Düşey eksenli hidrokinetik türbin için asimetrik kanal tasarımı
Asymmetric duct design for vertical axis hydrokinetic turbine
Yazar: OĞUZ SUSAM
Danışman: DR. HAKAN ÖKSÜZOĞLU
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Makine Mühendisliği Ana Bilim Dalı / Isı-Akışkan Bilim Dalı
Derece: Yüksek Lisans, 2019
Konu: Enerji = Energy ; Makine Mühendisliği = Mechanical Engineering
Dizin: Hidrolik enerji = Hydraulic energy ; Sayısal akışkanlar dinamiği = Computational fluids dynamic ; Su türbinleri = Water turbines ; Sıkıştırılamayan akışkan = Incompressible fluid ; Yenilenebilir enerji = Renewable energy

Özet:
Bu çalışmada, akış yönüne göre asimetrik geometriye sahip kanal takviyesinin, düşey eksenli hidrokinetik türbinlerinin performansına etkileri incelenmiştir. Kanallar hem rüzgar hem de hidrokinetik türbinler için oldukça önemli ve performans artırıcı etkileri olan eklentilerdir. İncelenen çalışmalarda simetrik kanal takviyesi ile h-rotor türbin üzerinde oluşan ortalama moment katsayısı yüzde 12 civarı artırılabilmiştir. Savonius türbini için yapılan çalışmalarda ise saptırma etkisi sayesinde türbin performansı oldukça artırılabilmektedir.
Performans artışına sebep olmasının dışında kanal takviyesinin pek çok avantajı bulunmaktadır. Öncelikle kanal takviyesi bir türbini dış etkilerden koruyacak bir yapı görevi görür. Kanal takviyesi ile türbin etrafındaki akış hızı artırılarak türbinde ve ve kanal içinde kalan diğer yapılar üzerinde korozyon gerçekleşme ihtimali azaltılır. H-rotor türbinlerde tork dalgalanma faktörünü ve negatif tork bölgesinin performansa olumsuz etkisini azaltır. Sualtı türbinleri için ise kanal yapısı demirleme yeri veya şamandıra görevi görür.
Düşey eksenli türbinler bir döngü boyunca dalgalanan güç üretirler. Türbin döngüsü boyunca bu güç dağılımı incelenmeli ve bu dağılım dikkate alınarak kanaldaki değişiklikler yapılmalıdır. Bu yüzden kanalda oluşturulacak geometrik anomaliler fırsatlar sağlayabilir. Akış yönüne göre asimetri bu anomaliler için iyi bir örnektir. Bu çalışmada h-rotor ve Savonius türbinleri için akış yönüne göre asimetrik kanallar oluşturulmuş ve HAD yardımıyla performansları incelenmiştir. H-rotor ve Savonius türbin için ikişer farklı yaklaşımla kanallar tasarlanmıştır. Sonuç olarak özellikle yüksek kanat uç hız oranlarında olumlu sonuçlar elde edilmiştir.
Tasarımlar Rhinoceros 5.0 adlı ticari yazılım yardımıyla hazırlanmıştır. Performans analizleri hesaplamalı akışkanlar dinamiği yardımıyla yapılmıştır.Hesaplamalarda Ticari bir yazılım olan ANSYS Fluent 19.2 öğrenci versiyonu ve İstanbul Teknik Üniveritesi Ulusal Yüksek Başarımlı Hesaplamalı Merkezi (UHeM) tarafından sağlanan ANSYS Fluent 18.2 akademik versiyonu kullanılmıştır. Hesaplamaların bir kısmı İTÜ-UHeM'in sağladığı imkanlar sayesinde gerçekleştirilebilmiştir. URANS türbülans modellerinden Spalart-Allmaras, k-$\epsilon$ ve k-$\omega$ SST kullanılmış ve problemler iki boyutlu ve zamana bağlı olarak çözülmüştür. Çözüm ağı ICEM-CFD adlı ticari program ile oluşturulmuştur.Çözüm ağları, yapılandırılmış ve yapılandırılmamış bölgelerden oluşan hibrit çözüm ağlarıdır. Türbinin hareketini simülasyona yansıtabilmek için dönel hacim yöntemi kullanılmıştır. Sayısal hesaplama yönteminin validasyonu için deneysel verileri elde edilen referans çalışmalardan faydalanılmıştır.Veriler, ANSYS'in HAD analizleri için kullanılan CFD-post adlı modülü kullanılarak işlenmiştir. Ayrıca Python 2.7 matplotlib adlı kütüphane ile iki boyutlu grafikler oluşturulmuştur.

