Asad Asghar

Influence of a Novel 3D Leading Edge Geometry on the Aerodynamic Performance of Low Pressure Turbine Blade Cascade Vanes

This paper addresses the issue of aerodynamic performance of a novel 3D leading edge modification to a reference low pressure turbine blade. An analysis of tubercles found in nature and used in some engineering applications was employed to synthesize new leading edge geometry. A sinusoidal wave-like geometry characterized by wavelength and amplitude was used to modify the leading edge along the span of a 2D profile, rendering a 3D blade shape. The rationale behind using the sinusoidal leading edge was that they induce streamwise vortices at the leading edge which influence the separation behaviour downstream. Surface pressure and total pressure measurements were made in experiments on a cascade rig. These were complemented with computational fluid dynamics studies where flow visualization was also made from numerical results. The tests were carried out at low Reynolds number of 5.5 × 104 on a well-researched profile representative of conventional low pressure turbine profiles. The performance of the new 3D leading edge geometries was compared against the reference blade revealing a downstream shift in separated flow for the LE tubercle blades; however, total pressure loss reduction was not conclusively substantiated for the blade with leading edge tubercles when compared with the performance of the baseline blade. Factors contributing to the total pressure loss are discussed.Copyright

Numerical Investigation of the Influence of Real World Blade Profile Variations on the Aerodynamic Performance of Transonic Nozzle Guide Vanes

This paper addresses the issue of aerodynamic consequences of variations in airfoil profile. An analysis of new and repaired airfoils was used to synthesize profiles representative of specific repair types. Five variations of a reference new low pressure turbine vane were obtained by changing the characteristic parameters of trailing-edge tweaking and laminate-repair methods used to refurbish turbine vanes. Flow visualization of shock structure and total pressure measurements were made by experimentation in a cascade rig and by calculations through Computational Fluid Dynamics (CFD). The performance of the modified profiles was compared with that of the reference new vane. The total pressure losses increased when the profile was bent at the trailing edge towards the pressure side. The losses for synthesized laminate repair profiles increased with an increase in the thickness of laminate repair. The numerical results were used to supplement experimental results in cases where the experimental conditions were not representative of typical design operating conditions.Copyright

Experimental Evaluation of Service-Exposed Nozzle Guide Vane Damage in a Rolls Royce A-250 Gas Turbine

A unique methodology and test rig was designed to evaluate the degradation of damaged nozzle guide vanes (NGVs) in a transonic annular cascade in the short duration facility at the Royal Military College. A custom test section was designed which featured a novel rotating instrumentation suite. This permitted 360 deg multispan traverse measurements downstream from unmodified turbine NGV rings from a Rolls-Royce/Allison A-250 turbo-shaft engine. The downstream total pressure was measured at four spanwise locations on both an undamaged reference and a damaged test article. Three performance metrics were developed in an effort to determine characteristic signatures for common operational damage such as trailing edge bends or cracked trailing edges. The highest average losses were observed in the root area, while the lowest occurred closer to the NGV tips. The results from this study indicated that multiple spanwise traverses were required to detect localized trailing edge damage. Recommendations are made for future testing and to further develop performance metrics.

Performance Evaluation of an S-Duct Diffuser of a Flight-Vehicle Inlet in High-Subsonic Flow

The characteristic aerodynamics of inlets in a fuselage-embedded propulsion system of an air-vehicle vary from one configuration to other, making it necessary to document the performance of each and every type of inlet in various flight conditions. This paper focuses on the internal performance evaluation of a baseline S-duct diffuser for a future parametric investigation of a generic S-duct inlet. The generic baseline was a rectangular-entrance, transitioning S-duct diffuser in high subsonic (Mach number > 0.8) flow. The test section was manufactured using rapid prototyping for facilitating a future parametric investigation of geometry. Streamwise static pressure and exit-plane total pressure were measured in a test-rig using surface pressure taps and a 5-probe rotating rake, respectively and was simulated through computational fluid dynamics. The investigation indicated the presence of streamwise and circumferential pressure gradients leading to three dimensional flow in the S-duct diffuser and distortion at the exit plane. Total pressure losses and circumferential and radial distortions at the exit plane were higher than that of the podded nacelle type of inlet. The work represents the beginning of the development of a database for the performance of a particular type of generic inlet. This database will be useful for predicting the performance of aero-engines and air vehicles in high subsonic flight.Copyright

Application of Complex Demodulation for Pseudo-Key-Phasor Recovery From Fast-Response Pressure Measurements

