Havuz kaynamasında bir mikro oyuktaki kabarcıklanmanın incelenmesi
Study of bubble nucleation in a microcavity in pool boiling
Yazar: TUĞBA TETİK
Danışman: DR. ÖĞR. ÜYESİ İSMAİL YALÇIN URALCAN
İstanbul Teknik Üniversitesi / Lisansüstü Eğitim Enstitüsü / Makine Mühendisliği Ana Bilim Dalı / Makine Mühendisliği Bilim Dalı
Derece: Doktora, 2022
Konu: Makine Mühendisliği = Mechanical Engineering
Dizin: Havuz kaynaması = Boiling pool ; Kabarcıklanma = Bubbling ; Matematiksel modelleme = Mathematical modelling

Özet:
Dünyadaki enerji kaynaklarının azalması, mevcut kaynakların daha verimli kullanılması üzerine yapılan çalışmaların önemini artırmaktadır. Isı geçiş miktarının iyileştirilmesi üzerine yürütülen çalışmalar da bu kapsamda yer almaktadır. 
Yüzeyden ısı çekilmesi/soğutulması için en etkin ısı geçiş yöntemlerinden biri olan kaynama olayı, güç üretim sistemlerinden günlük yaşama kadar geniş bir uygulama alanına sahiptir. Bu sebeple, bu alanda yapılacak herhangi bir iyileşme, enerjinin verimli kullanımı ve ekonomi açısından olumlu etkilerde bulunacaktır. Kaynama ısı geçiş iyileştirme yöntemleri aktif ve pasif yöntemler olmak üzere iki bölümde incelenmektedir. Aktif yöntemlerde genellikle yüzeye ve akışkana dışarıdan müdahale edilirken, pasif yöntemlerde yüzey yapıları üzerinde çalışılmaktadır. Bununla birlikte kabarcıkların rastgele hareketleri ve birbirleri ile olan etkileşimleri ısı geçiş performansını etkilemektedir. 
Kabarcıklanma olayının gerçekleştiği oyukların kaynama olayındaki ısı geçişine etkisinin anlaşılabilmesi, kaynama yüzeyi ve sistem ölçeğinde doğru çözüme ulaşılması açısından önemli bir adımı oluşturmaktadır. Bu çalışmada öncelikle gerçek yüzey yapısının etkilerinin en aza indirilerek kaynama olayına etki eden bilinmeyen parametrelerin sadeleştirilmesi amacıyla düz bakır yüzey üzerinde deneyler yapılmış ve yüzeyin havuz kaynama eğrisi elde edilmiştir. Daha sonra oyukların ısı geçişine etkisini araştırmak amacıyla üzerinde tek bir oyuğun bulunduğu ve 0.5 mm ve 2 mm aralıklarla yerleştirilmiş yapay oyukların bulunduğu üç ayrı yüzey daha oluşturulmuştur. Deneyler, yeni yüzeyler için aynı ısı akısı aralığında tekrarlanarak yapay oyukların ısı geçişine etkisi incelenmiştir. Kurulan deney tesisatında, aynı zamanda kabarcıklanma çevrimi ve kabarcık davranışları gözlemlenmiş, kabarcığın yüzeyden ayrılma çapı ve frekansı ölçülmüştür. 
Havuz kaynamasında gerçekleşen ısı miktarının belirlenmesi amacıyla yarı-ampirik bir matematik model geliştirilerek, sonuçlar deney verileri ile karşılaştırılmıştır. Isı geçiş mekanizmaları, etkin oldukları alanlar ile birlikte değerlendirilip modele dahil edilirken, ısı geçiş yüzeyi özellikleri deney verilerinden alınmıştır.
Sayısal çalışma kapsamında, düz yüzeyde ve konik biçimli bir kaynama odağında gerçekleşen kabarcıklanma olayı iki boyutlu modellenmiştir. Yapılan analizler sonucunda elde edilen kabarcık büyüme eğrisi, çapı ve şekli deneysel veriler ile ilişkilendirilerek sayısal modelin geçerliliği değerlendirilmiştir.

Abstract:
While energy sources became more scarce year by year, more efficient use of existing resources is prioritized throughout the world. Studies on improving heat transfer are considered equally important in this scope.
Boiling is one of the most effective heat transfer methods for heat extraction/cooling at the surface and has a wide range of applications from power generation systems to daily life. For this reason, any improvement in this area will have positive effects in terms of efficient use of energy and thermo-economy. Systems involving the boiling phenomenon are primarily designed and operated based on the nucleate boiling regime, which makes it possible to obtain high heat fluxes with low surface temperatures. 
