doktorsko delo
Matevž Zupančič (Author), Iztok Golobič (Mentor)

Abstract

Mehurčkasto vrenje je najbolj učinkovit in hkrati tehnično obvladljiv mehanizem prenosa toplote. Izboljšati prenos toplote pri vrenju pomeni zmanjšati pregretje vrelne površine pri danem sproščanju gostote toplotnega toka in povečati kritično gostoto toplotnega toka, kar je možno doseči z spremembo mikro- in nanostrukture ter omočljivosti vrelne površine. V disertaciji predstavljamo razvoj dveh tipov površin: bifilnih strukturiranih površin na osnovi premaza iz polidimetilsiloksana in silike ter lasersko strukturiranih hidrofilnih površin. Zasnovan eksperimentalni sistem nam omogoča spremljanje procesa vrenja na tankih kovinskih folijah, kjer s pomočjo hitrotekoče infrardeče kamere merimo nestacionarna temperaturna polja in z video kamero opazujemo rast in gibanje mehurčkov. Pri vrenju na bifilnih površinah smo pokazali, da hidrofobna mesta promovirajo nastop vrenja, velikosti hidrofobnih področji pa vplivajo na velikosti mehurčkov in frekvence nukleacij. Z ustreznim bifilnim vzorcem smo definirali področja aktivnih nukleacijskih mest in zakasnili pojav horizontalnih koalescenc, s čimer smo dosegli boljšo stabilnost vrelnega procesa. Izboljšave se pokažejo tudi v zmanjšanju pregretja ter in povečanju kritične gostote toplotnega toka. Ugotavljamo, da je optimalna razporeditev hidrofobnih mest na bifilnih površinah odvisna od točke obratovanja, torej od gostote toplotnega toka. Pri vrenju na lasersko strukturiranih površinah so bili najboljši rezultati doseženi na heterogeno omočljivi površini s prisotnimi mikro jamicami. Največje število aktivnih nukleacijskih mest sovpada z lokacijami mikro jamic, kar pojasnjujemo tudi s pomočjo obstoječih nukleacijskih kriterijev. Na tem vzorcu je bilo doseženo celo nižje pregretje kot na vseh ostalih bifilnih površinah. V sklopu analize eksperimentalnih podatkov smo s statistično obdelavo nestacionarnih temperaturnih polj določili porazdelitve gostot verjetnosti temperatur na vrelnih površinah. Na podlagi značilnosti teh porazdelitev je možno ovrednotiti stabilnost procesa vrenja, česar nam do sedaj uveljavljene vrelne krivulje niso omogočale. S tem odpiramo novo področje na poti boljšega razumevanja procesa vrenja in razvoja tehnologij za izboljšan prenos toplote.

Keywords

mehurčkasto vrenje v bazenu;izboljšan prenos toplote;bifilne površine;lasersko strukturirane površine;hitrotekoča infrardeča termografija;temperaturne porazdelitve na vrelnih površinah;disertacije;

Data

Language: Slovenian
Year of publishing:
Typology: 2.08 - Doctoral Dissertation
Organization: UL FS - Faculty of Mechanical Engineering
Publisher: [M. Zupančič]
UDC: 536.24:66.046.7:544.344.016.2(043.3)
COBISS: 15186715 Link will open in a new window
Views: 26
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Other data

Secondary language: English
Secondary title: Structured biphilic surfaces development for enhanced boiling heat transfer
Secondary abstract: Nucleate boiling is the most effective and technically controllable heat transfer mechanism. The main goal in boiling heat transfer enhancement is lowering the surface overheat for a given heat flux and increasing critical heat flux, which could be achieved by modifying surfaces' micro and nano structure and its wettability. In this dissertation we present development of (i) biphilic structured surfaces based on polydimethylsiloxane-silica coating and (ii) laser structured hydrophilic surfaces. Our designed experimental setup comprises high-speed infrared thermography for measuring transient temperature fields underneath thin metal foils and video camera to observe growing vapor bubbles. Results showed that hydrophobic spots on biphilic surfaces promote onset of nucleation and their size influences bubble detachment diameter as well as nucleation frequency. By varying biphilic pattern we managed to define positions of active nucleation sites, delay occurrence of horizontal coalescences and thus increase boiling stability. This resulted in lower overheat and higher critical heat flux. It was also concluded that the optimal biphilic pattern could only be determined for a particular heat flux. For laser structured surfaces the best results were achieved on the heterogeneously wettable sample with microcavities present on the surface. The highest number of active nucleation sites corresponded to the locations of microcavities, which is also supported by existing nucleation criteria. This surface demonstrated even lower superheat than all other tested biphilic surfaces. Finally, we propose a new approach to evaluating the boiling process on the basis of probability density of wall-temperature, calculated from spatio-temporal thermographs of the boiling surface. Instead of a single data point on the boiling curve, the presented wall-temperature distributions provide spectra of information that could be also used in evaluating boiling stability. This opens a new path towards a better understanding and the development of phase-change heat transfer technology.
Secondary keywords: nucleate pool boiling;enhanced heat transfer;biphilic surfaces;laser structured surfaces;high-speed infrared thermography;wall-temperature distributions;dissertations;Vrenje;Disertacije;Toplota;Prenos;
Type (COBISS): Doctoral dissertation
Thesis comment: Univ. v Ljubljani, Fak. za strojništvo
Pages: XXV, 119 str.
ID: 23459704