Journal of the Global Power and Propulsion Society

During the development of high by-pass aircraft engine fans, stall flutter often becomes a major issue because it restricts their part-speed operability at near-stall conditions. Stall flutter is usually experienced on the first flexure mode, where a few nodal diameters are dominant. However, our recent tests showed strong non-synchronous excitation for the second flexure (2F) mode containing multiple nodal diameters, which is a distinguishing characteristic from well-known stall flutter. This paper aims to understand the mechanism of the observed 2F excitation. Aerodynamic damping is evaluated by nonlinear transient CFD simulations across a range of fan massflow near stall. The aerodynamic damping turns negative when vortex shedding near the blade tip spills upstream and interacts with the leading edge of the neighbouring blade. Also, the vortex shedding behaviour and resulting aerodynamic damping clearly depend on the blade amplitude. The vortex shedding has broadband spectra for both frequency and circumferential modes under low amplitude, and they lock on the blades’ motion under large amplitude. From these observations, it is concluded that the observed 2F excitation can be categorized into non-synchronous vibrations found in recent literature. The coincidence of blade vibration and vortex shedding is also discussed from unsteady signal processing.

Atsushi Tateishi

Mizuho Aotsuka

Shinya Kusuda

Nozomi Tanaka

Tomonori Enoki

Takeshi Murooka

Aeroelastic assessment of the second flexure mode excitation in a low pressure ratio fan

低压比风扇中第二弯曲模态激励的气动弹性评估

The blade solidity, namely the blade chord-to-pitch ratio, largely affects the fluid-dynamic performance of turbomachinery and its cost. For turbo-machines operating with air or steam, the optimal value of the solidity which maximizes the efficiency is estimated with empirical correlations such as the ones proposed by &lt;xref ref-type="bibr" rid="fn19"&gt;Zweifel (1945)&lt;/xref&gt; and &lt;xref ref-type="bibr" rid="fn17"&gt;Traupel (1966)&lt;/xref&gt;. However, if the turbomachine operates with unconventional fluids, the accuracy of these correlations is questionable. Examples of such fluids are the organic compounds (e.g., hydrocarbons, siloxanes) used in organic Rankine cycle (ORC) power systems. This study concerns an investigation on how the working fluid, its thermodynamic state, and flow compressibility influence the optimum pitch-to-chord ratio of turbine stages. A first principle reduced-order model (ROM) for the computation of profile losses was developed for this purpose. The ROM results are compared with those obtained from numerical simulations of the flow over two axial turbine cascade geometries. The influence of both the working fluid, the flow compressibility, and the solidity value on both the boundary layer state at the blade trailing edge and the base pressure are evaluated. Models to compute mixing and passage losses in the compressible regime as a function of the axial solidity are proposed and discussed. Results show that the value of the optimal solidity of turbine cascades significantly increases with the flow compressibility, and mildly increases if the fluid is in the dense vapor state. Moreover, the optimal solidity value is strongly affected by the mixing process occurring downstream of the blade trailing edge. Therefore, currently available models for its estimation, which are solely based on the minimization of the profile losses, are inaccurate.

Francesco Tosto

Piero Colonna

Matteo Pini

The effect of working fluid and compressibility on the optimal solidity of axial turbine cascades

工作流体和可压缩性对轴流涡轮级最佳稠度的影响

In this paper, we present an experimental and numerical investigation into the flow disturbance by intrusive instrumentation in the form of spanwise pressure tubes mounted on the pressure and suction side of a compressor blade in a linear cascade wind tunnel at a moderate subsonic Mach number of 0.5 and a Reynolds number of 790,000. The pressure tubes were modelled as simplified bumps extending over the entire span with a width of 24% of the chord length and a height of 67% of the maximum blade thickness. The leading edge of the bumps is located at 30% of the streamwise surface length. We found that the total pressure losses generated by these pressure tubes are considerably larger when they are placed on the suction side of the blade. The suction-sided bump increases the total pressure loss coefficient by a factor of four, while the pressure-sided bump approximately doubles it. CFD simulations reveal that the suction-sided bump causes the flow to form a large, open recirculation zone, while for the pressure-sided bump, the flow reattaches near the trailing edge. In conclusion, we provide insights into the effects of pressure tubes on the losses generated by compressor blades. The mounted tubes significantly impact the flow and should be carefully considered during instrumentation design.

