Joao A. Teixeira

Parametric Study of Tip Clearance – Casing Treatment on Performance and Stability of a Transonic Axial Compressor

The control of tip leakage flow (TLF) through the clearance gap between the moving and stationary components of rotating machines is still a high-leverage area for improvement of stability and performance of aircraft engines. Losses in the form of flow separation, stall, and reduced rotor work efficiency are results of the tip leakage vortex (TLV) generated by interaction of the main flow and the tip leakage jet induced by the blade pressure difference. The effects are more detrimental in transonic compressors due to the interaction of shock-TLV. It has been previously shown that the use of slots and grooves in the casing over tip of the compressor blades, known as casing treatment, can substantially increase the stable flow range and therefore the safety of the system but generally with some efficiency penalties. This paper presents a numerical parametric study of tip clearance coupled with casing treatment for a transonic axial-flow compressor NASA Rotor 37. Compressor characteristics have been compared to the experimental results for smooth casing with a 0.356 mm tip clearance and show fairly good agreement. Casing treatments were found to be an effective means of reducing the negative effects of tip gap flow and vortex, resulting in improved performance and stability. The present work provides guidelines for improvement of steady-state performance of the transonic axial-flow compressors and improvement of the stable operating range of the system.Copyright

A numerical study of the effects of injection velocity on stability improvement in high-speed compressors

Abstract The current paper presents numerical simulations for a high-speed axial-flow compressor rotor, NASA Rotor-67, with continuous air injection upstream of the rotor. A parametric study relating injection velocity to compressor stability is undertaken. The effects of injector exit velocity on blade loading and flow field are investigated and discussed. Results indicate that the injection velocity has major effects on compressor stability. The injector effectiveness is found to be maximized when the injector exit Mach number is equivalent to unity. Significant range extension in terms of reduction in stalling flow coefficient equivalent to 13.23 per cent is obtained for the case of choked injector, whereas the injection mass flow is only a small percentage of the main flowrate.

Parametric study of tip clearance: Casing treatment on performance and stability of a transonic axial compressor

The control of tip leakage flow through the clearance gap between the moving and stationary components of rotating machines is still a high-leverage area for improvement of stability and performance of aircraft engines. Losses in the form of flow separation, stall, and reduced rotor work efficiency are results of the tip leakage vortex (TLV) generated by interaction of the main flow and the tip leakage jet induced by the blade pressure difference. The effects are more detrimental in transonic compressors due to the interaction of shock TLV. It has been previously shown that the use of slots and grooves in the casing over tip of the compressor blades, known as casing treatment, can substantially increase the stable flow range and therefore the safety of the system but generally with some efficiency penalties. This paper presents a numerical parametric study of tip clearance coupled with casing treatment for a transonic axial-flow compressor NASA Rotor 37. Compressor characteristics have been compared to the experimental results for smooth casing with a 0.356 mm tip clearance and show fairly good agreement. Casing treatments were found to be an effective means of reducing the negative effects of tip gap flow and vortex, resulting in improved performance and stability. The present work provides guidelines for improvement of steady-state performance of the transonic axial-flow compressors and improvement of the stable operating range of the system.

Parametric Study of Injection Angle Effects on Stability Enhancement of Transonic Axial Compressors

An effective approach in suppressing stall is mass injection upstream of the tip of an axial flow compressor. Developing reliable injection systems for high pressure ratio compressors requires an understanding of the effects of injection parameters, for example, injection angle, on the compressor stability and performance. This paper reports on a numerical investigation of injection angle on the operability of a high-speed compressor rotor. The injection port is set to be choked for all of the injection cases. Simulations indicate that injection angle has a significant impact on stability improvement. Maximum range extension is obtained when an injection yaw angle equal to -30 deg is applied. At this injection angle, the stalling mass flow coefficient was reduced by 17.4% by using an injected mass flow equivalent to 1.55% of the baseline annulus flow. Results also indicate that the best injection case creates an incidence angle of about 0 deg over the tip of the blade and adds the highest momentum in the relative frame of reference.

Performance enhancement in transonic axial compressors using blade tip injection coupled with casing treatment

Abstract The casing treatment and flow injection upstream of the rotor tip are two effective approaches in suppressing instabilities or recovering from a fully developed stall. This paper presents numerical simulations for a high-speed transonic compressor rotor, NASA Rotor 37, applying a state-of-the-art design for the blade tip injection. This is characterized by introducing a jet flow directly into the casing treatment machined into the shroud. The casing treatment is positioned over the blade tip region and exceeds the impeller axially by ∼30 per cent of the tip chord both in the upstream and in the downstream directions. To numerically solve the governing equations, the three-dimensional finite element based finite volume method CFD solver CFX-TASCflow (version 2.12.1) is employed. For a compressible flow with varying density, Reynolds-averaging leads to appearance of complicated correlations. To avoid this, the mass-weighted or Favre-averaging is applied. Using an injected mass flow of 2.4 per cent of the annulus flow, the present design can improve stall margin by up to 7 per cent when compared with a smooth casing compressor without tip injection. This research can lead to an optimum design of recirculating casing treatments or other mechanisms for performance enhancement applying tip flow injection.

