1,720,976 research outputs found
Discussion on “Entropy generation analysis of S825, S822, and SD7062 offshore wind turbine airfoil geometries” by Mamouri, A. R., Khoshnevis, A. B. and Lakzian, E
No full textThis discussion is based on the paper by Mamouri et al. (2019) (hereafter identified as “the original authors” or “the original paper”). The original authors employed computational fluid dynamics to calculate the flow field around several wind turbine airfoils, and combined this with an evaluation of the entropy generation to estimate the second law efficiency. It came to our attention that the equations used to estimate the entropy generation, and in particular the term that is used to estimate the entropy generated by turbulent fluctuations, are incorrect. In addition, the original authors claim that one of the main contributions of their work is the identification of a relation between entropy generation and drag, probably ignoring that the same relation has been known and used by several authors in the past, and examples can be found from at least 60 years ago
Discussion on “Review of CFD studies on axial-flow self-rectifying turbines for OWC wave energy conversion” by Cui, Y., Liu, Z., Zhang, X. and Xu, C
No full textThis discussion is based on the paper by Cui et al. (2019) (hereafter identified as “the original authors” or “the original paper”). The original authors present a review on the application of CFD in the analysis of self-rectifying turbines for OWC wave energy conversion devices. We feel that this review is incomplete, as it overlooks a number of papers published in recent years that gave a new explanation for a phenomenon largely studied in the last two decades and discussed in the original paper: the hysteresis in Wells turbine performance. In addition, the original authors, in their conclusions, envisage a larger use of CFD in the coming years, especially for the unsteady analysis of the chamber-turbine interaction, probably ignoring that several CFD studies focusing on this aspect have already been published. In fact, it is through three-dimensional time-dependent CFD studies of the unsteady interaction between chamber and turbine that the explanation of the cause of the hysteresis was found. The objective of this discussion is to clarify two important aspects: the chamber-turbine interaction and the turbine hysteresis, that in our opinion were not correctly analyzed in the original review paper by Cui et al
Wells turbines dynamic simulation using cfd and a lumped parameter model
No full textThe hydrodynamics and aerodynamics of oscillating columns systems have been largely studied both experimentally and numerically, especially through laboratory facilities which focused either on OWC or power unit performance, usually aWells turbine. For the latter, a common simplification is made by substituting the periodic wave motion with the alternative motion of a piston in a chamber that is connected to the duct that hosts the turbine. This setup led to detailed studies on the difference between turbine performance during acceleration and deceleration, usually attributed to a hysteretic behavior of the turbine. This work demonstrates, using both CFD and a lumped parameter model, that the OWC hysteresis is caused by compressibility effects in the air chamber and not, as previously assumed, by an aerodynamic hysteresis of the turbine
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
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
Optimization of blade profiles for the Wells turbine
No full textA Wells turbine, when coupled with an oscillating water column, allows the generation of power from the energy in waves on the surface of the ocean. In the present work, a tabu search is used to control the process of optimising the blade profile in the Wells turbine for greater performance, by maximising the torque coefficient. A free form deformation method is used as an efficient means of manipulating the blade profile and computational fluid dynamics in OpenFOAM are used to assess each profile in both two and three dimensions. Investigations into both the flow coefficient at which the optimization is performed and the number of control variables in the free form deformation tool are performed before optimisations are done on a two-dimensional blade at the hub and tip solidities. This results in increases to the torque coefficient of 34% and 32% at the tip and hub solidities, respectively. These results are then applied to the three-dimensional turbine, giving a 14% increase in the torque coefficient. The results are assessed and an improved method of optimising the blade in two dimensions is proposed
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
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
Effect of Boundary Conditions and Turbulence Treatment on the Simulated Performance of a Ribbed Heat Exchanger
No full textRibbed surfaces are widely employed in heat exchangers to enhance the convective heat transfer and hence the overall thermal efficiency. This study aims to investigate the effect of two important assumptions made in computational fluid dynamics simulations, i.e. the thermal boundary conditions and the turbulence modeling, using a popular test case for the heat transfer over a continuous ribbed plate was taken as a reference. Numerical simulations were performed both neglecting and considering the conduction within the solid, to verify the effect of different thermal boundary conditions on the fluid domain, and with several turbulence treatments, ranging from common Reynolds-averaged Navier-Stokes approaches to higher fidelity but more computationally intensive Large Eddy Simulations. The results demonstrate that both aspects are important for an accurate prediction of the thermal performance of ribbed channels
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