İki fazlı akış kanalının termo-hidrolik analizi

Abstract

Çalışma, Kaynar Sulu Reaktörlerde; yavaşlatıcı ve soğutucu olarak kullanılan yapan suyun bulunduğu akış kanalının termo-hidrolik analizini içermektedir. Radyal doğrultuda analitik, eksene! yönde ise sonlu farklar yaklaşımı çerçevesinde, kurulan yakıt soğutucu modeli esas alınarak, sürekli rejimde termo-hidrolik modelleme yapılmıştır. Modelleme öncesinde; kurulacak olan modelde kullanılacak olan bazı büyüklüklere temel teşkil edecek ve kullanılacak denklem takımlarının oluşturulmasına yönelik olarak da;tek fazlı akış sistemlerine ait korunum denklemleri ve bu denklemlerden istifade edilerek iki fazlı akışlarda kullanılan korunum denklemleri ve büyüklüklerin türetilmesi konularının yanında, faz dönüşümünün ısı geçişi açısından ele alındığı kaynama olayı, faz dönüşümünün temel mekanizması olan kabarcık oluşumu ve kabarcık dinamiği incelenmiştir. Kullanılan model; yakıt malzemesi, gaz boşluğu ve zarftan meydana gelen yakıt çubuğu ile yakıt çubuğunu çevreleyen soğurucu akış kanalım içine alan bir hücrenin termo-hidrolik davranışı esas alınarak gerçekleştirilmiş olup, ızgara hacminde akışkanın termo-hidrolik özellMerinin değişmediği kabul edilmektedir. Akışkanın her bir ızgara hacminde, sahip olduğu entaîpi değeri; doyma entalpisinin altoda ise akışın tek fazlı bölgede olduğu, akışkanın sahip olduğu entalpi değeri, doyma entalpisinin üzerinde ise akışın iki fazlı kaynama bölgesinde olduğu göz önüne alınmaktadır. Modelde; soğutucu akışkanın özellikleri sıcaklık ve basınca bağlı korelasyonlar yardımı ile hesaplanmaktadır. Tek fazlı bölgede `Dittus- Boelter` korelasyonu yardımı ile, iki fazlı bölgede ise `Jens-Lottes` korelasyonu kullanılarak ısı taşınım katsayıları hesaplanmaktadır. İki fazlı akış bölgesinde boşluk ve kayma oranlan için `Martmeîli-Nelson` bağıntılarından yararlanılmaktadır. Sürtünme faktörü için; `Fanning Sürtünme Faktörü` kullamlmaktadır. Yakıt çubuğunun ısı iletim katsayısı ise sıcaklığa bağlı olarak değişmektedir. vm

element. The thermal conductivities of the foel element depends on temperature. The correlations is ued to compute the properties of the coolant which is also the function of the temperature and pressure. Finally THCAN code is used to make thermal-hydraulic analysis of a two phase flow channel and the results are discussed at the end of the study. xmfinally detaching to form a bubly flow. With production of more vapour the bubble population increases with length and coalescence takes place to form slug flow which in turn gives way to annular flow further along the channel. Close to this point the formation of vapour at this sites on the wall may cease and further vapour formation will be as a result of evaporation at the liquid film-vapour core interface. Increasing velocities in the vapour core will cause entertainment of liquid in the form of droplets. The depletion of the liquid from the film by this entertainment and by evaporation finally causes the film to dry out completely. Droplets continue to exist and are slowly evaporated until only single phase vapour is present. The characteristic that distinguishes the boiling heat transfer process from other mechanism is the change of phase that occurs in the coolant. This change in phase can occur with in the fluid, due to a process known as homogenous nucleation, or more commonly, it can occur at nucleation sites with in the fluid eg. suspended foreign material in the fluid or on a heated surfaces. As a result it can be said that, the boiling process was a very efficent method for cooling heated surfaces. Because of these reasons the thermal -hydraulic analysis of the reactor core must play very important role in reactor design. In this study, the finite difference approximation in the axial direction and an analytical method in radial direction is used to make thermal-hydraulic analysis of two phase flow channel. In order to determine the convective heat transfer coefficents `Dittus- Boelter` correlation is used for single phase flow and `Jens- Lottes` correlation is used for two phase flow. `Martinelli Nelson` correlation is used to compute the slip ratio, quality and void fraction relationships. `Running Method` is used to determine the friction factor in each mesh. In this model, lineer power density is supplied as an input from the analytical solution of the thermal neutron flux in steady state for a fuel xnelement. The thermal conductivities of the foel element depends on temperature. The correlations is used to compute the properties of the coolant which is also the function of the temperature and pressure. Finally THCAN code is used to make thermal-hydraulic analysis of a two phase flow channel and the results are discussed at the end of the study. xmfinally detaching to form a bubly flow. With production of more vapour the bubble population increases with length and coalescence takes place to form slug flow which in turn gives way to annular flow further along the channel. Close to this point the formation of vapour at this sites on the wall may cease and further vapour formation will be as a result of evaporation at the liquid film-vapour core interface. Increasing velocities in the vapour core will cause entertainment of liquid in the form of droplets. The depletion of the liquid from the film by this entertainment and by evaporation finally causes the film to dry out completely. Droplets continue to exist and are slowly evaporated until only single phase vapour is present. The characteristic that distinguishes the boiling heat transfer process from other mechanism is the change of phase that occurs in the coolant. This change in phase can occur with in the fluid, due to a process known as homogenous nucleation, or more commonly, it can occur at nucleation sites with in the fluid eg. suspended foreign material in the fluid or on a heated surfaces. As a result it can be said that, the boiling process was a very efficent method for cooling heated surfaces. Because of these reasons the thermal -hydraulic analysis of the reactor core must play very important role in reactor design. In this study, the finite difference approximation in the axial direction and an analytical method in radial direction is used to make thermal-hydraulic analysis of two phase flow channel. In order to determine the convective heat transfer coefficents `Dittus- Boelter` correlation is used for single phase flow and `Jens- Lottes` correlation is used for two phase flow. `Martinelli Nelson` correlation is used to compute the slip ratio, quality and void fraction relationships. `Running Method` is used to determine the friction factor in each mesh. In this model, lineer power density is supplied as an input from the analytical solution of the thermal neutron flux in steady state for a fuel xnelement. The thermal conductivities of the foel element depends on temperature. The correlations is used to compute the properties of the coolant which is also the function of the temperature and pressure. Finally THCAN code is used to make thermal-hydraulic analysis of a two phase flow channel and the results are discussed at the end of the study. xm