Any prime mover exhibits the effects of wear and tear over time. The problem of predicting the effects of wear and tear on the performance of any engine is still a matter of discussion. Because the function of a gas turbine is the result of the fine-tuned cooperation of many different components, the emphasis of this paper is on the gas turbine and its driven equipment (compressor or pump) as a system, rather than on isolated components. We will discuss the effect of degradation on the package as part of a complex system (e.g., a pipeline, a reinjection station, etc.). Treating the gas turbine package as a system reveals the effects of degradation on the match of the components as well as on the match with the driven equipment. This article will contribute insights into the problem of gas turbine systent degradation. Based on some detailed studies on the mechanisms that cause engine degradation, namely, changes in blade surfaces due to erosion or fouling, and the effect on the blade aerodynamics; changes in seal geometries and clearances, and the effect on parasitic flows; and changes in the combustion system (e.g., which result in different pattern factors), the effects of degradation will be discussed. The study includes a methodology to simulate the effects of engine and driven equipment degradation. With a relatively simple set of equations that describe the engine behavior, and a number of linear deviation factors which can easily be obtained from engine maps or test data, the equipment behavior for various degrees of degradation will be studied. A second model, using a stage by stage model for the engine compressor, is used to model the compressor deterioration. The authors have avoided to present figures about the speed of degradation, because it is subject to a variety of operational and design factors that typically cannot be controlled entirely.

Degradation in Gas Turbine Systems

This paper summarizes the state of 3D CFD based models of the time average flow field within axial flow multistage turbomachines. Emphasis is placed on models which are compatible with the industrial design environment and those models which offer the potential of providing credible results at both design and off-design operating conditions. The need to develop models which are free of aerodynamic input from semi-empirical design systems is stressed. The accuracy of such models is shown to be dependent upon their ability to account for the unsteady flow environment in multistage turbomachinery. The relevant flow physics associated with some of the unsteady flow processes present in axial flow multistage machinery are presented along with procedures which can be used to account for them in 3D CFD simulations. Sample results are presented for both axial flow compressors and axial flow turbines which help to illustrate the enhanced predictive capabilities afforded by including these procedures in 3D CFD simulations. Finally, suggestions are given for future work on the development of time average flow models.

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Aerodynamic Analysis of Multistage Turbomachinery Flows in Support of Aerodynamic Design

This paper describes the introduction of 3D blade designs into the core compressors for the Rolls-Royce Trent engine with particular emphasis on the use of sweep and dihedral in the rotor designs. It follows the development of the basic ideas in a university research project, through multistage low-speed model testing, to the application to high pressure engine compressors. An essential element of the project was the use of multistage CFD and some of the development of the method to allow the designs to take place is also discussed. The first part of the paper concentrates on the university-based research and the methods development. The second part describes additional low-speed multistage design and testing and the high-speed engine compressor design and test.Copyright © 2002 by ASME

The Use of Sweep and Dihedral in Multistage Axial Flow Compressor Blading—Part I: University Research and Methods Development

In the early development of gas turbines, many empirical design rules were used; for example in obtaining fluid deflection using the deviation from blading angles, in the assumption of zero radial velocities (so-called radial equilibrium) and in expressions for clearance loss (the Lakshminarayana formulas). The validity of some of these rules, and the basic fluid mechanics behind them, is examined by use of modern ideas and computational fluid dynamics (CFD) codes. A current perspective of CFD in design is given, together with a view on future developments.

A Review of Some Early Design Practice Using Computational Fluid Dynamics and a Current Perspective

The operating range of turbomachines is limited in terms of the low flow rate by instabilities appearing in flow-leading parts of the machinery resulting in the creation of vortices. If the flow is further throttled, stall cells can start to propagate in the impeller at a fraction of the rotor speed. This article presents an investigation of rotating stall at different flow rates in a radial pump using time-resolved particle imaging velocimetry (PIV). This technique was used to investigate the flow field at the same position in every channel of the impeller during several revolutions. Frequency analysis was applied to the measured velocities to calculate the angular speed of the rotating stall in the impeller. The interest of time-resolved PIV to understand rotating stall is demonstrated, as it allows measurement of transient, irregularly appearing flow fields.

