• International Journal of Fluid Machinery and Systems
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• 제6권4호
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• pp.206-212
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• 2013

Self-excited pulse jet is a specific nozzle with a closed chamber which can change a continuous jet into a pulse one. Energy of the pulse jet can be output not only unevenly but also with multifrequency. With the peak pressure of pulse jet, the hitting power would be 2~2.5 times higher than that of continuous jet. In order to reveal the correlation between the self-excited pulse frequency and nozzle diameter ratio, nozzle spacing and operating pressure, the model of 3D unsteady cavitation model has been used. We found that with the same nozzle structure parameters and the different operating pressure, the self-excited frequency and the width of peak crest are different, but the wave profiles are similar. With FFT, we also found that the less bandwidth of amplitude in low frequency range will lead to the wider wave crest of outlet velocity in its time domain, and the larger force of the strike will be gained. By studying the St of self-excite nozzle, not only the frequency of a certain nozzle can be predicted, but also a nozzle structure with a certain frequency can be designed.

• 한국생산제조학회지
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• 제25권2호
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• pp.118-123
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• 2016

Overlap cutting is a fundamental method of applying abrasive water jet (AWJ) machining to milling to produce a wider surface because the nozzle outlet is approximately 1.0 mm wide. In this study, the effects of overlap cutting on the depth profile and surface roughness are investigated. The overlapping area depends on the amount of step over, which is controlled in the pick-feed direction. If the step over is equal to or larger than the diameter of the nozzle, no overlap cut occurs but large cusps remain between the cut paths. A step over as small as one-thirds of the nozzle diameter may lead to triple-overlap cutting resulting in an extraordinary depth. By using pocket milling experiments with a step over of 0.46 (or 0.47), it is verified that AWJ can produce a milled surface of titanium, one of the hard-to-cut materials, with $76{\mu}m$ Ra.

• 소성∙가공
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• 제21권5호
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• pp.312-318
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• 2012

With an increasing trend toward miniaturization, electrical discharge machining(EDM) has been receiving a lot of attention as a suitable production technology for micro-parts, since it enables the machining of hard conductive materials with a high degree of repeatability and without alteration to the material. When a micro-hole is fabricated by EDM, however, the diameter of the inlet hole is larger than that of the outlet region due to the additional discharge effect caused by the eroded particles. In this paper, a shot blasting surface treatment, in which an abrasive material is accelerated through a pressurized nozzle and directed at the surface of a part, is suggested as an effective method to reduce the tapered shape of EDM micro-hole. In addition, the influence of process parameters such as spark-on time and electrode diameter on the machining performance was investigated. It is shown quantitatively that the difference in diameter between the inlet and outlet holes decreases with the shot blasting treatment and with decreasing spark-on time.

• 한국자동차공학회논문집
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• 제6권5호
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• pp.174-181
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• 1998

Experimental and analytical studies are presented to characterize the break-up mechanism and atomization processes of the intermittent- impinging-type nozzle. Gasoline jets passing through the circular nozzle with the outlet diameter of 0.4mm and the injection duration of 10ms are impinged on each other. The impingement of fuel jets forms a thin liquid sheet, and the break-up of the liquid sheet produces liquid ligaments and droplets subsequently. The shape of liquid sheets was visualized at various impinging velocities and angles using the planer laser induced fluorescence (PLIF) technique. Based on the Kelvin-Helmholtz wave instability theory, the break-up length of liquid sheets and the droplet diameter are obtained by the theoretical analysis of the sheet disintegration. The mean diameter of droplet is also estimated analytically using the liquid sheet thickness at the edge and the wavelength of the fastest growing wave. The present results indicate that the theoretical results are favorably agreed with the experimental results. The size of droplets decreases after the impingement as the impinging angle or the injection pressure increase. The increment of the injection pressure is more effective than the increment of the impinging angle to reduce the size of droplets.

• 대한기계학회논문집
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• 제11권5호
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• pp.800-809
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• 1987

본 연구에서는 원통에 대하여는 Donnel 식과 Flugge식을 사용하여 원통의 유 한길이에 대한 일반적인 해를 구하였고, 평판에 대하여는 막이론과 굽힘이론을 사용하 여 일반해를 구한 후 중첩하였다. 평판과 원통에 발생하는 미정력계를 구하기 위하 여 가장 합리적이라고 생각되는 접합부에 모든 미정력계가 집중하여 작용한다고 가정 하고, 이 부분에서 평판과 원통의 각각에 대한 하중, 모우멘트, 기울기, 변위 등이 연 속하도록 접한조건식을 세웠다.그리고 이론해석의 타당성을 알아보기 위하여 S 45C 강재로 플러시타잎의 모델을 제작하여 실험을 행하였다.

