Havuz kaynaması sırasında kabarcık oluşumunun sayısal yöntemle incelenmesi
Numerical study of bubble growth during nucleate pool boiling
Yazar: MERT TÜRKELİ
Danışman: YRD. DOÇ. DR. İSMAİL YALÇIN URALCAN
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Makine Mühendisliği Ana Bilim Dalı / Isı-Akışkan Bilim Dalı
Derece: Yüksek Lisans, 2017
Konu: Makine Mühendisliği = Mechanical Engineering

Özet:
Kabarcıklı havuz kaynaması pek çok farklı mühendislik uygulama alanında çalışma imkanı sunmaktadır. Faz değişimi sırasında ısı geçiş katsayılarının tek falı ısı transferine göre fazla olması sebebiyle, yüksek ısı akısı gerektiren uygulamalarda kaynamalı ısı geçişi kullanılmaktadır. Kaynama sırasında faz değişimi olduğu için, sıvının gizli ısından faydalanılabilir. Kaynama sırasında çift fazlı bir akış söz konusudur ve Isı geçişi, yüzey üzerindeki girintilerde katı yüzey, sıvı ve gaz fazındaki akışkan arasında karmaşık bir mekaniğe dayalı olarak gerçekleşir. Bu nedenle problemin analitik olarak incelenmesi oldukça zor olmaktadır. Günümüze kadar kaynamalı ısı geçişi ile yapılan çalışmaların çoğunu deneysel ve sayısal modelleme çalışmaları oluşturmaktadır. Yapılan çalışmaların kapsamı mikro veya makro ölçekte olabilmektedir. Makro ölçekte yapılan çalışmalar arasında genellikle ısı transferini arttırmaya yönelik çalışmalar yer almaktadır. Isı geçişinin arttırıması için farklı yüzey şekilleri, kaplama malzemeleri ve akışkan tiplerinin kullanılması pek çok çalışmaya konu olmuştur. Mikro ölçekte yapılan çalışmalar ise ısı geçişinin detaylarının ortaya konması ve ısı geçişi sırasında gerçekleşen fiziksel olayların mekaniğinin incelenmesini amaçlamaktadır. Bu tez kapsamında bakır bir yüzeyden doymuş havuz kaynaması sırasında gerçekleşen kabarcık oluşumu, büyümesi ve ayrılması sürecleri sayısal olarak incelenmiştir. Buradaki amaç farklı sayısal modeller ve parametreler ile elde edilen çözümlemeleri deneysek veriler ile karşılaştırılarak, en uygun yöntemi belirlemektir.  Yapılan çalışmada 2 fazlı akış VOF ve CLSVOF yöntemleri ile ANSYS FLUENT versiyon 18 kullanılarak çözümlenmiştir. FLUENT içerisinde hazır olarak bulunan modeller kullanıldığı gibi, kaynama ve temas açısı ile ilgili modellemelerde ayrıca fonksiyonlar UDF ile tanımlanmıştır.   Yapılan sonuçlar deneysel veriler ile karşılaştırılarak modeller için uygun parametreler ve kullanılan yöntemin limitleri belirlenmiştir.

Abstract:
Pool boiling is used in many engineering applications in different disiplines. Due to phase change occuring in pool boiling regime, heat transfer coefficient is significantly higher compared to single phase heat transfer. Latent heat of liquid phase is come into account for boiling heat transfer causing increase in conduction. High heat transfer capability of such systems make it usefull in many high heat flux applications. Therefore heat transfer in multi-phase systems have been an extensive study field in mechanical engineering. 
In literature there are both numerical and experimental studies related to the topic. Analytical studies are limited due to complex nature of the heat and mass transfer characteristics of such systems. Because of the two-phase nature of boiling flows, heat and mass transfer occurs in a complex manner near close proximity of micro structure on heated surface. Because of the complexity in such systems, analytical solution of boiling heat transfer problems is not convinient. Therefore, most of the studies are relied on experimental results and emprical models to investigate and predict behaviour in boiling flows. 
