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Research Papers

J. Turbomach. 2018;140(6):061001-061001-9. doi:10.1115/1.4039181.

Fouling and erosion are two pressing problems that severely affect gas turbine performance and life. When aircraft fly through a volcanic ash cloud, the two phenomena occur simultaneously in the cold as well as in the hot section of the engine. In the high-pressure turbine (HPT), in particular, particles soften or melt due to the high gas temperatures and stick to the wet surfaces. The throat area, and hence the capacity, of the HPT is modified by these phenomena, affecting the engine stability and possibly forcing engine shutdown. This work presents a model for deposition and erosion in gas turbines and its implementation in a three-dimensional Navier–Stokes solver. Both deposition and erosion are taken into account, together with deposit detachment due to changed flow conditions. The model is based on a statistical description of the behavior of softened particles. The particles can stick to the surface or can bounce away, eroding the material. The sticking prediction relies on the authors' Energy Based FOulinG (EBFOG) model. The impinging particles which do not stick to the surface are responsible for the removal of material. The model is demonstrated on a HPT vane. The airfoil shape evolution over the exposure time as a consequence of the impinging particles has been carefully monitored. The variation of the flow field as a consequence of the geometrical changes is reported as an important piece of on-board information for the flight crew.

Commentary by Dr. Valentin Fuster
J. Turbomach. 2018;140(6):061002-061002-10. doi:10.1115/1.4038834.

During engine operation, fan casing abradable liners are worn by the blade tip, resulting in the formation of trenches. This paper describes the influence of these trenches on the fan blade tip aerodynamics. A detailed understanding of the fan tip flow features for cropped and trenched clearances is first developed. A parametric model is then used to model trenches in the casing above the blade tip and varying blade tip positions. It is shown that increasing clearance via a trench reduces performance by less than increasing clearance through cropping the blade tip. A response surface method is then used to generate a model that can predict fan efficiency for a given set of clearance and trench parameters. This model can be used to influence fan blade design and understand engine performance degradation in service. It is shown that an efficiency benefit can be achieved by increasing the amount of tip rubbing, leading to a greater portion of the tip clearance sat within the trench. It is shown that the efficiency sensitivity to clearance is biased toward the leading edge (LE) for cropped tips and the trailing edge (TE) for trenches.

Commentary by Dr. Valentin Fuster
J. Turbomach. 2018;140(6):061003-061003-8. doi:10.1115/1.4039180.

A fast-response pressure-sensitive paint (PSP) technique was applied to the measurement of unsteady surface pressure of an oscillating cascade blade in a transonic flow. A linear cascade was used, and its central blade was oscillated in a translational manner. The unsteady pressure distributions of the oscillating blade and two stationary neighbors were measured using the fast-response PSP technique, and the unsteady aerodynamic force on the blade was obtained by integrating the data obtained on the pressures. The measurements made with the PSP technique were compared with those obtained by conventional methods for the purpose of validation. From the results, the PSP technique was revealed to be capable of measuring the unsteady surface pressure, which is used for flutter analysis in transonic conditions.

Topics: Pressure , Blades
Commentary by Dr. Valentin Fuster
J. Turbomach. 2018;140(6):061004-061004-14. doi:10.1115/1.4039053.

The phenomena prior to rotating stall were investigated in a high-speed compressor test rig using optical and pneumatic measurement techniques. A number of throttling procedures were performed at transonic and subsonic speedlines with the aim to detect the unsteady effects initiating rotating stall or large amplitude blade vibrations. At transonic speed, radial vortices traveling around the circumference were detected in the upstream part of the rotor using phase-locked particle-image-velocimetry (PIV) measurements above 92% span and unsteady wall pressure measurements. When these radial vortices impinge on a blade leading edge (LE), they cause a forward spill of fluid around the LE. The effects are accompanied by a large-scale vortex breakdown in the blade passage leading to immense blockage in the endwall region. At subsonic speeds, the observed flow phenomena are similar but differ in intensity and structure. During the throttling procedure, blade vibration amplitudes were monitored using strain gauges (SG) and blade tip timing instrumentation. Nonsynchronous blade vibrations in the first torsional eigenmode were measured as the rotor approached stall. Using the different types of instrumentation, it was possible to align the aerodynamic flow features with blade vibration levels. The results show a clear correlation between the occurrence of radial vortices and blade vibrations.