Daniel Jung

Lukas Schäflein

Roland Wunderer

Peter Jeschke

Flow disturbance by intrusive instrumentation located on the surface of a compressor blade

压缩机叶片表面侵入式仪器引起的流动扰动

The blade tip is a particularly critical component of the high pressure turbine due to the hot overtip leakage gas being responsible for efficiency losses and thermal degradation, often limiting engine life. Squealer tip designs reduce the over-tip leakage mass flow, increasing efficiency, but the thin pressure side rims can be vulnerable to damage because of their large surface area and the difficulties of shielding them from hot overtip leakage flow. This paper considers a new approach to cooling these pressure side rims, by employing an inclined slot inside a recessed step. Compared to conventional cooling strategies, in which coolant is provided by multiple cylindrical holes, the slot feature improves cooling effectiveness by more than 50% in key regions of the pressure side rim, whilst also allowing for a substantial reduction in coolant mass flow. The concept design is developed and explored using Computational Fluid Dynamics (CFD) calculations, and the performance is validated experimentally in a linear cascade with representative Mach and Reynolds numbers.

Joao Antonio Coelho Vieira

John Coull

Peter Ireland

Implementation of cooling slots at the pressure side rims of squealer tips

在尖叫叶片压力面边缘实施冷却槽

Tip leakage flow (TLF) is an important flow structure in the compressor, and it has a significant effect on the efficiency and stability of the compressor. In this study, dynamic mode decomposition (DMD) is used to study the spatiotemporal flow structure of the tip leakage flow in a compressor rotor. The high-fidelity flow field calculated via the delayed-detached eddy simulation (DDES) of an axial compressor rotor is used as the input dataset for the DMD algorithm. The results show that the unsteady motion of the tip leakage vortex (TLV) is quite different at its generation, development, and dissipation phases. Additionally, three typical unsteady structures, the oscillation of the primary TLV, the TLV breakdown, and the unsteadiness of the unsteady blockage cell, is identified by the DMD modes. Unsteady motions also largely affect the vortex structure and loss generation, especially after the vortex breakdown. The spatiotemporal structure identified by DMD provides a new understanding, which could be helpful for the modeling or flow control of the TLF.

Xindi Wei

Yumeng Tang

Jiexuan Hou

Yangwei Liu

Analysis of unsteady tip leakage flow in an axial compressor rotor using dynamic mode decomposition

利用动态模态分解分析轴流压气机转子中的非定常叶尖泄漏流动

Abbreviation	J. Glob. Power Propuls. Soc.
Journal Impact	1.06
Quartiles(Global)	ENGINEERING, MECHANICAL(Q4)

Publication Information	Publisher: Global Power and Propulsion Society，Publishing cycle: ，Journal Type: journal，Open Access Journals: Yes
Basic data	Year of publication: 2019，Proportion of original research papers: 96.15%，Self Citation Rate:0.00%， Gold OA Rate: 100.00%
Journal Fees	$Details
Average review cycle	网友分享经验：8 Weeks

Journal of the Global Power and Propulsion Society

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Share Submission Experience

Aeroelastic assessment of the second flexure mode excitation in a low pressure ratio fan

The effect of working fluid and compressibility on the optimal solidity of axial turbine cascades

Flow disturbance by intrusive instrumentation located on the surface of a compressor blade

Implementation of cooling slots at the pressure side rims of squealer tips

Analysis of unsteady tip leakage flow in an axial compressor rotor using dynamic mode decomposition

Aeroelastic assessment of the second flexure mode excitation in a low pressure ratio fan

Flow disturbance by intrusive instrumentation located on the surface of a compressor blade

The effect of working fluid and compressibility on the optimal solidity of axial turbine cascades

Implementation of cooling slots at the pressure side rims of squealer tips

Analysis of unsteady tip leakage flow in an axial compressor rotor using dynamic mode decomposition

ISSN	2515-3080
h-index	8