• 천문학회보
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• 제36권2호
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• pp.112.1-112.1
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• 2011

We present numerical simulations of inverse energy cascade and in driven three-dimensional (3D) electron magnetohydrodynamic (EMHD) turbulence. It has been known that inverse energy cascade only occurs in two-dimensional (2D) turbulence. However, we demonstrate that inverse energy cascade occurs in 3D driven EMHD turbulence. When magnetic helicity is injected on a small-scale, magnetic energy goes up to larger scales. The energy spectrum clearly shows inverse energy cascade. At the same time, magetic helicity spectrum also shows that the helicity goes up to larger scales. We obviously confirm inverse energy cascade. Net magnetic helicity for scales larger than the driving scale shows linear growth, and magnetic energy shows non-linear growth. On the other hand, when we drived turbulence without magnetic helicity, we do not observe inverse energy cascade.

• 조명전기설비학회논문지
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• 제27권5호
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• pp.95-103
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• 2013

Protection of the electrical and electronic equipment against surges in low voltage AC power distribution systems is based on wide applications of surge protective devices(SPDs). Cascade application of SPDs located at the service entrance of a building and near sensitive equipment is intended to ensure the optimal voltage protection level and energy sharing among cascade SPDs. In this paper, when surges impinge at the service entrance of the building of interest, the protection characteristics of two-stage cascade SPD systems were investigated. The influence of the distance between the upstream and downstream SPDs on the voltage protection level and energy sharing of the two-stage cascade SPD systems were analyzed experimentally. It was found that the energy sharing of two-stage cascade SPD systems strongly depends on the distance between the two SPDs and the component of SPD. As the distance between the two SPDs increases, the energy absorbed by the upstream SPD increases while the energy absorbed by the downstream SPD decreases. Consequently, it is desirable to select the upstream and downstream SPDs having the proper energy capability with due consideration of the distance between the two SPDs.

• 한국전산유체공학회:학술대회논문집
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• 한국전산유체공학회 2008년도 학술대회
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• pp.355-358
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• 2008

In order to investigate the physical mechanism of the energy cascade in homogeneous isotropic turbulence, we introduce Galilean-invariant energy and its transfer rate in the real space as a function of position, time and scale. By using a database of direct numerical simulations (DNS) of homogeneous isotropic turbulence, it is shown that (i) fully developed turbulence consists of multi-scale coherent vortices of tubular shapes, (ii) the energy at each scale is mainly confined in vortex tubes with the radii of the same order of the length scale, and (iii) the energy transfer takes place around pairs (especially, anti-parallel pairs) of such vortex tubes. Based on these observations, it is suggested that the energy cascade can be caused, in the real space, by the process of the stretching and creation of smaller (i.e. thinner) vortex tubes by the straining field around pairs of larger (i.e. fatter) vortex tubes. Indeed, it is quite easy to find such events (in our DNS fields) which strongly support this scenario of the energy cascade.

• 한국전산유체공학회:학술대회논문집
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• 한국전산유체공학회 2008년 추계학술대회논문집
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• pp.355-358
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• 2008

In order to investigate the physical mechanism of the energy cascade in homogeneous isotropic turbulence, we introduce Galilean-invariant energy and its transfer rate in the real space as a function of position, time and scale. By using a database of direct numerical simulations (DNS) of homogeneous isotropic turbulence, it is shown that (i) fully developed turbulence consists of multi-scale coherent vortices of tubular shapes, (ii) the energy at each scale is mainly confined in vortex tubes with the radii of the same order of the length scale, and (iii) the energy transfer takes place around pairs (especially, anti-parallel pairs) of such vortex tubes. Based on these observations, it is suggested that the energy cascade can be caused, in the real space, by the process of the stretching and creation of smaller (i.e. thinner) vortex tubes by the straining field around pairs of larger (i.e. fatter) vortex tubes. Indeed, it is quite easy to find such events (in our DNS fields) which strongly support this scenario of the energy cascade.

• 천문학회보
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• 제38권2호
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• pp.60.2-60.2
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• 2013

Electron magnetohydrodynamic (EMHD) turbulence provides a fluid-like description of small-scale magnetized plasmas. Most EMHD turbulence studies consider "balanced" EMHD turbulence. However, imbalanced EMHD turbulence has never been studied. In this study, we numerically study "imbalanced" EMHD turbulence. Imbalanced turbulence means that wave packets moving in one direction have high amplitudes or strong perturbations than the others. In driven imbalanced EMHD turbulence, non-zero magnetic helicity is injected. When magnetic helicity is injected at a scale, we expect to have inverse cascade of magnetic helicity, as well as magnetic energy, in three-dimensional (3D) EMHD turbulence. For no helicity injection, we do not observe inverse energy cascade. However, when magnetic helicity is injected, inverse cascade of magnetic helicity is clearly observed. Magnetic energy also shows inverse cascade. In EMHD turbulence, it is well known that magnetic energy on scales smaller than the energy injection scale is forward-cascading quantity and the magnetic energy spectrum follows a k^{-7/3} one. On the other hand, the inverse-cascading entity on scales larger than the energy injection scale is uncertain. If the magnetic helicity is inverse-cascading quantity, we will obtain a k^{-5/3} magnetic energy spectrum. In our simulations, we do observe energy spectrum consistant with k^{-5/3} on large scales. Therefore, we confirm that magnetic helicity indeed is the inverse-cascading entity in 3D EMHD turbulence.

