• 대한기계학회논문집B
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• 제31권4호
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• pp.357-366
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• 2007

The present study investigated local heat/mass transfer characteristics on the surface of the rotating turbine blade with various incidence angles. The experiments are conducted in a low speed annular cascade with a single stage turbine. The blade has a flat tip with the mean tip clearance of 2.5% of the blade chord. A naphthalene sublimation method is used to measure detailed mass transfer coefficient on the blade. At design condition, the inlet Reynolds number is $Re_c=1.5{\times}10^5$ which results in the blade rotation speed of 255.8 rpm. Also, the effect of off-design condition is examined with various incidence angles between $-15^{\circ}$ and $+7{\circ}$. The results indicated that the incidence angle has significant effects on the blade surface heat transfer. In mid-span region, the laminar separation region on the pressure side is reduced and the laminar flow region on the suction side shrinks with increasing incidence angle. Near the tip, the effect of tip leakage flow increases in span wise and axial directions as the incidence angle decreases because the tip leakage flow is formed near the suction side surface. However, the effect of tip leakage flow is reduced with positive incidence angle.

• 대한기계학회논문집B
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• 제29권8호
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• pp.948-955
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• 2005

A three-dimensional computation was conducted to understand effects of the inlet boundary layer thickness on the internal flow in a low-speed axial compressor operating at the design condition($\phi=85\%$) and near stall condition($\phi=65\%$). At the design condition, the flows in the axial compressor show, independent of the inlet boundary layer thickness, similar characteristics such as the pressure distribution, size of the hub comer-stall, tip leakage flow trajectory, limiting streamlines on the blade suction surface, etc. However, as the load is increased, the hub corner-stall grows to make a large separation region at the junction of the hub and suction surface for the inlet condition with thick boundary layers at the hub and casing. Moreover, the tip leakage flow is more vortical than that observed in case of the thin inlet boundary layer and has the critical point where the trajectory of the tip leakage flow is abruptly turned into the downstream. For the inlet condition with thin boundary layers, the hub corner-stall is diminished so it is indistinguishable from the wake. The tip leakage flow leans to the leading edge more than at the design condition but has no critical point. In addition to these, the severe reverse flow, induced by both boundary layer on the blade surface and the tip leakage flow, can be found to act as the blockage of flows near the casing, resulting in heavy loss.

• Journal of Ship and Ocean Technology
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• 제8권1호
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• pp.10-30
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• 2004

High-fidelity RANS simulations are presented for a ducted marine propulsor, including verification ＆ validation (V＆V) using available experimental fluid dynamics (EFD) data. CFDSHIP-IOWA is used with $\textsc{k}-\omega$ turbulence model and extensions for relative rotating coordinate system and Chimera overset grids. The mesh interpolation code PEGASUS is used for the exchange of the flow information between the overset grids. Intervals V＆V for thrust, torque, and profile averaged radial velocity just downstream of rotor tip are reasonable in comparison with previous results. Flow pattern displays interaction and merging of tip-leakage and trailing edge vortices. In interaction region, multiple peaks and vorticity are smaller, whereas in merging region, better agreement with EFD. Tip-leakage vortex core position, size, circulation, and cavitation patterns for $\sigma=5$ also show a good agreement with EFD, although vortex core size is larger and circulation in interaction region is smaller.

• Journal of Mechanical Science and Technology
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• 제18권6호
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• pp.979-989
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• 2004

Numerical simulations have been performed to investigate tip clearance effect on through-flow and performance of a centrifugal compressor which has the same configuration of impeller with six different tip clearances. Secondary flow and loss distribution have been surveyed to understand the flow mechanism due to the tip clearance. Tip leakage flow strongly interacts with mainstream flow and considerably changes the secondary flow and the loss distribution inside the impeller passage. A method has been described to quantitatively estimate the tip clearance effect on the performance drop and the efficiency drop. The tip clearance has caused specific work reduction and additional entropy generation. The former, which is called inviscid loss, is independent of any internal loss and the latter, which is called viscous loss, is dependent on every loss in the flow passage. Two components equally affected the performance drop as the tip clearances were small, while the efficiency drop was influenced by the viscous component alone. The additional entropy generation was modeled with all the kinetic energy of the tip leakage flow. Therefore, the present paper can provide how to quantitatively estimate the tip clearance effect on the performance and efficiency.

