Enhancement of performance of wave turbine during stall using passive flow control: First and second law analysis

Shehata, Ahmed S.; Xiao, Qing; Mm, Selim; Elbatran, A. H.; Day, Sandy

doi:10.1016/j.renene.2017.06.008

Cited by 34 publications

(7 citation statements)

References 69 publications

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“…Nevertheless, in a large body of literature, entropy generation in the presence of turbulent flows is calculated considering only the contribution of the mean flow. This simplification has been adopted for Wells turbines [66,67,68,69,71], pipe flows [61,44], and turbine blades [51]. Herwig et al [34] demonstrated how neglecting the entropy produced by turbulent fluctuation may lead to errors in the global entropy production, as well as to misleading conclusions.…”

Section: Introductionmentioning

confidence: 99%

“…Wells turbines have been studied extensively, both experimentally [15,31,24,63,74,53,57,48] and numerically [40,62,17,76,30,72,32,27], but mainly from a first-law perspective. More recently, a number of authors [66,67,68,69,71] have focused on the irreversibilities in this component, trying to link the amount of available work (exergy) and the entropy produced.…”

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confidence: 99%

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Numerical evaluation of entropy generation in isolated airfoils and Wells turbines

et al. 2018

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In recent years, a number of authors have studied entropy generation in Wells turbines. This is potentially a very interesting topic, 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. Unfortunately, the approach used in these studies contains some crude simplifications that lead to a severe underestimation of entropy generation and, more importantly, to misleading conclusions. This paper contains a re-examination of the mechanisms for entropy generation in fluid flow, with a particular emphasis on RANS equations. An appropriate methodology for estimating entropy generation in isolated airfoils and Wells turbines is presented. Results are verified for different flow conditions, and a comparison with theoretical values is presented. 1Wells turbines have been studied extensively, both experimentally [15,31,24,63,74,53,57,48] and numerically [40,62,17,76,30,72,32,27], but mainly from a first-law perspective. More recently, a number of authors [66,67,68,69,71] have focused on the irreversibilities in this component, trying to link the amount of available work (exergy) and the entropy produced.The application of second-law analyses is not novel, as such analyses have been done by many authors, but mainly from a system perspective: Bejan [8,9] presents an extensive review of the application to different thermodynamic systems. Similar approaches have been used in the analysis of thermal power plants [39], wind turbines [56,6,7,60,52], and vortex generators [33].Computational Fluid Dynamics (CFD), aside from making possible a more rapid and economic evaluation of many systems (compared to experimental testing), can provide a better and more detailed understanding of many phenomena, by capturing a level of detail that would be extremely difficult in an experimental study. It allows a very fine decomposition of the overall problem that can be used to locate the sources of irreversibility within the system. Entropy generation by thermal and viscous sources can be calculated directly by post-processing the fields of thermo-and fluid-dynamic variables available from the numerical solution [8]. However, particular attention needs to be paid to the nature of the flow and to the solution approach. While in laminar flows all entropy generation is directly found in the CFD solution [18,2,70,33,79,59], this is not true for turbulent flows, unless a Direct Numerical Simulation (or DNS) is used. However, with current computational resources, the use of DNS is still impractical even for moderately large Reynolds numbers. To make high-Reynolds-number flows treatable, the governing equations are either averaged (leading to the Reynolds Averaged Navier Stokes equations, or RANS) or filtered (leading to the Large Eddy Simulation approach, or LES).Moore and Moore [49, 50] present a methodology for calculating entropy production in viscous flows, using RANS equations. They divide the mean entropy production into the contr...

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Section: Introductionmentioning

confidence: 99%

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Numerical evaluation of entropy generation in isolated airfoils and Wells turbines

et al. 2018

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“…counter-clockwise in Wells turbines and clockwise in oscillating airfoils (this statement is actually incorrect [18]). The presence of dynamic effects in Wells turbine was the focus of the recent numerical investigations by [19][20][21][22][23][24], who gave essentially very similar explanations. In this short note, it will be demonstrated how these results are in contradiction with well established literature on rapidly moving airfoils and wings, and are likely to be caused by errors in the analysis of the experiments and in the setup of the numerical simulations.…”

Section: Introduction Wells Turbines and Owc Systemsmentioning

confidence: 88%

“…In light of the discrepancies outlined in Section II, the authors of this note have considered important to verify the numerical results presented in older studies, noticing how none of them [11][12][13][14][15][16][17][19][20][21][22][23][24] presented an accurate analysis of the numerical errors due to temporal discretization. This appeared curious to say the least, given the importance of excluding any phase errors [41][42][43] in a study that deals with hysteresis.…”

Section: Revisiting the Cause Of The Hysteresismentioning

confidence: 95%

Revisiting Wells Turbine Hysteresis in Light of Existing Literature on Moving Airfoils

Ghisu¹,

Cambuli²,

Puddu³

et al. 2018

Preprint

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“…Techniques were developed to manipulate the boundary layer, either to increase the lift or decrease the drag, they are classified under the general heading of boundary layer control or flow control [56,57]. Methods of flow control to achieve separation postponement, lift enhancement and drag reduction have been considered in these studies.…”

Section: Introductionmentioning

confidence: 99%

Effect of passive flow control on the aerodynamic performance, entropy generation and aeroacoustic noise of axial turbines for wave energy extractor

et al. 2018

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Wells turbine is the simplest type of an axial flow self-rectifying air turbine that can be used in conjunction with Oscillating Water Column (OWC) system in the extraction of ocean wave energy. It has been noticed that this turbine is subjected to early stall. As a consequence, several attempts for improving the energy extraction performance of Wells turbine within the stall regime have been investigated. One of these attempts was using an inclined slot as a passive flow control to obtain a delayed stall. In the following study, the impact of varying the angle for the slot on the performance of Wells turbine in the stall regime was investigated. Furthermore, the first law of thermodynamics and the entropy analysis has been used to examine the effect of the slot angle on the entropy generation features around the turbine blade. Moreover, Investigation of slot angle effect on the aerodynamics noise emission from Wells turbine airfoil during the normal operation and the stall regime is covered in this study. The blade of turbine with optimum angle of slot was investigated using the OWC based on actual data from the Egyptian Northern Coast. It was found that the optimum slot angle is 10 degrees clockwise which results in 3% improvement in the torque coefficient before the stall and 15% after the stall as compared to the 0 degree slot. Otherwise, it gives a lower global entropy generation rate than the 0 degree slot by 4% before the stall and 3% after the stall. Furthermore, using airfoil of blade turbine with a slot resulted in a reduction of aeroacoustic noise by -21.2% at the stall regime under oscillating flow conditions.

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Enhancement of performance of wave turbine during stall using passive flow control: First and second law analysis

Cited by 34 publications

References 69 publications

Numerical evaluation of entropy generation in isolated airfoils and Wells turbines

Numerical evaluation of entropy generation in isolated airfoils and Wells turbines

Revisiting Wells Turbine Hysteresis in Light of Existing Literature on Moving Airfoils

Effect of passive flow control on the aerodynamic performance, entropy generation and aeroacoustic noise of axial turbines for wave energy extractor

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