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Za mikrofluidne naprave štejemo tiste pretočne naprave, ki se uporabljajo v kemiji oziroma kemijski proizvodnji, katerih vsaj ena dimenzija je manjša od enega milimetra. Zaradi te lastnosti je za njih značilno visoko razmerje med površino in volumnom, katerega posledica so boljša energijska učinkovitost, izboljšan nadzor procesa, izboljšan prenos toplote in snovi, ter zaradi njihovega majhnega volumna, manjša poraba topil. Modeliranje služi v kemijskem inženirstvu kot orodje za boljše razumevanje procesov, pomoč pri dimenzioniranju naprav in optimizaciji procesov ter kot pomoč pri vodenju procesov. Tradicionalni postopki modeliranja se v naši panogi praviloma poslužujejo makroskopskih metod, ki slonijo na predpostavki kontinuuma, torej zanemarjajo delčno sestavo snovi. Majhne karakteristične dimenzije mikrofluidnih naprav postavljajo upravičenost uporabe makroskopskih modelov pod vprašaj. Mezoskopska mrežna Boltzmannova metoda ima potencial za modeliranje mikrofluidnih naprav, saj z njenim statistično-mehanskim izvorom ne zanemarja delčne sestave snovi, hkrati pa je primerna za opis mehanike fluidov in transportnih pojavov tudi v kompleksnih geometrijah. Mrežno Boltzmannovo metodo je smiselno razširiti v večnivojski model, da na ta način omilimo porabo računalniške moči — tako uporabimo mrežno Boltzmannovo metodo za opis procesov v kompleksnih geometrijah, preprostejša področja mikrofluidne naprave pa opišemo z makroskopskim modelom. V tej nalogi predstavljamo naše delo z mrežno Boltzmannovo metodo. Najprej jo uporabimo za opis prenosa snovi v mikrobioreaktorju s strnjenim slojem sferičnih delcev. Nadaljujemo s primerjavo različnih modelov trka mrežne Boltzmannove metode in z uporabo mrežne Boltzmannove metode za modeliranje toka binarne mešanice med seboj netopnih fluidov, s pomočjo katerega teoretično načrtujemo brezmembranski mikroločevalnik. Na koncu mrežno Boltzmannovo metodo razširimo še v večnivojski model za opis reaktivne mešanice plinov v katalizirani reakciji suhega reforminga metana. Flow devices used in chemistry and chemical manufacturing, that have at least one dimension smaller than a millimeter, may be characterized as microfluidic devices. Among characteristics that arise from this property are: high area-to-volume ratio, better efficiency, improved process control, improved heat and mass transport, and due to their small volume, reduced solvent use. Chemical engineers use modelling to gain better understanding of a process, as a tool in process scaling and optimization, and even for process control. In our field we traditionally use continuum-based models to describe processes. These models inherently neglect the particle composition of matter. It is due to microfluidic devices’ small characteristic size that the continuum assumption may not be valid when modelling them. The lattice Boltzmann method has a lot of potential for modelling of microfluidic devices as with its statistical-mechanics background it does not fully neglect the particle composition of matter, and it may be used to model fluid mechanics, and transport phenomena even in complex geometries. Due to it not being computationally very cheap it is desired to use the lattice Boltzmann method in a multiscale model setup, where it serves to solve transport phenomena in complex geometries, and the rest of the microfluidic device is modelled with a simpler macroscopic method. In this dissertation we present our work with the lattice Boltzmann method. First we use it to model mass transport in a packed-bed microbioreactor with randomly packed spherical particles. This is followed up by a study in which we compare different lattice Boltzmann collision models, and a study where we theoretically design a membrane-free microseparator by simulating the flow of a binary mixture of immiscible fluids. Finally, we expand the lattice Boltzmann method to a multiscale model of dry-reforming of methane in a catalytic reactor.

