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Introduction: Magnetocaloric energy conversion represents an alternative to existing refrigeration, heat pump and energy harvesting technologies. A crucial part of a magnetocaloric device concerns the magnetic field source. It uses mainly rare-earth materials and consists of moving parts and a drive system while displaying a limited energy efficiency and unavailability of fast and variable control of the magnetic field. Recent advances in efficient heat transfer for high-frequency magnetic cooling call for new developments of magnetic field sources that can operate with high efficiency at high frequencies. Objectives: We report the concept of an electro-permanent magnetic (EPM) field source that efficiently recovers magnetic energy. In contrast to existing magnets, it allows very well-controlled operation without any moving parts. The main objective of this paper is to present a numerical and experimental study in which such an EPM was designed, built and tested. Methods: An extensive numerical investigation of the proposed design was carried out in terms of various geometrical and operating parameters. One of the design variations was built and experimentally evaluated for its energy efficiency and temperature increase at various operating frequencies. Results: We demonstrate an energy efficiency of these magnets of over 80% and operation with frequencies up to 50 Hz, which is crucial for future high-power-density and high-frequency magnetocaloric devices. Conclusions: Considering high energy efficiency at high operating frequencies, such EPMs would allow for miniaturization, making them a viable option for future compact magnetocaloric devices.

In the present paper, we investigate the heat transfer through the multilayer wall of aircraft cabins, as this process influences the comfortable conditions created for most passengers and crew members. The numerical modelling results and calculation of a multilayer wall with a hat-stringer in aircraft design are presented. The thermal characteristics evaluation and their relationship with the design parameters were made. The effect of the air layer size on the overall thermal resistance of the multilayer wall, taking into account the geometrical dimensions and properties of the surfaces, was studied. The relative temperature field in the insulation layer, which crosses the hat-stringer elements of the fuselage frame, is calculated. It is shown that the insufficient thickness of the layer of thermal insulation material in the zone of the hat-stringer at low temperatures leads to a significant deterioration in the multilayer wall characteristics, which can worsen the microclimatic conditions inside the aircraft cabin.

Heat transfer coefficient (HTC) was experimentally measured for saturated and subcooled pool boiling of binary mixtures of water and glycerin. Saturated boiling was studied for mixtures with water mass fractions ωw from 100% to 60% on horizontal flat nickel-plated surfaces at heat fluxes from 50 to 650kWm−2 at atmospheric pressure. Subcooled boiling was investigated in the range of subcooling from 0 to 30K at heat fluxes of approximately 250, 450 and 650kWm−2. It was found that mixture effects have a significant impact on saturated boiling HTC even for mixtures with very low content of glycerin as significant drops of HTC were observed for subtle changes in composition for mixtures of high ωw. Measured HTC was successfully correlated with the combination of Yagov (1999) and Inoue and Monde (2009) correlations with a mean relative error of 12%. A simple empirical HTC correlation is also proposed. For subcooled boiling, developed subcooled boiling regime was reached for all investigated heat fluxes. For this regime, correlations, which were able to predict HTC for saturated boiling, were employed to predict subcooled boiling HTCs for all investigated concentrations, heat fluxes and subcoolings. Effect of subcooling and effect of liquid composition on total HTC were of the same importance for mixtures with higher water content. With the increase in concentration of glycerin in the mixture, decrease in total HTC with increasing subcooling became more significant.

Nasl. z nasl. zaslona. Opis vira z dne 9. 1. 2023. Št. članka: 120916. Bibliografija: str. 7. Abstract.

Doktorsko delo obravnava prenos toplote pri magnetnem hlajenju z uporabo trdninskih toplotnih diod in tekočinskih toplotnih stikal. V prvem sklopu smo razvili 1D numerični model magnetokalorične naprave z vgrajenima dvema toplotnima diodama. Osredotočili smo se na lastnosti materialov, primernih za toplotne diode ter na toplotno rektifikacijo v tranzientnem režimu delovanja. Rektifikacijski koeficient smo določali tudi z uporabo nevronskih mrež. V drugem sklopu smo razvili 1D numerični model za simulacije statičnih toplotnih stikal. Z občutljivostno analizo smo določili najvplivnejše parametre, nadgraditev modela z genetskim algoritmom pa je omogočila optimizacijo dimenzij toplotnega stikala in magnetokaloričnega materiala glede na največjo temperaturno razliko med izvorom in ponorom toplote. Delovanje dveh konceptov toplotnih stikal smo analizirali eksperimentalno s principom elektroomočenja in ferohidrodinamike. Delovanje slednjega smo testirali tudi v magnetokalorični hladilni napravi. Rezultati numeričnih analiz potrjujejo sposobnost delovanja magnetokaloričnih naprav s toplotnimi stikali oz. toplotnimi diodami pri višjih delovnih frekvencah od trenutno uporabljenih aktivnih magnetnih regeneratorjev, česar pa v okviru dela tega nismo mogli eksperimentalno potrditi. Na področju toplotnih diod zaradi omejenega stanja tehnike in razpoložljivosti ustreznih materialov, pri toplotnih stikalih pa zaradi manjšega preklopnega razmerja ter daljšega odzivnega časa od pričakovanega. Kljub temu pa smernice hitrega razvoja mikrotehnologije, obdelave materialov in izdelave novih materialov kažejo na možnost eksperimentalne potrditve koncepta v prihodnjih letih. This doctoral thesis deals with the heat transfer in magnetic cooling using solid-state thermal diodes and fluidic thermal switches. In the first part we developed a 1D numerical model of a magnetocaloric device with two built-in thermal diodes. We focused on the properties of the materials suitable for thermal diodes and thermal rectification in a transient operating regime. The rectification ratio was also determined using neural networks. In the second part, we developed a 1D numerical model for the simulations of static thermal switches. We determined the most influential parameters with a sensitivity analysis, and the upgrade of the model with a genetic algorithm allowed the optimization of the dimensions of the thermal switch and the magnetocaloric material according to the maximum temperature span established between the heat source and the heat sink. The operation of two concepts of thermal switches was analysed experimentally: one was based on electrowetting and the other on ferrohydrodynamics. The operation of the latter was also tested in a magnetocaloric refrigeration device. The results of the numerical analyses confirmed that magnetocaloric devices with thermal switches or thermal diodes can operate at higher frequencies than the currently used active magnetic regenerators, but we were not able to confirm this experimentally within the scope of this work. The reason for this in the case of thermal diodes is the limited state of the art and the unavailability of suitable materials, and in the case of thermal switches, the switching ratio was smaller and the response time longer than expected. Nevertheless, the guidelines for the rapid development of micro-technology, new material processing and machining, indicate the possibility of an experimental confirmation of the concept in the coming years.

