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WOS: 000432736400010, Flow boiling researches in enhanced geometries practiced recently have been summarized in this review. Classical approximations in macroscale flows have not applied to the flows with small hydraulic diameters, so, this subject has been paid attention by the researchers. Application areas of these structures are increasing continuously due to cooling need in small confined spaces. The related studies have been reviewed in a chronological manner and their parameters have been given in a table in order for the researchers to observe the scientific duration regarding the progresses on this subject. Almost all papers on this subject have been produced after 2000, therefore, this subject is considered as an actual one to improve and worth to study in the literature. This study can not only be evaluated as the beginning argument for the researchers involved in flow boiling process in mini and microchannel, but it also consists of new works on the investigated subject., Department of Mechanical Engineering, King Mongkut's University of Technology Thonburi; "Research Chair Grant" National Science and Technology Development Agency; Thailand Research Fund; National Research University Project, All authors would like to thank the Department of Mechanical Engineering, King Mongkut's University of Technology Thonburi for the support during this research in Thailand The authors would like to thank the "Research Chair Grant" National Science and Technology Development Agency, the Thailand Research Fund and the National Research University Project for the support.

Tez (Yüksek Lisans) -- İstanbul Teknik Üniversitesi, Fen Bilimleri Enstitüsü, 2012, Thesis (M.Sc.) -- İstanbul Technical University, Institute of Science and Technology, 2012, Dünyadaki enerji talebi artışına bağlı olarak, francis tipi hidrolik türbinlerinin sayısı ve kapasitesi de gün geçtikçe artmaktadır. Ülkemizdeki hidroelektrik santrallerin birçoğunda Francis tipi türbinler kullanılmaktadır. Bu nedenle, bu tip tesislerin işletimi büyük önem taşımaktadır. Francis türbinli tesisler, termik santrallerin aksine, çoğu zaman optimum verim noktasının uzağında çalıştırılmaktadır. Bu durumun en önemli nedeni enerji piyasasındaki talebin esnekliğidir. Çalışamada, Francis türbini yayıcısındaki akış modellenmeye çalışılmıştır. Öncelikle, geometriyi basitleştirmek adına sadece genişleyen kanaldan ibaret olan dirseksiz bir yayıcı modellenmiştir. Ayrıca, kanal simetrik olduğundan, kanalın dörtte biri çizilmiş bu sadece bu alanda çalışılmıştır. Sınır şartlarında simetri yüzeyleri belirlenmiştir. Böylelikle, bilgisayar işlemci yükü büyük ölçüde azaltılarak daha kısa zamanda sonuçlar elde edebilmek mümkün olmuştur. Modelleme sırasında, türbin girişindeki hız değerlerinin hangi mertebelerde olduğu bilindiğinden, girişte hız sınır şartı kullanılmıştır. Çıkışta ise akışkanın hızının tanımlanması son derece zordur. Ancak, yayıcının çıkışının atmosfere açık olduğu unutulmamalıdır. Bu da çıkış basıncının atmosfer basıncına eşit olduğu, yani mutlak basıncın sıfır olduğu durumdur. Çıkışta bu nedenle basınç sınır şartı kullanılmıştır. Francis türbini yayıcısına gelen akışın, türbin çarkından çıktığı göz önüne alınacak olursa akışın modellenmesi konusunda bize önemli bir fikir verecektir. Bu bağlamda, suyun hızının, akışkanın kendiliğinden aşağıya doğru hareketi sırasında kazandığı bir eksenel bileşeni; türbin çarkının dönme etkisinden kaynaklanan da bir teğetsel (çevresel) bileşeni olduğu düşünülebilir. İşte bu noktada, silindirik koordinatlarda çalışılarak, akışkanın farklı eksenel ve çevrel hız bileşenlerine sahip olduğu durumlar irdelenmiştir. Ayrıca, çevresel hız bileşeninin şiddeti arttıkça akış ayrılmalarının ve girdap oluşumlarının arttığı da görülmüştür. Burada, literatür çalışmaları incelendiğinde, bu yöntemin sadece bir yaklaşım olarak kalabileceği ortaya çıkmıştır. Çünkü yayıcı içindeki akış hızının eksenel bileşeni üniform olarak kabul edilebilinirken; teğetsel bileşeninin başta giriş kesitinin merkezinden olan radyal uzaklık olmak üzere, birkaç değişkenin fonksiyonu olduğu görülmüştür. Bu nedenle çalışmanın daha sonraki aşamasında bu çevresel hız profili tanımlanmıştır. Çevresel