217 research outputs found
Structured copper surfaces for enhanced nucleate boiling heat transfer
No full textV delu obravnavamo vpliv mehanskega in laserskega strukturiranja bakrenih vzorcev na izboljšanje prenosa toplote pri mehurčkastem vrenju vode v bazenu. Vzorce smo mehansko obdelali z brušenjem ali peskanjem oz. lasersko strukturirali z namenom izdelave mikrojamic ali oksidne plastina eni površini smo po vzoru literature izdelali mikrokanale. Površine smo okarakterizirali z uporabo meritev topografije, izdelavo SEM posnetkov, EDS analizo kemijske sestave in meritvami kota omočenja. Na merilni progi lastne konstrukcije smo izvedli eksperimente z nasičenim vrenjem redestilirane vode v bazenu, pri čemer smo zabeležili vrelne krivulje posameznih površin pred prvim nastopom kritične gostote toplotnega toka in po njem ter izračunali koeficiente toplotne prestopnosti. Dosegli smo kritične gostote toplotnega toka do 1220 kW m-2 na mehansko obdelanih površinah, do 1580 kW m-2 na lasersko strukturiranih površinah in 1897 kW m-2 na površini z mikrokanali, kar predstavlja 46 %, 89 % oz. 127 % izboljšanje napram referenčni površiniizboljšanje koeficienta toplotne prestopnosti v primerjavi z referenčno površino znaša do 159 % (mehanska obdelava), 174 % (lasersko strukturiranje) oz. 296 % (mikrokanali). Ugotovili smo, da lasersko strukturiranje zagotavlja stabilne površine, ki se med zaporednimi meritvami minimalno spreminjajo. Prav tako smo opazili pojav spremembe površinske kemije kot posledice nastopa kritične gostote toplotnega toka, kar je imelo velik vpliv na prenos toplote z vrenjem.In the present work, the influence of mechanical and laser structuring of copper samples on enhanced nucleate pool boiling heat transfer is investigated. Samples were either mechanically treated using sanding or sandblasting or laser structured to produce microcavities or a surface oxide layermicrochannels were machined onto one sample. Characteristics of surfaces were determined through the use of topography measurement, SEM imaging, EDS surface chemical composition analysis and contact angle measurement. A pool boiling experimental setup of our own design was used to determine boiling heat transfer characteristics of structured surfaces during saturated boiling of twice-distilled water. Boiling curves were recorded before and after the first onset of critical heat flux and heat transfer coefficients were calculated. The following critical heat flux was reached: up to 1220 kW m-2 on mechanically treated surfaces, up to 1580 kW m-2 on laser treated surfaces and 1897 kW m-2 on the surface with microchannelsthis represents 46 %, 89% and 127 % improvement in regards to the reference surface, respectively. Heat transfer coefficient improvement ranged from 159 % (mechanical treatment) and 174 % (laser treatment) to 296 % (microchannels). It was determined that laser structuring produces stable surfaces with minimal deviation between consecutive measurements. Additionally, a change of surface chemistry was recorded after the first onset of the critical heat flux, which had a large influence on boiling heat transfer
Hybrid structured surfaces for enhanced nucleate boiling heat transfer
No full textV delu obravnavamo prenos toplote pri mehurčkastem vrenju in njegovo izboljšanje oz. intenzifikacijo preko modifikacije vrelne površine in njene interakcije z delovnim fluidom. V ta namen predstavljamo razvoj hibridnih strukturiranih površin, ki promovirajo mehurčkasto vrenje. Za funkcionalizacijo površin vzorcev iz aluminija, bakra in titana smo uporabili predobdelavo s kemičnim ali laserskim strukturiranjem in nanos hidrofobnega premaza v obliki fluoriranega silana. Na podlagi analize literature, v kateri obstaja velik raztros referenčnih vrednosti kritične gostote toplotnega toka kot posledica različnih metodologij procesiranja izmerkov, smo razvili novo merilno progo za spremljanje procesa vrenja z nizko merilno negotovostjo. Na razvitih površinah smo ovrednotili prenos toplote pri nasičenem vrenju vode pri atmosferskem tlaku, pri čemer smo zabeležili vrednosti kritične gostote toplotnega toka do 1943 kW m-2 (> 100 % izboljšanje v primerjavi z neobdelano površino) in koeficiente toplotne prestopnosti do 304,7 kW m-2 K-1 (> 500 % izboljšanje). Pokazali smo, da se zaradi večkratnega nastopa kritične gostote