•A second-order cooling concept “phantom cooling” was introduced.•Flat tip shows a better phantom cooling performance than squealer tip.•Increasing tip clearance weakens the tip phantom cooling ...performance.•Squealer tip induces smaller aerodynamic loss.
The blade tip region of the gas turbine is exposed to an extreme operating condition with a high heat load. The conventional cooling concept that places cooling holes on the tip would consume a lot of cooling air and lead to a reduction in thermal efficiency. Whereas phantom cooling was supposed to help cool the tip region without using additional cooling air. In this paper, computational comparisons were conducted to forecast the tip phantom cooling effects caused by the blade ejections by employing the standard k-ω model. Then, phantom cooling performance was presented under the flat tip (FT) and the squealer tip (ST), with three tip clearances. Results show that the FT shows a better phantom cooling performance compared to the ST. The phantom cooling effects on the FT are distributed at the tip forepart and near the pressure side (PS). Whereas, for the ST, traces of phantom cooling are barely detected on the PS rim. Increasing the tip clearance would weaken the phantom cooling performance on the FT forepart but enhance it on the rear part. For the ST, its phantom cooling effectiveness values decrease as the tip clearance increases. A lower aerodynamic loss is obtained for the ST under any coolant MFR or tip clearance.
Novel methods are proposed to expand the capability of a blade tip timing (BTT) system. Simultaneously measurement of blade tip clearance (BTC) and blade tip velocity (BTV) is realized. In terms of ...hardware, a convex lens is used at the emitting end of the laser fiber to adjust the optical path. In the software part, a new BTT protocol is designed where both the time-of-arrival and time-of-leaving as a blade sweeps across the detecting region are recorded. Meanwhile, new algorithms are developed to process the data. The methods are studied experimentally from various aspects. It is demonstrated that when a speed sensor is not available, the BTC and BTV can be measured by a double-lens sensor. The random error of BTC on a steady operational occasion is within 30 μm. Subtle dynamic responses during accelerations can be accurately captured. When a speed sensor is available, a single-lens sensor can be used to measure the BTC. Besides, a double-lens sensor can be automatically calibrated using a Kalman-filter-based algorithm to maintain high precision during long-time online measurement. All the methods have been validated on a compressor rig.
•Measure blade tip deflection, clearance, and velocity using optical blade tip timing.•Establish the linear relationship between the clearance and duration of trigger.•Reduce the rotational speed fluctuation error using a double-lens sensor.•Automatic calibrate the blade constants using a Kalman-filter-based algorithm.
Organic semiconductor gas sensor is one of the promising candidates of room temperature operated gas sensors with high selectivity. However, for a long time the performance of organic semiconductor ...sensors, especially for the detection of oxidizing gases, is far behind that of the traditional metal oxide gas sensors. Although intensive attempts have been made to address the problem, the performance and the understanding of the sensing mechanism are still far from sufficient. Herein, an ultrasensitive organic semiconductor NO2 sensor based on 6,13‐bis(triisopropylsilylethynyl)pentacene (TIPS‐petacene) is reported. The device achieves a sensitivity over 1000%/ppm and fast response/recovery, together with a low limit of detection (LOD) of 20 ppb, all of which reach the level of metal oxide sensors. After a comprehensive analysis on the morphology and electrical properties of the organic films, it is revealed that the ultrahigh performance is largely related to the film charge transport ability, which was less concerned in the studies previously. And the combination of efficient charge transport and low original charge carrier concentration is demonstrated to be an effective access to obtain high performance organic semiconductor gas sensors.
An ultrasensitive organic semiconductor NO2 sensor based on crystalline 6,13‐bis(triisopropylsilylethynyl)pentacene films is achieved with a sensitivity over 1000% ppm–1 and fast response/recovery within 200 s/400 s. The relationship between sensor performance and film charge transport is studied. The low original carrier concentration and efficient charge transport are demonstrated to be key factors for the ultrahigh performance.
In order to investigate the effect of impeller tip clearance on the internal flow fields and the hydraulic losses in mixed-flow pump, the entropy production method with computational fluid dynamics ...(CFD) is employed to analyze the energy losses in a low specified number mixed-flow pump with guide vane. The results show that the size of tip clearance is closely related to the external characteristic performance of mixed-flow pump, and the effect of tip clearance on the flow fields of mixed-flow pump is obvious at design flow rate condition. When the tip clearance raises from 0.2 mm to 1.1 mm, the head drop loss coefficient increases 1.62 times in the impeller. As the tip clearance augments from 0.2 mm to the 1.1 mm, the total entropy production in impeller increases by 142%. Whereas, the total entropy production in guide vane descends by 21.8% slightly. It indicates that the increase of tip leakage flow (TLF) may increase the energy losses in impeller but the hydraulic losses in guide vane is suppressed to some extent as a result of an existence of TLF. Therefore, for the sake of improving the energy performance of mixed-flow pump, it is necessary to take the scale of blade tip clearance into account and consider optimizing the hydraulic design structure of guide vanes comprehensively to match the tip clearance.