Commentary by Dr. Valentin Fuster
J. Turbomach. 2018;140(6):061005-061005-12. doi:10.1115/1.4038982.

Uncertainty quantification (UQ) is an increasingly important area of research. As components and systems become more efficient and optimized, the impact of uncertain parameters is likely to become critical. It is fundamental to consider the impact of these uncertainties as early as possible during the design process, with the aim of producing more robust designs (less sensitive to the presence of uncertainties). The cost of UQ with high-fidelity simulations becomes therefore of fundamental importance. This work makes use of least-squares approximations in the context of appropriately selected polynomial chaos (PC) bases. An efficient technique based on QR column pivoting has been employed to reduce the number of evaluations required to construct the approximation, demonstrating the superiority of the method with respect to full-tensor quadrature (FTQ) and sparse-grid quadrature (SGQ). Orthonormal polynomials used for the PC expansion are calculated numerically based on the given uncertainty distribution, making the approach optimal for any type of input uncertainty. The approach is used to quantify the variability in the performance of two large bypass-ratio jet engine fans in the presence of shape uncertainty due to possible manufacturing processes. The impacts of shape uncertainty on the two geometries are compared, and sensitivities to the location of the blade shape variability are extracted. The mechanisms at the origin of the change in performance are analyzed in detail, as well as the differences between the two configurations.

Commentary by Dr. Valentin Fuster
J. Turbomach. 2018;140(6):061006-061006-10. doi:10.1115/1.4039049.

In this paper, the effect of a novel honeycomb tip on suppressing tip leakage flow in turbine cascade has been experimentally and numerically studied. Compared to the flat tip cascade with 1%H blade height, the relative leakage flow in honeycomb tip cascade reduces from 3.05% to 2.73%, and the loss also decreases by 8.24%. For honeycomb tip, a number of small vortices are rolled up in the regular hexagonal honeycomb cavities to dissipate the kinetic energy of the clearance flow, and the fluid flowing into and out the cavities create aerodynamic interceptions to the upper clearance flow. As a result, the flow resistance in the clearance increased and the velocity of leakage flow reduced. As the gap height increases, the tip leakage flow and loss changes proportionally, but the growth rate in the honeycomb tip cascade is smaller. Considering its wear resistance of the honeycomb seal, a smaller gap height is allowed in the cascade with honeycomb tip, and that means honeycomb tip has better effect on suppressing leakage flow. Part honeycomb tip structure also retains the effect of suppressing leakage flow. It shows that locally convex honeycomb tip has better suppressing leakage flow effect than the whole honeycomb tip, but locally concave honeycomb tip is slightly less effective.

Commentary by Dr. Valentin Fuster
J. Turbomach. 2018;140(6):061007-061007-9. doi:10.1115/1.4039361.

This effort focuses on the comparison of unsteadiness due to as-measured turbine blades in a transonic turbine to that obtained with blueprint geometries via computational fluid dynamics (CFD). A Reynolds-averaged Navier–Stokes flow solver with the two-equation Wilcox turbulence model is used as the numerical analysis tool for comparison between the blueprint geometries and as-manufactured geometries obtained from a structured light optical measurement system. The nominal turbine CFD grid data defined for analysis of the blueprint blade were geometrically modified to reflect as-manufactured turbine blades using an established mesh metamorphosis algorithm. The approach uses a modified neural network to iteratively update the source mesh to the target mesh. In this case, the source is the interior CFD surface grid while the target is the surface blade geometry obtained from the optical scanner. Nodes interior to the CFD surface were updated using a modified iterative spring analogy to avoid grid corruption when matching as-manufactured part geometry. This approach avoids the tedious manual approach of regenerating the CFD grid and does not rely on geometry obtained from coordinate measurement machine (CMM) sections, but rather a point cloud representing the entirety of the turbine blade. Surface pressure traces and the discrete Fourier transforms (DFT) thereof from numerical predictions of as-measured geometries are then compared both to blueprint predictions and to experimental measurements. The importance of incorporating as-measured geometries in analyses to explain deviations between numerical predictions of blueprint geometries and experimental results is readily apparent. Further analysis of every casting produced in the creation of the test turbine yields variations that one can expect in both aero-performance and unsteady loading as a consequence of manufacturing tolerances. Finally, the use of measured airfoil geometries to reduce the unsteady load on a target blade in a region of interest is successfully demonstrated.