toplotnega toka ali dolgotrajnega vrenja spremenijo morfološke in kemične lastnosti vrelne površine, kar vpliva na prenos toplote pri vrenju. Ovrednotenje vrenja samoomočljivih fluidov na razvitih hibridnih površinah je pokazalo, da se parametri prenosa toplote v splošnem poslabšajo v primerjavi z vrenjem čiste vode na istih površinah, še vedno pa so zagotovljeni višji koeficienti toplotne prestopnosti kot pri vrenju istih fluidov na neobdelanih površinah. Rezultati doktorskega dela kažejo, da primerno razplinjene površine z nizko površinsko energijo in mikrostrukturo, ki promovira nukleacijo, zagotavljajo velike izboljšave parametrov prenosa toplote pri mehurčkastem vrenju, kar je v nasprotju s trenutno uveljavljenim prepričanjem, da takšne površine, ki na makroskali izkazujejo superhidrofobnost, niso primerne za izboljšanje prenosa toplote pri vrenju.This work deals with nucleate boiling heat transfer and its enhancement through modification of the boiling surface and the interaction of the latter with the working fluid. To this effect, we present the development of hybrid structured surfaces, which promote nucleate boiling. Surfaces of aluminum, copper and titanium samples are functionalized using chemical or laser surface texturing and subsequent hydrophobization through application of a fluorinated silane. Analysis of reference critical heat flux values from literature shows that considerable scatter results from different methodologies of processing measured values. This was used to develop a new experimental setup for boiling performance evaluation with reduced measurement uncertainty. Boiling heat transfer was evaluated on developed surfaces using saturated pure water at atmospheric pressure. Developed surfaces exhibited critical heat flux values of up to 1943 kW m-2 (> 100 % enhancement over the reference surface) and heat transfer coefficients of up to 304,7 kW m 2 K-1 (> 500 % enhancement). We have shown that repeated onset of critical heat flux or long-term boiling on the surface changes morphological and chemical properties of the boiling surface, which affects boiling heat transfer. Boiling of self-rewetting fluids on developed hybrid structured surfaces resulted in deterioration of heat transfer intensity compared to boiling of pure water on the same surfaces, although heat transfer is still enhanced compared to boiling of these fluids on untreated surfaces. The results of the doctoral thesis demonstrate that properly degassed surfaces with low surface energy and nucleate-boiling-promoting microstructure provide substantial enhancements of nucleate boiling heat transfer parameters, which challenges the currently established sentiment that such (macroscopically superhydrophobic) surfaces are unsuitable for boiling heat transfer enhancement
Boiling heat transfer enhancement using surface structuring with electrodeposition
No full textV zaključni nalogi predstavljamo izboljšanje prenosa toplote pri procesu mehurčkastega vrenja z uporabo strukturiranih površin, ki smo jih pripravili z elektrodepozicijo. Modifikacija lastnosti vrelnih površin je eden izmed glavnih pristopov k povečevanju intenzivnosti prenosa toplote pri vrenju, elektrodepozicija pa omogoča izdelavo poroznih površinskih struktur, ki pomagajo zagotavljati dotok kapljevine na vrelno površino in s tem preprečevati njeno izsušitev. S procesom elektrodepozicije smo obdelali bakrene površine, pri čemer smo spreminjali čas obdelave in preučevali vpliv naknadne obdelave s sintranjem izboljšanja lastnosti površine pri mehurčkastem vrenju. Prenos toplote na izdelanih površinah smo ovrednotili pod pogoji mehurčkastega vrenja vode v bazenu pri atmosferskem tlaku, pri čemer smo se osredotočili na kritično gostoto toplotnega toka in koeficient toplotne prestopnosti. Ugotovili smo, da čas elektrodepozicije nima signifikantnega vpliva na parametre prenosa toplote pri vrenjem na razvitih površinah, naknadna obdelava s sintranjem pa je neugodno vplivala na odpornost površin na termomehanske obremenitve med testiranjem. Najvišja zabeležena vrednost kritične gostote toplotnega toka je na strukturiranih površinah znašala 1735 kW/m2, na neobdelani površini pa 977 kW/m2. V primerjavi z neobdelano referenčno površino smo tako dosegli med 6% in 80%. Prav tako smo v primerjavi z referenčno površino izboljšali tudi vrednosti