• Title/Summary/Keyword: Saturation Length

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Numerical study on the effect of three-dimensional unsteady tunnel entry flow characteristics on the aerodynamic performance of high-speed train (터널진입시 비정상 유동특성이 고속전철의 공력성능에 미치는 영향에 관한 수치해석적 연구)

  • 정수진;김태훈;성기안
    • Journal of Advanced Marine Engineering and Technology
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    • v.26 no.5
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    • pp.596-606
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    • 2002
  • The three-dimensional unsteady compressible Euler equation solver with ALE, CFD code, PAM-FLOW based on FEM method has been applied to analyze the flow field around the high speed train which is entering into a channel. From the present study, the pressure and flow transients were calculated and analyzed. The generation of compression wave was observed ahead of train and the high pressure in the gap between the train and the tunnel was also found due to the blockage effects. It was found that abrupt fluctuation in pressure exists in the region from train nose to shoulder of train corresponding to 10% of total length of train during tunnel entry. Computed time history of aerodynamic forces of train during tunnel entry show that drag coefficient rapidly rises and saturates at about non-dimensional time 0.31. The total increase of drag coefficient before and after tunnel entry is about 1.1%. Transient profile of lift force shows similar pattern to drag coefficient except abrupt drop after saturation and lift force in the tunnel increases 0.08% more than that before tunnel entry.

Dead-Time for Zero-Voltage-Switching in Battery Chargers with the Phase-Shifted Full-Bridge Topology: Comprehensive Theoretical Analysis and Experimental Verification

  • Zhang, Taizhi;Fu, Junyu;Qian, Qinsong;Sun, Weifeng;Lu, Shengli
    • Journal of Power Electronics
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    • v.16 no.2
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    • pp.425-435
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    • 2016
  • This paper presents a comprehensive theoretical analysis and an accurate calculation method of the dead-time required to achieve zero-voltage-switching (ZVS) in a battery charger with the phase-shifted full-bridge (PSFB) topology. Compared to previous studies, this is the first time that the effects of nonlinear output filter inductance, varied Miller Plateau length, and blocking capacitors have been considered. It has been found that the output filter inductance and the Miller Plateau have a significant influence on the dead-time for ZVS when the load current varies a lot in battery charger applications. In addition, the blocking capacitor, which is widely used to prevent saturation, reduces the circulating current and consequently affects the setting of the dead-time. In consideration of these effects, accurate analytical equations of the dead-time range for ZVS are deduced. Experimental results from a 1.5kW PSFB battery charger prototype shows that, with the proposed analysis, an optimal dead-time can be selected to meet the specific requirements of a system while achieving ZVS over wide load range.

Electrical Characteristics of Single-silicon TFT Structure with Symmetric Dual-gate for Kink Effect Suppression

  • Kang Ey-Goo;Lee Dae-Yeon;Lee Chang-Hun;Kim Chang-Hun;Sung Man-Young
    • Transactions on Electrical and Electronic Materials
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    • v.7 no.2
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    • pp.53-57
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    • 2006
  • In this paper, a Symmetric Dual-gate Single-Si TFT, which includes three split floating n+ zones, is simulated. This structure drastically reduces the kink-effect and improves the on-current. This is due to the separated floating n+ zones, the transistor channel region is split into four zones with different lengths defined by a floating n+ region. This structure allows effective reduction in the kink-effect, depending on thy length of the two sub-channels. The on-current of the proposed dual-gate structure is 0.9 mA, while that of the conventional dual-gate structure is 0.5 mA, at both 12 V drain and 7 V gate voltages. This result shows an 80% enhancement in on-current. In addition, the reduction of electric field in the channel region compared to a conventional single-gate TFT and the reduction of the output conductance in the saturation region, is observed. In addition, the reduction in hole concentration, in the channel region, in order for effectively reducing the kink-effect, is also confirmed.

