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Spray drying is a widely used unit operation for producing particulate products directly from a liquid feed. Important processes that occur inside the spray dryer are droplet formation, droplet drying and interactions between droplets and recycled fines. Various studies have looked at the first two processes, but the latter phenomenon has received less attention. Literature on droplet-particle interaction which aims at quantitatively describing agglomeration in spray drying is scarce and mainly qualitative. For product quality the formation of agglomerates is often desirable. This thesis models and investigates the collisions of individual particles with single droplets. The surface tack of drying droplets has been identified as an important variable for the formation of agglomerates. In this thesis a novel method for measuring tack from the liquid phase has been further improved. The improvements are a more accurate load measurement, an automated control of the tack probe and an improved layout of the sample holder and probe. The key feature of the device is its ability to measure tack of drying droplets, whereas other devices measure tack by wetting a powder. Using our method the tack of a commonly spray dried product, yeast extract, has been measured. From these experiments it was found that with decreasing average moisture content the surface tack increases to a maximum. Below a critical average moisture content the surface of the droplet is dry and the tack rapidly decreases upon further drying. Another important parameter in determining the degree of agglomeration is the degree of penetration. If the particle penetrates the droplet too deeply, the agglomerate structure becomes too dense. To predict the penetration depth, a non-dimensional model has been developed. It describes the penetration of a particle into a liquid droplet during a head-on collision. It is based on a force balance and incorporates surface tension force, viscous force and capillary pressure force. The important parameters determining the collision outcome are the contact angle, the size of the droplet relative to the particle, the Reynolds and Weber numbers. For each contact angle an equilibrium penetration position exists, at this point the surface tension force vector is perpendicular to the penetration direction. Five different penetrations regimes are identified. At low Reynolds numbers, viscous forces dominate and the particle asymptotically travels towards the equilibrium position. Reducing the viscous drag force by increasing the Reynolds number results in initially overshooting the equilibrium position, but the surface tension force pulls the particle back, to attain the equilibrium in an oscillating motion. At even higher Reynolds numbers the particle fully penetrates the droplet, and reaches the centre of the droplet for even higher values for the Reynolds number. The ejection regime is found at high Reynolds number and low Weber numbers and the liquid should be non-wetting. Using the regime maps one is able to identify in which region a spray dryer is operating. Although the full penetration regimes are useful for capturing fines, it should be avoided when agglomeration is desired. The ejection regime should be avoided as well. To validate the model, impact experiments were carried out by dropping glass spheres on the surface of different liquids. These validation experiments were the first attempt to experimentally validate the collision of a single particle with a liquid surface. Besides yeast extract, which has non-Newtonian rheological properties, silicone oils with constant viscosities of 100 mPa•s and 1 Pa•s have been tested. The penetration over time for different impact velocities was determined by analysing high speed camera recordings. The typical penetration times ranged from 0.2 s to 2 s. To obtain accurate location data was recorded at frame rates up to 38 000 frames per second. Glass spheres, with a size of 2 mm were used to allow the visual tracking. Modelling the impacts showed that the model consistently predicted faster penetration times than were observed experimentally. The relative difference increased with increasing viscosity. A parameter fitting exercise showed that better agreement could be obtained by using a higher viscosity and a higher contact angle in the model. With this knowledge the most likely factor influencing the model-experiment mismatch was identified as being the dynamics of wetting of the particle surface. It was also found that using the dynamic contact angle in the model would improve its results. The non-Newtonian characteristics of the yeast extract resulted in the particle rebound and the formation of an air cavity upon impact. The tack measurement technique and penetration model presented in this thesis will be useful tools for the design of spray dryers. Recommendations are made for further model improvement. The experimental validation is the first attempt to validate the presented model. Future improvements are recommended and suggestions are presented.

Με τη διατριβή έγινε ανασκόπηση των πιο γνωστών θεωριών που διέπουν τις πολυφασικές ροές. Ακολούθησε συλλογή και κριτική αξιολόγηση των θεωρητικών μοντέλων. Εκπονήθηκαν δύο προγράμματα σε Η/Υ, τόσο για την ομογενή ροή όσο και τη χωριστή ροή. Έγινε σύγκριση των θεωρητικών με υπάρχοντα πειραματικά αποτελέσματα. Σχεδιάστηκε και κατασκευάστηκε πειραματική διάταξη. Λήφθηκαν πειραματικές μετρήσεις και έγινε αξιολόγησή τους και σύγκριση με θεωρητικά αποτελέσματα. Προτάθηκε μέθοδος διόρθωσης της πρόβλεψης λειτουργίας μιας υδροπνευματικής αντλίας με τη βοήθεια του μοντέλου ‘drift-flux’ (μέθοδος CoSM). Προτάθηκαν νέοι ροϊκοί χάρτες κατάλληλοι για την παρουσίαση της λειτουργίας των υδροπνευματικών αντλιών, τον υπολογισμό του κλάσματος κενού και την μετάβαση των ροϊκών καταστάσεων. Προτάθηκε τρόπος προσδιορισμού της μετάβασης ‘slug-churn’ με απλή φωτογραφική μέθοδο (camera) και χρήση του μοντέλου ‘drift-flux’. Τα αποτελέσματα της διατριβής (πειράματα, μέθοδος CoSM, ροϊκοί χάρτες και μετάβαση slug-churn) παρουσιάστηκαν σε συνέδρια και επιστημονικά περιοδικά. / This PhD thesis deals with multiphase flows and air-lift pumps. All well-known theories concerning these two scientific fields are presented and analyzed in detail. Two computational codes were developed for homogeneous two-phase and separated three-phase flow. A comparison between theoretical results and experimental data followed. An experimental investigation was performed in a lab scale air-lift pump installation at Fluid Mechanics Laboratory, University of Patras. A new method for the precise prediction of the performance of a two-phase air-lift pump with the aid of drift-flux model was presented (CoSM method). New regime maps were introduced suitable for air-lift pump presentations. That means the direct view of the flow behaviour inside the air-lift pump, void fraction calculation and the regime transitions. An experimental method was presented for the prediction of slug-churn transition in two-phase flow, using a camera and ‘drift-flux’ model. The results of this work were presented in International Conferences and Journals.

