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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

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