1,720,967 research outputs found
EXPERIMENTAL INVESTIGATION on A SPEED CONTROLLED WELLS TURBINE for WAVE ENERGY CONVERSION
No full textOcean wave energy represents one of the most attractive re-newable sources due to its high availability and predictability. Solutions based on the Oscillating Water Column (OWC) princi-ple are one of the most promising for sea-wave energy conver-sion. The system is composed of two main units, an open cham-ber that converts the sea wave motion into an alternating airflow, and a turbine driven by this flow. The typical alternating airflow inside the OWC chamber requires a turbine with self-rectifying behavior. The Wells turbine is the simplest and most reliable tur-bine for this purpose in virtue of its rotor with symmetric blades staggered at 90 degrees relative to the axis of rotation. The non-stationary operating conditions of theWells turbine strongly affect its performance when working away from its opti-mal efficiency point. By controlling the turbine rotational speed, the operating conditions can be kept closer to the maximum effi-ciency point. Recent works, based on dynamic simulations, have proposed control strategies for the turbine rotational speed, to avoid stall occurring under variable wave conditions. The present work investigates a rotational speed control in order to keep the operating conditions closer to the turbine's maximum efficiency point. The analyses have been conducted in an experimental facility capable to simulate an OWC system with regular (sinusoidal)wave motion. Wells turbine performance has been evaluated for different control laws and it is compared to not-controlled turbine performance in order to evaluate the ef-fectiveness of the control action
Detailed investigation of the local flow-field in a Wells turbine coupled to an OWC simulator
Full text linkOne of the most promising technologies for sea-wave energy conversion is the one based on the OWC principle. In a system of this type, the oscillatory motion of the sea waves is converted into a bi-directional air flow which is commonly exploited by means of a self-rectifying turbine such as the Wells turbine, the simplest and most reliable device for this purpose. The vast majority of experiments on Wells turbines and OWC devices has analyzed their performance from a global point of view, often in experimental facilities where the turbine was operated under stationary flow conditions. This paper presents the results of the experimental investigation carried out on a Wells turbine, by measuring the flow field both upstream and downstream of the rotor, in a laboratory set-up capable to reproduce the bi-directional airflow typical of an OWC system. The investigation aims to evaluate the local performance of the Wells turbine under unsteady flow conditions. The experimental measurements allow the identification of the loss components that affect the performance of the turbine. Viscous losses, due to the aerodynamic of the rotor cascade, represent the main contribution to the total losses, and appear larger than kinetic energy losses at the machine exhaust
EXPERIMENTAL ANALYSIS OF THE THREE DIMENSIONAL FLOW IN A WELLS TURBINE ROTOR
Full text linkAn experimental investigation of the local flow field in a Wells turbine has been conducted, in order to produce a detailed analysis of the aerodynamic characteristics of the rotor and support the search for optimized solutions. The measurements have been conducted with a hot-wire anemometer (HWA) probe, reconstructing the local three-dimensional flow field both upstream and downstream of a small-scale Wells turbine. The multi-rotation technique has been applied to measure the three velocity components of the flow field for a fixed operating condition.
The results of the investigation show the local flow structures along a blade pitch, highlighting the location and radial extension of the vortices which interact with the clean flow, thus degrading the turbine’s overall performance. Some peculiarities of this turbine have also been shown, and need to be considered in order to propose modified solutions to improve its performance
Multi-Fidelity Modelling of the Effect of Combustor Traverse on High-Pressure Turbine Temperatures
Full text linkAs turbine entry temperatures of modern jet engines continue to increase, additional thermal stresses are introduced onto the high-pressure turbine rotors, which are already burdened by substantial levels of centrifugal and gas loads. Usually, for modern turbofan engines, the temperature distribution upstream of the high-pressure stator is characterized by a series of high-temperature regions, determined by the circumferential arrangement of the combustor burners. The position of these high-temperature regions, both radially and circumferentially in relation to the high-pressure stator arrangement, can have a strong impact on their subsequent migration through the high-pressure stage. Therefore, for a given amount of thermal power entering the turbine, a significant reduction in maximum rotor temperatures can be achieved by adjusting the inlet temperature distribution. This paper is aimed at mitigating the maximum surface temperatures on a high-pressure turbine rotor from a modern commercial turbofan engine by conducting a parametric analysis and optimization of the inlet temperature field. The parameters considered for this study are the circumferential position of the high-temperature spots, and the overall bias of the temperature distribution in the radial direction. High-fidelity unsteady (phase-lag) and conjugate heat transfer simulations are performed to evaluate the effects of inlet clocking and radial bias on rotor metal temperatures. The optimized inlet distribution achieved a 100 K reduction in peak high-pressure rotor temperatures and 7.5% lower peak temperatures on the high-pressure stator vanes. Furthermore, the optimized temperature distribution is also characterized by a significantly more uniform heat load allocation on the stator vanes, when compared to the baseline one
Discussion on “Influence of incoming wave conditions on the hysteretic behavior of an oscillating water column system for wave energy conversion” by J. Peng, C. Hu and C. Yang
Full text linkRecently, Peng, Hu and Yang presented a lumped parameter model to quantify the hysteresis in Oscillating Water Column systems. We noticed that the model they presented is remarkably similar to the one we introduced in some of our previously published works. The similarity extends not only to the assumptions, derivation and methodology used to obtain an analytical solution, but even to the almost totality of the symbols chosen for the many model variables. None of the papers where we introduced the model and its solution were referenced by Peng and his coauthors, who therefore claimed for themselves the credit due to the original authors of the model. Peng and his coauthors have then applied the lumped parameter model to a test case different from the one that we had validated it on. This gives further confirmation of the validity of the model, which we feel the responsibility to reestablish the scientific property of
Evaluation of entropy generation methods in wells turbines
No full textEntropy generation analyses have been applied, in recent years, to a variety of systems, including Wells turbines. This can be a very powerful method, as it can provide important insights into the irreversibilities of the system, as well as a methodology for identifying, and possibly minimizing, the main sources of loss. However, some of the simplifications used in recent studies raise more than a concern on the validity of the approach. This work proposes a method based on RANS equations to evaluate the en-tropy production in Wells turbines. An estimation of the second-law efficiency of different Wells turbine rotors is also presented, under conditions representative of the air flow inside an OWC device. The main sources of entropy generation are highlighted and compared for the different geometries
A comparison of different approaches to estimate the efficiency of wells turbines
Full text linkWells turbines are among the most interesting power takeoff devices used in oscillating water column (OWC) systems for the conversion of ocean-wave energy into electrical energy. Several configurations have been studied during the last decades, both experimentally and numerically. Different methodologies have been proposed to estimate the efficiency of this turbine, as well as different approaches to evaluate the intermediate quantities required. Recent works have evaluated the so-called second-law efficiency of a Wells turbine, and compared it to the more often used first-law efficiency. In this study, theoretical analyses and numerical simulations have been used to demonstrate how these two efficiency measures should lead to equivalent values, given the low pressure ratio of the machine. In numerical simulations, small discrepancies can exist, but they are due to the difficulty of ensuring entropy conservation on complex three-dimensional meshes. The efficiencies of different rotor geometries are analyzed based on the proposed measures, and the main sources of loss are identified
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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