When digitizing the output of fast-response pressure transducers installed on rotating machinery, it is often desirable to use a phase-synchronized method. The mechanical design of many turbomachines, particularly those not originally designed for a research application, can make it difficult to install the physical key-phasor needed to acquire phase-synchronized measurements.A method of phase-synchronization using a pseudo-key-phasor is presented in this paper. The technique applies complex demodulation to recover a pseudo-key-phasor signal from the blade-passing signal recorded in sampled data. The recovered pseudo-key-phasor is then used to digitally resample the data at a constant phase angle, removing the effect of small rotor speed variations. Example applications of this technique to vibration measurements can be found in the literature; however, examples of application to rotor pressure measurement were not.The technique has been applied to fast-response pressure measurements taken on the shroud of a high speed centrifugal compressor. It was found that this technique was able to remove the effect of rotor speed variations from data sampled with equal time intervals, making them suitable for phase averaging.Copyright

Influence of Freestream Turbulence on the Aerodynamic Performance of Transonic Vanes

The present authors have reported a noticeable reduction in the aerodynamic performance of turbine vanes which had been modified as a result of commonly applied repair processes. These tests were done at a low turbulence level to isolate the profile-only effect. In the present research, the effect of the same profile modification on the performance of the vanes was investigated at engine representative flow conditions by increasing the turbulence level and length scale. Since the tested vane profiles in the present research were synthesized using the profile of LPT vanes, the turbulence level was maintained at around 4% and the length scale was set at 2 cm. In the present investigation, calculations with computational fluid dynamics and measurements in a transonic cascade rig were carried out. The high turbulence level in the cascade rig was produced using a passive turbulence-generating grid and in CFD by specifying the desired level and length scale. Coordinates of the baseline profile were obtained from the LPT vanes of an in-service turboshaft engine using 3D optical scanning and digital modeling. The repaired vanes were synthesized using profiles representative of two specific repair types. In both methods, flow visualization was carried out using axial density gradient or schlieren and exit total pressure was obtained numerically or using a multihole probe. Further insight into the flow phenomenon was obtained by surface flow visualization in the cascade rig using a graphite and paraffin oil mixture and by computed surface pressure distributions on the vane. The shock pattern in the cascade for low and high turbulence flows was similar; however, the surface flow pattern exhibited a significant difference for the two conditions. The total pressure ratio and cascade loss also showed some differences.Copyright

Characterization of the Discharge Flow of a Dual Volute Axi-Centrifugal Gas Turbine Compressor

To contribute to further understanding of the unique dual-discharge volute design of the Rolls-Royce Allison 250-C20B engine, a rig has been constructed to characterize the compressor discharge flow. A compressor module for this gas turbine engine was powered by an automotive engine in parallel with a modified turbine module operating in the cold compressor discharge flow. Continuous operation at 70% and 80% speed has been achieved while conducting simultaneous discharge pressure traverses. A non-uniform discharge Mach number profile was observed in each port with no significant asymmetry. Evidence of the influence of the volute geometry was observed in the discharge Mach number profile. The discharge mass flow rate from each port was observed to differ by as much as 4%. This observation could offer an explanation for the evidence of regions of higher turbine inlet temperature that has been observed by others.

Influence of a Novel Three-Dimensional Leading Edge Geometry on the Aerodynamic Performance of Transonic Cascade Vanes

This paper addresses the issue of aerodynamic performance of a novel 3D leading edge modification to a reference vane. An analysis of tubercles found in nature and some engineering applications was used to synthesize new leading edge geometry. Three variations of the reference low pressure turbine vane were obtained by changing the characteristic parameters of the tubercles. Shock structure, surface flow visualization and total pressure measurements were made through experiments in a cascade rig, as well as through computational fluid dynamics. The tests were carried out at design zero incidence and off-design ±10-deg and ±5-deg incidences. The performance of the new 3D leading edge geometries was compared against the reference vane. Some leading edge tubercle configurations were effective at decreasing total pressure losses at positive inlet incidence angles. Numerical results supplemented experimental results.Copyright

Assessment of the Flow Quality of a Transonic Turbine Cascade

The assessment of flow quality through a newly constructed transonic turbine cascade is presented. Although the main objective of this research was to investigate the effect of the modification of a vane profile due to repair on pressure loss, only the results for checking the flow periodicity, two-dimensionality of the flow and transonic exit flow condition are described in this paper. The cascade blades were constructed using the profiles of nozzle guide vanes of a low pressure turbine of an in-service turboshaft engine. The assessment of the flow quality in the cascade was carried out using three methods: wall static pressure measurements at the inlet and exit of each flow passage of the cascade to check the flow periodicity, surface flow visualization using blackened paraffin oil to check the two dimensionality of the flow and thirdly, Schlieren flow visualization to verify the periodicity and transonic flow conditions at the exit of the cascade. The cascade inlet and exit wall pressure showed that the flow was nominally periodic in the cascade. The surface flow visualization of the suction surface showed that the flow was two-dimensional on approximately 70% of the central span and also indicated flow separations on the suction surface. The Schlieren flow visualization confirmed the flow periodicity and revealed the existence of shock waves on the suction surface and near the trailing edge of the blades.Copyright