Abstract:
In this study, the effects of duct augmentation on the performance of vertical axis hydrokinetic turbines were investigated. The performances of many duct augmented vertical axis hydrokinetic and wind turbines systems  already are in used. 
The performance improvement is obtained because the average velocity magnitude of the incoming flow to the turbine is increased. In other words, the pressure difference through the flow direction is increased as a result of using the duct augmentation. The mean moment coefficient on the h-rotor turbine which is taken as base design in validation study was increased by around 12 percent by means of symmetric duct augmentation. For some ducted turbine designs in this study, the mean moment coefficient is increased by about forty percent.
In addition to the performance increment, the duct augmentation has other advantages. First, the duct augmentation serves as a structure to protect a turbine from external influences. By means of that duct augmentation increase velocity around the turbine,  the possibility of corrosion occurrence on the turbine and other structures positioning in the channel is reduced. The use of ducts reduces the torque fluctuation factor in the h-rotor turbines and the performance of the negative torque zone. 
Although the duct augmentation makes the design requirements and criteria more difficult, the diffuser makes the turbine get the same power with a smaller turbine. For hydrokinetic turbines, the channel structure serves as a mooring.
When the graph of the change of the torque coefficient during a single rotation of the vertical axis turbines is examined, the torque generating by a blade is highly variable along its cycle and the power oscillation is very high. Considering the distribution, ducts are also very useful extensions for vertical axis turbines. 
For the ducted turbines, it is possible to improve the distribution of the moment coefficient by changing the flow of the blade by interfering with the flow through the modifications made in the channel design.
In this study, the suitable numerical method to obtain the moment coefficient distribution along the one cycle of a blade was chosen, after the computational fluid dynamics (CFD) methods, had been examined and evaluated. There are some potential flow methods to analyze the performance of h-rotor turbine such as discrete vortex method, cascade method, blade element theory, etc. 
Although these methods are efficient to get information about the performance of the h-rotor turbine, potential flow methods are not efficient performance analyses on the Savonius turbine which is mostly driven by drag forces. In addition, potential methods can be insufficient for high tip speed ratios and low tip speed ratios in which the dynamic stall effect is very crucial.		
In the validation study, the numerical method, commercial CFD software, turbulence method, discretization scheme, and grid generations were presented. The commercial software,  ANSYS Fluent student version 19.2 which has mesh size restrictions, 512000 nodes or cell limit, and ANSYS Fluent academic version 18.2 which is supplied by Istanbul Technical University National Center of High-Performance Computing were used for CFD simulations. Grids are generated as structured around the wall and unstructured together calling hybrid grid by using ANSYS ICEM-CFD mesh generation tool. In addition, Paraview, Ansys CFD-Post, Python 2.7 Matplotlib library were used for post-processing.   
All simulations are run with using second-order discretization divergence and gradient terms of momentum, mass and turbulence equations. Time derivation is obtained with the second-order implicit formulation. Simulations are made by two-dimensional transient SIMPLE scheme. Rotational effect of the turbine is simulated by rotation of the region called as a rotor which contains all cells around the turbine and by rotation of the no-slip wall boundary modeling the turbine blade. In the literature, this method is called as the sliding mesh method. 
Validation is made by two different experimental studies. First one is an experiment of a ducted h-rotor hydrokinetic turbine conducted in University of British Columbia Towing Tank. The second one is an experiment of freestream Savonius turbine conducted in Indian Technology Institute, Guwahati. The steps of the validation studies are stated.
