The Effect of Radial Skew Angles of Blade Angle Slots on the Stability and Performance of an Axial Flow Compressor

The parametric numerical study was performed to explore the effect of radial skew angles of blade angle slot casing treatment (CT) on the stability and performance of an axial flow subsonic compressor. Five kinds of blade slot casing treatment with difference radial skew angles (0 degree, 30 degrees, 45 degrees, 60 degrees, and 75 degrees) were designed in the numerical investigations. The unsteady calculated results show that among the radial skew angles of 0 degree, 30 degrees, 45 degrees, 60 degrees, the bigger the radial skew angle of the slots is, the greater the stall margin improvement (SMI) generated by the slots is, and the slots with 60 degrees radial skew angle can generate 58.86% SMI. Moreover, the SMI for the slots with 60 degrees radial skew angle is 29.64% more than that for the slots with 75 degrees radial skew angle. Besides, the slots with radial skew angle of 75 degrees cause the least penalty in peak efficiency among five kinds of radial skew angle, and the peak efficiency for the slots with 75 degrees radial skew angle is 0.88% higher than that for smooth wall casing treatment. The slots with 0 degree radial skew angle generate the biggest peak efficiency loss of 5.94% among five kinds of radial skew angle. The flow field analyses show that the recirculated flows formed in the slots can generate momentum transport effects on the flow field in the blade tip. Under the effects of the momentum transport, the momentum balance between the tip leakage flow (TLF) and main flow (MF) is changed, and the momentum balance determines the trajectory of the TLF and the mainstream/tip leakage flows interface. As a result, the flow condition in the tip channel is also changed. By changing the radial skew angle of slots, the slots behave different capacities of momentum transport on the tip flow field, and the momentum balance between tip leakage flow and main flow is changed differently. So, the trajectory of the TLF and the mainstream/tip leakage flows interface are deflected differently. Different improvements of the compressor stability are obtained by the slots with different radial skew angles. The slots with radial skew angle of 60 degrees can largely deflect the trajectory of TLF and the mainstream/tip leakage flows interface to the blade suction surface. It improves the flow condition of the blade tip channel, and the slots generate 58.86% SMI. However, after the slots with radial skew angle of 0 degree and 30 degrees are applied, the tip leakage vortex breakdown is occurred. So, they generate few improvements of the stall margin. Furthermore, the interaction between recirculated flows inside slots and main flows inevitably causes additional flow losses. With the increase of radial skew angle, the efficiency loss caused by slot CT decreases. Thus, the slots with 75 degrees radial skew angle generate the least peak efficiency loss among five kinds of radial skew angle.

Focusing Schlieren Visualization of Transonic Turbine Tip-Leakage Flows

This paper presents a focusing schlieren system designed for the investigation of transonic turbine tip-leakage flows. In the first part, the functional principle and the design of the system are presented. Major design considerations and necessary trade-offs are discussed. The key optical properties, e.g., depth of focus, are verified by means of a simple bench test. In the second part, results of an idealized tip-clearance model as well as linear cascade tests at engine representative Reynolds and Mach numbers are presented and discussed. The focusing schlieren system, designed for minimum depth of focus, has been found to be well suited for the investigation of three-dimensional transonic flow fields in turbomachinery applications. The schlieren images show the origin and growth of the tip-leakage vortex on the blade suction side. A complex shock system was observed in the tip region, and the tip-leakage vortex was found to interact with the suction side part of the trailing edge shock system. The results indicate that transonic vortex shedding is suppressed in the tip region at an exit Mach number around M 2 , i s = 0.8.

Controlling Leakage Flows Over a Rotor Blade Tip Using Air-Curtain Injection: Part II — Rotor Casing Film Cooling

In modern gas turbine engines, the rotor casing region experiences high thermal loads due to complex flow structures and aerothermal effects. Thus, casing cooling is one of essential measures to ensure turbine service lifetime and performance. However, studies on heat transfer and cooling over the rotor casing with tip leakage flows are limited in the open literature during the past decades. The present work aims at controlling leakage flows over the blade tip and decreasing heat loads on the rotor casing. A novel approach proposed in a companion paper (GT2019-90232) is adopted in this paper as Part II by introducing an air-curtain injection from the rotor casing through a pair of inclined rows of discrete holes positioned in the range of 30% and 50% axial chord downstream of the blade leading edge in the casing. This air-curtain injection approach is applied to flat and recessed tips with and without tip injection to evaluate its sealing capability on tip leakage flows and film cooling effectiveness on the casing for two injection ratios of 0.7% and 1.0%. In this paper, Reynolds-averaged Navier-Stokes (RANS) simulations with Shear Stress Transport (SST) k-ω turbulence model and γ-Reθ transition model, which are validated with relevant experimental data, are performed to investigate tip leakage flows and film cooling effectiveness on the casing in a single-stage, high-pressure gas turbine engine. Results show that casing injection can reduce tip leakage mass flow effectively by changing the development and migration of tip leakage mass flows, especially when the recessed tip is applied. Adding tip injection would further reduces the tip leakage. The casing injection also provides an excellent cooling effect on the casing across rotor middle chord through trailing edge regions. In the presence of the recessed tip, coolant spreads out well on the rotor tip and the casing surfaces, resulting in better film cooling effectiveness on the casing over rotor tip leading edge. In addition, the tip injection could provide an extra cooling effect in some other regions of the casing.