This dissertation includes a theoretical and numerical development to simulate liquid-vapor flows and the applications to microchannels. First, we obtain a consistent non-local pressure equation for simulating liquid-vapor interfacial flows using mean-field free-energy theory. This new pressure equation is shown to be the general form of the classical van der Waals" square-gradient theory. The new equation is implemented in two-dimensional (2D) D2Q7, D2Q9, and three-dimensional (3D) D3Q19 lattice Boltzmann method (LBM). The three LBM models are validated successfully in a number of analytical solutions of liquid-vapor interfacial flows. Second, we have shown that the common bounceback condition in the literature leads to an unphysical velocity at the wall in the presence of surface forces. A few new consistent mass and energy conserving velocity-boundary conditions are developed for D2Q7, D2Q9, and D3Q19 LBM models, respectively. The three LBM models are shown to have the capabilities to successfully simulate different wall wettabilities, the three typical theories or laws for moving contact lines, and liquid-vapor channel flows. Third, proper scaling laws are derived to represent the physical system in the framework of the LBM. For the first time, to the best of the author's knowledge, we obtain a flow regime map for liquid-vapor channel flows with a numerical method. Our flow map is the first flow regime map so far for submicrochannel flows, and also the first iso-thermal flow regime map for CO₂ mini- and micro-channel flows. Our results show that three major flow regimes occur, including dispersed, bubble/plug, and liquid strip flow. The vapor and liquid dispersed flows happen at the two extremities of vapor quality. When vapor quality increases beyond a threshold, bubble/plug patterns appear. The bubble/plug regimes include symmetric and distorted, submerged and non-wetting, single and train bubbles/plugs, and some combination of them. When the Weber number<10, the bubble/plug flow regime turns to a liquid strip pattern at the increased vapor quality of 0.5~0.6. When the Weber number>10, the regime transition occurs around a vapor quality of 0.10~0.20. In fact, when an inertia is large enough to destroy the initial flow pattern, the transition boundary between the bubble and strip regimes depends only on vapor quality and exists between x=0.10 and 0.20. The liquid strip flow regimes include stratified strip, wavy-stratified strip, intermittent strip, liquid lump, and wispy-strip flow. We also find that the liquid-vapor interfaces become distorted at the Weber number of 500~1000, independent of vapor quality. The comparisons of our flow maps with two typical experiments show that the simulations capture the basic and important flow mechanisms for the flow regime transition from the bubble/plug regimes to the strip regimes and from the non-distorted interfaces to the distorted interfaces. Last, our available results show that the flow regimes of both 2D and 3D fall in the same three broad categories with similar subdivisions of the flow regimes, even though the 3D duct produces some specific 3D corner flow patterns. The comparison between 2D and 3D flows shows that the flow map obtained from 2D flows can be generally applied to a 3D situation, with caution, when 3D information is not available. In addition, our 3D study shows that different wettabilities generate different flow regimes. With the complete wetting wall, the flow pattern is the most stable. / Ph. D.

In-situ vapor extraction is performed by applying a pressure differential across a hydrophobic porous membrane that forms a wall of the channel as a means of reducing the local quality of flow boiling within the channel. As the local quality is reduced, the heat transfer capability can be improve while large pressure drops and flow instability can be mitigated. The present study investigates the potential of vapor extraction, by examining the characteristics and mechanisms of extraction. The physics based models for transition among extraction regimes are developed which can be used as a basis for a regime-based vapor extraction rate model. The effects of vapor extraction on flow boiling in a microscale fractal-like branching network and diverging channels are studied by using a one-dimensional numerical model based on conservation of mass and energy, along with heat transfer and pressure drop correlations. The results show the improvement in reduced pressure drop and enhanced flow stability, and show the potential of heat transfer enhancement. / Graduation date: 2013 / Access restricted to the OSU Community at author's request from March 25, 2013 - March 25, 2014