Time-resolved particle imaging velocimetry for the investigation of rotating stall in a radial pump

Effects of tip clearance, secondary flow, skew, and corner stall on the performance of a multistage compressor with controlled diffusion blading have been studied experimentally Measurements between 1 and 99 percent annulus height were carried out in both the first and the third stages of a four-stage low-speed compressor with repeating-stage blading Measurements were obtained at a datum rotor tip clearance and at a more aerodynamically desirable lower clearance The consequences of the modified rotor tip clearance on both rotor and stator performance are examined in terms of loss coefficient and gas exit angle Stator losses close to the casing are found to increase significantly when the clearance of an upstream rotor is increased These increased stator losses cause 30 percent of the stage efficiency reduction that arises with increased rotor tip clearance

Endwall Effects at Two Tip Clearances in a Multistage Axial Flow Compressor With Controlled Diffusion Blading

An existing throughflow method for axial compressors, which accounts for the effects of spanwise mixing using a turbulent diffusion model, has been extended to include the viscous shear force on the endwall. The use of a shear force, consistent with a no-slip condition, on the annulus walls in the throughflow calculations allows realistic predictions of the velocity and flow angle profiles near the endwalls. The annulus wall boundary layers are therefore incorporated directly into the throughflow prediction. This eliminates the need for empirical blockage factors or independent annulus boundary layer calculations. The axisymmetric prediction can be further refined by specifying realistic spanwise variations of loss coefficient and deviation to model the three-dimensional endwall effects. The resulting throughflow calculation gives realistic predictions of flow properties across the whole span of a compressor. This is confirmed by comparison with measured data from both low and high-speed multistage machines. The viscous throughflow method has been incorporated into an axial compressor design system. The method predicts the meridional velocity defects in the endwall region and consequently blading can be designed that allows for the increased incidence, and low dynamic head, near the annulus walls.

Viscous Throughflow Modeling for Multistage Compressor Design

The performance of a single-stage low-speed compressor has been measured both before and after the introduction of certain features of the multistage flow environment. The aim is to make the single-stage rig more appropriate for developing design rules for multistage compressors. End-wall blockage was generated by teeth on the hub and casing upstream of the rotor. A grid fitted upstream produced free-stream turbulence at rotor inlet typical of multistage machines and raised stage efficiency by 1.8 percent at the design point. The potential field that would be generated by blade rows downstream of an embedded stage was replicated by introducing a pressure loss screen at stage exit. This reduced the stator hub corner separation and increased the rotor pressure rise at flow rates below design, changing the shape of the pressure-rise characteristic markedly. These results highlight the importance of features of the flow environment that are often omitted from single-stage experiments and offer improved understanding of stage aerodynamics.

Simulating the multistage environment for single-stage compressor experiments

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Closure to “Discussion of ‘Endwall Effects at Two Tip Clearances in a Multistage Axial Flow Compressor With Controlled Diffusion Blading’” (1994, ASME J. Turbomach., 116, pp. 645–646)

The performance of a single-stage low-speed compressor has been measured both before and after the introduction of certain features of the multi-stage flow environment. The aim is to make the single-stage rig more appropriate for developing design rules for multi-stage compressors. End-wall blockage was generated by teeth on the hub and casing upstream of the rotor. A grid fitted upstream produced free-stream turbulence at rotor inlet typical of multi-stage machines and raised stage efficiency by 1.8% at the design point. The potential field that would be generated by blade rows downstream of an embedded stage was replicated by introducing a pressure loss screen at stage exit. This reduced the stator hub corner separation and increased the rotor pressure rise at flow rates below design, changing the shape of the pressure-rise characteristic markedly. These results highlight the importance of features of the flow environment that are often omitted from single-stage experiments and offer improved understanding of stage aerodynamics.Copyright © 1995 by ASME

M. A. Howard

Papers

Endwall Effects at Two Tip Clearances in a Multistage Axial Flow Compressor With Controlled Diffusion Blading

Viscous Throughflow Modeling for Multistage Compressor Design

Simulating the multistage environment for single-stage compressor experiments

Closure to “Discussion of ‘Endwall Effects at Two Tip Clearances in a Multistage Axial Flow Compressor With Controlled Diffusion Blading’” (1994, ASME J. Turbomach., 116, pp. 645–646)

Simulating the Multi-Stage Environment for Single-Stage Compressor Experiments