When experimental stuides review in literature, papers on heat transfer enchancement draws attention because of the wide range of applications. To be able increase heat transfer coefficient, many methods involing surface modifications, changing mechanical properties of working liquid and applying varius coatings over heated surface, are introduced into application. By investigating these methods in macro scale researchers studied benefit over heat transfer efficiency. Another point of view is to investigate boiling in micro scale, there are many studies to investigate characteristics of heat transfer and boiling mechanics in such scale by using both numeraical and analytical methods. Such studies provide insight over the complex mechanics of boiling. 
Boiling heat transfer have different characteristics based on superheat of the boiling surface. Superheat value indicates the tempraute rise on the surface beyond saturation temperature during boiling process. Initial phase of boiling named as nucleate boiling cause boiling process occurs on small distributed cavities along the surface. Vapor bubbles begin to from on this nucleation sites during this period. When heat flux increase on the surface, temperature of the surface also increase further above the saturation temperature. This rise on surface temperature eventually alters the characteristic of the boiling process. With more nucleation sites activated by excess heat more bubbles start to form and grow, creating larger bubbles and vapor colums. This phase called transition boiling. With further increase in surface temperature, solid liquid contact at the interface begins to disapper due to significant evaporation rates. This results in a thin vapor phase region trapped between solid and liquid substances causing loss of contact totally. Without surface being wetted by the liquid continiously surface temperature rises further and radiation affects become significant. Thisphase is called film boiling. Within the scope of the study, nucleate boiling regime is investigated for water under atmospheric conditions. 
There are two major methods to evaluate pool boiling flows in literature based on the scale of the analysis. Studies done for macro scale evaluation generally considers pool or flow boiling temperature and velocity distributions on component scale to determine the heat transfer charactristics of manufactured surfaces. Other possible analysis scale is to conduct analysis on micro scale to determine characterictics of formation, growth and the movement of vapor phase bubbles in flow field. There are many experimental and numerical studies applying these two different approaches. 
This thesis focuses on the numerical evaluation of a single bubble growth during nucleate pool boiling of water. Numerical simulations of two phase flow has been carried out by VOF and CLSVOF methods using commercial software ANSYS FLUENT version-18. Simulations done in a cylinderical domain around one nucleation site with dimensions 0.150mm in diameter and 0.150mm in depth. Two different approach has been used to model the nucleation site; first problem was solved over a flat plate and bubble nucleation introduced numerically on the simulation. Then actual cavity is modeled in fluid domain and actual nucleation behaviour due to entraped bubble examined. Since fluid domian is cylindirical, 2D axissymetric domain was used in solutions. Simulations has been run trasiently with a time stepsize around 10-6s. Gravity is included in simulations since this is one of the major forces to determine bubble departure diameter. Other importance aspect of bubble departure is surface tension between phases. Departure occurs when bouyancy forces overcome the effect of surface tension. 
To be able determine bubble growth, mass transfer between liquid and vapor phases needs to be modeled into simulation. For the purpose of this thesis, mass transfer model based on superheat temperature is used. This model relates the rate of mass transfer over interphase to temperature difference above saturation point. With mass transfer and surface tension modelled into simulation bubble growth is succesfully modeled within fluid domain. Bubble departure problem also require modelling contact angle between phases. Contact angle has significant impact on bubble departure frequency which has been identified a major contributor to total heat transfer coeficient.
Numerical calculations made for different heat fluxes for different surface geometries. Two variation of VOF solution method is used. Based on results capability of the numerical model to predict bubble growth and departure is tested against experimental data. Initial solutiıns has been made for flat surface geometry with varius heat fluxes using VOF model. Results indicate succesful prediction of bubble growth rate over a limited range of heat flux. Bubble departure could not been achived using VOF model. To overcome the difficulties VOF model is coupled with level-set methodology. When simulations run for same heat fluxes, results indicate similar level of confidence on bubble growth with VOF method but with level-set included it was possible to model ubble departure over flat surface. Since surface geometry is flat there were no nucleation sites modeled to trap some remainin vapor after departure. Due to this limitaion, repetative buble growth and departure could not be modelled. Final trials are done with a cavity modeled on the surface to allow some vapor remain after departure. Simulations done with a cavity resulted in succesfull modeling of bubble growth and departure repetitively. 
On conclusion of all trials made with varius geometries and modelling approaches coupled VOF and level set method resulted in best results when compared to experimental data with succes in capturing all important aspects of bubble formation growth and departure.