koeficientov toplotne prestopnosti za skoraj 100 %.In this thesis, we present the heat transfer enhancement during nucleate boiling using structured surfaces prepared by electrodeposition. Modification of the surface properties is one of the main approaches to increase the intensity of boiling heat transfer, and electrodeposition induces porosity of the surface structure, which helps to ensure the inflow of liquid towards the boiling surface and thus prevents its dryout. We treated copper surfaces using the electrodeposition process, varying the treatment time and studying the influence of post-treatment by sintering. The heat transfer on the manufactured surfaces was evaluated under pool boiling conditions using saturated water at atmospheric pressure and focusing on the critical heat flux and heat transfer coefficients. We found that the electrodeposition time has no significant effect on the boiling heat transfer parameters on the developed surfaces, and the subsequent sintering treatment had an adverse effect on the surface resistance to thermomechanical stress during testing. The highest recorded value of the critical heat flux was 1735 kW/m2 on the structured surfaces, and 977 kW/m2 on the untreated surface. Compared to the untreated reference surface, we thus achieved an increase between 6% and 80% in the critical heat flux. We also improved the values of heat transfer coefficients by almost 100% compared to the reference surface
Superhydrophobic surfaces for enhanced resistance against freezing of impacting water droplets
No full textNabiranje ledu na površinah povzroča težave na številnih področjih. To problematiko lahko učinkovito rešujemo s superhidrofobnimi površinami, ki povečajo odpornost proti zmrzovanju. V okviru zaključne naloge smo s kombinacijo laserskega strukturiranja in hidrofobizacije na aluminijastih substratih razvili superhidrofobne površine, ki uspešno povečajo odpornost proti zmrzovanju padajočih vodnih kapljic. Učinkovitost površin smo določili z meritvami kotov omočenja in ovrednotenjem obstoja odboja vodnih kapljic pri različnih temperaturah podhladitve površine. Dobljene rezultate smo primerjali z neobdelano površino. Pri tem smo se osredotočili na vpliv vlažnosti, temperature in hitrosti kapljice ob trku. Vsi izmerjeni koti omočenja so višji od 160°, histereze kota omočenja pa ne presegajo 5°. Relativna vlažnost v zraku ima velik vpliv na rezultate. Povečana vlažnost zmanjša sposobnost odboja vodnih kapljic pri padcu na podhlajeno površino. Nižanje temperature površine najbolj vpliva na fazo krčenja kapljice, ki se skrajša zaradi hitrejše zamrznitve kapljice. Večje hitrosti kapljice ob trku so pomenile večje maksimalne premere med trkom in tudi večje končne premere kapljic po zamrznitvi. Večja, kot je bila hitrost, bolj se je kapljica razpršila in razpadla na manjše satelitske kapljice. Najboljšo odpornost proti zmrzovanju je izkazala površina s povprečno globino in širino kanalov 26,8 µm oz. 26,8 µm ter razmakom med posameznimi kanali 50 µm.Ice accumulation on surfaces poses challenges in many fields. This issue can be effectively addressed with superhydrophobic surfaces that enhance the resistance against freezing. In this diploma thesis, we developed superhydrophobic surfaces on aluminum substrates using a combination of laser-texturing and hydrophobization, which successfully increased the resistance against freezing of impacting water droplets. The effectiveness of the surfaces was determined by measuring the contact angles and evaluating droplet rebound on them at various subcooled surface temperatures. The obtained results were compared with an untreated surface, focusing on the influence of humidity, temperature, and droplet impact velocity. All measured contact angles were above 160°, and the contact angle hysteresis did not exceed 5°. Relative humidity in the air significantly impacted the results. Increased humidity reduced the droplet rebound capability upon impact on a supercooled surface. Lowering the surface temperature mainly affected the droplet retraction phase, which was shortened due to faster droplet freezing. Higher droplet impact velocities resulted in larger maximum diameters during the impact and larger final diameters after freezing. The higher the velocity, the more the droplet splashed and disintegrated into smaller satellite droplets. The best resistance against freezing was demonstrated by a surface with an average channel depth and width of 26.8 μm and 26.8 μm, respectively, and a spacing between the channels of 50 μm