V zaključni nalogi se na primeru stavb z dvojno stekleno fasado posvečamo problematiki učinkovite rabe energije. Omenjeni tematiki v zadnjih letih posvečamo vedno več pozornosti, ker predstavlja pomemben vzrok za globalno segrevanje in posledično podnebne spremembe. Stavbeni sektor je odgovoren za porabo kar 40 % energije in je hkrati vir 36 % emisij CO_2, zaradi česar ima tudi največji potencial za izboljšave. Eden izmed možnih izboljšav je uporaba dvojnih steklenih fasad, s katero bi lahko energetsko učinkovitost stavb povečali. V zaključni nalogi se tako seznanimo z direktivami Evropske unije in Zakonom o učinkoviti rabi energije (ZURE) in nato predstavimo delovanje stavb z dvojno stekleno fasado. Energy efficiency presents an increasingly important topic, as it is closely tied to the ever more pressing issues of global warming and climate change. The building sector is just one of many areas striving to optimise its energy efficiency. The fact that this particular sector accounts for 40% of final energy use and generates 36% of annual global CO_2 emissions, makes it an area with vast opportunities for improvements regarding energy efficiency. One of many possibilities for the optimisation and improvement of energy efficiency in the aforementioned sector is the use of double skin facade. Consequently, the aim of this diploma thesis was to provide an overview on the energy efficiency of double skin facade. In the thesis we introduce the EU’s directives and the Act on Energy Efficiency (RS, št. 158/20) as well as present techinical aspects of double-skin façades.

Zaključna naloga zajema vpliv stropnega panelnega ogrevanja in hlajenja na toplotno ugodje v prostoru in analizo temperatur površin v prostoru, ki jo izvedemo s termokamero. Paneli so se razvili kot bolj varčna in za toplotno ugodje boljša alternativa klasičnim sistemom hlajenja in ogrevanja. Za zaključno nalogo v toplotni komori ustvarimo zimske oz. poletne pogoje in eksperimentalno ugotovimo toplotno ugodje pri uporabi panelov, s katerimi ogrevamo oz. hladimo prostor. Za določitev indeksov toplotnega ugodja PMV in PPD uporabimo dve metodi, ki ju nato primerjamo. This paper studies the effect of ceiling heating and cooling panels on indoor thermal comfort and presents an indoor surface temperature analysis conducted by a thermal camera. Panels have been developed as a more economic and better alternative, in terms of thermal comfort, to classic heating and cooling systems. For the purpose of this paper, we create winter and summer conditions in a thermal chamber to experimentally determine thermal comfort using heating and cooling panels. In order to determine PMV and PPD thermal comfort indexes, we use two methods, which are then compared to one another.

Diplomska naloga obravnava cevne prenosnike toplote, ki bi bili primerni za uporabo v tehnologiji magnetnega hlajenja. Poleg vira magnetnega polja in magnetokaloričnega materiala, sta v magnetnih hladilnikih pomembna tudi prenosnika toplote, ki skrbita za učinkovit odvod toplote iz hlajene okolice v magnetokalorični material ter nadaljnji odvod toplote v okolje. Zasnovali in izdelali smo dva različna cevna prenosnika toplote s prekati. Prvi je bil v celoti izdelan s postopkom 3D tiskanja s stereolitografijo, drugi pa je imel cevke iz bakra in ohišje izdelano s postopkom stereolitografije. Postavili smo merilno progo in eksperimentalno verificirali in medsebojno primerjali termo-hidravlične lastnosti izdelanih prenosnikov toplote. Na podlagi eksperimentalne analize smo učinkovitejši koncept prenosnika toplote še dodatno nadgradili, ga izdelali in eksperimentalno ovrednotili. Delo na koncu podaja ključne značilnosti za dosego čim višje učinkovitosti prenosnikov toplote za magnetno hlajenje in podaja smernice za nadaljnje delo. The thesis deals with shell and tube heat exchangers, which would be suitable for use in magnetic cooling technology. In addition to the magnetic field source and the magnetocaloric material, heat exchangers are also important elements of magnetic refrigerators. They are utilized for extraction of heat from the cooled environment to the magnetocaloric material and further heat dissipation to the ambient. We designed and developed two different shell and tube heat exchangers. The first was made entirely by a 3D printing process using stereolithography, while the second had copper tubes and a housing made by using a stereolithography process. We set up a measuring track and experimentally verified and compared the thermo-hydraulic properties of the investigated heat exchangers. On the basis of the experimental analysis, we further upgraded and experimentally evaluated the more efficient concept of the heat exchanger. At the end, the work gives the key features for achieving the highest possible effectiveness of heat exchangers for magnetic cooling and provides guidelines for further work.

Toplotni kontrolni elementi so naprave, s katerimi lahko reguliramo jakost in smer toplotnega toka. Kadar spreminjamo toplotno prevodnost z zunanjim vnosom energije, ki ni toplota, gre za toplotno stikalo, če pa se toplotna prevodnost spreminja s temperaturo, pa gre za toplotno diodo ali toplotni regulator. Med prve spadajo tudi Peltier naprave, pri katerih gre za neposredno pretvorbo električne napetosti v temperaturno razliko in obratno. Na podlagi Peltier učinka termoelektrična toplotna stikala omogočajo črpanje toplote v želeni smeri. V diplomski nalogi smo izvedli eksperiment, na katerem smo testirali tankoslojne Peltier elemente z možnostjo kasnejše integracije v magnetno hladilno napravo. Peltier elementi v tem primeru predstavljajo toplotno stikalo, s katerim omogočamo oziroma prekinjamo toplotni tok. To omogočimo na način, da Peltier elementi delujejo na principu pulzacij. Za izvedbo praktičnega eksperimenta smo zasnovali preklopno stikalo Peltier elementov, ki nam je omogočalo reguliranje poljubne frekvence in amplitude pulzirajočega signala. Preklopno stikalo smo nadgradili v krmiljenje, ki nam je omogočalo enostavno upravljanje Peltier elementov. Rezultati kažejo, da lahko toplotno stikalo transportira večje količine toplote v izjemno kratkih časovnih intervalih. Thermal control elements are devices with which we can regulate the intensity and direction of heat flow. When we change the thermal conductivity with an external input of energy other than heat, it is a thermal switch, but if the thermal conductivity changes with temperature, it is a thermal diode or heat regulator. Among the first are also Peltier devices, where it is a direct conversion of electric voltage into a temperature difference and vice versa. Based on the Peltier effect, thermoelectric thermal switches enable heat to be pumped in the desired direction. In the diploma thesis, we performed an experiment in which we tested thin-film Peltier elements in the way of later possible integration into a magnetic cooling device, as part of a structure with magnetocaloric material. In this case, the Peltier elements represent a heat switch with which we enable or interrupt the heat flow. This is made possible by the Peltier elements operating on the principle of the frequency of thermal pulsations. To perform a practical experiment, we designed a switching switch of Peltier elements, which allowed us to regulate any frequency and amplitude of the pulsating signal. We switched the toggle switch to a control that allowed us to easily operate the Peltier elements. The results show that a heat switch can transport large amounts of heat in extremely short time intervals.

The enhancement of boiling heat transfer has been extensively shown to be achievable through surface texturing or fluid property modification, yet few studies have investigated the possibility of coupling both enhancement approaches. The present work focuses on exploring the possibility of concomitant enhancement of pool boiling heat transfer by using TiO2-water nanofluid in combination with laser-textured copper surfaces. Two mass concentrations of 0.001 wt.% and 0.1 wt.% are used, along with two nanoparticle sizes of 4–8 nm and 490 nm. Nanofluids are prepared using sonification and degassed distilled water, while the boiling experiments are performed at atmospheric pressure. The results demonstrate that the heat transfer coefficient (HTC) using nanofluids is deteriorated compared to using pure water on the reference and laser-textured surface. However, the critical heat flux (CHF) is significantly improved at 0.1 wt.% nanoparticle concentration. The buildup of a highly wettable TiO2 layer on the surface is identified as the main reason for the observed performance. Multiple subsequent boiling experiments using nanofluids on the same surface exhibited a notable shift in boiling curves and their instability at higher concentrations, which is attributable to growth of the nanoparticle layer on the surface. Overall, the combination of nanofluids boiling on a laser-textured surface proved to enhance the CHF after prolonged exposure to highly concentrated nanofluid, while the HTC was universally and significantly decreased in all cases.