The Effect of Local Casing Features and External Dummy Flanges on Casing Contraction for Thermal Control of Blade Tip Clearance

Thermal closure of the engine casing is widely used to minimise undesirable blade tip leakage flows and thus improve jet engine performance. While this may be achieved using an external cooling scheme on the casing wall, the geometry of the casing itself may have considerable influence on the contraction. In this paper, key controllable design parameters such as the thickness of the casing, the extent (radial height) of the annular dummy flanges and their axial position have been examined to identify how the geometric features may be manipulated to obtain an optimised system for both contraction and weight. Finite element modelling has been used to simulate the contraction of a range of casing geometries using external cooling schemes, adapted previous work by the authors. The displacement-coolant exchange rate of each casing configuration is reported in mm / kg s−1. The results show the relative sensitivity of the contraction of the engine casing to the casing thickness, the radial height, and thickness profiling of the flanges, and also to the balance between jets impinging on the casing and towards the root of the flanges.

Influences of Tip Leakage Flows Discharged From Main and Splitter Blades on Flow Field in Transonic Centrifugal Compressor Stage

A transonic centrifugal compressor impeller is generally composed of the main and the splitter blades which are different in chord length. As a result, the tip leakage flows from the main and the splitter blades interact with each other and then complicate the flow field in the compressor. In this study, in order to clarify the individual influences of these leakage flows on the flow field in the transonic centrifugal compressor stage at near-choke to near-stall condition, the flows in the compressor at four conditions prescribed by the presence and the absence of the tip clearances were analyzed numerically. The computed results clarified the following noticeable phenomena. The tip clearance of the main blade induces the tip leakage vortex from the leading edge of the main blade. This vortex decreases the blade loading of the main blade to the negative value by the increase of the flow acceleration along the suction surface of the splitter blade, and consequently induces the tip leakage vortex caused by the negative blade loading of the main blade at any operating points. These phenomena decline the impeller efficiency. On the other hand, the tip clearance of the splitter blade decreases the afore mentioned acceleration by the formation of the tip leakage vortex from the leading edge of the splitter blade and the decrease of the incidence angle for the splitter blade caused by the suction of the flow into the tip clearance. These phenomena reduce the loss generated by the negative blade loading of the main blade and consequently reduce the decline of the impeller efficiency. Moreover, the tip clearances enlarge the flow separation around the diffuser inlet and then decline the diffuser performance independently of the operating points.

An Experimental and Numerical Study of Tip-Leakage Flows in an Idealized Turbine Tip Gap at High Mach Numbers

This paper presents results of a detailed investigation of turbine tip-leakage flows at high Mach numbers. The experimental work was carried out using a small blow-down wind tunnel. An idealized blade test section was used to study blade tip-clearance effects in transonic conditions. Unshrouded blade tips are considered and different tip gap heights are investigated. A high blade exit Mach number of Me = 2 was selected deliberately. While conventional transonic turbine stages generally operate at lower supersonic exit Mach numbers, the conditions are representative for ORC turbines. Both experimental and numerical results are presented in this contribution. The results indicate, that tip leakage flow under transonic conditions leads to a complex three-dimensional flow field. A strong interaction between tip gap vortex and trailing edge shocks was observed, that also had a profound effect on the base region. While no final statement on losses could be made in the present configuration, the results indicate a weakened shock system.

Oscillatory Tip Leakage Flows and Stability Enhancement in Axial Compressors

Rotating stall axial compressor is a difficult research field full of controversy. Over the recent decades, the unsteady tip leakage flows had been discovered and confirmed by several research groups independently. This paper summarizes the research experience on unsteady tip leakage flows and stability enhancement in axial flow compressors. The goal is to provide theoretical bases to design casing treatments and tip air injection for stall margin extension of axial compressor. The research efforts cover (1) the tip flow structure at near stall that can explain why the tip leakage flows go unsteady and (2) the computational and experimental evidences that demonstrate the axial momentum playing an important role in unsteady tip leakage flow. It was found that one of the necessary conditions for tip leakage flow to become unsteady is that a portion of the leakage flow impinges onto the pressure side of the neighboring blade near the leading edge. The impediment of the tip leakage flow against the main incoming flow can be measured by the axial momentum balance within the tip range. With the help of the theoretical progress, the applications are extended to various casing treatments and tip air recirculation.