A New Design for Tip Injection in Transonic Axial Compressors

This paper presents a state of the art design for the blade tip injection. The design includes the means to inject high-pressure gas jet directly into a circumferential casing groove formed in the shroud adjacent to the blade tip. The casing groove is positioned over the blade tip and exceeds 30% of the blade axial chord beyond the impeller to both upstream and downstream directions. In order to validate the multi block model used in the tip gap region, main flow characteristics are verified with the experimental data for smooth casing with a design clearance of 0.5% span. Three arbitrary mass flow rates (1.75%, 2.45%, and 4.35% of choked mass flow) have been studied. The results indicate remarkable advantageous effects on the compressor stability margin. Further, compared to classical design for tip injection, the current design can significantly improve the compressor stall margin due to direct injection of flow. An increase of the injected air may enhance the stall margin improvement. Furthermore, results for injection at different angles, shows that the compressor stability margin reaches a maximum when the bleed air in the relative coordinates is aligned with the mean camber line of the blade leading edge. The main objective of this research is to present an improved design for tip injection as well as to determine its effect on the stability enhancement of the compressor. The current research also provides guidelines to an optimum design of tip injection.Copyright

The degradation of abradable honeycomb labyrinth seal performance due to wear

This feature presents work undertaken to improve the understanding of the behaviour and performance of worn abradable honeycomb labyrinth seals. An experimental programme, backed up by CFD analysis, has investigated a range of clearances and pressure ratios plus the effect of tooth to groove location against a baseline using a flat metal stator. Significant increases in mass flow have been recorded when using a worn honeycomb stator. However, these increases can be reduced by running with the labyrinth teeth displaced with respect to the groove.

Stall inception in a transonic axial fan

Abstract The current paper reports a numerical investigation of stall inception in a transonic compressor rotor, NASA Rotor-67, by using the whole flow passages in the computations. Surface roughness is added to one of the blades in order to trigger rotating stall. During stall inception, the tip clearance vortex moved away from the suction surface of the roughened blades upper neighbour, leading to vortex breakdown. The stall cell was found to propagate opposite the direction of the blade rotation at about 30 per cent of rotational speed. The effect of air bleeding on stabilizing the compressor is also studied in this work. The mean mass flowrate removed from the bleed valves was ∼1.2 per cent of the mainstream flowrate. This amount of bleeding was found to effectively suppress the stalling disturbances.

Investigation of Closed Valve Operation Using Computational Fluid Dynamics

Predicting the head of a centrifugal pump operating at closed valve remains a difficult task. The nature of the flow regime and the influence of geometric features on this flow is uncertain. In this paper both the flow regime and the influence on that regime by geometry is investigated using a commercially available CFD RANS code. A CFD methodology is presented that takes account of the difficult boundary conditions. This methodology is then used to present the flow regime in a volute pump against the background of available research. The theory of solid body rotation, as a major influence on the closed valve head, is shown by the CFD simulations to be analogous to but not representative of the actual flow regime. The nature of the flow is impulsive and unsteady with fluid interchange occurring between the pump collector and the impeller vane passages. At the inlet the pump impeller experiences a strong steady outflow from the impeller blade tip. This spiralling flow must be accommodated within the computational solution. The impeller outlet is filled with a vortex driven by the flow which cannot be accommodated within the stalled stator passageways. The annular gap between the impeller and collector is filled with a pulsating flow whose frequency is determined by the number of vanes within the impeller. Model validation was carried out by reference to experimental papers and time averaged closed valve head values obtained under standard performance testing.© 2009 ASME

Experimental Investigation of the Influence of Fouling on Compressor Cascade Characteristics and Implications for Gas Turbine Engine Performance

This article describes the findings of a study which examined the influence of fouling on the behaviour of a cascade and by making use of these results the performance implications for gas turbine engines of exposure to airborne foulants. A suction-type compressor cascade tunnel with a plenum chamber was employed for investigating fouling blade effects. The tests showed that such a testing arrangement allows the extraction of pressure and corrected velocity distribution data downstream of the blades that is comparable with what can be obtained from blow-type cascade tunnels. This study presents experimental results for smooth clean cascade blades and for uniformly fouled blades. For all the cases considered, mid-span-corrected velocity distributions and pressure losses taken one chord downstream of the blades were investigated in order to identify the effects of fouling on the blades. The result of fouling on exit flow angle was investigated as well. In the present study, cascade clean and fouled cases were used to predict real engine performance. Results are obtained in terms of stage polytropic efficiency, thermal efficiency, useful power, and compressor efficiency deterioration. Roughening the cascade blades uniformly with particles of 254 μ m size, the compressor efficiency dropped by 7.7 percentage points.