Nucleate pool boiling regime starts from the onset of nucleate boiling to fully developed nucleate boiling. Boiling heat transfer enhancement methodologies can be classified into active and passive groups.  While external power is applied to the surface and fluid in active methods; passive methods include surface structure and modifications.
In boiling systems, a large amount of heat is transferred from the surface within small temperature differences between the surface and saturation temperature of the fluid. The energy transfer depends on the bubble characteristics, i.e., nucleation, growth period, departure diameter, and frequency. To obtain the most efficient heat transfer, it is important to fill the space with optimum amount of saturated liquid after the bubbles leave the surface. The continuous growth of bubbles by merging on the surface makes the amount of liquid flow to the surface insufficient. This causes dry out of the surface, and induces critical heat flux. In contrast, early release of the bubbles reduces the heat transfer by cooling effect of continuous liquid feeding to the surface.
Understanding the effect of the cavities on boiling heat transfer constitutes an essential step in reaching the exact solution at the boiling surface and system scale. The objective of the thesis is to study pool boiling phenomena experimentally and numerically from both macroscopic and microscopic viewpoints. A semi-empirical mathematical model was developed using experimental data.
This study investigates the effects of evenly spaced artificial cavities on a plane surface on the nucleate boiling heat transfer. An experimental facility was designed and constructed to visualize nucleate pool boiling phenomena. The experimental work can be classified into two groups: nucleation site density/distribution and bubble dynamics.  Experiments have been conducted on the top surface of the copper rod with distilled water at atmospheric pressure. The bottom end of the copper rod is screwed into a copper heating block which encompasses the electric heater powered with a 15 V x 200 A DC supply. In the pool, saturation conditions were maintained by auxiliary heaters. 
Axial temperature distribution along the rod, determined via thermocouple readings, is extrapolated to specify the boiling surface temperature. The product of the gradient of the temperature profile and the thermal conductivity of copper gives the heat flux at the boiling surface. Before starting the experiments, the working fluid was boiled for at least two hours to degas and reach the steady-state conditions. Afterward, the main heater was adjusted to yield a specific heat flux at the boiling surface. 
Initial experiments were carried out on a flat copper surface in order to minimize the effects of the surface structure and simplify the unknown parameters affecting the boiling phenomenon. From the test results, the pool boiling curve of the surface was obtained. Then, three more surfaces with cavities, which are 180±30 μm in diameter and 160±10 µm in depth, were produced on the copper surface by mechanical methods; a single cavity, and artificial cavities placed at 0.5 mm and 2 mm intervals respectively. The effects of artificial cavities on heat transfer were investigated by repeating the experiments in the same heat flux range for new surfaces. The surfaces' boiling performance was evaluated by measured surface temperatures and wall heat flux.
During the experiments, bubbling cycle and bubble behavior were also observed. To understand the bubbling mechanism, detailed measurements of the bubbles have been made using a high-speed camera (Photron 512-PCI) with up to eight thousand frames per second and a long-distance microscope (K2 Distamax). Qualitative observations like bubble shapes and coalescence processes were made, and quantitative information like diameter and frequency was obtained from the recorded images and videos. At least thirty bubble cycles were examined to determine the bubble growth period, departure diameter, and frequency. Average values were used to compare the surface's boiling performance and the mathematical model. 
In order to measure bubble diameter, an image processing code was developed in MATLAB. The bubble images were imported into MATLAB and read as a two-dimensional array of color values. After removing noise, Canny method was applied with specified threshold values to detect edges of the bubbles. The results were given in pixels and then scaled to millimeters. The results were compared with the literature.
Measured diameter and frequency values were used to calculate latent heat transport by vapor bubbles and compared with the boiling curves obtained by temperature measurements.
A semi-empirical mathematical model, including heat transfer mechanisms and bubble behaviors, was developed to determine the amount of heat generated in the pool boiling, and the results were compared with the experimental data.  Boiling surface area is examined in two parts: bubble effective and natural convection areas. Relevant heat transfer mechanisms were utilized for related areas while carried out the calculations. Empirical constants, representing the characteristic of the heat transfer surfaces were derived from the experimental data. Suggested model can predict the experimental trend within an error of ±30%.
A numerical model has been developed to simulate bubble growth and departure. Boiling is considered as two-phase flow for liquid and vapor phases of water. 2D axisymmetric simulations were conducted for water at equilibrium with its vapor phase. The study aims to examine the similarity of the bubble shape and growth rate with the experimental results. the bubbling phenomenon on a flat surface and in a conical-shaped cavity is modeled. The interface separating the liquid and vapor phases from each other is considered as a thin surface. In the analysis, CLSVOF method was used. The numerical model was validated by experimental results, i.e., bubble growth curve, diameter and shape.