Doğrusal kesitli olmayan pertürbatörlerle periyodik aralıklarla kaplanmış bir türbin iç soğutma kanalındaki akışın RSM ile sayısal analizi
Numerical RSM analysis of flow in turbine internal cooling channel roughened at periodic intervals with nonlinear cross section perturbator
Yazar: GÖKMEN KIDIK
Danışman: DR. ÖĞR. ÜYESİ MURAT ÇAKAN
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Makine Mühendisliği Ana Bilim Dalı / Isı-Akışkan Bilim Dalı
Derece: Yüksek Lisans, 2018
Konu: Makine Mühendisliği = Mechanical Engineering

Özet:
Modern gaz türbinlerinin performansını etkileyen en önemli parametrelerden biri türbin giriş sıcaklığıdır. Türbin giriş sıcaklığının yüksek olması daha verimli türbin sistemlerinin olmasını sağlar. Günümüzde türbin giriş sıcaklıkları kullanılan malzemenin dayanım değerlerini aşmıştır. Bundan dolayı türbin malzemesi makul seviyelere soğutulmalıdır.
Türbinlerde kullanılan çeşitli soğutma yöntemleri vardır ve bunların en önemlileri film soğutma, çaptırmalı soğutma ve iç soğutmadır. İç soğutma sisteminde türbin kanatlarının içerisinde çeşitli geometrilere sahip soğutma kanalları bulunmaktadır. Kanat dış ortam sıcaklığına göre daha soğuk akışkan bu kanallardan geçerek kanadı soğutmaktadır. Kanallarda ısı transferi yüzeyini ve türbülansı artırıcı pertürbatörler kullanılmaktadır. Bu sayede malzemeden uzaklaştırılan ısı akısı da belirli miktarda artmaktadır ve daha verimli sistemler elde edilmektedir. 
Mevcut çalışmada tek yüzeyi % 30 blokaj oranına sahip pertürbatörler ile kaplı soğutma kanalının performansı incelenmiştir. Bu tarz yüksek blokaj oranına sahip kanallar daha çok küçük türbin kanatlarında ve kanadın firar kısmına yakın bölgelerde bulunmaktadır. 
Literatürde daha önce deneysel ve LES analizi yapılmış pertrübatörlü kanal  ele alınarak sayısal olarak RANS modeli ile analiz edilmiştir. Yapılan sayısal çalışma konjuge olarak ve metalden  akışkana doğru sabit ısı akısı verildiği kabul edilerek yapılmıştır. Kanal içerisinde 7 adet pertürbatör bulunmaktadır ve pertürbatör adımı ve Reynold sayısı hiçbir modelde değişmemektedir. 
En uygun RANS modeli seçilerek blokaj oranı aynı kalmak şartıyla 3 adet bombeli 3 adet de çukurlu pertürbatörden oluşan kanalın hem akış performansı hem de ısı transferi performansı incelenmiştir. Bombe ve çukurların eğrilik yarıçapları pertürbatör yüksekliği ile orantılı olarak değişmektedir. Bu eğrilik yarıçapları pertürbatörlerin yüzey alanlarında meydana gelen değişimler göz önüne alınarak belirlenmiştir. 
Yapılan sayısal çalışmada bir adımlık pertürbatör bölgesindeki ortalama EF katsayısı hem genel hem de bölgesel olarak incelenmiştir. Bombeli ve çukurlu pertürbatörlerden elde edilen bölgesel ve genel sonuçlar kare kesitli pertürbatörlü sonuçlar ile karşılaştırılmıştır. Buna ilave olarak boyutsuz basınç düşüşü katsayısı da her yeni pertürbatörlü kanal için değerlendirilmiştir. Bu sayede hem ısı transferi hem de basınç kaybı konusunda önerilen  yeni pertürbatörler mevcut kare kesitli pertürbatörün RANS sonuçlarıyla karşılaştırılmıştır.

Abstract:
One of the most important parameters affecting the performance of modern gas turbines is the turbine inlet temperature. The high turbine inlet temperature ensures more efficient turbine systems. Today, the turbine inlet temperature exceed the material strenght value. Hence, the türbine material must be cooled down to acceptable levels.
There are various cooling methods used in turbines, and the most importants one are film cooling, impingement cooling and internal cooling. In the internal cooling system, the türbine blades have cooling channel with various geometries. The fluid which is colder than blade outher temperature passes through these channels and cools the turbien blade. The heat transfer surfaces and turbulence enhancing ribs are used in the channels. In this case, the heat flux removed from the material also increases in certain amounts and more efficient systems are obtained.
