• 대한기계학회논문집B
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• 제34권10호
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• pp.933-939
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• 2010

본 연구는 타원형 노즐과 원형 노즐 내부에서 발생되는 cavitation의 발생 및 성장을 실험적으로 관찰하였다. 원형 노즐과 타원형 노즐의 cavitation 특성을 가시화 하기위해 투명한 아크릴로 노즐을 제작하였다. 실험에 사용된 노즐들은 같은 단면적으로 제작되었으며, 타원형 노즐의 경우 형상비(a/b)를 다르게 하였다. 분사압력의 증가에 따라 노즐내부 유동은 no cavitation, cavitation, hydraulic flip 영역으로 나뉘어졌다. 노즐의 형상에 상관없이 no cavitation과 cavitation 영역에서는 분사압력의 증가에 따라 유량은 증가하며, 유출계수는 감소하는 경향을 나타냈다. 그러나 hydraulic flip 영역에서의 유량계수는 일정한 값을 나타냈다. 타원형 노즐은 원형 노즐에 비해 높은 cavitation number에서 cavitation이 성장, 발달하였다. 특히 타원형 노즐에서는 장축의 cavitation length가 단축보다 길게 나타났다.

• 대한기계학회논문집B
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• 제31권3호
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• pp.264-272
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• 2007

The aim of this study is to investigate the effects of nozzle orifice shapes and the nozzle length-to-diameter ratio(L/D) on the nozzle cavitation formation inside the orifice and the external flow pattern. The nozzle used in this work was tested the taper orifice nozzle and the rectangular orifice nozzle which was made from the transparent acrylic acid resin. For studying the effect of the nozzle L/D ratio, it was used to three L/D ratios of 3.33, 10, and 20. The cavitation flow of nozzle was visualized by using the ICCD camera and optical system. This work revealed that the flow rate and discharge coefficient($C_d$) of the taper orifice nozzle was larger than those of the rectangular orifice nozzle at the same injection pressure. The cavitation flow was observed in the nozzle orifice at the low injection pressure and the breakup of liquid jet was promoted as the L/D ratio is decreased. The cavitation of biodiesel fuel was formed at the lower injection pressure than that of diesel fuel because of higher viscosity and density.

• 한국자동차공학회논문집
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• 제15권4호
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• pp.94-100
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• 2007

The purpose of this work is to investigate the effect of properties of diesel and biodiesel fuels on the nozzle cavitation and the effect of the length/diameter(L/D) ratio on internal and external flow pattern of nozzle at the various injection conditions. In order to study the effect of the L/D ratio on the nozzle cavitation characteristics of diesel and biodiesel, the characteristics of cavitation flow in the nozzle are visualized and analyzed at the injection pressure of 0.1 MPa to 0.7 MPa by using the visualized images. It was founded that the cavitation was formed in the nozzle orifice at the low injection pressure and the breakup of the issuing liquid jet was promoted at the low L/D ratio. When the L/D ratio decrease, cavitation beginning and growth were affect by cavitation number and Reynolds number.

• 한국분무공학회지
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• 제13권4호
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• pp.200-205
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• 2008

Spray formation mechanism was controlled by a cavitation inside an injection nozzle. Nozzle geometry affects spray characteristics and formation behavior, which could determine engine performance and pollutant formation. A study was carried out on the influence of aspect ratio on cavitation inside a nozzle. The cavitation model available in Star-CD code was used to obtain cavitation behavior inside nozzle, which was compared with previous experimental results. In this paper, a CFD approach combining multiphase Volume-of-Fluid(VOF) and k-model was applied. The numerical results are similar with the experimental results.

• International Journal of Automotive Technology
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• 제7권3호
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• pp.283-288
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• 2006

A mathematical methodology is proposed for designing nozzle hole shapes producing controlled geometric cavitation. The proposed methodology uses an unstructured RANS flow solver, with the ability to compute sensitivity derivatives via an adjoint algorithm. The adjoint formulation for the N-S equations is presented while variation of the turbulence viscosity is not taken into account during the geometry modifications. The sensitivities are calculated in a mode independently of the shape parameterisation. The method is used to develop and evaluate conceptual shapes for nozzle hole cavitation reduction. The localized region at the hole inlet producing cavitation, is parameterised using its radius of curvature, while a cost function is formulated to eliminate the negative pressures present at this location. Sensitivity derivatives are used to assess the dependence of the localized region on the minimum pressure, and to drive the geometry to the targeted shape. The results show that the computer model can provide nozzle hole entry shapes that produce predefined flow characteristics, and thus can be used as an inverse design tool for nozzle hole cavitation control.

