• 대한기계학회논문집B
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• 제31권4호
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• pp.349-356
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• 2007

The present study investigated local heat/mass transfer characteristics on the tip of the rotating turbine blade with various incoming flow incidence angles. The experiments are conducted in a low speed annular cascade with a single stage turbine. The blade has a flat tip with a mean tip clearance of 2.5% of the blade chord. The incoming flow Reynolds number is $1.5{\times}10^5$ at design condition. To examine the effect of off-design condition, the experiments with various incidence angles ranging between $-15^{\circ}$ and $+7{\circ}$ were conducted. A naphthalene sublimation method is used to measure detailed mass transfer coefficient on the blade. The results indicated that the incidence angle strongly affects the behavior of tip leakage flow around the blade tip and consequently plays an important role in determining heat transfer characteristics on the tip. For negative incidence angles, the heat/mass transfer in the upstream region on the tip decreases by up to 20%. On the contrary, for positive incidence angles, much higher heat transfer coefficients are observed even with small increase of incidence angle.

• 대한기계학회논문집B
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• 제27권12호
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• pp.1750-1757
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• 2003

This paper describes the flow measurements inside the blade passage of an axial flow fan by using a rotating hot-wire probe sensor from a relative flame of reference fixed to the rotor blades. The validity of fan rotor designed by a streamline curvature equation was performed by the measurement of the three-dimensional flow upstream and downstream of the fan rotor using a 5-hole pitot tube. The vortical flow structure near the rotor tip can be clearly observed by the measurements of a relative velocity and its fluctuation on quasi-orthogonal planes to a tip leakage vortex. Larger vortical flow, which results in higher blockage in the main flow, is formed according to decrease a flow rate. The vortical flow spreads out to the 30 percent span from the rotor tip at near stall condition. In the design operating condition, the tip leakage vortex is moved downstream while the center of the vortex keeps constant in the spanwise direction. Detailed characteristics of a velocity fluctuation with relation to the vortex were also analyzed.

• 대한기계학회논문집B
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• 제28권2호
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• pp.181-188
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• 2004

The frequency characteristics in an axial flow fan operating at a design and three off-design operating conditions have been investigated by measuring the velocity fluctuation of a tip leakage vortex and a wake flow. Two hot-wire probe sensors rotating with the fan rotor. a fixed and a moving ones, were introduced to obtain a cross-correlation coefficient between two sensors as well as the fluctuating velocity. The results show that the spectral peaks due to the fluctuating velocity near the rotor tip are mainly observed in the reverse flow region of higher flow rates than those in the peak pressure operating condition. However, no peak frequency presents near the rotor tip for near stall condition. Detailed wake flow just downstream of the rotor blade was also measured by the rotating hot-wire sensor. The peak frequency of a high velocity fluctuation due to Karman vortex shedding in the wake region is mainly observed at the higher flow rate condition than that in the design point.

• 대한기계학회논문집B
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• 제40권10호
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• pp.645-651
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• 2016

본 연구에서는 압력면익단소익의 폭이 터빈 동익 압력면스퀼러팁 하류의 팁누설유동 및 압력손실에 미치는 영향에 대하여 연구하였다. 팁간극비 h/s = 1.36%에 대하여, 흡입면스퀼러의 높이는 $h_p/s=3.75%$로 일정하게 유지하고, 압력면익단소익의 폭은 w/p = 2.64%, 5.28%, 7.92%, 10.55% 등으로 변화시키면서 실험을 수행하였다. 일반적으로 압력면익단소익의 폭이 증가할수록, 통로와류 영역에서의 압력손실은 감소하였지만 팁누설유동 영역에서는 압력손실이 오히려 증가하였다. 그 결과 익단소익의 폭이 증가할수록, 질량평균 압력손실은 매우 소폭 감소하는 경향을 보였다. 본 연구 결과, 압력면스퀼러팁에 설치된 압력면익단소익은 압력손실 저감에 거의 기여를 하지 못함을 확인할 수 있었다.

• 대한기계학회논문집B
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• 제29권4호
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• pp.485-494
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• 2005

For the extensive investigation of local heat/mass transfer on the near-tip surface of turbine blade, experiments were conducted in a low speed stationary annular cascade. The turbine test section has a single stage composed of sixteen guide vanes and blades. The chord length and the height of the tested blade are 150 mm and about 125 mm, respectively. The blade has flat tip geometry and the mean tip clearance is about $2.5{\%}$ of the blade chord. Detailed mass transfer coefficient on the blade near-tip surface was obtained using a naphthalene sublimation technique. The inlet flow Reynolds number based on chord length and incoming flow velocity is changed from $1.0{\times}10^{5}\;to\;2.3{\times}10^{5}.$ Extremely complex heat transfer characteristics are observed on the blade surface due, to complicated flow patterns, such as flow acceleration, laminarization, transition, separation bubble and tip leakage flow. Especially, the suction side surface of the blade has higher heat/mass transfer coefficients and more complex distribution than the pressure side surface, which is related to the leakage flow. For all the tested Reynolds numbers, the heat/mass transfer characteristics on the turbine blade are the similar. The overall averaged $Sh_{c}$ values are proportional to $Re_{c}^{0.5}$ on the stagnation region and the laminar flow region such as the pressure side surface. However, since the flow is fully turbulent in the near-tip region, the heat/mass transfer coefficients are proportional to $Re_{c}^{0.8}.$

• 한국추진공학회:학술대회논문집
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• 한국추진공학회 2008년 영문 학술대회
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• pp.25-34
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• 2008