IR thermographic investigation of pool boiling process on vertically oriented metal foils
No full textVrenje je eden najučinkovitejših načinov prenosa toplote, ki pri majhnih temperaturnih razlikah omogoča odvod velikih gostot toplotnega toka. Proces nukleacije predstavlja ključen fenomen pri tem mehanizmu prenosa toplote, zato je pomembno poznavanje in razumevanje faktorjev, ki vplivajo na nukleacijo. Magistrsko delo obravnava proces vrenja v bazenu na vertikalno orientiranih površinah iz nerjavnega jekla z uporabo IR termografske analize. Na površinah smo z laserskim teksturiranjem in uporabo hidrofobnega premaza izdelali vzorce s hidrofilnim, hidrofobnim nukleacijskim mestom in enega s tremi hidrofilnimi nukleacijskimi mesti. Ob uporabi eksperimentalne proge smo za tri vzorce opravili meritve pri treh različnih podhladitvah vode – 1, 4 in 9 K pri izpostavljenosti vzorca gostotam toplotnega toka med 70 in 130 kW m-2. Ugotovili smo, da se lokalne temperature vrelne površine in gostote toplotnega toka na mestu nukleacije, pri zmanjšanju podhladitve in povečanju gostote toplotnega toka, ki mu je vzorec izpostavljen povečujejo, hkrati pa se povečuje tudi premer nastalih mehurčkov.Boiling is considered as one of the processes with the most effective heat removal. It enables high heat fluxes at relatively low temperature differences. Bubble nucleation reperesents the key mechanism of the processtherefore, it is vital to be familiar with factors that influence nucleate boiling process and understand them. This thesis investigates the pool boiling process on vertically oriented, stainless steel surfaces, using IR thermographic analysis. Laser texturing and the hydrophobic coating was used to fabricate three samples – one with hydrophilic, one with hydrophobic nucleation spot and one with three hydrophilic spots. Using an experimental setup, we performed measurements on samples for three different subcooling, of water bulk – 1, 4, and 9 K. The sample was exposed to heat fluxes between 70 and 130 kW m-2.. We found that local boiling surface temperatures and heat fluxes increase with the decrease of subcooling and the increase of the heat flux that the sample is exposed to. At the same time, with the increase of subcooling and decrease of exposure heat flux, bubble diameters are reduced
Evaluating the growth of a hydrogen bubble on a microelectrode using a high-speed camera
No full textVodik se trenutno omenja kot prihodnji energetski vir, saj ne temelji na ogljiku, zato pri uporabi ne proizvaja neposrednih ogljikovih emisij. Večina vodika se trenutno proizvede iz zemeljskega plina, le manjši del pa z elektrolizo. Elektroliza vode, tj. postopek razgradnje vode na vodik in kisik z uporabo električne energije, predstavlja obetavno metodo za pridobivanje vodika ob pogoju uporabe elektrike iz obnovljivih virov energije. S pomočjo hitrotekoče kamere smo v elektrolitski celici opazovali rast vodikovih mehurčkov na elektrodah različnih velikosti in v različno koncentriranih raztopinah H2SO4. Ugotovili smo, da se premer mehurčkov povečuje z večanjem napetosti, hkrati pa potrebujejo tudi več časa za nastanek. Manjša površina mikroelektrode in nižja koncentracija raztopine H2SO4 sta ločeno vplivali na nastajanje večjih mehurčkov, ki rastejo počasi. Pokazali smo, da velikost mikroelektrode, napetost in koncentracija elektrolita pomembno vplivajo na proces elektrolize in rast vodikovih mehurčkov. Rezultati prispevajo k boljšemu razumevanju dinamike elektrolitskih procesov ter pripomorejo k optimizaciji elektrolitskih sistemov za učinkovitejšo proizvodnjo vodika, kar predstavlja potencial za razvoj novih tehnologij na področju energetske učinkovitosti in trajnostnih virov energije in okolju prijaznejših energetskih rešitev.Hydrogen is nowadays mentioned as a future energy source because it does not contain carbon and thus causes no direct carbon compound emissions when used for energy purposes. Most hydrogen is currently produced from natural gas, with only a fraction produced by electrolysis. Water electrolysis, the process of decomposing water into hydrogen and oxygen using electricity, is