• 해양환경안전학회지
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• 제24권7호
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• pp.939-944
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• 2018

청정소화약제는 지구오존층 보호를 위해 발효된 몬트리올 의정서에 따라 할론 1211 및 할론 1310을 대체할 수 있는 약제를 말한다. 국 내외적으로는 청정소화약의 시스템 표준화와 성능평가가 수행되고 있다. 본 논문은 일반적인 청정소화약제 시스템을 바탕으로 다양한 노즐의 형상에 대한 모델링 및 수치해석을 수행하여 최적의 노즐 형상을 제안하였다. Type A와 B의 2가지 형상에 대한 노즐의 3차원 모델링을 통해 노즐의 분출속도가 개선될 수 있도록 하였다. 2가지 형상의 노즐에 대하여 유동해석을 실시하였으며 노즐의 홀 직경을 다르게 하여 가스속도 및 압력분포를 측정하였다. 측정결과 노즐 홀 수 및 직경에 따라 노즐출구에서 분출속도가 달라지는 것을 확인 할 수 있었으며 노즐 홀 직경에 관계없이 유량은 압력이 증가함에 따라 증가하는 경향을 나타내었다. 실험을 통해 얻어진 결과를 바탕으로 노즐 직경이 5 mm인 경우의 K-factor값이 $101.8l/min{\cdot}bar^{-0.5}$임을 확인하였으며, 최종적으로 노즐 홀 5 mm인 12개의 홀이 2층 구조로 되어 있는 형상의 노즐을 제안하였다.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2002년도 학술대회지
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• pp.155-158
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• 2002

In general, TVC(Thermal Vapor Compressor) is used to boost/compress a low pressure vapor to a higher pressure for further utilization. The one-dimensional method is simple and reasonably accurate, but cannot realize the detail as like the back flow and recirculation in the mixing chamber, viscous shear effect, and etc. In this study, the axisymmetric How simulations have been performed to reveal the detailed flow characteristics for the various ejector shapes. The Navier-Stokes and energy equations are solved together with the continuity equation In the compressible flow fields. The standard $k-{\epsilon}$ model is selected for the turbulence modeling. The commercial computational fluid dynamic code FLUENT software is used for the simulation. The results contain the entrainment ratio under the various motive, suction and discharge pressure conditions. The numerical results are compared with the experimental data, and the comparison shows the good agreement. The three different flow regimes (double chocking, single chocking and back flow) have been clearly distinguished according to each boundary pressure values. Also the effects of the various shape variables (nozzle position, nozzle outlet diameter, mixing tube diameter, mixing tube converging angle, and etc.) are quantitatively discussed.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2004년도 추계학술대회
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• pp.1196-1200
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• 2004

Micropump is very useful component in micro/nano fluidics and bioMEMS applications. In this study, a bubble-powered micropump was fabricated and tested. The micropump consists of two-parallel micro line heaters, a pair of nozzle-diffuser flow controller and a 1 mm in diameter, 400 ${\mu}m$ in depth pumping chamber. The two-parallel micro line heaters with 20 ${\mu}m-width$ and 200 ${\mu}m-length$ were fabricated to be embedded in the silicon dioxide layer of a wafer which serves as a base plate for the micropump. The pumping chamber, the pair of nozzle-diffuser unit and microchannels including the liquid inlet and outlet port were fabricated by etching through another silicon wafer. A glass wafer (thickness of $525{\pm}15$ ${\mu}m$) having two holes of inlet and outlet ports of liquid serve as upper plate of the pump. Finally the silicon wafer of the base plate, the silicon wafer of pumping chamber and the glass wafer were aligned and bonded (Si-Si bonding and anodic bonding). A sequential photograph of bubble nucleation, growth and collapse was visualized by CCD camera. Clearly liquid flow through the nozzle during the period of bubble growth and slight back flow of liquid at the end of collapsing period can be seen. The mass flow rate was found to be dependent on the duty ratio and the operation frequency. As duty ratio increases, flow rate decreases gradually when the duty ratio exceeds 60%. Also as the operation frequency increases, the flow rate of the micropump decreases slightly.

• 대한기계학회논문집
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• 제15권6호
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• pp.2109-2124
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• 1991

본 연구에서는 축류 터빈의 최적 설계 계산에서 사용 용도에 따라 달리 적용 될 수 있는 특정 제한조건, 즉 유량 계수, 압력비, 출력 그리고 하중 계수를 각각 고 려하였을 경우에 최대 효율을 가지기 위한 최적 조건을 계산하고자 한다. 또한 단일 설계 변수의 민감도(sensitivity) 뿐만 아니라, 단일 민감도에서 성능에 큰 영향을 주 는 설계 변수들에 대하여 복수 민감도를 나타내어 설계 변수 및 설계 제한 조건이 축 류 터빈의 성능에 미치는 영향을 조사하고자 한다.

• 대한설비공학회지:설비저널
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• 제17권6호
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• pp.708-716
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• 1988

Local heat transfer coefficients were measured on semi-cylinders on which a circular water jet impinged in crossflow. The ratio of the semi-cylinder's diameter and the nozzle outlet diameter were varied parametrically, as were the Reynolds number and the supplementary water heights. The measurements showed that the circumferential distribution of the heat transfer coefficient peaked at the stagnation point. For a fixed supplementary water height, the peak heat transfer coefficient was not depend on the curvature of test specimen(d/D). Optimum height of supplementary water which brought about the augmentation of heat transfer at the stagnation point was S/D=1. The Nusselt number decreased as the circumferential distance or angle increased. The circumferential distribution of dimensionless heat transfer (Nu/Nus) was independent of d/D ($d/D{\geq}8.33$), but for the d/D<8.33, it was depended on d/D. At a fixed angle of specimen, dimensionless heat transfer (Nu/Nus) decreased as the ratio d/D increased. The extent of the decrease between d/D=6.67 and 8.33 was markedly greater than that between d/D=8.33 and 10, or d/D=10 and 11.67.