Multilayer Laue lenses (MLLs) made from WC and SiC were previously used to focus megahertz X-ray pulse trains of the European XFEL free-electron laser, but suffered damage with trains of 30 pulses or longer at an incident fluence of about 0.13 J/cm2 per pulse. Here, we present numerical simulations of the heating of MLLs of various designs, geometry and material properties, that are exposed to such pulse trains. We find that it should be possible to focus the full beam of about 10 J/cm2 fluence of XFEL using materials of a low atomic number. To achieve high diffraction efficiency, lenses made from such materials should be considerably thicker than those used in the experiments. In addition to the lower absorption, this leads to the deposition of energy over a larger volume of the multilayer structure and hence to a lower dose, a lower temperature increase, and an improved dissipation of heat.

V magistrski nalogi smo raziskali uporabo hidrofobnih in hidrofilnih lasersko strukturiranih površin za izboljšanje prenosa toplote pri mehurčkastem vrenju vode. Na površinah smo z uporabo vlakenskega bliskovnega laserja pripravili različne mikrostrukture, s hidrofobizacijo izbranih vzorcev smo zasledovali dodatno znižanje pregretja površine. Pokazali smo, da na vseh funkcionaliziranih površinah zabeležimo izboljšanje intenzivnosti prenosa toplote pri vrenju, kar se odrazi v dvigu kritične gostote toplotnega toka in povečanju koeficienta toplotne prestopnosti. Dodatno smo zabeležili zgodnejši prehod v režim mehurčkastega vrenja, kar je bilo izrazito predvsem na hidrofobiziranih površinah. Dosegli smo koeficiente toplotne prestopnosti do 287 kW/m2K, kar pomeni 9-kratno izboljšanje v primerjavi z neobdelano površino. Najvišja kritična gostota toplotnega toka je znašala 1660 kW/m2, kar predstavlja 83 % izboljšanje v primerjavi z neobdelano površino. Z razvitimi površinami bi lahko pomembno izboljšali intenzivnost odvoda toplote v aplikacijah, kjer nastopajo koncentrirane toplotne obremenitve zaradi generacije toplote na majhnih površinah. In this master's thesis, we have researched the use of hydrophobic and hydrophilic laser structured surfaces for enhanced boiling heat transfer. We fabricated different surface microstructures using a pulsed fiber laser combined with selective hydrophobization to pursue an additional decrease in wall superheat. All functionalized surfaces exhibited increased heat transfer intensity under pool boiling conditions through increased critical heat flux and higher heat transfer coefficients. Additionally, an earlier onset of nucleate boiling was observed, which was especially true for hydrophobic surfaces. We achieved heat transfer coefficients of up to 287 kW/m2K, which is a 9x improvement in comparison with an untreated reference surface. The highest measured critical heat flux was 1660 kW/m2, which represents an 83 % increase with respect to the treated surface. With developed surfaces, it is possible to substantially increase the intensity of heat transfer in applications, where heat loads are highly concentrated.

Zaradi visoke porabe električne energije električnih naprav in s tem povezanih stroškov se izdelovalci gospodinjskih aparatov, med drugim tudi sušilnih strojev, vedno bolj trudijo izboljšati njihovo učinkovitost s hkratnim zmanjšanjem porabe energije. Cilj diplomskega dela je bil izvesti in analizirati meritve sušenja perila na prototipu sušilnega stroja. Na podlagi rezultatov meritev smo želeli proces sušenja optimizirati. Iz desetih meritev smo pridobili najboljše rezultate pri meritvi z nastavljeno vrtilno frekvenco elektromotorja bobna 3000 obr/min ter vrtilno frekvenco elektromotorja ventilatorja 2700 obr/min (korigiran čas sušenja je bil v tem primeru 124,2 min in korigirana poraba električne energije 1,38 kWh) ter pri meritvi z nastavljeno vrtilno frekvenco elektromotorja bobna 3000 obr/min ter vrtilno frekvenco elektromotorja ventilatorja 2900 obr/min (korigiran čas sušenja je bil v tem primeru 122,6 min in korigirana poraba električne energije 1,36 kWh) in tako ugotovili, kateri so najboljši parametri delovanja elektromotorjev. Na podlagi teh dveh meritev smo nato poskusili opraviti optimizacijo sušilnega cikla. To smo izvedli tako, da smo vrtljaje elektromotorjev spreminjali glede na faze v sušilnem ciklu in tako poskusili izboljšati rezultate. Optimizacija se je izkazala za neuspešno, kar pripisujemo predvsem spremenjeni kinematiki perila v bobnu. Due to high electricity consumption and the associated costs of household appliances, including tumble dryers, manufacturers are striving to increasingly improve efficiency by reducing energy consumption. The aim of the thesis was to execute and analyse measurements made on a prototype tumble dryer. Based on managed measurements we tried to optimize the drying process. From the ten made measurements, we obtained the best results in the measurement where the revolutions of the electric motor of the drum were set on 3000 rpm and the revolutions of the electric motor of the fan set at 2700 rpm (in this case the corrected time of drying was 124,2 min and the corrected electric consumption was 1,38 kWh) and the measurement where the revolutions of the electric motor of the drum were set on 3000 rpm and the revolutions of the electric motor of the fan set at 2900 rpm (in this case the corrected time of drying was 122,6 min and the corrected electric consumption was 1,36 kWh) thus obtained the best parameters for the operation of electric motors. Based on these two measurements, we then tried to perform the drying cycle optimization. We did this by changing the revolutions of electric motors according to phases in the drying cycle and thus trying to obtain better results. The optimisation was unsuccessful mainly due to the different laundry kinematics in the drum.

We present the results of a study aimed at investigating the effects of electric fields on quasi-static bubble departure dynamics during pool boiling of perfluorohexane (FC-72) in microgravity conditions. Analysis was performed through an alternative formulation of the bubble momentum balance in which the contribution of non-uniform electric stress distributions at the bubble interface can be quantified through high-speed video measurements without having to numerically solve the laws of electrostatics. Data used in this study were obtained in the scope of the Multiscale Boiling Project, which included advanced single bubble growth experiments performed aboard the International Space Station. Our results confirm that bubble departure counterintuitively begins before the force resulting from electric stresses starts to pull the bubble up from the wall. When this occurs, it is shown that the shrinking process of the contact line accelerates, in agreement with known theoretical results. It is concluded that the electric force is essentially determined by the electric stress distribution at the bubble cap above the contact area. Furthermore, we show that the electric stress at the bubble interface is also responsible for the increase in bubble internal overpressure, which explains the early departure of the bubble while increasing the intensity of the electric field. The results of this study provide an important step in achieving a more comprehensive understanding of the bubble behavior at the heated surface in the presence of an electric field, which is essential to optimally design electrodes and two-phase heat transfer devices for future space applications.