In this paper, three-dimensional multiblade row unsteady Navier-Stokes simulations at a hot streak temperature ratio of 2.0 have been performed to reveal the effects of rotor tip clearance on the inlet hot streak migration characteristics in low pressure stage of a Vaneless Counter-Rotating Turbine. The hot streak is circular in shape with a diameter equal to 25% of the high pressure turbine stator span. The hot streak center is located at 50% of the span and the leading edge of the high pressure turbine stator. The tip clearance size studied in this paper is 2.0mm(2.59% high pressure turbine rotor height, and 2.09% low pressure turbine rotor height). The numerical results show that the hot streak is not mixed out by the time it reaches the exit of high pressure turbine rotor. The separation of colder and hotter fluid is observed at the inlet of low pressure turbine rotor. Most of hotter fluid migrates towards the rotor pressure surface, and only little hotter fluid migrates to the rotor suction surface when it convects into the low pressure turbine rotor. And the hotter fluid migrated to the tip region of the high pressure turbine rotor impinges on the leading edge of the low pressure turbine rotor after it goes through the high pressure turbine rotor. The migration of the hotter fluid directly results in very high heat load at the leading edge of the low pressure turbine rotor. The migration characteristics of the hot streak in the low pressure turbine rotor are dominated by the combined effects of secondary flow and leakage flow at the tip clearance. The leakage flow trends to drive the hotter fluid towards the blade tip on the pressure surface and to the hub on the suction surface, even partial hotter fluid near the pressure surface is also driven to the rotor suction surface through the tip clearance. Compared with the case without rotor tip clearance, the heat load of the low pressure turbine rotor is intensified due to the effects of the leakage flow. And the numerical results also indicate that the leakage flow effect trends to increase the low pressure turbine rotor outlet temperature at the tip region.

• 대한기계학회논문집B
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• 제28권3호
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• pp.330-337
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• 2004

Analysis of flow phenomena in a centrifugal compressor impeller has been carried out with numerical simulation to understand the physics of flow near stall. Near stall point, tip leakage flow spills ahead of the leading edge of adjacent blade and other leakage flow passes over the clearance of the adjacent blade instead of rolling up into vortex within the passage. The tip leakage flow at the mid chord of impeller blade impinges against the pressure surface of the adjacent blade and then rolls up into vortex within the passage, which blocks the flow passage and generates viscous loss. The spillage of leakage flow ahead of the adjacent blade generates the recirculation of flow entering the impeller, which causes the power transferred into the flow by the impeller to decrease and blocks the flow passage. Near diffuser hub wall, flow recirculation occurs. As operating point goes to stall point, the core of recirculation approaches the impeller exit The length rises to peak point and then drops with mass flow reduction, while the height steadily rises.

• 한국분무공학회지
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• 제12권4호
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• pp.204-210
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• 2007

The worldwide energy problem and global warming cause the need of alternative fuels which feature low carbon-dioxide emission and another energy source. Liquefied Petroleum Gas (LPG) is one of the alternative fuels widely used as domestic and transportational fuel. The third generation LPLi fuel supply system has merits in the increase of engine power and low emissions. The injectors used in LPLi system should overcome a leakage problem and satisfy the durability conditions. Therefore, 1000 hour durability test of the injectors was carried out throughout this research. First, the spray pattern and the penetration length of the selected injectors is graphically shown. Next, the leakage amount with respect to the injection cycle is introduced. Finally, the shapes of nozzle holder and nozzle tip after durability test was investigated by analyzing the microscopic image of the injector tip. The variation in the shape of nozzle tip mainly due to the residue of rubber materials is found to be the reason for leakage.

• 마이크로전자및패키징학회지
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• 제7권3호
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• pp.27-29
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• 2000

Lift-off를 이용한 DLC-coated Si-tip FEA 제조에 있어서 gate 절연막의 측면에 DLC가 coating되는 것을 방지하기 위해 기판 상에 Al 희생층을 경사-회전 증착한 뒤 DLC를 coating하고, 다음으로 희생층을 식각하여 tip 이외의 DLC를 제거하는 방법을 제안하였다. 이러한 Al희생층을 이용한 lift-off공정에 의해 제조된 DLC-coated Si-tip FEA의 전류전압 특성과 전류 표동 특성을 조사하였으며, gate 누설 전류의 감소와 방출 전류의 안정성을 확인하였다.

• 한국추진공학회:학술대회논문집
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• 한국추진공학회 1998년도 제10회 학술강연회논문집
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• pp.13-13
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• 1998

A simple model of the tip leakage flow models of the rotor downstream flow is developed, based on Lakshminarayana's theoretical concept on the tip clearance flow and the experimental data published in open literature. And new spanwise distribution models of deviation angle and pressure loss coefficient due to the tip leakage flow are formulated for use in association with the streamline curvature method as a through flow analysis. Combining these new models and previous deviation and loss models due to secondary flow, a robust streamline curvature method is established for flow analysis of single-stage, subsonic axial turbines with wide ranges of turning angle, aspect ratio and blading type. At the exit from rotor rows, the flow variables are mixed radially according to a spanwise transport equation. The proposed streamline curvature method is tested against a forced vortex type turbine as well as a free vortex type one. The results show that the spanwise variations of flow angle, axial velocity and loss coefficients at rotor exit are predicted with good accuracy, being comparable to a steady three-dimensional Navier-Stokes analysis. This simple and fast flow analysis is found to be very useful for the turbine design at the initial design phase.