• 한국전산유체공학회지
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• 제18권1호
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• pp.7-12
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• 2013

This paper presents a numerical investigation of the influences of various non-uniform tip clearances on the performance and flow field in a centrifugal compressor. Numerical simulations were conducted for three centrifugal compressor impellers in which the tip clearance height varied linearly from the leading edge to the trailing edge. The numerical result was compared with the experimental data for validation. Although the performance improved for low tip clearances, a smaller tip clearance at the trailing edge reduced the overall tip leakage flow more effectively than a smaller tip clearance at the leading edge. Therefore, a smaller tip clearance at the trailing edge lowered the mixing loss caused by interactions between the tip leakage flow and the main passage flow.

• 유체기계공업학회:학술대회논문집
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• 유체기계공업학회 2006년 제4회 한국유체공학학술대회 논문집
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• pp.161-164
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• 2006

Three-dimensional flow and aerodynamic loss in the tip-leakage flow region of a high-turning first-stage turbine rotor blade with a squealer tip have been measured with a straight miniature five-hole probe for the tip gap-to-chord ratio, h/e, of 2,0%. This squealer tip has a indent-to-chord ratio, $h_{st}/c$, of 5.5%. The results are compared with those for a plane tip ($h_{st}/c\;=\;0.0%$). The squealer tip tends to reduce the mass flow through the tip gap and to suppress the development of the tip-leakage vortex. Therefore, it delivers lower aerodynamic loss in the near-tip region than the plane tip does. At the mid-span, however, the aerodynamic loss has nearly the same value for the two different tips.

• 대한기계학회논문집B
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• 제38권5호
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• pp.399-406
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• 2014

본 연구에서는 압력면윙렛/흡입면스퀼러형 터빈 동익 팁에 대하여 익렬 하류에서 3차원 유동 및 압력손실을 측정하였다. 팁간극비 h/s = 1.36%에 대하여, 흡입면스퀼러의 높이는 $h_s/s$ = 3.75%로 일정하게 유지하고, 압력면윙렛의 폭은 w/p = 2.64%, 5.28%, 7.92%, 10.55% 등으로 변화시키면서 실험을 수행하였다. 그 결과, 본 연구의 팁은 평면팁 대비 통로와류 영역에서 압력손실의 저감 효과가 매우 뛰어났지만, 팁누설와류 영역에서는 오히려 압력손실을 크게 증가시켰다. 본 연구의 질량평균 압력손실은 평면팁에 비해 크게 낮았지만, 전면스퀼러팁보다는 더 크게 나타났다. 압력면윙렛의 폭이 증가할수록 질량평균 압력손실은 감소하는 경향을 보였다.

• 대한기계학회논문집B
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• 제27권3호
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• pp.389-397
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• 2003

Effect of tip leakage flow on through flow and performance of a centrifugal compressor impeller was numerically studied using CFX-TASC flow. Seven different tip clearances were used to consider the influence of tip clearance on performance. Secondary flow and loss factor were evaluated to understand the loss mechanism inside the impeller due to tip leakage flow. The calculated results were circumferentially averaged along the passage and at the impeller exit for quantitative discussion. Tip clearance effect on Performance could be decomposed into inviscid and viscous components using one dimensional equation. The inviscid component is related with the specific work reduction and the viscous component is related with the additional entropy generation. Two components affected Performance equally. while efficiency drop was mainly influenced by viscous loss. Performance and efficiency drop due to tip clearance were proportional to the ratio of tip clearance to exit blade height. A simple model suggested in the present study predict performance and efficiency drop quite successfully.

• 대한기계학회논문집B
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• 제30권9호
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• pp.913-920
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• 2006

Three-dimensional flow and aerodynamic loss in the tip-leakage flow region of a high-turning first-stage turbine rotor blade with a squealer tip have been measured with a straight miniature five-hole probe for the tip gap-to-chord ratio, h/c, of 2.0%. This squealer tip has a indent-to-chord ratio, $h/{st}/c$, of 5.5%. The results are compared with those for a plane tip $(h_{st}/c=0.0%)$. The squealer tip tends to reduce the mass flow through the tip gap and to suppress the development of the tip-leakage vortex. Therefore, it delivers lower aerodynamic loss in the near-tip region than the plane tip does. At the mid-span, however, the aerodynamic loss has nearly the same value for the two different tips.

• 한국전산유체공학회:학술대회논문집
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• 한국전산유체공학회 2002년도 춘계 학술대회논문집
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• pp.102-108
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• 2002

Numerical analysis has been conducted in order to simulate the flow characteristics by tip clearance of turbine cascades. A 3-D Navier-Stokes CFD code based on body-fitted coordinate system, pressure-correction and finite volume method has been used along with a commercial CFD code. The present results have showed that the development and generation of leakage vortex, vortex within tip clearance, etc. are clearly simulated, consistent with the generally known tendency. The leakage vortex occurs mainly by a separation of leakage flow that arises due to a pressure difference between two surfaces of the blade at the tip.