V tlačnem litju aluminija se pojavlja problem hlajenja gravur kompleksnih izdelkov, saj je število injektorskih hladilnih naprav (»jet cooler« – sistem kombiniranega hlajenja voda-zrak) v obravnavanem orodju omejeno. Trenutno se v praksi, v primeru prekoračitve števila dovoljenih JC, uporablja vzporedna vezava, kar zmanjša vodni tok in povzroči padec hladilne učinkovitosti. Zato smo razvili alternativno metodo hlajenja reber, in sicer z vstavljanjem bakrene sredice, ki bo pomagala pri hitrejšem odvodu toplote. Konstruirali smo tri variante reber ter na njih opravili analitični preračun za potrditev predvidevanj. Nato smo na treh modelih opravili še numerične analize z metodo končnih elementov za vrednotenje učinkovitosti odvajanja toplote v rebrih. Rebro z bakreno sredico je manj učinkovito od rebra s hladilnimi kanali in rebro z debelejšo sredico je učinkovitejše od rebra s tanjšo sredico. In the process of high pressure die casting a problem occurs when cooling the dies of a highly complicated product, as the number of “jet cooling” (combined water-air cooling system) cores is limited to 16 per tool. That is why we designed an alternative to cooling thin ribs with inserting copper cores in the centre, which will help in faster heat dissipation. We constructed three different variations of ribs and completed an analytical calculation to confirm our predictions. Then we performed numerical analysis with finite element method on all three models, to evaluate the effectiveness of heat transfer within the ribs. The ribs with copper cores are less effective than the one with cooling channels. The thicker copper core is more effective than the thinner core.

Magistrsko delo obravnava konceptualni sistem digitalne mikrofluidike, ki ob dovajanju električne napetosti omogoča manipulacijo diskretnih kapljic. Električni potencial doveden elektrodam, na katerih leži kapljica, povzroči spremembo kota omočljivosti, kar rezultira v premiku kapljice. Razvoj digitalne mikrofluidike se v panogi strojništva osredotoča predvsem na področje upravljanja s toploto kot odziv na hiter napredek v elektronski industriji. Običajne metode upravljanja s toploto namreč ne zadovoljujejo več toplotnih zahtev naprednih sistemov. Digitalna mikrofluidika na osnovi manipuliranja diskretnih kapljic velja za obetavno rešitev odvajanja toplotnih tokov visoke gostote in neenakomerne temperaturne porazdelitve v zaprtih prostorih elektronskih naprav. V začetnih poglavjih magistrskega dela so opisane teoretične osnove površinske napetosti in elektro-omočljivosti. Predstavljeni so tipično uporabljeni materiali za izdelavo ter najpogostejši koncepti naprave, ki za svoje delovanje izkoriščajo pojav elektro-omočljivosti. V nadaljevanju je podrobneje opisana zasnova in izdelava naprave odprtega koncepta, ki deluje po načelu elektro-omočljivosti na dielektriku. Postopek izdelave naprave obsega zasnovo geometrije elektrod in izdelavo le teh na površini. Pri izdelavi elektrod je bil uporabljen postopek fotolitografije. Na površino elektrod je bil nanešen sloj električnega izolatorja in hidrofobni sloj. Na koncu magistrskega dela so predstavljeni eksperimentalno pridobljeni rezultati, primerjani z rezultati simulacije. Na podlagi analize meritev je bila izvedena validacija numeričnega modela simulacije. Zasnovana je bila naprava, ki pri uporabi vode dovoljuje začetni kot omočljivosti 115° in dosega minimalni kot omočljivosti 80°. Mejna napetost naprave znaša 175 V. Rezultati meritev dokazujejo doseganje hitrosti nad 80 mm/s, kar potrjuje tudi numerični model simulacije. The master's thesis addresses conceptual system of digital microfluidics that allows the discrete droplet manipulation by applying an electrical voltage. Applied electrical potential to the electrodes, on which the droplet is positioned, causes a change in contact angle, resulting in a droplet movement. The development of digital microfluidics in the mechanical engineering industry focuses primarily on the field of thermal management as a response to rapid advances in the electronics industry. Conventional thermal management methods no longer fulfill thermal requirements of advanced systems. Digital microfluidics based on the manipulation of discrete droplets is considered a promising solution for the dissipation of high-density heat fluxes and non-uniform temperature distribution inside closed electronic devices. In the beginning, chapters of the master's thesis theoretical fundamentals of surface tension and electrowettability are described. Typically used materials and the most common concepts of devices exploiting the phenomenon of electrowetting on dielectric (EWOD) are presented. The design and fabrication process of an open EWOD device using the principle of electrowetting on dielectric is described in detail. The process of fabricating a device includes the geometry design of the electrodes and the fabrication itself on the surface. In the fabrication of the electrodes, the technique of photolithography was used. An electrical insulation layer and a hydrophobic layer were applied on the surface of the electrodes. At the end of the master thesis, the experimentally obtained results are presented and compared with the simulation results. Based on the measurement analysis, a validation of the numerical model was performed. A device was designed to work with water, allowing an initial contact angle of 115° and reaching a contact angle of 80°. The device voltage limit is 175 V. Measurement results prove reaching velocity above 80 mm/s, which is also confirmed by the simulations of the numerical model.

Vse pogostejša uporaba električnih naprav na različnih področjih pomeni vse večjo uporabo električno prevodnih žic, ki so prevlečene z izolacijsko plastjo. Slednja mora pogosto biti pred spajanjem odstranjena. V industriji se za različne obdelovalne postopke vse pogosteje uveljavljajo laserski sistemi. Z njihovo pomočjo lahko tudi odstranimo izolativni sloj iz površine obdelovanca. V zaključnem delu smo zasnovali fizikalni model laserskega odstranjevanja izolativnega sloja, temelječega na prenosu toplote, preko polneskončnega telesa in ga na podlagi izvedenih eksperimentov, kjer smo osvetljevali čelno površino bakrene žice, postopoma prilagajali. Z opravljenimi eksperimenti smo ugotovili obratno sorazmernost parametrov laserske moči in trajanja bliska v kombinaciji z dolžino odstranjenega izolativnega premaza žice ter pridobili nadzor nad samim procesom. Natančnost fizikalnega modela smo validirali z izračunom standardne deviacije, ki je pri določanju trajanja laserskega bliska za pričetek pretaljevanja izolativnega sloja, znašala 14 ms, raztros koeficienta absorptivnosti je znašal 7,7 %, natančnost izračuna dolžine odstranjenega laka pa 0,40 mm. The increasing use of electrical devices in various areas results in higher demand of coil wires, which are coated with an insulating layer. Often insulation must be removed before welding. Laser systems are increasingly being used for different industrial purposes. In this work we designed a physical model of copper wire insulation removal process via semi-infinite body and gradually adjusted it based on the results of the experiments, where we illuminated the front surface of copper wire. With the experiments carried out, we found the inverse proportionality of the parameters of laser power and the duration of laser pulse in combination with the removed length of the wire insulation coating while simultaneously gaining control of the process itself. The accuracy of the physical model was validated by calculating multiple standard deviations, which were evaluated at 14 ms when determining the duration of the laser pulse for initiation of insulation layer melting, standard deviation for calculated coefficient of absorbtivity was 7.7 % and the accuracy of insulation removal length was 0,40 mm.