a promising method for hydrogen production if renewable electricity is used. Using a high-speed camera, we observed the growth of hydrogen bubbles on microelectrodes of different sizes and in different concentrations of the H2SO4 in an electrolytic cell. The bubble diameters increased with increasing voltage, while at the same time, they also took longer to form. A smaller electrode surface and lower H2SO4 concentration separately contributed to larger bubble diameters, which grew slowly. We demonstrated that the size of the microelectrode, voltage, and electrolyte concentration significantly influence the electrolysis process and the growth of hydrogen bubbles. The results contribute to a better understanding of the dynamics of electrolytic processes and aid in optimizing electrolytic systems for more efficient hydrogen production, which represents a potential to develop new technologies in the field of energy efficiency, sustainable energy sources, and environmentally friendly energy solutions
Determining the need for an expanded fitness component in the Fire Science Program at Milwaukee Area Technical College
No full textIncludes bibliographical references
Laser-textured aluminum surfaces with anti-icing properties
No full textV zaključni nalogi smo obravnavali razvoj lasersko strukturiranih superhidrofobnih površin s protizmrzovalnimi lastnostmi. Razvoj na področju zakasnitve in preprečevanja zmrzovanja je trenutno usmerjen v uporabo pasivnih metod, ki temeljijo na ustreznih obdelavah površine. S pregledom literature smo postavili temelje za eksperimentalni del, v katerem smo na aluminijastih vzorcih iz zlitine 1050A razvili lasersko strukturirane površine z ugodnimi protizmrzovalnimi lastnostmi. Površine smo ovrednotili iz vidika omočljivosti, nukleacijske temperature, časovnega zamika zmrzovanja, spremljanja procesa zmrzovanja in korozijske odpornosti ter jih primerjali z neobdelano referenčno površino in hidrofobizirano neobdelano površino. Lasersko obdelane površine, ki so izkazale najboljše superhidrofobne lastnosti, so bile površine z globokimi in plitkimi kanali, z razmiki med linijami 25 µm in 50 µm. Na površini s plitkimi kanali in razmiki med linijami 25 µm smo dosegli najnižjo nukleacijsko temperaturo, in sicer -23,2 °C. Časovni zamik zmrzovanja smo beležili pri temperaturi površine med -20 °C in -12,5 °C ter potrdili, da se časovni zamik zmrzovanja z nižjo temperaturo površine skrajša. Na razvitih površinah smo izvedli elektrokemične teste za ovrednotenje korozijske odpornosti površin, pri čemer je površina s plitkimi kanali in razmiki med linijami 25 µm izkazala najboljšo korozijsko odpornost. V primerjavi z referenčno površino smo z lasersko obdelavo in hidrofobizacijo znatno izboljšali protizmrzovalne lastnosti površine.Within the framework of the diploma thesis, we focused on the development of laser-textured superhydrophobic surfaces with anti-icing properties. The development in the field of icing delay and prevention is currently focused on using passive methods based on surface modification. By reviewing the available literature, we provided the framework for the empiric part of the research, where we developed laser-textured surfaces with favorable anti-icing properties on samples from 1050A aluminium alloy. We evaluated their surface wettability, nucleation temperature, and icing onset delay whilst also tracking the icing process and evaluating the corrosion resistance before comparing them with a reference untreated surface and a solely hydrophobized surface. Laser-textured surfaces that showed the most favorable superhydrophobic properties were surfaces with both deep and shallow channels, with the spacing between consecutive laser passes of 25 µm and 50 µm. We achieved the lowest nucleation temperature of -23,2 °C on a surface with shallow channels with 25µm line spacing. We measured the icing delay at surface temperatures between -20°C and -12,5 °C and confirmed our hypothesis that the icing delay becomes shorter with decreasing surface temperature. On the developed surfaces, we conducted electrochemical tests to evaluate the corrosion resistance of the surfaces. The surface with shallow channels and a 25µm line spacing showed the best corrosion resistance. Compared to the reference surface, we notably improved the anti-icing properties with the hydrophobized laser-textured surfaces