hız profilinin tanımlanması için kullanıcı tanımlı fonksiyonlar kullanılmıştır. Kullanıcı tanımlı fonksiyonlar C programlama dili ile yazılan ve mevcut kodun özelliklerini iyileştirmek adına, kullanıcı tarafından programlanarak FLUENT’in çözücüsüne dinamik olarak yüklenebilen fonsiyonlardır. Bu yöntem ile çevrel hız profili oluşturularak FLUENT’in çözücüsüne yüklenmiş ve çevresel hız değerleri belirlenen fonksiyona bağlı olarak elde edilmiştir. Çalışmanın son aşamasında dirsekli bir francis türbini çizilerek modelleme yapılmıştır. Burada da, tıpkı dirseksiz yayıcının modellenmesinde olduğu gibi, xvi yayıcının simetrik özelliğinden faydalanılarak, yarısı üzerinde sayısal çözüm ağı oluşturulmuştur. Bir önceki çalışmada yapılan, çevresel hızın belirlenmesinde, kullanıcı tanımlı fonksiyonların kullanılması işi burada da yapılmıştır. Dirsekli francis türbininin modellenmesinde gerçek bir türbinin model deneyi verileri kullanılmıştır., Depending to the increase of enery demand in the world, the number and capacity of hydraulic turbines are increasing day after day. Most of the hydraulic turbines in Turkey are Francis type hydraulic turbine so, operation and maintenance of francis turbines is an important issue. On the contrary to thermal power plants, hydraulic turbines operates mostly away from their best efficiency point. This occurs mostly due to the flexibility in the enegy market. This study is related with the fluid flow modelling in francis turbine draft tubes. Two types of dfrat tube are examined: Draft tube as divergent channel and draft tube with elbow. In order to simplfy the geometry, first of all draft tubes without elbow (divergent channel only) are investigated. The study starts with forming the geometry. The geometrical shape of the draft tube is a truncated cone. However, only the quarter of the model is generated. Due to the symmetric geometry of the truncated cone draft tube, the quarter of the model can be constructed. This will be helpfull to decrease the processor load becouse only the one fourth amont of mesh will be generated. Determining the boundary conditions is also an important issue. The order of velocity at inlet is known aproximatelly in this kind of flows so, as inlet boundary condition “velocity inlet” is used. On the other hand, measurement of velocity quantities is very difficult. Fortunately, there is another information that can bu used as a bounday condition. The outlet of the draft tube is open to the atmosphere. This means that the outlet pressure is atmospheric pressure, and gage pressure is zero. So, “pressure outlet” boundary condition is set. The exterior face is defined as “wall” and symmetry faces are defined as “symmetry” while defining the other boundary conditions. These modelling and mash generation processes are done by the help of “Gambit”. During meshing procedure, first off all boundary layer creation operation is done. This is a vital approach in order to get the expected turbulance values like dimensionless wall distance (y+). In order to work with near wall treatment, dimensionless wall distance should not exceed 5. This requires finer mesh at near wall regions. As a result mesh number and computer work increases. After generating boundary bondition at channel wall surface mesh generation procedure continues with meshing inlet and outlet surfaces. These surface meshes are very important becouse the whole geomery will be meshed by the help of these surfaces. These surface meshes will be helpfull for base generation for the mesh. Another important issue in meshing procedure is setting mesh element types. There are three corners at inlet and outlet surfaces. The corner vertex is set as “trielement” xviii and the other two corners are set as “end”. As a result quadrilateral mesh elements except first row of the mesh surface is obtained easily. So, mapped mesh elements are acquired. While modelling the flow in draft tube, it should be considered that the fluid coming to the inlet of the draft tube comes from the runner of the hydraulic turbine. In this respect, it can be seen that water falling from the dam forms an axial velocity and the runner of the hydraulic turbine forms a tangential component for the velocity. Simply, it