V zaključni nalogi smo obravnavali generator pare, kot sestavni del preprostega parnega postroja. Predstavljen je enostaven generator pare z gelnim gorivom, ki se dan danes vedno bolj uveljavlja. Za nas je bil zanimiv predvsem iz vidika ohranjanja kvalitete zraka. Določili smo zgornjo in spodnjo kurilnost dveh gelnih goriv, pri čemer smo izhajali iz meritev opravljenih na kalorimetru. Določili smo oddano toplotno energijo ter količino uparjene vode pri vretju z gelnima gorivoma. Tako smo lahko definirali oddano toploto ter toploto, ki se je dejansko uporabila za segrevanje vode. Glavni cilj je bil zagotoviti ustrezne vhodne podatke za določitev količine in hitrosti pare, ki priteka na turbino. In final thesis, we examined steam generator as a part of a simple steam engine. Simple steam generator with gel fuel was presented, which is becoming more and more popular. For us it was interesting mainly from the point of view of maintaining air quality. We determined the upper and lower heating values of two gel fuels, which was determinded from measurements performed on the calorimeter. Thermal energy of the already mentioned fuels was determinded and the amount of evaporated water during the boiling. Thus, we were able to define heat emitted from the fuels and heat that was accually used to boil the water. The main aim was to provide relevant data to determine the amount and velocity of steam flowing to the turbine.

This protocol describes the use of a simple 1D numerical model to evaluate a single-stage (magneto)caloric refrigerating device with static thermal switches. The model can be used to find appropriate values of parameters that lead to a significant refrigerating effect and COP of the device. The modeled device can comprise any type of static thermal switch in combination with any kind of magnetocaloric or electrocaloric material. Simulation parameters need to be set with care for acceptable computational time. For complete details on the use and execution of this protocol, please refer to Klinar et al. (2022).

Increasing power densities of electric machines in e-vehicles in addition to the resulting quest for enhanced cooling concepts are bringing forward the importance of defining adequate heat transfer correlations in air gaps. This is a highly challenging topic, as there exist no generally applicable flow and heat transfer phenomena descriptions for air gaps due to their highly variable geometrical properties and operating conditions. As an answer to this challenge, this paper presents a workflow that defines an adequate 3D CFD model for an arbitrary air-gap design that includes its system-dependent boundary conditions. The workflow is built on the recognition of underlying air-gap flow phenomena, which are used to steer the subsequent design of the 3D CFD model in a systematic step-by-step manner. Consequently, the complexity of the 3D CFD model gradually increases to the point where it provides an adequate flow and heat transfer description. Validation of the workflow is presented for a wide range of air-gap designs and flow conditions. It is demonstrated that the 3D CFD models obtained with the workflow match the experimentally obtained data from various flow cases that have been documented in the literature. Considerable optimization of computational costs, offering potentially an order-of-magnitude reduction in computational time, is achieved as a result of computational domain span optimization and transient simulations being applied only when required. The validation confirms that this workflow facilitates construction of valid 3D CFD models without the prior knowledge of flow and heat transfer phenomena in a specific air gap. This workflow thus provides a reliable and computationally efficient tool for valorization of convective heat transfer, and opens up prospects for time- and cost-efficient optimizations of electric machines’ cooling system designs.

Z miniaturizacijo elektronskih vezij ter povečevanjem njihove zmogljivsti se povečuje potreba po odvodu odpadne toplote. V prihodnosti trenutne tehnologije zračnega ter vodnega hlajenja tem potreban ne bodo več zmožne slediti. Poiskati moramo nove rešitve, ki bodo za hlajenje izkoriščale latentno toploto. V okviru te naloge je bila opravljena eksperimentalna in teoretična analiza dveh miniaturnih hladilnih sistemov. Prvega na osnovi parno-kompresijskega in drugega na osnovi črpalno-uparjalnega hlajenja. Eksperimentalni rezultati so pokazali, da je slednji v primerjavi z vodnim hlajenjem sposoben odvajati 3-krat večjo količino toplote na osnovi enakega masnega toka. With the miniaturization of electronic circuits and increase in their performance, the need for waste heat dissipation is increasing. In the future, the current air and water cooling technologies will no longer be able to keep up. We need to find new solutions that will use latent heat. As part of this task, an experimental and theoretical analysis of two miniature cooling systems was performed. The first on the basis of steam-compression and the second on the basis of pumped-evaporative cooling. Experimental results have shown that the latter is able to dissipate 3 times the amount of heat based on the same mass flow compared to water cooling.

Over the past decade, solid‐state thermal control devices have emerged as potential candidates for enhanced thermal management and storage. They distinguish themselves from traditional passive thermal management devices in that their thermal properties have sharp, nonlinear dependencies on direction and operating temperature, and can lead to more efficient circuits and energy conversion systems than what is possible today. They also distinguish themselves from traditional active thermal management devices (e.g., fans) in that they have no moving parts and are compact and reliable. In this article, the recent progress in the four broad categories of solid‐state thermal control devices that are under active research is reviewed: diodes, switches, regulators, and transistors. For each class of device, the operation principle, material choices, as well as metrics to compare and contrast performance are discussed. New architectures that are explored theoretically, but not experimentally demonstrated, are also discussed.

Caloric energy conversion is an emerging field of cooling, heat-pumping and power-generation technologies. The potentially high energy efficiency and use of environmentally friendly and safe solid-state working substances in the form of refrigerants has stimulated increased research activity in the past two decades. Most of today's caloric devices apply so-called active regeneration, which involves the oscillation of the working fluid through the matrix of the caloric material – the caloric regenerator. However, the unavoidable, irreversible viscous and heat-transfer losses apply limits to the caloric device's performance as well as its size. The quest for better caloric-device performance has led to the development of thermal control elements, which control the heat flux on different size and time scales. In this paper we describe the working principles of these elements: thermal switches, thermal diodes and thermal regulators. This is followed by the first up-to-date critical review of the research activities and applications of thermal control elements in all types of caloric devices. We show that thermal control elements have the potential to improve the power density of caloric devices. Finally, we propose target features for these elements with respect to future research activities in the field of caloric technologies.

Pojem industrijska energetika obsega razmeroma široko področje tehnike, s katero se inženir pogosto srečuje v različnih proizvodnih procesih, zato je prav, da je seznanjen vsaj z osnovami teh področij. V kratkem času, ki ga študijski program Lesarsko inženirstvo na Biotehniški fakulteti Univerze v Ljubljani odmerja tej temi, jo je nemogoče poglobljeno predstaviti. Po drugi strani pa je obilica domače in tuje literature, ki je na voljo, preveč obsežna za potrebe študija. Zbirka rešenih nalog s kratkimi teoretičnimi osnovami je zato zasnovana kot pregled osnov različnih področij, ki jih obravanava predmet Industrijska energetika (študijski program Lesarsko inženirstvo na Biotehniški fakulteti Univerze v Ljubljani), in praktičnih primerov, ki prikažejo uporabo teorije v inženirski praksi. Naj bo študentom v pomoč na poti k novemu znanju.

