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Research Papers

J. Turbomach. 2015;137(10):101001-101001-12. doi:10.1115/1.4030308.

The present work is a contribution to understanding the windmilling operation of low-speed fans. Such an operating situation is described in the literature, but the context (mainly windmilling of aero-engines) often involves system dependence in the analysis. Most of the time, only regimes very close to the free-windmilling are considered. A wider range is analyzed in the present study, since the context is the examination of the energy recovery potential of fans. It aims at detailing the isolated contribution of the rotor, which is the only element exchanging energy with the flow. Other elements of the system (including the stator) can be considered as loss generators and be treated as such in an integrated approach. The evolution of the flow is described by the use of theoretical and experimental data. A theoretical model is derived to predict the operating trajectories of the rotor in two characteristic diagrams. A scenario is proposed, detailing the local evolution of the flow when a gradual progression toward free and load-controlled windmilling operation is imposed. An experimental campaign exerted on two low-speed fans aims at the analysis of both the local and global aspects of the performance, for validation. From a global point of view, the continuity of the operating trajectory is predicted and observed across the boundary between the quadrants of the diagrams. The flow coefficient value for the free-windmilling operation is fairly well predicted. From a local point of view, the local co-existence of compressor and turbine operating modes along the blade span is observed as previously reported. It is further demonstrated here that this configuration is not exclusive to free-windmilling operation and occurs inside a range that can be theoretically predicted. It is shown that for a given geometry, this local topology strongly depends on the value of the flow coefficient and is very sensitive to the inlet spanwise velocity distribution.

Commentary by Dr. Valentin Fuster
J. Turbomach. 2015;137(10):101002-101002-11. doi:10.1115/1.4030258.

During extreme low volume flow conditions, the last stages of a low pressure steam turbine operate in ventilation conditions that can cause a significant temperature increase of critical regions of the last stage moving blade (LSB). Under some conditions, the blade temperature may rise above a safe operating temperature, requiring the machine to be shut down. Limiting the heating effect on the LSB increases the allowable operating range of the low pressure turbine. One common method is to spray water droplets into the low pressure exhaust. As the length of LSBs continues to increase, this method reaches its limit of practical operating effectiveness due to the amount of water required and its impact on the erosion of the LSB. An investigation into complimentary solutions to limit the temperature increase was conducted using CFD. An appropriate CFD setup was chosen from a sensitivity study on the effects from geometry, mesh density, turbulence model, and time dependency. The CFD results were verified against steam turbine data from a scaled test facility. The proposed solutions include low temperature steam extraction, targeted for critical regions of the moving blade. From the test turbine and CFD results, the drivers of the temperature increase during ventilation conditions are identified and described.

Commentary by Dr. Valentin Fuster
J. Turbomach. 2015;137(10):101003-101003-8. doi:10.1115/1.4030259.

Temperature profiling of components in gas turbines is of increasing importance as engineers drive to increase firing temperatures and optimize component’s cooling requirements in order to increase efficiency and lower CO2 emissions. However, on-line temperature measurements and, particularly, temperature profiling are difficult, sometimes impossible, to perform due to inaccessibility of the components. A desirable alternative would be to record the exposure temperature in such a way that it can be determined later, off-line. The commercially available thermal paints are toxic in nature and come with a range of technical disadvantages such as subjective readout and limited durability. This paper proposes a novel alternative measurement technique which the authors call thermal history paints and thermal history coatings. These can be particularly useful in the design process, but further could provide benefits in the maintenance area where hotspots which occurred during operation can be detected during maintenance intervals when the engine is at ambient temperature. This novel temperature profiling technique uses optical active ions in a ceramic host material. When these ions are excited by light they start to phosphoresce. The host material undergoes irreversible changes when exposed to elevated temperatures and since these changes are on the atomic level they influence the phosphorescent properties such as the life time decay of the phosphorescence. The changes in phosphorescence can be related to temperature through calibration such that in situ analysis will return the temperature experienced by the coating. A major benefit of this technique is in the automated interpretation of the coatings. An electronic instrument is used to measure the phosphorescence signal eliminating the need for a specialist interpreter, and thus increasing readout speed. This paper reviews results from temperature measurements made with a water-based paint for the temperature range 100–800 °C in controlled conditions. Repeatability of the tests and errors are discussed. Further, some measurements are carried out using an electronic hand-held interrogation device which can scan a component surface and provide a spatial resolution of below 3 mm. The instrument enables mobile measurements outside of laboratory conditions. Further, a robust thermal history coating is introduced demonstrating the capability of the coating to withstand long term exposures. The coating is based on thermal barrier coating (TBC) architecture with a high temperature bondcoat and deposited using an air plasma spray process to manufacture a reliable long lasting coating. Such a coating could be employed over the life of the component to provide critical temperature information at regular maintenance intervals for example indicating hot spots on engine parts.