Landslide characteristics for Hoengseong area in 2006 (2006년 횡성지역 산사태 발생특성)

  • Yoo, Nam-Jae;Choi, Joon-Sik
    • Land and Housing Review
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    • v.2 no.2
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    • pp.157-162
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    • 2011
  • This paper presents the landslide characteristics occurred in Hoengseong, Gangwondo and around July 16 in 2006, caused by heavy rainfall and antecedent precipitation by two typhoons of Ewiniar and Bilis. The main causes of landslides were antecedent precipitation between July 12 to 13, resulting in weakening grounds by increasing the degree of saturation previously, and the additional heavy rainfall between July 15 to 16. Most of landslides at natural slopes were transitional failures occurred along the boundary between residual weathered soil in shallow depth and hard mother rock. From the results of conclusive analyses for 100 sites in Hoengseong region where landslides occurred, the slope length of landslide and slope width were less than 100m with 87% of frequency and 30m with 74% of frequency, respectively. The average value of slope angles was $24^{\circ}$.

Effect of Surface Pyramids Size on Mono Silicon Solar Cell Performance

  • Kim, Hyeon-Ho;Kim, Su-Min;Park, Seong-Eun;Kim, Seong-Tak;Gang, Byeong-Jun;Tak, Seong-Ju;Kim, Dong-Hwan
    • Proceedings of the Materials Research Society of Korea Conference
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    • 2012.05a
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    • pp.100.2-100.2
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    • 2012
  • Surface texturing of crystalline silicon is carried out in alkaline solutions for anisotropic etching that leads to random pyramids of about $10{\mu}m$ in size. Recently textured pyramids size gradually reduced using new solution. In this paper, we investigated that texture pyramids size had an impact on emitter property and front electrode (Ag) contact. To make small (${\sim}3{\mu}m$) and large (${\sim}10{\mu}m$) pyramids size, texturing times control and one side texturing using a silicon nitride film were carried out. Then formation and quality of POCl3-diffused n+ emitter in furnace compare with small and large pyramids by using SEM images, simulation (SILVACO, Athena module) and emitter saturation current density (J0e). After metallization, Ag contact resistance was measured by transfer length method (TLM) pattern. And surface distributions of Ag crystallites were observed by SEM images. Also, performance of cell which is fabricated by screen-printed solar cells is compared by light I-V.

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Enhanced magnetic properties of FeCo alloys by engineering crystallinity and composition (FeCo의 결정성 및 조성 제어를 통한 자기 특성 향상)

  • Kim, Dan-Bi;Kim, Ji-Won;Eom, Nu-Si-A;Park, Seong-Heum;Im, Jae-Hong
    • Proceedings of the Korean Institute of Surface Engineering Conference
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    • 2018.06a
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    • pp.32.1-32.1
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    • 2018
  • Novel soft magnetic materials can be achieved by altering material properties such as morphology, composition, crystallinity, and grain size of soft magnetic alloys. Especially, magnetic properties (i.e., saturation magnetization, coarcivity) of soft magnetics are significantly affected by grain boundaries which act as a control of magnetic domain wall movement. Thus, we herein develop a two-step electroless plating method to control morphology and grain size of FeCo films for excellent magnetic properties. Accordingly, the chemical composition to control the degree of polarization of FeCo alloys was altered by electroless deposition parameters; for example, electrolyte concentration and temperature. The grain size and crystallinity of FeCo alloys was dramatically affected by the reaction temperature because the grain growth mechanism dominantly occurs at $90^{\circ}C$ where as the neucleation only happens at $50^{\circ}C$. By simply controlling the temperature, the micron-sized FeCo grains embedded FeCo film was synthesized where the large grains allow high magnetization originated from larger magnetic domain with low corecivity and the nano-sized grains allow excellent soft magnetic properties due to the magnetic correlation length.

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The Heart Rate Monitoring System using Inverted Photoplethysmography (반전 용적맥파 신호를 이용한 심박 모니터링 시스템)

  • Lee, Jun-Yeon
    • Journal of the Korea Society of Computer and Information
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    • v.17 no.3
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    • pp.105-111
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    • 2012
  • Photoplethysmogram is widely used to measure heart rate and arterial blood oxygen saturation in human. This paper describes implementation a photoplethysmography monitoring system that uses 780nm and 940nm length infrared light in radial artery. That system used combinations of 8 LEDs and 2 photoelectricities. When recorded on the skin over radial artery, the radial pulse wave was inverted. The mechanism of inverted pulse wave skin, which was reduced during systolic period and increase during diastolic period of the cardiac cycle. Through this system, we discovered optimal environments and combinations of sensors in both penetration type and reflection type. These results suggest that radial arterial wall way reflect infrared ray.