V magistrskem delu predstavimo dinamično modalno dekompozicijo tokovnih struktur, ki nastanejo pri naravni konvekciji nenewtonskih tekočin v zaprti kotanji. Fizikalni model vsebuje ohranitev mase, prenos toplote in gibalne količine, Ostwald-de Waele potenčni model za opis nenewtonske viskoznosti in Boussinesqov približek temperaturne odvisnosti gostote. Fizikalni model rešimo numerično s posplošeno metodo končnih diferenc, eksplicitnim Eulerjevim korakanjem in Chorinovo projekcijsko metodo. Implementiran rešitveni postopek preverimo preko primerjave z objavljenimi rezultati za več različnih primerov. Pokažemo dobro ujemanje z referenčnimi rešitvami in konvergentno obnašanje metode. V nadaljevanju vpeljemo dinamično modalno dekompozicijo, metodo za določanje glavnih elementov dinamike sistema, in jo demonstriramo na sintetičnem primeru. V glavnem delu metodo uporabimo za analizo načinov dinamike naravne konvekcije nenewtonske tekočine v zaprti kotanji. Analiziramo več različnih nenewtonskih tekočin v dveh različnih geometrijah in pokažemo spremembo glavnih elementov dinamike pri prehodu med stacionarnimi in nihajočimi rešitvami. Identificiramo elemente, ki pri newtonskih tekočinah niso prisotni in pokažemo, da je dinamika za psevdoplastične nenewtonske tekočine bogatejša. In this work, we present the dynamic mode decomposition of natural convection in a non-Newtonian fluid within a closed cavity. The physical model considers mass and momentum transport, Ostwald-de Waele power-law for a description of non-Newtonian viscosity, and Boussinesq approximation for description of density-temperature dependency. The model is numerically solved with generalised finite differences, explicit Euler stepping and Chorin's projection method. The implemented solution procedure is tested by comparison of computed results with published data with good agreement achieved. We introduce dynamic mode decomposition and demonstrate its performance on a synthetic case. The main part of this work is focused on a mode analysis of natural convection of non-Newtonian fluid in a closed cavity. We analyse different non-Newtonian fluids in two different geometries where we are in particular interested in the transition from stationary to oscillatory solutions. In addition, we identify elements of dynamics that are not present in Newtonian fluids and show that the pseudoplastic non-Newtonian fluids exhibit richer dynamics in comparison with Newtonian fluids.

Jekla iz skupine PROTAC odlikujejo dobre mehanske lastnosti in odlične protibalistične lastnosti tudi že pri majhnih debelinah. Mehanske lastnosti jekla PROTAC 600 so poznane, medtem ko podatki o toplotnih lastnostih (toplotni prevodnosti, specifični toploti in temperaturni prevodnosti) niso na voljo, zato smo v okviru diplomskega dela določili toplotne lastnosti tega jekla. V okviru diplomskega dela je bila izvedena študija in ocena delovanja naprave za določevanje toplotnih lastnosti Hot Disk TPS 2200, danes enega izmed najsodobnejših in najkakovostnejših instrumentov za določevanje toplotnih lastnosti. V okviru dela smo izvedli meritve toplotnih lastnosti v skladu s standardom ISO 22007-2 pri sobni temperaturi. Vrednosti toplotnih lastnosti jekla PROTAC 600 pri sobni temperaturi (pribl. 25°C) znašajo: toplotna prevodnost 28,69 W/mK, specifična toplota 3,94 MJ/m3K in temperaturna prevodnost 7,29 mm2/s. Ugotovili smo, da je jeklo PROTAC 600 za približno 12% boljši toplotni prevodnik v primerjavi z jeklom prejšnje generacije PROTAC 500. Steels from the PROTAC group are distinguished by good mechanical properties and excellent anti-ballistic properties even at small thicknesses. The mechanical properties of PROTAC 600 steel are known, while data on thermal properties (thermal conductivity, specific heat and temperature conductivity) are not available, so we determined the thermal properties of steel in the diploma thesis. As part of the diploma work, a study and evaluation of the operation of the device for determining the thermal properties of Hot Disk TPS 2200, today one of the more modern and high-quality instruments for determining thermal properties. As part of the work, we performed measurements of thermal properties in accordance with the ISO 22007-2 standard at room temperature. The values of thermal properties of PROTAC 600 steel at room temperature (approx. 25 ° C) are: thermal conductivity 28.69 W/mK, specific heat 3.94 MJ/m3K and temperature conductivity 7.29 mm2/s. We found that PROTAC 600 steel is about 12% better heat conductor compared to the previous generation PROTAC 500 steel.

Toplotne črpalke predstavljajo v primerjavi s kurilnimi napravami za proizvodnjo toplote energijsko učinkovitejšo in okolju prijaznejšo tehnologijo. Za prenos toplote med virom toplote in hladivom v uparjalniku ter hladivom v kondenzatorju in ponorom toplote je potrebna temperaturna razlika, ki je eden pomembnejših virov nepovračljivosti v sistemu toplotne črpalke. Zmanjšanje temperaturne razlike vodi v izboljšanje učinkovitosti delovanja sistema toplotne črpalke. Za namene vrednotenja energetske učinkovitosti toplotne črpalke pri različnih temperaturnih razlikah med virom toplote in temperaturo uparjanja je bil izdelan transparentni uparjalnik, ki omogoča modifikacijo uparjalne površine ter izkazuje možnost vizualizacije toka hladiva. V diplomskem delu so predstavljene osnove delovanja toplotne črpalke ter opis posameznih komponent. Velik poudarek smo posvetili preverjanju kompatibilnosti materialov, uporabljenih pri izdelavi uparjalnika ter preračunih za zagotovitev tesnjenja in s tem varne rabe uparjalnika. Na podlagi znanih robnih pogojev delovanja eksperimentalne toplotne črpalke so bili izvedeni tudi preračuni teoretičnega izboljšanja energetske učinkovitosti obstoječe toplotne črpalke. Preračuni so osnova za potrditev osnovnega cilja, predstavljenega v diplomskem delu, pri čemer z ustrezno modifikacijo površine uparjalnika zasledujemo vpliv na energetsko učinkovitost pri delovanju toplotne črpalke. Heat pumps represent a more energy-efficient and environmentally friendly technology compared to combustion plants used for heat generation. Heat transfer between the heat source and the refrigerant in the evaporator and the refrigerant and the heat sink in the condenser requires a temperature difference, which is one of the main sources of irreversibility of the heat pump system. The reduction of the temperature difference has a great influence on the improvement of the efficiency of the heat pump system. In order to evaluate the energy efficiency of the heat pump at different temperature differences between the heat source and the evaporation temperature, a transparent evaporator has been designed, which allows the modification of the evaporating surface and the possibility of visualizing the refrigerant flow. In the diploma thesis the basics of heat pump operation and a description of the individual components are presented. Great importance was attached to checking the compatibility of the materials used in the production of the evaporator and to the calculations, necessary to ensure the tightness and thus the safe operation of the evaporator. Based on the known boundary conditions of the experimental heat pump, we carried out calculations to theoretically improve the energy efficiency of the existing heat pump. The calculations serve as a basis for confirming the basic objective presented in the thesis, where we are observing the effects of the modified surface of the evaporator on the energy efficiency of a heat pump.

