Zorlanmış salınımlı dikey akışta gözenekli ortamın ısı geçişine etkisinin deneysel incelenmesi
Experimental investigations on the effect of porous media on heat transfer from vertical forced oscillated fluid flow
Yazar: ESRA KEŞAF
Danışman: ÖĞR. GÖR. ERSİN SAYAR
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Makine Mühendisliği Ana Bilim Dalı / Isı-Akışkan Bilim Dalı
Derece: Yüksek Lisans, 2015
Konu: Makine Mühendisliği = Mechanical Engineering
Dizin: Boru akışı = Pipe flow ; Kabarcıklı kaynamalı akım = Nucleate flow boiling ; Salınımlı akım = Oscillatory flow

Özet:
Bu çalışmada, sürekli olduğu varsayılan düşey halkasal sıvı kolonunda, sinüzoidal salınımlı ve zorlanmış laminer akış için, tel örgü katmanlar ile gözenekli ortam oluşturarak kabarcıklı kaynama rejiminde ısı geçişi deneysel ve teorik olarak incelenmiştir. Daha önce Ünal AKDAĞ, Mustafa ÖZDEMİR ve Ersin SAYAR'ın yaptıkları deneysel ölçmeler ve değerlendirmeler sonucunda, sabit ısı akısına sahip bir yüzeyden, salınımlı akışta ısı geçişine etki eden parametrelerin frekans, genlik ve yüzey sıcaklığı olduğu görülmüştü. Deneylerden elde edilen değerler için bir hesap tarzı geliştirilmiş ve Nusselt sayıları bulunmuştu.  Bulunan Nusselt sayıları için boyutsuz sayılara bağlı olarak bir korelasyon bağıntısı verilmişti. Bu çalışmadaki fark ise temelde Ersin SAYAR'ın yapmış olduğu "Sinüzoidal Salınımlı Akış İçin Hareketli Dikey Sıvı Kolonundaki Kaynamaya Ait İnceleme"nin, tel örgü katman oluşturarak gözenekli ortam için irdelenmesidir. Çalışma sonucu elde edilen bağıntılar literatürde bulunan tek fazlı çalışmalarla karşılaştırılmıştır. Böylelikle gözenekli ortamın ısı transferine etkisi tespit edilmiştir. 

Deney tesisatı ve salınımlı akışa uygun olarak kurulan matematiksel model iki farklı yaklaşımla ele alınmıştır.  İlk olarak diferansiyel yaklaşımla genel olarak ele alınan kütle, momentum ve enerji denklemleri salınımlı akış şartlarına göre sadeleştirilerek geçerli denklemler bulunmuştur. Hız profili için halkasal kesitte sınır şartları yazılarak momentum denklemi analitik olarak çözülmüştür.  İkinci olarak kontrol hacmi yöntemiyle integral formda süreklilik, momentum ve enerji denklemi yazılarak hareketli sınıra sahip kontrol hacminde ısıl enerji dengesi yazılmıştır. 

Tek fazlı bölgede, ısı geçişinde etkili olan mekanizmanın akışın merkezini takip edemeyen hidrodinamik sınır tabakadan kaynaklandığı, bu durumun ısı geçişini artırdığı doğrulanmıştır. Salınımlı akışta kısmi soğutulmuş kaynamanın ve tam gelişmiş soğutulmuş kaynamanın etkin olduğu rejimde ısı geçişi tek fazlı durum ile kıyaslanmış, kabarcıkların ısı geçiş katsayısında keskin bir artış sağladıkları doğrulanmıştır. Gözenekli ortamın saydam ve akışkanla termal dengede olduğu kabul edilmiştir. Akışkan ve gözenekli ortamın viskozite, termal iletkenlik, özgül ısı ve geçirgenlik gibi özellikleri sabit alınmıştır. Gözenekli ortamın homojen ve izotropik olduğu kabul edilmiştir. 