Commentary by Dr. Valentin Fuster
J. Turbomach. 2015;137(10):101004-101004-9. doi:10.1115/1.4030260.

Existing thermal barrier coatings (TBCs) can be adapted enhancing their functionalities such that they not only protect critical components from hot gases but also can sense their own material temperature or other physical properties. The self-sensing capability is introduced by embedding optically active rare earth ions into the thermal barrier ceramic. When illuminated by light, the material starts to phosphoresce and the phosphorescence can provide in situ information on temperature, phase changes, corrosion, or erosion of the coating subject to the coating design. The integration of an on-line temperature detection system enables the full potential of TBCs to be realized due to improved accuracy in temperature measurement and early warning of degradation. This in turn will increase fuel efficiency and will reduce CO2 emissions. This paper reviews the previous implementation of such a measurement system into a Rolls-Royce jet engine using dysprosium doped yttrium-stabilized-zirconia (YSZ) as a single layer and a dual layer sensor coating material. The temperature measurements were carried out on cooled and uncooled components on a combustion chamber liner and on nozzle guide vanes (NGVs), respectively. The paper investigates the interpretation of those results looking at coating thickness effects and temperature gradients across the TBC. For the study, a specialized cyclic thermal gradient burner test rig was operated and instrumented using equivalent instrumentation to that used for the engine test. This unique rig enables the controlled heating of the coatings at different temperature regimes. A long-wavelength pyrometer was employed detecting the surface temperature of the coating in combination with the phosphorescence detector. A correction was applied to compensate for changes in emissivity using two methods. A thermocouple was used continuously measuring the substrate temperature of the sample. Typical gradients across the coating are less than 1 K/μm. As the excitation laser penetrates the coating, it generates phosphorescence from several locations throughout the coating and hence provides an integrated signal. The study successfully proved that the temperature indication from the phosphorescence coating remains between the surface and substrate temperature for all operating conditions. This demonstrates the possibility to measure inside the coating closer to the bond coat. The knowledge of the bond coat temperature is relevant to the growth of the thermally grown oxide (TGO) which is linked to the delamination of the coating and hence determines its life. Further, the data are related to a one-dimensional phosphorescence model determining the penetration depth of the laser and the emission.

Commentary by Dr. Valentin Fuster
J. Turbomach. 2015;137(10):101005-101005-9. doi:10.1115/1.4030498.

The main focus of this work is on the geometrical modifications can be made to the fan wheel and to the volute tongue of a radial fan to reduce the tonal noise. The experimental measurements are performed by using the in-duct method in accordance with ISO 5136. In addition to the experimental measurements, CFD (computational fluid dynamics) and CAA (computational aeroacoustics) simulations are carried out to investigate the effects of different modifications on noise and performance of the fan. It is shown that by modifying the blade outlet angle, the tonal noise of the fan can be reduced without impairing its aerodynamic performance. Moreover, it is indicated that increasing the number of blades leads to a significant reduction in the tonal noise and also an improvement in the aerodynamic performance. However, this trend is only valid up to a certain number of blades, and a further increment might reduce the aerodynamic performance of the fan. Besides modifying the impeller geometry, new volute tongues are designed and tested on the rig. It is demonstrated that the shape of the volute tongue plays an important role in the tonal noise generation of the fan. Moreover, in order to find out whether or not it is possible to reduce the tonal noise level through a destructive phase-shift generation, stepped tongues are comprehensively investigated by means of numerical simulations and experimental measurements.

Commentary by Dr. Valentin Fuster
J. Turbomach. 2015;137(10):101006-101006-1. doi:10.1115/1.4030395.