Flow Boiling Heat Transfer Characteristics of R22 Alternative Refrigerants in a Horizontal Microfin Tube (R22 대체 냉매의 마이크로 핀관내 흐름 비등 열전달 특성)

  • 한재웅;김신종;정동수;김영일
    • Korean Journal of Air-Conditioning and Refrigeration Engineering
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    • v.13 no.8
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    • pp.692-700
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    • 2001
  • Flow boiling heat transfer coefficients(HTCs) of R22, R134a, R407C, and R410A were measured experimentally for a horizontal plain and a microfin tube. Experimental apparatus was composed of 3 main parts: a refrigerant loop, a water loop and a water-glycol loop. The test section in th refrigerant loop was made of a copper tube of 9.52 mm outer diameter and 1 m length for both tubes. The refrigerant was heated by passing hot water through an annulus surrounding the test section. Tests were performed at a fixed refrigerant saturation temperature of $5^{\circ}C$ with mass fluxes of 100~300 kg/$m^2$s. Test results showed that at similar mass flux the flow boiling HTCs of R134a were similar to those of R22 for both plain and microfin tube. HTCs of R407C were similar to those of R22 for a plain tube but lower than those of R2 by 25~48% for a microfin tube. And HTCs of R410A were higher than those of R2 by 20~63% for a plain tube and were similar to those of R22 for a microfin tube. In general, HTCs of a microfin tube were 1.8~5.7 times higher than those of a plain tube.

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Comparative Study of Condensation Heat Transfer Coefficients between R404A and R152a Flow in a Horizontal Smooth Tube (수평 평활관내 R404A와 R152a 냉매 유동의 응축 열전달 계수에 대한 비교 연구)

  • Lee, Sang-Yong;Kim, Man-Hoe;Lee, Chi-Young
    • Proceedings of the SAREK Conference
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    • 2005.11a
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    • pp.256-261
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    • 2005
  • In the present experimental study, condensation heat transfer coefficients between R404A and R152a flow in a horizontal smooth tube were compared. The outer and inner diameters of the tube were 9.52 mm and 7.55 mm, respectively, and the heated length was 1045 mm. The mass flux ranged from 150 to 400 $kg/m^{2}s$ and the test section were uniformly heated from 8 to 12. $kW/m^2$. The quality range was from 0.2 to 0.8 at the saturation temperature from 27.3 to $34^{\circ}C$. Experimental condensation heat transfer coefficients increased as the quality and mass flux increased. Modified Dobson and Chato correlation reduced the mean deviation of 5.1% for R404A and 9.4% for R152a than the original correlation$^{(2)}$.

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Flow Condensation Heat Transfer Characteristic of R245fa in a Horizontal Plain Tube (수평 평활관내 R245fa의 흐름 응축 열전달 특성)

  • Park, Hyun-Shin;Park, Ki-Jung;Jung, Dong-Soo
    • Korean Journal of Air-Conditioning and Refrigeration Engineering
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    • v.20 no.2
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    • pp.87-96
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    • 2008
  • Flow condensation heat transfer coefficients(HTCs) of R123 and R245fa are measured in a horizontal plain tube. The main test section in the experimental flow loop is made of a plain copper tube of 9.52 mm outside diameter and 530 mm length. The refrigerant is cooled by passing cold water through an annulus surrounding the test section. Tests are performed at a fixed saturation temperature of $50\;{\pm}\;0.2\;^{\circ}C$ with mass fluxes of 50, 100, $150\;kg/m^2s$ and heat flux of $7.3{\sim}7.7\;kW/m^2$. Heat transfer data are obtained in the vapor quality range of $10{\sim}90%$. Test results show that the flow condensation HTCs of R245fa are overall 7.9% higher than those of R123 at all mass fluxes. The pressure drop of R245fa is smaller than that of R123 at the same heat flux. In conclusion, R245fa is a good candidate to replace ozone depleting R123 currently used in chillers from the view point heat transfer and environmental properties.