In this present study, performance of the cooling channel roughed single surface with % 30 blockage ratio ribs was investigated. Such high blockage ducts are mostly found in small gas turbine blades and in areas close to the trailing edge of the blade.
That experimental and LES analysis of ribbed roughned channel has been worked in literature is analyzed numerically with RANS model. The numerical study was carried out as a conjugate and assuming that constant heat flux from meatal to fluid. There are 7 ribs inside the channel and pitch to rib ratio and Reynolds number have not changed any model.
A proper RANS turbulence model was chosen for channel whose experimental work and LES analysis were done. Ansys Fluent version 18.1 was used for two and tree dimensional validation and benchmark works. First of all, entire channel which has 7 ribs was simulated with proper flow properties in two dimensions. Besides, K-Epsilon Standard, K-Epsilon RNG, K-Epsilon Realizable, K-Omega, K-Omega SST, Spalart Allmaras ve Reynolds Stress Model ( RSM ) were tried with software in two dimensional model. 
In experimental work, the periodic flow in the channel commences after 4th rib and measurements were made between 4th and 5th ribs. The reason for modelling of entire channel, not periodically, to observe how the flow develop at the channel inlet and determine that the periodic flow begins at which ribs due to new rib geometries. All analysis results obtained between 4th and 5th ribs. 
The validation studies were carried out by considering dimensionless pressure drop, enhancement factor, velocity profiles, bubble dimension. When results were obtained RANS simulations examined, It is seen that RSM is the most suitable sutbulence model. Later, validation studies were carried out in 3 dimensional. Moreover, 3 dimensonal model simulated simetrically and results were obtained symetry plane compared with 2 results of 2 dimensional RANS model, LES and experimental results. In addition this, mesh independency was achieved by tried 3 mesh structure with 3 dimensional RSM model.  
Heat transfer mechanism of conjugate and non-conjugate conditions were considered in this work.  For a more accurate analysis of the internal cooling system of turbine blade, It is a more correct approach to introduce heat flux from the metal into the flow. Temperature gradient on the channel surface is different due to the effect of the metal. The temperature gradient of conjugate system is lower than  gradient of non-conjugate system. The conjugate and non-conjugate condition for channel was carried out and compared each other for channel. 
Generally in literature studies, parameters affecting the heat transfer in the channel was emphasized. Numerous experimental and numerical studies have beed carried out on the cooling performance and aerodynamic effects of the variation of these parameters. Some of these parameters are blockage ratio, pitch to rib height ratio, channel aspect ratio, angle attack of rib, reynols number and cruolis effects. 
While new rib geometries creating, previous studies in the literatüre have been examined and a different rib geometry was introduced. Curved surface at various radii of curvature on the rib surface were formed. This Radius of curvature strats from half of the rib height and goues to infinity. The change of heat transfer surface area is taken into consideration while determining the radius of curvature to be investigated. In this study, the radius of curvature of the curved surface was determined to be 0.75, 1 and 1.5 times the height of the rib. 
Dimple application on the turbine blades are generally applied on the trailing edge section. In some cases it is used in internal cooling channels. In this study, a dimple is applied on the rib surface and radius of curvature of these dimples is the same as that of the curved ribs. Both curved and hollow rib geometries were examined with a blockage rate of % 30.  
In the las phase of the study, 3 curved and 3 hollow ribs were analyzed in 3 dimensional with RSM. Firstyl, the dimensionles pressure drop and heat transfer enchancement factor along the symmerty region were compared with the results of the rectangular rib. The results obtained in the new geometries were compared only with the RSM results of the rectangular ribs because 3 dimensional RSM result was considered as reference work.
The contur distribution on the channel was examined and heat transfer enhancement factor differences was observed between symmeryt axis and channel wall. For this reason, heat trasnher enhancement factor were calculated on total surfaces and area weight average method was used for calculation. The front ,top and back of rib, the distance between the ribs and a total one step region taken into account separately.
At the end of the thesis, the curved ribs give better result than the rectanguler rib in terms of heat transfer. It seems that there is not much change about heat transfer when the hallow ribs are considered. In terms of dimensionless pressure drop, curved ribs give more favorable results than rectangular ribs. However; hallow ribs led to more pressure drop than rectangular ribs. 
While turbine blade producing, there may be production error in the ribs are lcoated cooling channel besides, sharp edges and surfaces can have various radii of curvature. Therefore, in this thesis, an aero-thermal study has been crried out on the new rib geometries that may be formed during the production process or are formed advertently.