• Journal of Hydrogen and New Energy
• /
• v.29 no.1
• /
• pp.105-110
• /
• 2018

The flow boiling heat transfer of water was experimentally investigated on plain and sintered microporous surfaces in a mini-channel. The effects of microporous coating on flow boiling heat transfer of subcooled water were investigated in a 300 mm long mini-channel with a cross section of $20{\times}10mm^2$. The test section has sufficiently long entrance length of 300 mm which provides a fully-developed flow before the channel inlet. The bottom side of the channel was heated by a copper block assembled with a high-density cartridge heater and other sides of the channel were insulated. The microporous surface was fabricated by sintering copper particles with the average particle size of $50{\mu}m$ on the top side of the copper block. Heat transfer measurement was conducted at the mass flux of $208kg/m^2s$ and the heat flux up to $500kW/m^2$. Microporous coated surface showed an earlier boiling incipience compared with plain surface regardless of the mass flux. Microporous coating were significantly attributed to local wall temperature and local heat transfer coefficient for flow boiling.

• Nuclear Engineering and Technology
• /
• v.40 no.6
• /
• pp.477-488
• /
• 2008

The objective of the paper is to analyze the thermally induced density wave oscillations in water cooled boiling water reactors. A transient thermal hydraulic model is developed with a characteristics-based implicit finite-difference scheme to solve the nonlinear mass, momentum and energy conservation equations in a time-domain. A two-phase flow was simulated with a one-dimensional homogeneous equilibrium model. The model treats the boundary conditions naturally and takes into account the compressibility effect of the two-phase flow. The axial variation of the heat flux profile can also be handled with the model. Unlike the method of characteristics analysis, the present numerical model is computationally inexpensive in terms of time and works in a Eulerian coordinate system without the loss of accuracy. The model was validated against available benchmarks. The model was extended for the purpose of studying the flow-induced density wave oscillations in forced circulation and natural circulation boiling water reactors. Various parametric studies were undertaken to evaluate the model's performance under different operating conditions. Marginal stability boundaries were drawn for type-I and type-II instabilities in a dimensionless parameter space. The significance of adiabatic riser sections in different boiling reactors was analyzed in detail. The effect of the axial heat flux profile was also investigated for different boiling reactors.

• Journal of Electrical Engineering and Technology
• /
• v.10 no.3
• /
• pp.1075-1080
• /
• 2015

A hybrid electric vehicle (HEV) powertrain has more than one energy source including a high-voltage electric battery. However, for a high voltage electric battery, the average current is relatively low for a given power level. Introduced to increase the voltage of a HEV battery, a compact, high-efficiency boost converter, sometimes called a step-up converter, is a dc-dc converter with an output voltage greater than its input voltage. The inductor occupies more than 30% of the total converter volume making it difficult to get high power density. The inductor should have the characteristics of good thermal stability, low weight, low losses and low EMI. In this paper, Mega Flux^® was selected as the core material among potential core candidates. Different structured inductors with Mega Flux^® were fabricated to compare the performance between the conventional air cooled and proposed potting structure. The proposed inductor has reduced the weight by 75% from 8.8kg to 2.18kg and the power density was increased from 15.6W/cc to 56.4W/cc compared with conventional inductor. To optimize the performance of proposed inductor, the potting materials with various thermal conductivities were investigated. Silicone with alumina was chosen as potting materials due to the high thermo-stable properties. The proposed inductors used potting material with thermal conductivities of 0.7W/m·K, 1.0W/m·K and 1.6W/m·K to analyze the thermal performance. Simulations of the proposed inductor were fulfilled in terms of magnetic flux saturation, leakage flux and temperature rise. The temperature rise and power efficiency were measured with the 40kW boost converter. Experimental results show that the proposed inductor reached the temperature saturation of 107℃ in 20 minutes. On the other hand, the temperature of conventional inductor rose by 138℃ without saturation. And the effect of thermal conductivity was verified as the highest thermal conductivity of potting materials leads to the lowest temperature saturations.

• Design & Manufacturing
• /
• v.13 no.2
• /
• pp.22-29
• /
• 2019

This study conducted a research as to condensation heat transfer by using three types of flat micro multi-channel tubes with different processing of micro-fin and number of channels inside the pipes and different sizes of appearances. In addition, identical studies were conducted by using smoothing circular tubes with 5mm external diameter to study heat transfer coefficient. The condensation heat transfer coefficient showed an increase as the vapor quality and mass flux increased. However, each tube shows little differences compared to 400kg/m2s or identical in case the mass flux are 200kg/m2s and 100kg/m2s. The major reason for these factors is increase-decrease of heat transfer area that the flux type of refrigerant is exposed to the coolant's vapor with the effect of channel aspect ratio or micro-fin. In addition, the heat transfer coefficient was unrelated to the heat flux, and shows a rise as the saturation temperature gets lower, an effect that occurs from enhanced density. The physical factor of heat transfer coefficient increased as the channel's aspect ratio decreased. Additionally, the micro pin at the multi-channel type tube is decided as a disadvantageous factor to condensation heat enhancement factor. That is, due to the effect of aspect ratio or micro-fin, the increase-decrease of heat transfer area that the flux type of a refrigerant is exposed to the vapor is an important factor.