Bu yüksek lisans tez çalışmasında, literatürde yapılan çalışmalar doğrultusunda, akışkana doymuş gözenekli ortam ve bu ortama komşu akışkan tabakasından oluşan bileşik bir sistemde akış analitik olarak incelenmiştir.  Tel örgü katmanlardan oluşan bir gözenekli ortam kabulü yapılmıştır. Hem salınımlı akışın varlığı hem de gözenekli ortamın ısı geçişine etkisi incelenmiştir. Gözenekli ortamın ısı geçişini artırdığı gözlenmiştir.  Ayrıca, arayüzeyde sinüzoidal bir hız ifadesi kullanılarak ve akışkan tabakada momentum denklemi yatay ve düşey eksen için yazılarak elde edilen denklemler analitik olarak çözülmüştür. Sonuçlar, gözeneklilik, akışkan tabakası kalınlığı ve Darcy sayısına bağlı olarak hız ve sıcaklık dağılımları şeklinde gösterilmiştir.

Abstract:
In this study, in quasi-steady state conditions, heat transfer in a laminar oscillating vertical annular liquid column flowing in the bubbly flow regime investigated experimentally and theoretically.  
It was before observed by Unal AKDAG, Mustafa OZDEMIR and Ersin SAYAR. Their studies proved that the frequency, displacement, amplitude and wall temperature were important parameters affecting heat transfer from a uniform heat flux surface to reciprocating flow. For the experimental investigation a new approach was considered in the calculation of Nusselt number. 
Heat transfer effect is very important in industrial usage with porous media such as space craft design, inuclear reactors, pollution of environment etc. Flow through porous media have numerous engineering problems, for example, in the study of underground water resources, the movement of oil and natural gas through oil reservoirs,  purification of crude oil, pulp. 
Heat and mass transfer in oscillating flow has been fundamental investigation field in 
recent years. Oscillation-induced heat transport processes maintain an effective heat 
enhancement comparable with heat pipes. 
It has many important applications in the compact heat exchangers, cooling processes of nuclear plants, the design of Stirling heat machines and heat transport in the internal combustion machines.
The two dimensional Oscillatory flow in thin channels with a porous wall has numerous applications in various branches of Engineering and Technology such as filtration, boundary layer control and Lubrication approximation problems. It plays an important role in the study of problems which involve porous media, adhesion, biological flows and some manufacturing flows.
For flow in porous media, the Darcy equation has been applied. The Darcy equation is based on the principle of a linear relation between the velocity and the pressure gradient in the porous media. The linear factor is expressed as porosity and is representing the resistant to flow in a solid media. The flow process in porous media is governed by several physical phenomena such as viscous forces and also the forces coming from surface tensions between solid and fluid, but also surface tensions between different phases of the fluid. The flow process would in principle be best modeled by use of the momentum equation, but it takes more simulation effort to solve the momentum equation than use the Darcy equation. For this reason the Darcy equation is most commonly applied in simulations of fluid flow through porous media. 