•Energy losses induced by tip leakage flow are analyzed by entropy production method.•The effect of tip clearance size on head loss of pump is analyzed in detail.•Entropy production in impeller is positively correlated with tip clearance.•Entropy production in guide vane is negatively correlated with tip clearance.•Tip clearance size and structure of guide vane are essential to optimize the pump.
•A comprehensive review on developments of BTT probes, BTT signal preprocessing and BTT vibration monitoring is carried out, which can help future newcomers to quickly overlook this field.•Three main ...challenges in the BTT technique for blade health monitoring are summarized.•Future directions of the BTT technique are deeply discussed, including but not limited to uncertainty calibration, deep compressed sensing, deep learning, fusion of BTT and BTC.•It is appealed that actual and raw BTT data can be shared in the academic and industrial communities in the future.
As a promising way of on-line and non-intrusive blade vibration monitoring, the blade tip-timing (BTT) technique is currently being explored by almost all gas turbine or aero-engine manufacturers as replacing strain gauges. The principle of BTT seems to be simple, but how to make it effective in reality is indeed an interdisciplinary problem. In particular, aeroengines always operate under extremely complex environment, so that the BTT technique still faces many potential challenges. The main purpose of this paper is to review key developments, summarize possible issues and discuss the trends. After the brief introduction of BTT, three main challenges in BTT-based health monitoring are refined. Then developments of BTT probes, signal preprocessing and vibration monitoring are reviewed comprehensively. Finally, detailed prospects are discussed on future directions including but not limited to uncertainty calibration, deep compressed sensing, deep learning, fusion of BTT and blade tip clearance (BTC). In particular, an appeal is made that actual and raw BTT data can be shared in the academic and industrial communities in the future. This paper should provide a quick guide for future newcomers into this field.
An experimental study of the near wake up to four rotor diameters behind a model wind turbine rotor with two different wing tip configurations is performed. A straight‐cut wing tip and a ...downstream‐facing winglet shape are compared on the same two‐bladed rotor operated at its design tip speed ratio. Phase‐averaged measurements of the velocity vector are synchronized with the rotor position, visualizing the downstream location of tip vortex interaction for the two blade tip configurations. The mean streamwise velocity is found not to be strongly affected by the presence of winglet tip extensions, suggesting an insignificant effect of winglets on the time‐averaged inflow conditions of a possible downstream wind turbine. An analysis of the phase‐averaged vorticity, however, reveals a significantly earlier tip vortex interaction and breakup for the wingletted rotor. In contradistinction, the tip vortices formed behind the reference configuration are assessed to be more stable and start merging into larger turbulent structures significantly further downstream. These results indicate that an optimized winglet design can not only contribute to a higher energy extraction in a rotor's tip region but also can positively affect the wake's mean kinetic energy recovery by stimulating a faster tip vortex interaction.
•The comprehensive performances of VAWT blades with different tips are analyzed.•Twenty types of VAWT blade tips are modeled to perform a fair comparison.•The streamlined endplate further improves ...the blade lift by reducing the drag.•The winglet tip tapered to tailing edge shows significant performance improvements.
Vertical axis wind turbines are receiving renewed attention due to the challenges faced by horizontal axis wind turbines in recent years. To achieve commercial applications, a key aspect is to improve the relatively low power coefficient of vertical axis wind turbines. Among several factors that affects output efficiency, the three-dimensional tip loss effect is essential because each vertical axis wind turbine blade has two tops (e.g., the H-type wind turbine). Although several blade tips have been designed, a comprehensive and fair analysis is necessary to find a suitable one. Therefore, in this paper, a total of 20 blade tips have been analyzed using the three-dimensional computational fluid dynamic method. The results indicate that the improved winglet and endplate we proposed are relatively optimal when the synthetic performance of aerodynamic efficiency, structural loads and start-up torque is considered. However, the improvement mechanisms of these two tips are different. Based on the performance curves and the fluid fields at specific tip speed ratios, we find that the endplate increases the blade torque in the upwind region by reducing the spanwise flow, but its negative effect in the downwind region partially counteracts the improvement. For the improved winglet, it reduces the tip loss in the upwind region and suppress separation in the downwind region at the appropriate tip speed ratio, thus significantly improving its performance in one rotor revolution. The winglet also contributes to the reduction of rotor thrust. In addition, the influence of the blade tip on wake is not remarkable and is mainly concentrated in the tip area affected by the tip vortex.
PDF