Studies of film cooling holes embedded in craters and trenches have shown significant improvements in the film cooling performance. In this paper, a new design of a round film cooling hole embedded in a contoured crater is proposed for improved film cooling effectiveness over existing crater designs. The proposed design of the contour aims to generate a pair of vortices that counter and diminish the near-field development of the main kidney-pair vortex generated by the film cooling jet. With a weakened kidney-pair vortex, the coolant jet is expected to stay closer to the wall, reduce mixing, and therefore increase cooling effectiveness. In the present study, the performance of the proposed contoured crater design is evaluated for depth between 0.2D and 0.75D. A round film cooling hole with a 35 deg inclined short delivery tube (l/D = 1.75), freestream Reynolds number ReD = 16,000, and density ratio of coolant to freestream fluid ρj = 2.0 is used as the baseline case. Hydrodynamic and thermal fields for all cases are investigated numerically using large eddy simulation (LES) technique. The baseline case results are validated with published experimental data. The performance of the new crater design for various crater depths and blowing ratios are compared with the baseline case. Results are also compared with other reported crater designs with similar flow conditions and crater depth. Performance improvement in cooling effectiveness of over 100% of the corresponding baseline case is observed for the contoured crater.

Commentary by Dr. Valentin Fuster
J. Turbomach. 2015;137(10):101007-101007-15. doi:10.1115/1.4030790.

This paper presents a computational study for a high-speed centrifugal compressor stage with a design pressure ratio equal to 4, the stage consisting of a splittered unshrouded impeller and a wedged vaned diffuser. The aim of this paper is to investigate numerically the modifications of the flow structure during a surge cycle. The investigations are based on the results of unsteady three-dimensional, compressible flow simulations, using large eddy simulation (LES) model. Instantaneous and mean flow field analyses are presented in the impeller inducer and in the vaned diffuser region through one surge cycle time intervals. The computational data compare favorably with the measured data, from the literature, for the same compressor and operational point. The surge event phases are well detected inside the impeller and diffuser. The time-averaged loading on the impeller main blade is maximum near the trialing edge and near the tip. The amplitude of the unsteady pressure fluctuation is maximum for the flow reversal condition and reaches values up to 70% of the dynamic pressure. The diffuser vane exhibits high-pressure fluctuation from the vane leading edge to 50% of the chord length. High-pressure fluctuation is detected during the forward flow recovery condition as a result of the shock wave that moves toward the diffuser outlet.

Commentary by Dr. Valentin Fuster
J. Turbomach. 2015;137(10):101008-101008-7. doi:10.1115/1.4030791.

Geometric variability increases performance variability and degrades the mean performance of turbomachinery compressor blades. These detrimental effects can be reduced by using robust optimization to design the blade geometry or by imposing stricter manufacturing tolerances. This paper presents a novel computational framework for optimizing compressor blade manufacturing tolerances and incorporates this framework into existing robust geometry design frameworks. Optimizations of an exit guide vane geometry are conducted. When the design is optimized to improve performance at a single operating point, the optimal geometry is found to depend on the manufacturing tolerances due to a switch in the dominant loss mechanism. Including multiple operating points in the optimization avoids this switch so that the geometry and tolerance optimization problems are decoupled.

Commentary by Dr. Valentin Fuster
J. Turbomach. 2015;137(10):101009-101009-9. doi:10.1115/1.4030809.

In this paper, the influence of nonuniform bleed extraction on the stability of an axial flow compressor is quantified. Nonuniformity can be caused by several geometric factors (for example, plenum chamber size or number of off-take ducts), and a range of configurations is examined experimentally in a single stage compressor. It is shown that nonuniform bleed leads to a circumferential distribution of flow coefficient and swirl angle at inlet to the downstream stage. The resultant distribution of rotor incidence causes stall to occur at a higher flow coefficient than if the same total bleed rate had been extracted uniformly around the circumference. A connection is made between the analysis of nonuniform bleed extraction and the familiar DCθ criterion used to characterize inlet total pressure distortion. The loss of operating range caused by the nonuniform inlet flow correlates with the peak sector-averaged bleed nonuniformity for all the bleed configurations tested.

Commentary by Dr. Valentin Fuster

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