For the single phase region of flow,  it is understood that,  the effective heat transfer 
mechanism is due to the hydrodynamic boundary layer which can not follow the core 
flow. Because of this condition, it is found that the heat flow has been increasing. 
Multiphase flow through porous media is important for a various applications such as CO2 sequestration, and enhanced oil recovery. These often involve the displacement of a nonwetting invading fluid from a porous medium by a wetting fluid, a physical phenomenon known as imbibition. Modeling of multiphase flow, on the other hand is still an enormous technical challenge. To capture the best model of multiphase flow, true description of fluid interaction such as capillary pressure and relative permeability is inevitable. 
A correlation equation was obtained for the Nusselt number depending on dimensionless numbers.  The basic difference of this study from Ersin Sayar's observation of Investigation of Heat Transfer in Reciprocating Boiling Flow is that the analysis on heat transfer in a laminar oscillating vertical annular liquid column flowing in the bubbly flow regime was changed by using foam layers to produce a porous medium in the system. 
The increment of heat transfer coefficient for the studied working regime was obtained comparing correlation with ones for single phase flows. 
According to the experimental results, bubbles induce highly efficient heat transfer mechanisms. The effect of the porous media in transport mechanics are analysed with this study. Experimental setup and mathematical model related to reciprocating flow were examined with two different approximations.  
Firstly mass, momentum and energy equations examined generally by using differential approximation. Then equations were simplified according to reciprocating flow and governing equations were obtained.  
In order to obtain velocity profile, using the appropriate boundary conditions in an annular cross-section, momentum equations were solved analytically.  Secondly, using the control volume approach mass, momentum and energy equations were written in integral form, then energy balance on the moving control volume were written.  
For the single phase region of flow, it is understood that, the effective heat transfer mechanism is due to the hydrodynamic boundary layer which can not follow the core flow. Because of this condition, it is found that the heat flow has been increasing. 
In reciprocating flow, the heat transfer coefficient which is strongly affected by single phase flow and nucleate-bubbly flow boiling conditions was studied. According to the experimental results, bubbles induce highly efficient heat transfer mechanisms. 
The two-phase flowmodel considered here is based on the following assumptions:
1. Pore air and pore water are single-component fluids.
2. Mass transfers between the fluids, i.e. the dissolution of air in water and the
evaporation of water, are neglected.
3. The flow is isothermal.
4. Both fluid phases are barotropic, i.e. each phase density depends only on the
pressure in the respective phase.
5. The solid phase is homogeneous, materially incompressible and does not react
with the pore fluids.
6. The solid skeleton is rigid.
7. The flow of each fluid can be described by the extended Darcy formula including
the relative permeability coefficient. An analysis of the flow of a viscous, incompressible fluid through a highly porous medium is presented when the free stream velocity oscillates in time about a non-zero constant mean. The permeability of the medium is assumed to be periodic because of which the problem becomes three-dimensional. 
The skin friction at the plate in the direction of the flow, rate of heat transfer in terms  of Nusselt number, the velocity field and the temperature field are obtained in nondimensional forms . 
Expressions for the transient velocity, transverse velocity component and skin friction have been obtained by series expansion method. During the course of discussion, it is found that in the porous medium the separation is prevented considerably despite larger frequency of the free stream oscillations.
It was assumed that porous media is in thermal equilibrium with fluid an liquid. Viscosity, thermal conductivity, heat capacity and permeability of the fluid and porous media is thought to be constant. 
Stainless steel wire layers were used as porous media. It was assumed to be homogeneous and isotropic. Fluid flow of a compact system which was full of saturated porous media and fluid layer in contact with this medium was analysed by the literature. 
There were made sixty experiments. Heat flux was changed four times, radial frequency was changed five times and amplitude was changed three times. According to experiment results some graphics were drawn. 
These were the ones determining the relationship between Reynolds numbers and Nusselt numbers or Reynolds numbers and heat fluxes and temperatures of the system or heat fluxes and amplitude of oscillations and temperatures of the system etc. 
Some correlations according to experimental results are gained. These correlations give an opinion about getting Nusselt numbers in such kind of a system  under the conditions in the project. 
In literature there are some investigations on porous media by using foam layers. Layers were generally inside paralel walls or cylinders. In this study, there are two coaxial cylinders and positioned as vertical and  porous layer is just wounded by the inside heater cylinder. Water was used as liquid in the project. 
System has both a heater and cooler pipe and adiabatic parts itself. To minimize the heat losses insulation with foam layers wounded by the outside cylinder was used. 
The effect of both oscillated flow and porous media on the heat transfer was observed. It is understood that porous media increases heat transfer. 
Sinusoidal velocity expression was used at interface and momentum equation is written on horizontal and vertical faces with the equations that are analytically solved.  Results were shown as Reynolds numbers and temperature diagrams related to Nusselt numbers.