• Journal of computational fluids engineering
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• v.20 no.4
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• pp.7-13
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• 2015

Vortex tube is a thermal static device that separates compressed air into hot and cold streams. In general, the cooling efficiency of vortex tubes is lower than that of traditional air conditioning equipment and vortex tubes are mainly used for industrial spot cooling applications because of their quick responses. In this study, conical frustums are employed in the nozzle chamber to improve the cooling performance. Conical frustums can be used to decrease the ineffective mass fraction that directly passes through the cold exit without energy separation. The shape optimization of conical frustums has been performed using full factorial design. It is found that the height of frustums has the largest main effects on the cooling performance. Computational results show that the cooling performance can be increased by about 10% within the considered range of the design parameters. This is because the ineffective mass fraction toward the cold exit is decreased by about 20%.

• Proceedings of the Korean Vacuum Society Conference
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• 2011.02a
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• pp.484-485
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• 2011

반사방지(Anti-Reflection, AR) 특성은 태양전지, LED, 광검출기 등의 광전소자와 디스플레이의 효율과 투과도를 향상시키기 위해 적용되고 있다. 또한 최근에 네비게이션, 스마트폰의 보급 증가로 인해 소형 디스플레이에 지문방지와 동시에 반사방지 기능을 갖는 필름이 사용되고 있다. 현재 적용되고 있는 반사방지 필름은 다층박막 코팅으로 형성된 필름[1]으로 생산단가와 박막의 내구성 및 신뢰성에 문제점을 가지고 있다. 이런 문제점을 해결하기 위해 나노구조로 제작 되는 반사방지 필름에 관한 연구가 활발히 진행되고 있다[2]. 나노구조로 형성된 반사방지 구조는 moth-eye 구조라고 하며, 기본 원리는 원뿔 형태를 형성된 나노 구조를 통해 공기와 나노구조 사이의 유효 굴절률을 서서히 변화시켜 반사를 줄이는 것이다. 그러므로 moth-eye 나노구조는 파장 이하의 pitch와 파장 크기의 높이를 갖도록 구조가 제작되어야 한다[3]. Photo-lithography[4], e-beam lithography[5], interference lithography[6], dip-pen nanolithography[7], hybrid nano-patterning lithography[8] 등 여러 가지 방법으로 나노 구조를 제작하고 있으나, 네비게이션이나 스마트폰 등에 적용될 수 있는 대면적으로 제작하기 위해서는 roll-to-roll printing과 같은 대면적 공정을 이용하여 제작하는 것이 필요하다. 본 논문에서는 원통형 알루미늄 rod에 양극산화를 통해 다공성 AAO(anode aluminium oxide) template를 제작하고, roll-to-roll printing 기술을 사용하여 moth-eye 나노구조를 갖는 반사방지 필름을 제작하는 것에 대해 기술하였다.

• Laser Solutions
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• v.18 no.3
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• pp.14-19
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• 2015

A helical fabricating method using $CO_2$ laser was utilized for producing cone-shaped structure on a silica substrate. Output power and the number of scanning radiation were modified in order to control the structure. The experiment shows that the depth and width of cone-shape were increased with higher output power of the laser and the number of scanning. We demonstrate fabrication of multidirectional side-firing optical fiber with diameter of 440 um using the $CO_2$ laser fabrication technique.

• Journal of Educational Research in Mathematics
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• v.18 no.1
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• pp.81-101
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• 2008

This study points out that the way of teaching varies according to the kinds of figures in explaining the volume and the surface area of solid figures in the seventh grade curriculum. Especially, the study discusses the limitation of the explanation depending on the experimental method using physical objects. Considering the study of Archimedes' research about measuring sphere, we investigate its educational implications and, based on this result, suggest the complementary approach and the considerations for applying to current school mathematics curriculum.