Na Odseku za reaktorsko tehniko Inštituta Jožef Stefan se postavlja nov eksperiment v Laboratoriju za termohidravliko večfaznih tokov (THELMA). Namen eksperimenta je študija temperaturnih fluktuacij na greti kovinski foliji, ki je hlajena s turbulentnim tokom. Načrtovanje eksperimenta temelji na numeričnih simulacijah, v katerih smo analizirali vpliv različnih aproksimacij na temperaturne fluktuacije na greti foliji. To delo opisuje rezultate simulacij. Tok in prenos toplote v novem eksperimentu smo napovedali s simulacijami z metodo velikih vrtincev (ang. Large Eddy Simulation) s pomočjo računalniškega programa OpenFOAM. Prevajanja toplote v foliji nismo modelirali. Folijo smo opisali z idealiziranim robnim pogojem konstantnega toplotnega toka. Simulacije so predpostavljale nestisljiv, polno razvit turbulentni tok Newtonske tekočine z Reynoldsovim številom 10000. V prvih simulacijah je bila dodana temperatura kot pasivni skalar, torej brez povratne zanke na enačbo za ohranitev gibalne količine. Drugi del simulacij pa je upošteval spreminjanje viskoznosti vode s temperaturo ter vzgonske sile. Določili smo ustrezno toplotno moč folije v eksperimentu ter moč, pri kateri je mogoče zanemariti vpliv temperature na materialne lastnosti vode. Posebej nas je zanimala porazdelitev temperature na vroči foliji v turbulentnem režimu. Z analizo koherentnih termičnih struktur v simulacijah smo uspeli določiti potrebno resolucijo in frekvenco visokohitrostne termične kamere pri danemu Reynoldsovemu in Prandtlovemu številu. V zaključku je predstavljena kratka primerjava naših simulacij s preliminarnimi meritvami temperaturnih fluktuacij na greti foliji. At Reactor Engineering Division of Jožef Stefan Institute a new experiment is being designed in the THELMA laboratory to study temperature fluctuations at the heated foil cooled by turbulent flow. Detailed design of the experiment is based on Computational Fluid Dynamics (CFD) studies which are described in the present work. The flow and heat transfer in the new experiment predicted with a wall-resolved Large Eddy Simulation (LES) using a wall-adapting local eddy-viscosity (WALE) model in OpenFOAM computer code. Foil was modeled with approximation of the constant heat flux. Simulations assumed incompressible, fully developed turbulent flow of Newtonian fluids with Reynolds number of 10000. In the first part of the study, heat transfer has been included to the study using temperature as a passive scalar, so there is no feedback loop on mass and momentum conservation equation. The second part of the simulations took into account the influence of the temperature on the water viscosity and density. We have identified the power of the foil in the experiment and the maximum foil temperatures at which the water temperature can be described with the passive scalar approximation. The most relevant result of our simulations were temperature fluctuations on the foil. With observation and analysis of coherent thermal structures in simulations we have predicted the required resolution and frequency needed by thermographic high-speed camera at given Reynolds and Prandtl numbers. In the Conclusions we present the comparison of our simulations with the preliminary measurements.

Svet se zadnja desetletja vedno bolj zaveda problematike porabe energije in z njo povezanih izpustov toplogrednih plinov, zato ljudje razvitejših držav vedno bolj stremimo k bolj čistim tehnologijam in manjši količini porabljene energije. Eden izmed načinov, kako varčevati z energijo, je tudi rekuperacija odpadne toplote s pomočjo rekuperatorja in posrednega hlapilnega hlajenja. To je tudi osrednja tema diplomske naloge, v kateri je predstavljena eksperimentalna metoda merjenja temperaturne učinkovitosti rekuperatorja. Pri tem so bile opravljene potrebne meritve zračnih tokov in temperaturnih ter vlažnostnih pogojev svežega in odpadnega zraka. Meritve so bile opravljene za IMP Klimat, postavljen v laboratoriju LOSK. Končni rezultati temperaturne učinkovitosti prikazujejo, da učinkovitost merjenega rekuperatorja brez hlapilnega hlajenja znaša do 58 %, medtem ko s hlapilnim hlajenjem temperaturna učinkovitost znaša do 55 %, odvisno od robnih pogojev. S primerjavo med hlapilnim hlajenjem rekuperatorja in brez njega pa je bilo ugotovljeno, da s hlapilnim hlajenjem dosežemo večje temperaturne razlike, sama temperaturna učinkovitost pa je zaradi različne metodologije računanja, ki ne upošteva spremembe vlažnosti, nižja. The understanding of problematics about energy consumption in all aspects of our life is rising. Therefore, the tehnology related to energy efficiency is getting improved each day. One of the most effective way to save energy is to upgrade buildings and houses with instalation of recuperators and to use indirect adiabatic cooling. That is also the main topic of this thesis, where experimented methods of calculating thermal efficiency of recuperator with and without adiabatic cooling will be presented. Therefore, the necessary measurements of the air currents, the temperature and humidity conditions of outlet and inlet air will be performed. Measurements were made for IMP Climate installed in the LOSK laboratory. The final thermal efficiency results show that the efficiency of the measured recuperator is up to 58 % without, and up to 55 % with the use of adiabatic cooling, depending on boundary conditions. However, by comparing the recuperator using evaporative cooling with the one that does not, it was found that with evaporative cooling we can achieve greater temperature differences, but the thermal efficiency itself is lower due to the different calculation methodology.

V nalogi je predstavljeno delovanje in mehanizmov hlajenja zaprtega hladilnega sistema električnega izvenkrmnega pogona za manjše čolne. Analizo sistema smo razčlenili na problematiko hidravličnega sistema in načine prenosa toplote v okolje. S pomočjo podatkovnih listov komponent sistema smo potrdili ustreznost konstrukcije in izpostavili potencialne težave. Na osnovi meritev in preračunov smo popisali tlačne razmere v sistemu. Lastnosti hidravličnega sistema so nam pomagale pri izbiri primerne črpalke, ki smo jo kot glavno komponento sistema testirali na trajnostnem testu. Kot drugo pomembno komponento sistema smo podrobneje analizirali še hladilni medij in alternative zanj. Izvedli smo pregled glavnih virov in ponorov toplote v sistemu in računsko določili razmerja med posameznimi izhodnimi toplotnimi tokovi. Na podlagi analiz sistema smo v nadaljevanju predlagali optimizacije kritičnih komponent z vidika hidravlike in prenosa toplote. Predstavili smo še načine testiranja pogonov z vidika hladilnega sistema na terenu in na preizkuševališču, namenjenem trajnostnim testom pogonov. Različni metodi testiranja smo primerjali in analizirali pridobljene rezultate. Rezultati analize tlačnega stanja v sistemu so kot ključne izpostavili peto in nogo pogona, ki se je za najpomembnejšo izkazala tudi pri analizi toplotnih tokov. Iz tega razloga se večina optimizacijskih predlogov nanaša na spremembo njene konstrukcije. The thesis is an introduction to the operation and understanding of the cooling mechanisms of a closed cooling system of an electric outboard drive for smaller boats. The analysis of the system was divided into the problems of the hydraulic system and the way the heat is transferred to the environment. Using the data sheets of the system components, we confirmed the adequacy of the design and pointed out possible problems. On the basis of measurements and calculations, we described the pressure conditions in the system. The characteristics of the hydraulic system helped us to select a suitable pump, which we tested in a durability test as the main component of the system. As another important component of the system, we analysed the refrigerant and alternatives to it in more detail. We made an overview of the main heat sources and sinks in the system and calculated the ratios between the individual heat outputs. Based on the system analyses, we then proposed optimizations of critical components from the hydraulic and heat transfer point of view. We also presented the methods for testing drives from the viewpoint of the cooling system in the field and in the test facility designed for sustainable testings of drives. We compared and analysed the results obtained with different test methods.

