CFD Analysis of a Wells Wave Turbine

Abstract

Sun heats up differently the zones of Atmosphere. This causes the temperature dependent pressure gradients between the various regions of air. Pressure gradients generate the winds. Those winds can occur at different layers of atmosphere in different formations. If formations are on the surface layer of oceans or seas these situations trigger the creation of the waves. It is clear that, there is an energy circulation cycle in this period. This cycle starts from sun lights energy ends at wave stored energy. This last energy source (wave) becomes ready for daily usage as an electrical energy after being processed. The energy that stored in the waves converted in to the electrical energy by wave turbines. That conversion is commonly made by Oscillating water columns (OWC). These systems are becoming popular while their efficiency values increase. OWC systems are studied in two main methods, first one is experimental method, the other one is CFD. There are important differences between the test results and CFD analysis results about efficiency values of OWC’s. These differences reach up nearly to 40%. At the literature, there is a big discussion about that gap between the efficiencies. The strongest hypothesis on this subject can be described like that Mesh should realize the real situation of the turbine and at the setup section of CFD, the turbulence model must be suitable. These are thought to be the cause. In this study, the focused point is that the differences between the efficiencies of numerical and experimental analysis which are referred commonly in the international literature. The literature is described and in those studies one is selected as a reference. Methods and the turbulence models are selected from that reference. Generally OWC’s analysis is a matter of turbine efficiency. Developments are done especially in the turbine section. These are special turbines called Wells turbine. The importance comes from their bidirectional rotation. It means for the both inlet and outlet flow of air, they rotates in the same direction. The properties of the studied Wells turbine are known from the selected reference study. For numerical analysis the same Wells turbine was used. This turbine system consist of 7 NACA 0015 aerofoils with c=74 mm chord length, with Rhub= 101 mm hub radius and diffuser radius Rtip=155 mm. The length of the diffuser is 1000 mm. To get the data for efficiency of numerical analysis, ANSYS Fluent software was used. There are big differences up to 40% between the referred study’s experimental efficiency values and the Spalart Allmaras turbulance modeled numerical analysis studied in this paper. To describe the methods in details, Ø variable should be identified as the ratio of rotational velocity to circumferential speed. Under the different Ø values, the speed of inlet air is taken constant, 7 m/s in numerical analysis method. Related to that speed, torque changes and pressure gradients observed. As it clear in the definition of efficiency, these are important components. Shortly, the change of the torque and pressure are effecting efficiency directly. To get that torque and pressure gradient values, ANSYS software used for three main parts first one is modeling, secondly preprocessing called meshing, and post processing done in setup section.

Özet

Enerji kaynakları, tükenme durumuna göre dünyanın var olduğu süreç içerisinde tükenen ve tükenmeyen, kendini devamlı yenileyen yenilenebilir enerji kaynakları olarak iki kısımda incelenebilir. Tükenen enerji kaynakları, kömür, doğalgaz, petrol gibi fosil kaynaklar ve nükleer enerjiden oluşurken, yenilenebilir enerji kaynakları ana kaynağa göre üç grupta incelenebilirler: Güneş kaynaklı, Ay kaynaklı ve Dünya kaynaklı. Ay’ın çekim gücü ile gel git olayı ve Gel-Git Güç Santralleri yardımıyla elektrik enerjisi, Dünya kaynaklı olarak ise Jeotermal enerji ve Jeotermal enerji santralleri yardımıyla ısı ve elektrik enerjisi üretilmektedir. Güneşten ise, su, rüzgar, güneş ışınları ve biyokütle enerjileri türemektedir. Güneşin atmosferdeki noktaları farklı ısıtmasıyla rüzgarlar, rüzgarların deniz serbest su yüzeylerini sürtünme ile hareketlendirmesinden dalgalar oluşur. Dalgalardaki enerji, dalga türbinleri aracılığı ile elektrik enerjisine dönüştürülürler. Salınan su sütunu (OWC) tipi dalga türbinleri, bu dönüşümü gerçekleştiren en yaygın türbinlerdendir. OWC dalga türbinlerinin deneysel olarak elde edilen verimleri ile, hesaplamalı akışkanlar dinamiği yöntemini kullanan paket programlar yardımıyla hesaplanan verimleri arasında %40’lara varan büyük farklar bulunmaktadır. Bu çalışmada, literatürde çok sık atıf alan ve deney-nümerik yöntem verimleri arasında %40’a varan bir çalışmada kullanılan ve özellikleri bilinen bir OWC türbininin sonlu hacimler yöntemini kullanan ANSYS Fluent paket programında analizi gerçekleştirilmiş ve baz alınan çalışmaya göre, verimde deney sonuçlarına göre negatif yönde %40’a varan farklara ulaşılmıştır.