• Journal of the Korean Society of Propulsion Engineers
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• v.10 no.1
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• pp.23-29
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• 2006

In an annular injection supersonic ejector, the supersonic primary flow is injected along the side wall, therefore a funnel-shaped shock wave is generated by the contraction angle of the mixing chamber. In the present study, we developed a simple funnel shock wave model using 2-D wedge and conical shock wave relations. In result, the secondary flow pressure can be predicted more accurately than using a simple 2-D wedge shock wave model. Through the same analysis, the compression ratio and the adiabatic efficiency according to the entrainment ratio were calculated.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.9 no.1
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• pp.55-63
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• 1997

A numerical investigation has been made for flows in an axisymmetric circular cylinder with an impulsively rotating cone located at the bottom of the container. The axisymmetric container is completely filled with a viscous fluid. Major parameter for the present research is only the vertex angle of the cone, otherwise Reynolds number and aspect ratio of the vessel are fixed. Main interest concerns on the vortex breakdown of meridional circulation by impulsive rotation of the cone with respect to the longitudinal axis of the cylinder. Numerical method has been used to integrate momentum and continuity equations on a generalized body-fitted grid system. The pattern of vortex breakdown is quite different from that in a right circular cylinder with flat endwall disks. The flow visualization photograph of the preceeding work by Escudier is compared with the present numerical results and the two results are in good agreements. Also flow data are plotted to gain a deep understanding for the present phenomena of the vortex breakdown. The conclusions of this work are clearly explained by the classical theory of the vortex flows in a finite geometry.

• Proceedings of the KIEE Conference
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• 2007.11a
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• pp.132-133
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• 2007

$In_xGa_{1-x}N/In_yGa_{1-y}N$ 다중 양자우물 녹색 발광 다이오드에 2차원 광자 결정을 이용하여 음극선 발광의 향상을 관찰 하였다. 정사각형 배열의 2차원 광자 결정의 주기와 격자 상수는 200/500 nm 이고 전자빔 리소그래피로 광자결정 패턴을 제작한 후, 플라즈마 건식 식각법으로 패턴을 구현하였다. 식각 시간의 차이를 둔 구현된 패턴의 홀 깊이는, 각각 ${\sim}69nm,\;{\sim}99nm,\;{\sim}173nm$ 이었다. 전계 방사 주사 현미경 측정 결과, 형성된 홀은 끝이 잘린 역전된 원뿔 모양으로 식각 되었다. 식각 된 홀의 깊이에 따라 광자 결정이 있는 부분이 없는 부분보다 최대 ${\sim}30$배 많은 광자가 검출 됨을 확인하였다.

• Proceedings of the KIEE Conference
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• 2007.07a
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• pp.1271-1272
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• 2007

Experimental results regarding to the structural properties of carbon nanotubes (CNTs) and the field-emission characteristics of CNT-coated tungsten (W) tips are presented. CNTs are successfully grown on conical-type W-tips by inductively coupled plasma-chemical vapor deposition (ICP-CVD) with or without inserting a TiN-buffer layer prior to the formation of Ni catalysts. For all the CNTs grown, their nanostructures, morphologies, and crystalline structures are analyzed by FESEM, HRTEM, and Raman spectroscopy. Furthermore, the emission properties of CNT-based field-emitters are characterized to estimate the maximum current density and the threshold voltage. The results obtained in this study indicate that the emission current level of the CNT-emitter without using a TiN buffer is desirable for the application of micro-focused x-ray systems.

• Journal of Korea Society of Waste Management
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• v.34 no.7
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• pp.685-696
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• 2017

This study focuses on computational particle fluid dynamics (CPFD) modeling for the fast pyrolysis of biomass in a conical spouted bed reactor. The CPFD simulation was conducted to understand the hydrodynamics, heat transfer, and biomass fast pyrolysis reaction of the conical spouted bed reactor and the multiphase-particle in cell (MP-PIC) model was used to investigate the fast pyrolysis of biomass in a conical spouted bed reactor. A two-stage semi-global kinetics model was applied to model the fast pyrolysis reaction of biomass and the commercial code (Barracuda) was used in simulations. The temperature of solid particles in a conical spouted bed reactor showed a uniform temperature distribution along the reactor height. The yield of fast pyrolysis products from the simulation was compared with the experimental data; the yield of fast pyrolysis products was 74.1wt.% tar, 17.4wt.% gas, and 8.5wt.% char. The comparison of experimental measurements and model predictions shows the model's accuracy. The CPFD simulation results had great potential to aid the future design and optimization of the fast pyrolysis process for biomass.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.50 no.11
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• pp.771-781
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• 2022

Sequential convex programming based performance analysis of the designed UAV is performed. The nonlinear optimization problems generated by aerodynamics are approximated to socond order program by discretization and convexification. To improve the performance of the algorithm, the solution of the relaxed problem is used as the initial trajectory. Dive trajectory optimization problem is analyzed through iterative solution procedure of approximated problem. Finally, the maximum final velocity according to the performance of the actuator model was compared.