Mikropretočne naprave predstavljajo učinkovito platformo za gojenje celic in študij njihovega obnašanja in medsebojnih interakcij. Izdelati jih je mogoče iz različnih materialov, med njimi pa prevladujejo polimeri, za spremljanje procesov v njih pa so potrebne miniaturizirane merilne naprave. Nadzor nad procesnimi spremenljivkami v mikropretočnih napravah je precej boljši kot pri velikih reaktorjih, hkrati pa te naprave omogočajo hitrejše in cenejše izvajanje raznih študij, kot sta preučevanje kemotaksije in testiranje učinkovitosti antimikrobnih sredstev. V mikropretočnih napravah je mogoče gojiti različne vrste mikroorganizmov, od bakterij in gliv do mikroalg, mikroorganizmi pa v njih pogosto uspevajo bolje kot v konvencionalih reaktorjih. Poseben tip mikropretočnih naprav so kapljične mikropretočne naprave, pri katerih se mikroorganizme ujame v kapljice tekočine, ki delujejo kot samostojni reaktorji. Microfluidic devices provide an effective platform for cell culture and the study of their behavior and interactions. They can be made of various materials, among which polymers predominate, and miniaturized measuring devices are needed to monitor the processes in them. The control of process variables in microfluidic devices is much better than in large reactors, and at the same time these devices enable faster and cheaper implementation of various studies, such as studying chemotaxis and testing the effectiveness of antimicrobial agents. Various types of microorganisms can be grown in microfluidic devices, from bacteria and fungi to microalgae, and microorganisms often thrive better in them than in conventional reactors. A special type of microfluidic devices are droplet microfluidic devices, in which microorganisms are trapped in liquid droplets that act as stand-alone reactors.

Opis nizkogravitacijskega okolja in raziskovalnih platform, ki ga omogočajo. Predstavitev osnovnih fluidnih pojavov v mikrogravitaciji in kratka predstavitev mikrofluidike z osnovnim obnašanjem mikrofluidov. Na koncu sledi še pregled nekaterih mikrofluidnih naprav, ki se uporabljajo v vesolju in primerjava mikrogravitacijskega in mikrofluidnega okolja. In this paper a description of the low-gravity environment and the research platforms that enable it is presented. Furthermore, basic fluid phenomena in microgravity and short presentation of microfluidics with basic behavior of microfluids is disscused. Finally, an overview of some microfluidic devices used in space and a comparison of the microgravity and microfluidic environment is offered.

V diplomskem delu so v tretjem poglavju predstavljene osnove kompleksnih fluidov in matematični modeli, ki so pomembni za opis kompleksnih fluidov. V četrtem poglavju sem s pomočjo literature raziskal in opisal področje mikrofluidike, mikrofluidnih naprav ter najpomembnejše transportne pojave v mikrofluidnih sistemih. Predstavljeni so bili tudi materiali za izdelavo mikrofluidnih čipov ter fabrikacija mikrofluidnih naprav. V poglavju pet, sem se posvetil pregledu literature o pasivnih in aktivnih mikromešalnih napravah in njihovi uporabi. V zadnjem poglavju diplomske naloge je bilo glavno področje raziskovanja večfazni tok fluidov. Pri tem sem se osredotočil na pregled literature s področja nastanka nestabilnosti v dvokomponentnem toku v mikrofluidnih napravah. Ugotovljeno je bilo, da eden izmed ključnih dejavnikov za nastanek nestabilnosti v večfaznem toku, predvsem razlika v viskoznosti in gostoti komponent in da je mogoče s spreminjanjem volumenske frakcije fluidov, ter uravnavanjem hitrosti pretokov, nestabilnosti bodisi ojačati ali pa omejiti. Temeljna metoda dela pri izdelavi diplomske naloge je bil pregled literature, tako člankov kot knjig o omenjenih temah. In the third chapter of my diploma thesis some basic principles of complex fluids and mathematical models for their modelling are presented. The fourth chapter concentrates on the field of microfluidics, microfluidic devices and transport phenomena in microfluidics. In the chapter I also present different materials and methods for fabrication of microfluidic devices. In the chapter five the main goal was to present passive and active micromixers and their use in microfluidics. The last chapter of my diploma thesis was driven by research on multiphase flow of fluids. I also focused my research on summarizing literature on the formation of instabilities in the multiphase flow, from which I concluded that difference in viscosity and density of two fluids is one of the main reasons for formation of instabilities in a fluid network. From literature review I concluded that instabilities can be either enhanced or weakened by adjusting volume fraction of inlet fluids and flow rate. The main method of my research was literature review from articles, as well as books.