• 대한기계학회논문집B
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• 제20권6호
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• pp.1971-1982
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• 1996

The breakup of liquid jet is the result of competing, unstable hydrodynamic forces acting on the liquid jet as it exit the nozzle. The nozzle geometry and up-stream injection conditions affect the characteristics of flow inside the nozzle, such as turbulence and cavitation bubbles. A set of calculation of the internal flow in a single hole type nozzle were performed using a two dimensional flow simulation under different nozzle geometry and up-stream flow conditions. The calculation showed that the turbulent intensity and discharge coefficient are related to needle position. The diesel nozzle with sharp inlet under actual engine condition has possibility of cavitation, but round inlet nozzle has no possibility of cavitation.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2001년도 추계학술대회논문집B
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• pp.952-957
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• 2001

The objectives of this investigation were to obtain an excellent spray by cavitation under the low injection pressure. When cavitation occurs in the nozzle hole, the atomization of the liquid jet enhanced considerably. In this experiments, a acrylic nozzle made the gap and installed the bypass in the nozzle hole was used to enhance the atomization of the liquid jet at the low injection pressure. The liquid flow in the nozzle hole was photographed by a transmitted light using a micro flash. The spray angle was measured macroscope images of PMAS and the Sauter mean diameter was measured PDA system. To measure the pressure of the nozzle hole, pressure transducer was used. The results of this study indicated that enhanced atomization of the liquid jet at the low injection pressure was obtained by making the gap and installing the bypass at the single hole nozzle.

• 한국신재생에너지학회:학술대회논문집
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• 한국신재생에너지학회 2010년도 춘계학술대회 초록집
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• pp.162.1-162.1
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• 2010

Cavitation phenomenon has long been a difficult problem that regarded as negative event to fluid machines or industrial facilities. In the latest, however, some engineers became to understand the power of cavitation and use it to cleaning wall after developing cavitation nozzle. In this paper, we introduce new concept for power-generation system using cavitation jet flow maid by nozzle and impulse turbine in vacuum condition. The vacuum needed to make cavitation is generated naturally by Torricelli's vacuum, 10.23m effective head drop without additional power. We analyzed water's boiling and the steam's mean free path according to vacuum purity levels for nozzles and turbine blades. The nozzles make water accelerate in the neck and boil in expansion section of the nozzles. The shape of the impulse turbine is designed for absorption of the molecule's kinetic energy of the steam.

• International Journal of Fluid Machinery and Systems
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• 제10권2호
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• pp.176-188
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• 2017

The nozzle-flapper valves are widely applied as a pilot stage in aerospace and military system. A subject of the analysis presented in this work is to find out a reasonable range of null clearance between the nozzle and flapper. This paper has presented a numerical flow coefficient simulation. In every design point, a parameterized model is created for flow coefficient simulation and cavitation under different conditions with varying gap width and inlet pressure. Moreover, a new test device has been designed to measure the flow coefficient and for visualized cavitation. The numerical simulation and test results both indicate that cavitation intensity gets fierce initially and shrinks finally as the gap width varies from small to large. From the curve, the flow coefficient mostly has experienced three stages: linear throttle section, transition section and saturation section. The appropriate deflection of flapper is recommended to make the gap width drop into the linear throttle section. The flapper-nozzle null clearance is optionally recommended near the range of $D_N/16$. Finally through simulation it is also concluded that the inlet pressure plays a little role in the influence on the flow coefficient.

• 한국전산유체공학회지
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• 제18권3호
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• pp.94-101
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• 2013

In the present study, a practical method to predict cavitation erosion, which caused a critical damage on hydraulic machineries, was developed. Impact and critical velocities were defined to develop a practical method for the prediction of cavitation erosion. To develope the practical method, the computational fluid dynamics (CFD) was introduced. Cavitating flows with erosion in a converging-diverging nozzle and around a hydrofoil were simulated by developed and validated code. Based on the CFD results, the cavitation erosion coefficient was derived by a curve fitting method. The cavitation erosion coefficient was formulated as the function of the cavitation and Reynolds numbers. A cavitating flow in an axisymmetric nozzle followed by radial divergence was simulated to validate the developed practical method. For the application to a propeller, a cavitating flow around a propeller was simulated. Predicted damage extent showed similar with damaged full-scale propeller blade.