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Acoustic cavitation, generated by a piezo-driven transducer, is a commonly used technique in a variety of processes, from homogenization, emulsification, and intensification of chemical reactions to surface cleaning and wastewater treatment. An ultrasonic horn, the most commonly used acoustic cavitation device, creates unique cavitation conditions under the horn tip that depend on various parameters such as the tip diameter, the driving frequency of the horn, its amplitude, and fluid properties. Unlike for hydrodynamic cavitation, the scaling laws for acoustic cavitation are poorly understood. Empirical relationships between cavitation dynamics, ultrasonic horn operating conditions, and fluid properties were found through systematic characterization of cavitation under the tip. Experiments were conducted in distilled water with various sodium chloride salt concentrations under different horn amplitudes, tip geometries, and ambient pressures. Cavitation characteristics were monitored by high-speed (200,000 fps) imaging, and numerous relations were found between operating conditions and cavitation dynamics. The compared results are discussed along with a proposal of a novel acoustic cavitation parameter and its relationship to the size of the cavitation cloud under the horn tip. Similar to the classical hydrodynamic cavitation number, the authors propose for the first time an acoustic cavitation parameter based on experimental results.

A sudden decrease in pressure triggers the formation of vapour and gas bubbles inside a liquid medium (also called cavitation). This leads to many (key) engineering problems: material loss, noise, and vibration of hydraulic machinery. On the other hand, cavitation is a potentially useful phenomenon: the extreme conditions are increasingly used for a wide variety of applications such as surface cleaning, enhanced chemistry, and wastewater treatment (bacteria eradication and virus inactivation). Despite this significant progress, a large gap persists between the understanding of the mechanisms that contribute to the effects of cavitation and its application. Although engineers are already commercializing devices that employ cavitation, we are still not able to answer the fundamental question: What precisely are the mechanisms how bubbles can clean, disinfect, kill bacteria and enhance chemical activity? The present paper is a thorough review of the recent (from 2005 onward) work done in the fields of cavitation-assisted microorganismʼs destruction and aims to serve as a foundation to build on in the next years.

Wastewater recycling and sludge removal in the paper industry account for about 60 % of all process costs. New and environmentally friendly wastewater treatment techniques are, therefore, continually being developed. Cavitation exploitation is currently a well-investigated topic that is also interesting for the paper production industry. This study investigates efficiency of hydrodynamic cavitation alone and in combination with the addition of NaOH for the treatment of secondary pulp and paper mill sludge in order to enhance nutrient release. First, two laboratory-scale devices were tested: the blow-through and the rotating hydrodynamic cavitation generator. The latter set-up proved to be more efficient: therefore, further experiments were performed on its pilot-scale version. The results showed an increase of soluble chemical oxygen demand (CODs) by 514 mg/L, total nitrogen (Nt) by 17.4 mg/L, and total phosphorous (Pt) by 2.3 mg/L. To further increase nutrient release combination of cavitation and sludge sample alkalinization was tested. The addition of NaOH and 30 min cavitation of secondary sludge (500 L) significantly improved CODs and Nt release by 2400 mg/L and 120 mg/L, respectively. Microbiological photos revealed a definite disintegration of sludge flocks. According to our estimates, 1.9 kg of released CODs from alkaline pre-treated and cavitated sludge would cost only one euro.

V sklopu zaključne naloge smo opravili eksperimentalne meritve za potrebe razvoja merilnega testnega dela, kjer bo formiran superkavitacijski oblak. Različne geometrije smo zasnovali v modelirniku (SolidWorks), jih dali izdelati iz akrilnega stekla in zasnovali eksperimentalno progo, na kateri smo izvajali meritve, rezultate pa smo nato analizirali. Merilna proga je omogočala hitro in enostavno menjavo različnih geometrij testnih delov. Kavitacijski oblak formiran znotraj testnega dela je bil okarakteriziran s pomočjo tlačnih razmer pred in za testnim delom, meritvami pretoka tekočine skozi testni del in vizualizacijo s pomočjo hitre kamere. Cilj naloge je bil okarakterizirati različne geometrije Venturijevih zožitev za formacijo superkavitacijskega oblaka. Želeli smo razviti superkavitacijski oblak, ki bi zavzemal čim večji volumen pretočnega kanala znotraj zožitve. As part of the final task, we performed experimental measurements for the needs of the development of the measurement test part, where a supercavitation cloud will be formed. Different geometries were designed in the model (SolidWorks) from acrylic glass. We designed an experimental track on which we performed measurements, then analyzed the results. The measuring track made it possible to quickly and easily change the different geometries of the test parts. The cavitation cloud formed in the test section was characterized by pressure conditions before and after the test section, measurements of fluid flow through the test section, and visualization using a high-speed camera. The purpose of the task was to describe the different geometries of the Venturi constrictions for the formation of a supercavitation cloud. We wanted to develop a supercavitation cloud that would occupy as much of the flow channel volume as possible within the constriction.