• 한국원자력학회:학술대회논문집
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• 한국원자력학회 1996년도 추계학술발표회논문집(2)
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• pp.571-576
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• 1996

The 3-stage cascade composed of the multisection-type bithermal $H_2$/$H_2O$-exchange columns was suggested for heavy water separation. In order to study the separation characteristics for the cascade, a matrix equation with 18 simultaneous equations was composed and the concentrations and flow rates were calculated for the all parts of the cascade. Product D-concentration decreases and extraction yield increases with increasing cut in each stage, which is one of the principal parameters of the separation characteristics. The optimization of the 3-stage cascade can be made by case study using the matrix equation.

• 대한설비공학회:학술대회논문집
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• 대한설비공학회 2009년도 하계학술발표대회 논문집
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• pp.544-549
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• 2009

The purpose of this study is to investigate the field Operation Characteristics of a sea water heat source cascade heat pump system and system applicable to Building. Cascade heat pump system is composed R410A compressor, R134a compressor, EEV, cascade heat exchanger, Plate heat exchanger etc. Building area is $890m^2$ and has five floors above ground. R410A is used for a low-stage working fluid while R134a is for a high-stage. The system could runs at dual mode. One is mode of general R410A refrigeration cycle in summer and the other is cascade cycle. In order to gain a high temperature supply water in winter season, the system is designed to perform a cascade cycle. The filed test results show that the sea water heat source heat pump system exhibits a COP of about 5.5 in cooling mode along with a heating COP of about 4.0 in 1-stage heating mode. Cascade 2-stage heat pump system is enough to supply $60^{\circ}C$ water and heating COP is about 3.0

• Journal of Korean Society for Atmospheric Environment
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• 제17권E1호
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• pp.9-16
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• 2001

To evaluate the correct separation capacity of cascade impactor for liquid aerosol, theoretical and experimental calculations of 50% cut-off diameter(ECOD(sub)50) were performed. A recalculation method of original diameter for hemispheric liquid aerosol collected on casecad impactor is also proposed newly using fixation technique. Calculated values for theoretical (ECOD(sub)50) of 40stage cascade impactor are 20, 6.4, 2.8, and 1.4$\mu\textrm{m}$ at 1st- ,2nd-, 3rd- and 4th-stage, respectively. A good agreement between the result of theoretical (ECOD(sub)50) and that og experimental ones was obtained at Stage 2 and 3. On the other hand, relatively large differences were found at Stage 1 and 4. Fixation for liquid aerosols using ${\alpha}$-cyanoacrylate monomer was performed successfully. The orignal diameter of liquid aerosols collected on each stage was calculated. The maximum levels of number size distribution curves at each stage are 19.8, 6.5, 3.1 and 1.5 $\mu\textrm{m}$ at 1st-, 2nd-, 3rd- and 4th-stage, respectively. The distortion of separation capacity of cascade impactor due to the split, merger, disappearance, and evaporation of liquid aerosols in the fluid did not occur.

• 한국지열·수열에너지학회논문집
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• 제9권2호
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• pp.1-7
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• 2013

Most researches done on heat pumps have been on heat pumps for refrigeration, cooling and heating. There is therefore the need for more research on hot water heat pumps, especially for high temperature. Even though the cascade heat pump cycle has a great potential more efficient hot water generation even at low evaporating temperatures, it has been researched least for this purpose. In this study, the heating performance of a variable speed cascade heat pump was investigated by varying operating conditions. For the same heating capacity values, it was found that increasing the low stage compressor speed was more suitable for enhancing the performance of the system to get a higher temperature.

• 한국지열·수열에너지학회논문집
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• 제11권1호
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• pp.15-20
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• 2015

In this study, the optimization and performance characteristics of a cascade heat pump system was analyzed with the variation of low stage refrigerant charge amount. The cascade heat pump was designed and constructed with R134a and R410A as the refrigerant for high stage and low stage cycle, respectively. Experiments were conducted by varying the low stage charge amount and the performance characteristics of the cascade heat pump were studied. The refrigerant charge amount of the low stage cycle was varied between the ranges of -15% and +10% of the optimum charge amount. The performance variation experienced in the cascade heat pump due to the variation of refrigerant charge amount shows greater effect in the undercharge regions than the overcharge regions. COP reduction in the undercharge region is larger than the decrease in the overcharge region. Some cycle variation such as power consumption and cycle pressure according to low stage refrigerant charge amount showed different trends comparing with those according to high stage refrrgerant charge amount. Therefore, the optimum charge amount of the cascade heat pump should be determined based on the experimental data obtained by the variation of high and low stage refrigerant charge amount.