 18.7 K is recorded and the cooling power of 0.4 W is measured at 20 K. The experimental results are analyzed in dynamic and thermal aspects by using the numerical model. The model can well explain how much losses are distributed in the system.</P> <P><B>Highlights</B></P> <P> <UL> <LI> “Cold compressor and colder expander” concept is proposed. </LI> <LI> Stirling-type PTR with cold linear compressor is tested as a proof of the concept. </LI> <LI> Mass flow is generated by the compressor near 80 K with 85% compression efficiency. </LI> <LI> 18.7 K-no load temperature is recorded and 0.4 W-cooling power is measured at 20 K. </LI> <LI> Dynamic and thermal characteristics of the Stirling-type PTR are investigated. </LI> </UL> </P>
Transient cooling operation of multistage thermoelectric cooler (TEC)
Jiho Park,Sangkwon Jeong 한국초전도저온공학회 2021 초전도와 저온공학 Vol.23 No.3
A thermoelectric cooler (TEC) is promising as an alternative refrigeration technology for the sake of its inherent advantages; no-moving parts and refrigerant-free in its operation. Due to the compactness, reliability and excellence in temperature stability, TECs have been widely used for small cooling devices. In recent years, thermoelectric devices have been attractive technologies that not only serve the needs of cooling and heating applications but also meet the demand for energy by recycling waste heat. In this research paper, multistage TEC is proposed as a concept of demonstrating the idea of transient cooling technology. The key idea of transient cooling is to harnesses the thermal mass installed at the interfacial level of the stages. By storing heat temporally at the thermal mass, the multistage TEC can readily reach lower temperatures than that by a steady-state operation. The multistage TEC consists of four different sizes of thermoelectric modules and they are operated with an optimized current. Once the cold-part of the uppermost stage is reached at the no-load temperature, the current is successively supplied to the lower stages with a certain time interval; 25, 50 and 75 seconds. The results show the temperatures that can be ultimately reached at the cold-side of the lowermost stage are 197, 182 and 237 K, respectively. It can be concluded that the timing or total amount of the current fed to each thermoelectric module is the key parameter to determine the no-load temperature.
Concept of Cold Energy Storage for Superconducting Flywheel Energy Storage System
Jisung Lee,Sangkwon Jeong,Young Hee Han,Byung Jun Park IEEE 2011 IEEE transactions on applied superconductivity Vol.21 No.3
<P>A superconducting flywheel energy storage (SFES) system is an energy storage device with unprecedented small kinetic energy loss by utilizing diamagnetic levitation property of superconductor. The system, therefore, is expected to be one of the most promising candidates in the application of renewable energy field such as PV (photovoltaic) or wind energy development where the power generation is intermittent. An innovative concept to store cold thermal energy as well as kinetic energy in the SFES system is proposed in this paper to decrease required cooling energy during the energy storage period. We have found that the cooling energy can be considerably decreased by the suggested cooling concept. The methodology of cold thermal energy storage is introduced, and the experimental validation is carried out. A specially designed thermosiphon is adopted as a thermal bridge between the high temperature superconductor (HTS) bulks and the cold head of cryocooler, and the working fluid of the thermosiphon is utilized as the thermal energy storage material. Solid nitrogen is generated in the thermosiphon by surplus electricity, and then the mock up HTS bulks are successfully cooled around 64 K by the existence of solid nitrogen even though the implemented cryocooler is turned off.</P>
Park, Inmyong,Jeong, Sangkwon Elsevier 2017 Cryogenics Vol.88 No.-
<P><B>Abstract</B></P> <P>The experimental investigation of an active magnetic regenerative refrigerator (AMRR) operating between 77 K and 20 K is discussed in this paper, with detailed energy transfer analysis. A multi-layered active magnetic regenerator (AMR) is used, which consists of four different rare earth intermetallic compounds in the form of irregular powder. Numerical simulation confirms that the AMR can attain its target operating temperature range. Magnetic field alternation throughout the AMR is generated by a high temperature superconducting (HTS) magnet. The HTS magnet is cooled by a two stage Gifford-McMahon (GM) cryocooler. Helium gas was employed as a working fluid and its oscillating flow in the AMR is controlled in accordance with the magnetic field variation. The AMR is divided into two stages and each stage has a different mass flow rate as needed to achieve the desired cooling performance. The temperature variation of the AMR during the experiment is monitored by temperature sensors installed inside the AMR. The experimental results show that the AMRR is capable of achieving no-load temperature of 25.4 K while the warm end temperature is 77 K. The performance of the AMRR is analyzed by observing internal temperature variations at cyclic steady state. Furthermore, numerical estimation of the cooling capacity and the temperature variation of the AMR are examined and compared with the experimental results.</P> <P><B>Highlights</B></P> <P> <UL> <LI> The magnetic refrigerator operating between 77 K and 20 K is fabricated and tested. </LI> <LI> The AMR reached the lowest temperature of 25 K and the temperature span is 52 K. </LI> <LI> The numerical estimation of the performance is compared with the experimental result. </LI> </UL> </P>