Development of superhydrophobic surfaces with laser-fabricated pillars for prevention of water droplet impact freezing
No full textV okviru zaključne naloge obravnavamo razvoj superhidrofobnih površin z lasersko izdelanimi stebrički za preprečevanje zmrzovanja padajočih vodnih kapljic. Zmrzovanje kapljic, ki padejo na podhlajene površine, je pogost problem na mnogih področjih, vključno z letalskim in ladijskim prometom, daljnovodi, uparjalniki toplotnih črpalk in vetrobranskimi stekli. Za preprečevanje zmrzovanja kapljic se pospešeno razvijajo pasivne metode zaščite v obliki superhidrofobnih površin, ki odbijajo vodne kapljice. Za izdelavo superhidrofobnih površin na aluminijastih vzorcih smo uporabili lasersko obdelavo z nanosekundnimi laserskimi bliski, površine pa smo hidrofobizirali z nanosom tankoslojne prevleke z nizko površinsko energijo. Pri različnim temperaturah površine in hitrostih kapljice smo ovrednotili obstoj odboja kapljice in spremljali njeno širjenje po površini. Ugotovili smo, da tako razmik med lasersko izdelanimi stebrički kot njihove dimenzije pomembno vplivajo na zmožnost odboja vodnih kapljic tudi pri neugodnih kombinacijah parametrov, to je pri nizkih temperaturah površine in visokih hitrostih kapljice.Within this thesis, we are focusing on the development of superhydrophobic surfaces with laser-fabricated pillars to prevent the freezing of water droplets impacting the surface. The freezing of droplets that impact subcooled surfaces is a common problem in many areas, including aviation and maritime transport, power lines, heat pump evaporators, and windshields. To prevent droplet freezing, passive protection methods in the form of superhydrophobic surfaces that repel water droplets are being rapidly developed. For the fabrication of superhydrophobic surfaces on aluminum samples, we used laser processing with nanosecond laser pulses, while the surfaces were rendered hydrophobic by applying a thin low-surface-energy coating. We evaluated the droplet repellency capability and monitored droplet spreading on the surface at various surface temperatures and droplet velocities. We found that both the spacing between the laser-fabricated pillars and the pillar dimensions significantly influence the ability to repel water droplets, especially under unfavorable combinations of parameters, such as low surface temperatures and high droplet velocities
Effect of time delay between impacts of droplets on the wetted surface area of a tablet
No full textV diplomski nalogi smo se osredotočili na analizo omočene površine pri procesu oblaganja tablete. Pripravili smo merilno progo, pri kateri smo spremljali, kako se spreminja omočenost tablete, ko na površje le-te pade najprej ena kapljica, nato pa z različnimi časovnimi zamiki še druga kapljica. Določili smo volumne kapljic za različne pretoke kapljevine, časovne zamike padca druge kapljice ter analizirali spreminjanje velikosti omočene površine ob spreminjanju teh dveh parametrov. Za večjo omočeno površino potrebujemo večji pretok, posledično velik volumen kapljice ali kratek časovni zamik padca druge kapljice. Največja omočena površina pri igli G30 je bila pri pretoku 550 µl/min, najmanjša pri pretoku 10 µl/min, pri igli G34 pa je bila največja omočena površina pri pretoku 450 µl/min, najmanjša pa pri pretoku 6 µl/min.In the diploma thesis, we focused on analyzing the wetted surface during the tablet coating process. A measuring setup was prepared comprising of a high speed camera, a strong light, a syringe pump and a computer, which was used to evaluate how the wetted areas of the tablet changes when water drops fall on its surface. Volumes for different liquid flow rates and the time delays of the drop of the second droplet were determined, and the change in the size of the wetted surface when changing these two parameters were analyzed. For a larger wetted surface a larger flow is needed, resulting in a large droplet volume or a short time delay for the second droplet to fall. The largest wetted surface for the G30 needle was at a flow rate of 550 μl/min, the smallest at a flow rate of 10 μl/min, the largest wetted area for the G34 needle was at a flow rate of 450 μl/min and the smallest at a flow rate of 6 μl/min
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