Commentary by Dr. Valentin Fuster
J. Turbomach. 2018;140(6):061008-061008-9. doi:10.1115/1.4039727.

Computational fluid dynamics (CFD) is nowadays extensively used for turbomachinery design and performance prediction. Nevertheless, compressors numerical simulations still fail in correctly predicting the stall inception and the poststall behavior. Several authors address such a lack of accuracy to the incomplete definition of the boundary conditions and of the turbulence parameters at the inlet of the numerical domain. The aim of the present paper is to contribute to the development of compressors CFD by providing a complete set of input data for numerical simulations. A complete characterization has been carried out for a state-of-art 1.5 stage highly loaded low-pressure compressor for which previous CFD analyses have failed to predict its behavior. The experimental campaign has been carried out in the R4 facility at the Von Karman Institute for Fluid Dynamics (VKI). The test item has been tested in different operative conditions for two different speed lines (90% and 96% of the design speed) and for two different Reynolds numbers. Stable and unstable operative conditions have been investigated along with the stalling behavior, its inception, and the stall-cell flow field. Discrete hot-wire traverses have been performed in order to characterize the spanwise velocity field and the turbulence characteristics.

Commentary by Dr. Valentin Fuster
J. Turbomach. 2018;140(6):061009-061009-11. doi:10.1115/1.4039362.

This two-part paper deals with the influence of high-pressure turbine (HPT) purge flows on the aerodynamic performance of turbine center frames (TCF). Measurements were carried out in a product-representative one and a half-stage turbine test setup. Four individual purge mass flows differing in flow rate, pressure, and temperature were injected through the hub and tip, forward and aft cavities of the unshrouded HPT rotor. Two TCF designs, equipped with nonturning struts, were tested and compared. In this first part of the paper, the influence of different purge flow rates (PFR) is discussed, while in the second part of the paper, the impact of the individual hub and tip purge flows on the TCF aerodynamics is investigated. The acquired measurement data illustrate that the interaction of the ejected purge flow with the main flow enhances the secondary flow structures through the TCF duct. Depending on the PFR, the radial migration of purge air onto the strut surfaces directly impacts the loss behavior of the duct. The losses associated with the flow close to the struts and in the strut wakes are highly dependent on the relative position between the HPT vane and the strut leading edge (LE), as well as the interaction between vane wake and ejected purge flow. This first-time experimental assessment demonstrates that a reduction in the purge air requirement benefits the engine system performance by lowering the TCF total pressure loss.

Commentary by Dr. Valentin Fuster
J. Turbomach. 2018;140(6):061010-061010-8. doi:10.1115/1.4039363.

The aerodynamic behavior of turbine center frame (TCF) ducts under the presence of high-pressure turbine (HPT) purge flows was experimentally investigated in this two-part paper. While the first part of the paper demonstrated the impact of varying the purge flow rates (PFR) on the loss behavior of two different TCF designs, the second part concentrates on the influence of individual hub and tip purge flows on the main flow evolution and loss generation mechanisms through the TCF ducts. Therefore, measurements were conducted at six different operating conditions in a one and a half stage turbine test setup, featuring four individual purge flows injected through the hub and tip, forward and aft cavities of the HPT rotor. The outcomes of this first-time assessment indicate that a HPT purge flow reduction generally benefits TCF performance. Decreasing one of the rotor case PFRs leads to an improved duct pressure loss. The purge flows from the rotor aft hub and tip cavities are demonstrated to play a particularly important role for improving the duct aerodynamic behavior. In contrast, the forward rotor hub purge flow actually stabilizes the flow in the TCF duct and reducing this purge flow can penalize TCF performance. These particular HPT/TCF interactions should be taken into account whenever high-pressure turbine purge flow reductions are pursued.

Commentary by Dr. Valentin Fuster

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