Problem obravnavanih volumetričnih črpalk iste generacije je, da ne dosegajo enakega pretoka v izbranih, enakih obratovalnih točkah. Magistrska naloga obravnava zasnovo merilne postaje za merjenje aksialnih pomikov rotorja elektromotorja in fluktuacij tlaka na izstopu iz črpalke. Merjenje aksialnih pomikov je zasnovano z mikrooptiko s pripadajočo hitro kamero do natančnosti ±2 µm, fluktuacije tlaka na izstopu iz črpalke pa z visokofrekvenčnim merilnikom dinamičnega tlaka. Sledila je analiza in povezava rezultatov meritev ter razlaga procesa črpanja v obravnavani volumetrični črpalki. Pri črpalkah iste generacije, ki dosegajo slabši pretok od ostalih, je bilo ugotovljeno da je velikost posameznega poskoka rotorja elektromotorja pri enem pripadajočem obratu največja pri vseh izmerjenih obratovalnih točkah, glede na ostale črpalke. The problem of volumetric pumps of the same generation is that they do not achieve the same flow at selected, certain operating points. This Master's degree discusses the design of measurement station for measuring axial movement of the electric motor rotor and pressure fluctuations on the pump outlet. The measurement of axial movements is designed by microoptics with an according fast frame rate camera to precision ±2 µm. The measurement of the aforementioned pressure fluctuations is measured with a high-frequency pressure sensor. These measurements were followed by an analysis and synthesis and explanation of pumping process Concerning pumps of the same generation which achieved lesser flow than others, the conclusion was that the size of a single electric motor rotor hop in one corresponding revolution was the biggest at all measured operating points, according to other pumps. The size of pressure fluctuations at the pump outlet differs at different operating points for each pump.

Delo obravnava zasnovo in izdelavo merilnika vlažnosti, ki bo sposoben določevanja relativnih vlažnosti okoli 100 % s prisotnimi vodnimi kapljicami. Merilnik deluje kontinuirano tako, da mešanico zraka in vodnih kapljic vzorči izokinetično. Prisilni vlek skozi merilnik zagotavlja ventilator. Princip delovanja merilnika temelji na dvigu temperature vzorčenega zraka, s tem mu znižamo relativno vlažnost. Izločanje in uparjanje kapljic zagotavljajo stene ciklona, ki so ogrevane. Tako imamo na izstopu le zrak z relativno vlažnostjo okoli 80 %, takšno relativno vlažnost lahko določimo s komercialno dostopnim merilnikom vlažnosti. V delu je opisana zasnova in sestava merilne proge za določitev tlačnega padca na ciklonu in določitev napajalne napetosti ventilatorja pri posameznih pretokih zraka. Pokazano je, kako različni parametri vplivajo na tlačni padec v ciklonu in njegovo učinkovitost. The thesis presents the design and production of a humidity meter capable of determining relative humidity around 100% with the presence of water droplets. The meter operates continuously with isokinetic sampling of a mixture of air and water droplets. Forced draft through the meter is produced by the fan. The mechanical principle is based on the rise of temperature of the sampled air which lowers the relative humidity. The release and evaporation of the water droplets occurs because of the heated walls of the cyclones. We thus have relative humidity around 80% in the outlet air. We determine this relative humidity with a commercially accessible humidity meter. The thesis describes the design and composition of the measurement setup which determines the pressure drop on the cyclone and determines the fan feed voltage for individual air flows. The way how different parameters affect the pressure drop in the cyclone and its effectiveness is also shown in the thesis.