can be said that, the inlet velocity has two components: axial and tangential. In this study the various combination of different axial and tangential velocity values are analyzed. It is obviously seen that, as the tangential component of the velocity increses, flow seperations and vortex generations increases. Tangential velocity component varied from 1 m/s to 4 m/s. On the other hand, its seen that constant tangential velocity component approximation is not realistic and reliable. After literature review, it is clear that the axial componenet of the can be modelled as uniform and constant but, the tangential velocity component is a function of the radial distance from the center of the inlet “r”. Therefore, from now on it is a must to determine this tangential profile. In order to define the tanmgential velocity profile user defined functions (UDF) are used. A user defined function, or UDF, is a function that you program that can be dynamically loaded with the FLUENT solver to enhance the standard features of the code. For example, you can use a UDF to define your own boundary conditions, material properties, and source terms for your own regime, as well as specify customized model parameters (e.g., DPM, multiphase models), initialize a solution, or enhance post-processing. UDFs are written in the C programming language using any text editor. By the help of the compiled UDF’s study continued with the variable tangential velocity at same geometry. The tangential velocity profile defined as a function of radial distance (r), angular velocity (Ω) and characteristic vortex radius (R). After several analyses, dimensionless tangential velocity versus dimensionless radius graphics is obtained at different angular velocities and characteristic vortex radii. It is seen that the results are in good agreement with the results taken from literature review. As a last step of the study, a draft tube with elbow is simulated. It is obvious that the geometry of draft tube with elbow is much more complicated becouse the geometry starts with diverging circular cross section, continues with varying elliptical cross section at elbow and ends up with diverging quadrilateral cross section. So, encountering more and more complicated flow conditions must be expected by a simple overview. Becouse of the hardness of the geometry, modeling had done with “Solid Works” and meshing was carried on by ICEM. On the other hand, there is no option for boundary layer mesh in ICEM so, it would not be possible to have fine mesh near wall regions. Due to this problem, while modellig a real and bigger draft tube instead of enhanced wall treatment, standart wall functions are used. Moreover, flow analyses are done by “FLUENT”. xix After several analyses, case and data files saved and post-processing stage started off. During post-processing procedure, CFD Post is used which is a workbench under ANSYS. The draft tube is divided into three stages: entrance, elbow and outlet. At each part of the draft tube, at least three surfaces created which are perpendicular to the flow direction. At those surfaces velocity fields, pressure fields and streamlines are examined. It is seen that the 90° elbow disturbs uniform fluid flow. The velocity vectors and 2- D streamlines at the symmetry surfaces obviously showed the characteristic features of the flow. The flow seperations and vortex formations were clearly seen. Especially these flow seperations and vortex formations occur after the elbow section at partial load. As a result, it can be said that besides the tangential velocity component of the fluidn at inlet, the elbow of the draft tube causes flow seperations and vortex formations. Moreover, pressure fields, vorticity profiles and the streamlines at the whole computational domain are also visualized. In conclusion, when hydraulic Francis turbines works far away from their operating points many unwelcome consequences occurs like flow seperations and vortex rope formations. These consequences led to noise and vibrations during operation. As a result, the lifetime of that equipment sharply decreases. Due to these problems, these plants should not be worked far away from their designed operating points., Yüksek Lisans, M.Sc.