Akışkan yatak enerji depolama sistemi hesaplamalı akışkanlar dinamiği modellemesi
Fluidized bed energy storage system computational fluid dynamics modeling
Yazar: MUHAMMED BAHADDİN KOCATAŞ
Danışman: PROF. DR. ÜNER ÇOLAK
İstanbul Teknik Üniversitesi / Enerji Enstitüsü / Enerji Bilim ve Teknoloji Ana Bilim Dalı
Derece: Yüksek Lisans, 2020
Konu: Enerji = Energy
Dizin: Sayısal akışkanlar dinamiği = Computational fluids dynamic

Özet:
Akışkan yatak, yatak içerisinde bulunan katı taneciklerin verilen gaz etkisiyle akışkan gibi davrandıkları duruma verilen isimdir. Bu olay basınçlı bir gazın, ortamı oluşturan katı tanecikler arasından geçmesiyle oluşur. Bu durumda katılar akışkan karakteristiği göstermeye başlar. Verilen gaz akışının hızı artırıldıkça taneciklere uygulanan kaldırma kuvveti de artırılmış olur. Bu gaz akışının artırılmasına taneciklere uygulanan kaldırma kuvveti ile yerçekimi arasında denge oluncaya kadar devam edilir. Denge durumunda ise yukarı doğru akış halindeki havanın içerisinde parçacıklar asılı kalırlar ve akışın özelliklerini sergilemeye başlarlar. Bu noktadan sonra akışkanlaşma başlamış olur. Daha sonra hızın artırılmasına bağlı olarak kabarcıklar ortaya çıkar ve bu çalışmada analizi yapılan kabarcıklı akışkan yatak modeli ortaya çıkmış olur.
Bu tez çalışması akışkan yatağın enerji depolama özelliklerini belirlemek için yapılan hesaplamalı akışkanlar dinamiği simülasyonlarından elde edilen, akışkan yataklı bir enerji depolama sisteminde yatak malzemelerinin hidrodinamik ve termal davranışlarını sunmaktadır. Bu amaçla hazırlanan modelde, Euler laminer akış modeli benimsenmiştir. Katı ve gaz fazları için sürekliliği, momentumu ve enerji denklemleri de bu modelde dikkate alınmıştır. Sürükleme katsayısı için Syamlal-O'Brien modeli kullanılmıştır. Yatağın ağ yapısı yine kullanılan paket program ANSYS Mesh içinde oluşturulduktan sonra hesaplamalı akışkan dinamiği analizine (ANSYS Fluent 19.3) geçerek, önceden belirlediğimiz yatak malzemelerinin de özellikleri ve birbirleri arasındaki etkileşimler de göz önünde bulundurularak, simülasyon sonuçlarına ulaşıldı.
Enerji depolamada kıyaslanacak yatak malzemeleri olarak, kullanılan diğer yatak malzemelerine göre, yoğunluğu daha yüksek olan Zirkonyum Dioksit, özgül ısısı daha yüksek olan Silisyum Dioksit ve ısıl iletkenliği daha yüksek olan Alüminyum Oksit kullanılmıştır. Simülasyon sonuçlarında her bir akışkan yatak modelinin katı hacim fraksiyonlarındaki 5 saniyelik değişimleri gösterilmiştir. Termal performansa baktığımızda simülasyonlar yapılarak enerji ve sıcaklık değişimleri incelenmiştir. Sonuç olarak en kısa sürede en fazla sıcaklık değişimi gözlenen ve ısı transferi imkanı sağlayan malzeme, yoğunluğu ve özgün ısısı kullanılan diğer malzemelere düşük ancak ısıl iletkenliği kullanılan diğer malzemelere göre yüksek olan, Alüminyum Oksit olmuştur.

Abstract:
Fluidized bed is the name given to the situation where the solid particles in the bed behave like a fluid with the effect of the given gas. This event occurs when a pressurized gas passes through the solid particles forming the bed. In this case, the solids begin to show fluid characteristics. As the speed of the gas flow given is increased, the buoyant force applied to the particles is increased. Increasing this gas flow continues until there is a balance between the buoyant force applied to the particles and gravity. In the case of equilibrium, particles remain suspended in the upstream air and begin to exhibit the properties of the flow. After this point, fluidization begins. Then, bubbles appear due to the increase in speed, and the bubbling fluidized bed model analyzed in this study is revealed.