Firstly, some URANS turbulence methods are evaluated. In fact, most useable and accurate turbulence methods are Large Eddy Simulations(LES) and Detached Eddy Simulations (DES). However, LES and DES are very expensive turbulence methods. Even though URANS turbulence methods are not efficient and accurate as much as LES and DES, they are sufficient to evaluate the performance of the turbine with respect to moment coefficient distribution or other parameters which are acquired by integrating over the blade. Hence, URANS methods are efficient and useable to analyze the vertical axis hydrokinetic or wind turbines performance. These URANS methods are Spalart-Allmaras, Realizable k-$\epsilon$ and k-$\omega$ SST.
Secondly, the study of grid convergence and time step independency has been carried out. With different time steps varying between 1e-4 second and 1e-2 second, simulations are made. 
Then, a convenient and logical time step was chosen. Grid convergence study was implemented with the enlargement factor which is √(2 )for h-rotor turbine simulations and 2 for Savonius turbine simulations. For simulations had been completed the grids having about 130000 cell number for h-rotor turbine and Savonius turbine models. 
Thirdly, simulations at different tip speed ratios were carried out on referenced h-rotor and Savonius turbines. The results over a blade and all the turbine are presented and mean power coefficient values along a cycle are compared with referenced experimental data.
In addition, distribution of moment coefficient along a cycle at optimum tip speed ratio obtained by CFD is compared with experimental data. Thereafter, asymmetric ducts according to flow direction which is composed of two geometrically nonequivalent parts are designed for h-rotor and Savonius turbines. 
Asymmetric duct augmentation in two design sets is implemented to h-rotor turbines. Designs of the first set are created with respect to two different parameters ; the duct
length and height of the main parts of the duct. The ratio of widths of duct inlet and throat is kept the same. 
Every duct design sets implemented on h-rotor and Savonius turbine are created by taking into account this ratio except for the second design set of Savonius turbine. The first set has ducts having two different duct lengths and while the height of a part of the duct is decreased as much as the height of another part of the duct is increased to keep the same throat width and channel inlet width. Performance analyses on the first set of asymmetric ducted h-rotor turbines have not resulted in significant improvements. Actually, the modifications do not affect moment coefficient distribution of designs of the first set or the improvements are not important considering with error margin. Further, the results of the first set at low tip speed ratios are unfavorable. 
Second design set of asymmetric duct augmented h-rotor turbines contains three duct parts and there are two mouths of the ducts. The second mouth is placed on outside of a main part of the duct and actually, a part of the duct is separated with an inner channel. The slope of a wall of the inner channel is defined as the angle between the wall tangential and freestream flow direction.  Second design set is created considering the slopes of the inner channel walls and heights of duct involving inner channel. Secondary mouth allows occurring secondary flow from outside of the ducts. The secondary flow increases the mean moment coefficient of the turbine at tip-speed-ratio 3 and 3.5 such that the average moment coefficient increase ten percent according to referenced design at optimum tip-speed-ratio. However, at tip-speed-ratio 2 and 2.5 the performance gets worse.The first design set appended to Savonius turbine is the same as the first design set of h-rotor turbine. Similarly, the set has not any contribution to turbine performance.
The second group of designs of asymmetric duct augmented Savonius turbines contain a deflector duct part. The deflector duct part is placed to deflect or inject the flow to the positive torque region. While Savonius turbine blades moving up the stream, the flow generates the negative torque on the blade. The deflector can decrease the magnitude of the negative torque. The set is created in terms of two parameters such as the length of the deflector part and interval of the mouth of the duct. Clearly, to increase the length of the deflector affect the performance in a negative way especially at low tip speed ratios. On the other hand, the duct augmentation with deflector increment the mean moment coefficient between ten percent and forty percent.
Consequently,  the vertical axis turbines generate power fluctuating along a cycle. This power distribution along the cycle of the turbine should be examined and modifications on the duct are made taking into consideration this distribution. The geometrical anomalies on the duct provide opportunities for this reason. The asymmetry according to the flow direction is a good example for these anomalies. However,  geometrical asymmetry according to flow direction has not been very interesting.  
In the present study, many ducts are designed for h-rotor and Savonius turbines and different concepts are improved. In literature,  there are a lot of duct designs for vertical axis hydrokinetic turbines and those are very inspirational and efficient. As the contribution of the study, the moment coefficient distribution of a turbine blade might be manipulated via appending the asymmetric duct according to the free-stream flow direction in order to enhance the performance of vertical axis hydrokinetic turbines.