• The Journal of the Acoustical Society of Korea
• /
• v.39 no.4
• /
• pp.303-317
• /
• 2020

Extracorporeal Shock Wave Therapy (ESWT) is an innovative treatment in chronic musculoskeletal pain management and cardiovascular diseases. In this study, we surveyed the acoustic shock wave outputs from the domestically used focal type ESWT devices. The survey data were collected through 30 technical documents registered to the Ministry of Food and Drug Safety (MFDS), Rep. Korea. The results show that the focusing geometry varies largely, 5 mm to 65 mm in the focal length, 3 mm to 30 mm in focal width, and 4 mm to 108 mm in focal depth. The maximum positive pressure (P+) ranges from 7 MPa to 280 MPa, the focal Energy Flux Density (EFD) from 0.0035 mJ/㎟ to 35 mJ/㎟, and the energy per pulse (E) from 0.737 mJ to 80.86 mJ. All domestic PE-type (five) and one EM-type domestic devices included in the analysis of the correlation between P+ and EFD are shown to be far beyond the usual ranges and do not comply with expected correlation so that the reliability of their data was suspected. For the suspected, post-performance tests are required by a recognized testing agency. MFDS guidelines need to be revised so that the pass criteria for the shock wave acoustic outputs can be based on the clinical tests for indications.

• Journal of Korean Society of Water and Wastewater
• /
• v.23 no.6
• /
• pp.825-832
• /
• 2009

A series of laboratory experiments has been performed to investigate the flow characteristics of 2-dimensional density currents induced by selective withdrawal, which is commonly suggested as a measure for removal of high turbid water from reservoirs. Saltwater has been used to simulate the density stratification over depth and PIV(Particel Image Velocimetry) for observing the velocity structure. Experimental conditions have been established according to Richardson number, which is the dimensionless number that expresses the ratio of potential to kinetic energy. From the experiments, the patterns of longitudinal decay of centerline axial velocity induced by the withdrawal have been distinguished from other experimental cases. The rate of longitudinal decay increase as the Richardson number increases. The variations of volumetric and momentum flux along the longitudinal axis have also shown to be dependent on Richardson number.

• Journal of the Korean Chemical Society
• /
• v.17 no.5
• /
• pp.324-331
• /
• 1973

We study a mixture of ideal gases by use of recently developed methods in continuum thermodynamics of irreversible processes. A complete form of the free energy function and the gas law for each component are derived directly from an entropy production inequality by assuming that: (1) Constitutive functions depend on the mass densities, the diffusion velocities, the temperature and its gradient only. (2) Phenomenological coefficients appearing in an extra entropy flux are material constants. (3) The internal energy density per unit mass is independent of the total mass density (Joule).

• Journal of The Korean Astronomical Society
• /
• v.37 no.5
• /
• pp.597-600
• /
• 2004

A substantial number of processes have been suggested as possible contributors to the extragalactic $\gamma$-ray background (EGRB). Yet another contribution to this background will be emission produced in hadronic interactions of cosmic-ray protons with the cluster thermal gas; this class of cosmic rays (CRs) has been shown to be responsible for the EUV emission in the Coma Cluster of galaxies. In this paper we assume the CRs in the Coma Cluster is prototypic of all clusters and derive the contribution to the EGRB from all clusters over time. We examine two different possibilities for the scaling of the CR flux with cluster size: the number density of the CRs scale with the number density of the thermal plasma, and alternatively, the energy density of the CRs scale with the energy density of the plasma. We find that in all scenarios the EGRB produced by this process is sufficiently low that it will not be observable in comparison with other mechanisms that are likely to produce an EGRB.

• Bulletin of the Korean Space Science Society
• /
• 2008.10a
• /
• pp.25.2-25.2
• /
• 2008

Polar rain is a spatially uniform precipitation of electrons with energies around 100eV that penetrate into the polar cap region where geomagnetic field lines are connected to the Interplanetary Magnetic Fields (IMF). Since their occurrences depend on the IMF sector polarity, they are believed to originate from the field aligned component of the solar wind. However, statistically direct correlation between polar rain and solar wind has not been shown. In this presentation, we examined specifically the IMF strength influence on the polar rain flux variation by classifying of IMF sector polarities. For this study, we employed the polar rain flux data measured by STSAT-1 and compared them with the solar wind parameters obtained from the WIND and ACE satellites. We found the direct mutuality between polar rain flux and IMF strength with correlation coefficient above 0.5. This proportional tendency appears stronger when the northern hemisphere is in the away sector of the IMF, which could be associated with a favorable geometry for magnetic reconnection. Simple particle trajectory simulation clearly shows why polar rain intensity depends on the IMF sector polarity. These results are consistent with the direct entry model of Fairfield et al.(1985), while low correlation coefficient with solar wind density, the similarity between slops of both energy spectra shows that transport process occur without acceleration.

• Design & Manufacturing
• /
• v.13 no.2
• /
• pp.37-43
• /
• 2019

This study conducted a research as to condensation heat transfer friction loss headby using three types of flat micro multi-channel tubes with different processing of micro-fin and number of channels inside the pipes and different sizes of appearances. In addition, identical studies were conducted by using smoothing circular tubes with 5mm external diameter to study heat enhancement factor and pressure drop penalty factor. 1) The friction head loss showed an increase as the vapor quality and mass flux increased. In case of saturation temperature, it shows an increase as it gets lower. These factors are the reason occurring as the lower the saturation temperature is, the higher the density of refrigerant vapor gets. The influence of heat flux is similar as the dryness is low, but as it gets higher, it lowers in heat flux, and as the high temperature of high heat flux, it is a factor that occurs as the density gets lower. 2) RMS error of the in case of friction head loss, it showed to be predicted as 0.45~0.67 by Chisholm, Friedel, Lockhart and Martinelli. 3) As forfriction head loss penalty factor, the smaller the aspect ratio is, the larger the penalty factor gets, and as for the effect of micro-fin, the penalty factor increased because it decreases to the gas fluid the way groove for the refrigerant's flow.