Proces vrenja je zaradi svoje učinkovitosti eden najbolj priljubljenih načinov prenosa toplote. Zaradi kompleksnosti procesa in velikega števila vplivnih parametrov so eksperimenti vrenja, ki so izvedeni v mikrogravitaciji, zelo ugodni, saj skoraj popolnoma izničimo vpliv težnosti, ter tako lažje preučujemo vpliv ostalih parametrov. V tem delu so predstavljeni eksperimenti vrenja, ki so bili izvedeni na Mednarodni vesoljski postaji pod pokroviteljstvom Evropske vesoljske agencije. Opisana je izvedena pretvorba surovih podatkov in strojna obdelava slik, ki je temu sledila. Popisano je nukleacijsko vrenje v treh različnih eksperimentalnih konfiguracijah v mikrogravitaciji in sicer mehurčkasto vrenje pri naravni konvekciji brez prisilnega toka tekočine, s prisilnim tokom tekočine ter mehurčkasto vrenje ob prisotnosti električnega polja. Predstavljen je vpliv posameznih procesnih parametrov na dinamiko rasti mehurčka, kot tudi vpliv obdelave slikovnih podatkov. Dobljeni rezultati so razloženi in primerjani z nekaterimi predhodnimi ugotovitvami ostalih raziskovalnih skupin. Obravnavana je ponovljivost meritev. Boiling is one of the most widely used methods of heat transfer, due to its high effectiveness. Because of its complexity and a high number of influential parameters the experiments conducted under microgravity conditions are favorable, as the influence of gravity can be almost completely negated. As a result, the influence of other parameters can be studied. This paper describes the boiling experiments that were conducted on the International Space Station under the patronage of the European Space Agency. Conversion of raw data and image processing is presented. Three different boiling configurations in microgravity are described and analyzed, namely nucleate boiling with natural convection without shear flow, nucleate boiling in the presence of shear flow and nucleate boiling in the presence of an electric field. The influence of experimental parameters on bubble growth dynamics and the effects of image processing is discussed. Results are explained and compared to already existing data on boiling in microgravity. Measurement repeatability is also given.

Diverter fuzijskega reaktorja je izpostavljen stalnemu toku visoko energijskih delcev z gostoto toplotnega toka okoli 10 MW/m2. V fazi razvoja materialov in konceptov aktivnega hlajenja tarč diverterja je potrebno, običajno na pomanjšanih modelih, najprej izvesti numerične simulacije in eksperimente v okolju podobnem razmeram v fuzijskem reaktorju. V sklopu te naloge smo izvedli predhodne računalniške simulacije modela vodno-hlajene tarče diverterja pri pogojih, ki ustrezajo eksperimentalni napravi v raziskovalnem centru Juliech v Nemčiji, kjer bo kasneje opravljen eksperiment. V nalogi je predstavljena zasnova monobloka tarče, izdelava geometrije, računske mreže in računalniškega modela. Rezultati opravljenih računalniških simulacij so pokazali, da maksimalna temperatura monobloka tarče preseže 1300 °C, ob upoštevanju hlajenja s sevanjem okoliških površin, pa je lahko tudi za 50 °C nižja. Rezultate padca tlaka v hladilni cevi smo preverili z empiričnimi korelacijami. The divertor in fusion reactor is exposed to a constant flow of high-energy particles with a heat flux density of about 10 MW/m2. In the development phase of materials and concepts of active cooling of diverter targets, numerical simulations and experiments in an environment similar to the conditions in a fusion reactor must be performed first, usually on scale-down models. In the frame of this work, preliminary numerical simulations of a water-cooled diverter target model were performed, under the conditions corresponding to an experimental facility at the Juliech Research Center (FZJ) in Germany, where an experiment will be also performed later. In this work the design of the target monoblock, the geometry layout, generation of computional mesh and setup of numerical model are presented. The results of the performed computional simulations showed that the maximum temperature of the target monoblock exceeds 1300 °C, and by taking into account the thermal radiation cooling of the surrounding surfaces, it can also be 50 °C lower. The results of the pressure drop in the cooling pipe were verified by empirical correlations.

Thermal control elements, i.e., thermal diodes, switches, and regulators, can control the heat flow in an analogous way in how electronic devices control electrical currents. In particular, a thermal diode allows a larger heat flux in one direction than in the other. This has aroused the interest of researchers working on the thermal management of electronics, refrigeration, and energy conversion. Solid-state thermal diodes are attractive because they are silent, reliable, lightweight, and durable. While some solid-state thermal diodes have been developed at the nano- and microscale, the leap to the macroscale has yet to be made. A macroscale thermal diode would play a crucial role in the future development of applications related to caloric refrigeration and heat pumping. Additionally, the temperature changes of caloric materials (due to the caloric effect) are ideal for testing these thermal devices. This paper aims to numerically evaluate the influence of a macroscopic solid-state thermal diode in a magnetocaloric refrigeration device under transient and quasi-steady-state conditions. Materials with different temperature-dependent properties were analyzed, and the most promising ones were selected for the operating range of a magnetocaloric device (290–296 K). The highest achieved magnetocaloric thermal rectification ratio under transient conditions was up to 295-times higher than with quasi-steady-state operation. This shows that transient operation should be considered for future progress with this technology.

V doktorski disertaciji predstavljamo hitro robno-območno integralsko metodo, ki smo jo uporabili za reševanje prenosnih pojavov v tekočini. Hitra robno-območna integralska metoda izhaja iz robno-območne integralske metode. Beseda hitra pove, da smo jo pospešili. Računska kompleksnost robno-območne integralske metode raste s kvadratom števila neznank. Zaradi tega hitro dosežemo zmogljivost razpoložljive računalniške opreme in smo tako omejeni na preproste primere. Z aproksimacijsko metodo zmanjšamo računsko kompleksnost robno-območne integralske metode. Uporabili smo križno aproksimacijo s hierarhično razdelitvijo matrik. Razvit algoritem smo primerjali s primeri, ki so v svetu priznani. V naših raziskavah smo rešili hitrostno-vrtinčno obliko Navier-Stokesovih enačb. Simulirali smo tok nanotekočine, ki ga žene naravna konvekcija v zaprti kotanji. Nanotekočina je zmes tekočine in delcev, ki imajo velikost nekaj nanometrov. Zapisali smo model, s katerim smo rešili tok nanotekočine, ki je vsebovala nanodelce aluminijevega oksida. In this dissertation, we present a Fast Boundary-Domain Integral Method that was used to solve transport phenomena in fluids. The Fast Boundary-Domain Integral Method has derived from the Boundary-Domain Integral Method. The word fast states that an acceleration method was employed. The computational cost of the Boundary-Domain Integral Method increases with the square of the number of unknowns. Thus, the computational cost quickly reaches the maximal computational power. Therefore, we are limited to smaller cases. In order to reduce the computational cost of the Boundary-Domain Integral Method, an approximation procedure has to be employed. We approximated the matrices that evolve from the Boundary-Domain Integral Method with a hierarchical decomposition method and an adaptive cross approximation algorithm. The developed algorithm was tested on test cases that are recognized in the world. In this study, we solved the velocity-vorticity formulation of the Navier-Stokes equations. We simulated a buoyancy-driven flow of a nanofluid in a closed cavity. The nanofluid is a mixture of a base fluid in nanoparticles. The nanoparticles are of the size of few nanometers. We present a model to solve the flow of a nanofluid. The nanofluid contained nanoparticles of alumina.