The formation of gas bubbles is one of the prominent features of fluidized beds and studies have been conducted about it for a long time. Many unique properties of fluidized beds can be directly related to the presence of bubbles. And these properties may vary with the behavior of the bubbles. For this reason, estimating the bubble properties such as size distribution and the distribution of the rising speed of the bubble by regions makes our job greatly easier. However, these distributions are dependent on the properties of the first bubble produced at the place where the gas is produced. The formation of bubbles significantly affects mass and heat transfer processes.
Heat energy occurs as a result of atomic or molecular vibrations in materials. It is stored as sensible heat, latent heat, reaction heat or a combination of all of these with the change in the internal energy of the material. In sensible heat storage, sensible heat resulting from the change in the temperature of the material from which the heat is stored is used. Heat storage can be done in materials where liquid, solid and liquid and solid are together. In latent heat storage, phase changing materials are used. In the temperature range suitable for storage, the latent heat caused by the phase change of the material from which the heat will be stored is determined. In this method, the volume of the energy storage unit is smaller than in the sensible heat. In thermochemical storage, heat energy is stored as bond energy in the compound and this stored energy is released by reversible reactions.
Heat energy storage systems can be divided into two as short-term storage and long-term storage, depending on their duration of use. Short-term ones are divided into day-night and long-term ones as summer-winter seasonally. In addition, these systems are divided into hot storage, cold storage or hot and cold storage depending on the purpose of use. With long-term storage, it is possible to store the heat of summer and use it in winter or store it in winter and use it in summer.
Due to the high heat transfer rate caused by the increased interaction area of the two phases in the gas-solid fluidized beds, it is a better option in thermal heat storage applications than other types of fluidized beds (eg liquid-gas). Thanks to its prominent mass and heat transfer characteristics, its use in industry is wide. Some prominent application areas are coating, drying, granulation and fuel synthesis.
Multiphase flows show different regimes depending on the geometry of the equipment used, the ambient conditions during operation and the properties of the gas and solid phases. Industrial-scale fluidized beds operating in the bubble regime have gas bubbles formed and rising in solid-gas insoluble in each other. Many such gas bubbles affect the heat and mass transfer rate. Depending on whether the reactions are exothermic or endothermic, the heat exchange of the fluidized particles varies from the emulsion phase to the bubble phase or vice versa.
As an alternative in energy storage systems, we encounter many systems. When fluidized beds used for different purposes are used as energy storage system, the properties of the material to be stored in the fluidized bed investigated.
Various materials are found in the use of fluidized beds as energy storage systems. By classifying these materials according to their various properties, materials having different values according to the selected properties were investigated.
The aim of the study is to obtain the most efficient result by selecting the materials with the desired properties after modeling the fluidized bed.
With the use of technology in the modeling of multiphase flow regimes, the multiphase flow is modeled with ANSYS Fluent software, which is one of the computational fluid dynamics programs. Based on similar studies to reach a numerical solution, the results have been reached by using appropriate methods.
This thesis study presents the hydrodynamic and thermal behavior of bedding materials in a fluidized bed energy storage system obtained from computational fluid dynamics simulations to determine the energy storage properties of the fluidized bed. In the model prepared for this purpose, Euler laminar flow model has been adopted. Continuity, momentum and energy equations for solid and gas phases are also taken into account in this model. Syamlal-O'Brien drag model was used for the drag coefficient. After the mesh structure of the bed was created in the ANSYS Mesh software, the simulation results were reached by the computational fluid dynamics analysis (ANSYS Fluent 19.3), taking into account the properties of the bedding materials we previously determined and the interactions between each other.
The mesh structure was created by preparing a 2D model of the fluidized bed. The width of the bed is 0.2 meters and the height is 1.2 meters. Silicon Dioxide, Aluminum Oxide and Zirconium Dioxide were determined as bearing materials. The height of these materials in the bed was determined as 0.24 meters. In addition, the created mesh structure consists of 9600 elements.
The reason for comparing these materials is because of the differences in these values between the properties shown in the list. It will be determined which types of materials are suitable for the purpose by considering these parameters.
Zirconium Dioxide, Silicon Dioxide and Aluminum are used as bedding materials to be compared in energy storage. Simulation results show the 5-second changes of each fluidized bed model in the solid volume fractions. 
As a result, the material with the highest temperature variation and heat transfer in the shortest period of time was Aluminium Oxide, which is low in density and specific heat capacity, but high in thermal conductivity in comparison with other materials.
The thermal conductivity of Aluminium Oxide, which is the material with which we get the highest performance in the shortest time in the temperature increase and heat transfer rate, is significantly higher than other materials. 
As a result, the types of materials that we can reach the thermal equilibrium in energy storage systems and get the fastest results in energy transfer are prominent as Aluminium Oxide type materials.