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The basis of the doctoral thesis is the analysis of flow and heat transfer of one or two electrically conducting micropolar fluids in the presence of magnetic and electric fields. The first part of the dissertation is the development of mathematical models for described problems of MHD flow and heat transfer of micropolar fluid, where the research is conducted in inductionless approximation and for the case of coupled equations. After that, in the next chapters, the problem of walls conductivity of the channel is considered, also the effects of induced magnetic field and interface boundary conditions for the flow of two fluids. Special attention is paid to the flow of two fluids between inclined plates where one fluid is micropolar and the other one is Newtonian. Analytical solutions are given for momentum, microrotation, energy and magnetic induction equations for the problems of MHD flow of one and two fluids. Next part of the dissertation has a detailed analysis of physical phenomena that occur in MHD and EMHD flow of micropolar fluids, and comparison of analytical solutions obtained in closed form with the results obtained by numerical simulations. Using the numerical simulations the case of the flow of micropolar fluid in the horizontal and vertical channels under the influence of magnetic and electric fields are solved. At the end, within the dissertation detailed analysis of results are given for micropolar fluids, which is significant in the technical practices, for different values characteristic dimensionless numbers that describes MHD and EMHD problems of flow and heat transfer of micropolar fluids. Special attention is also given to the applicability of the research in the development of new technologies.

The aim of this dissertation is to define mathematical models that describe flow and heat transfer in porous media by generalizing the classical Navier-Stokes model in order to model the influence of porous media properties. Considering the importance of both theoretical and applied research of magnetohydrodynamic (MHD) flow, the dissertation includes the analysis of MHD one-fluid and two-fluidflow and heat transfer within channels containing a porous medium. The first chapter begins with the research overview, followed by the explanation of what motivated the research, the introduction of the subject matter, and the definition of research objectives.The second chapter deals with the mathematical modelling of MHD flow and mass and heat transfer. The third chapter first discusses a case of MHD one-fluid flow and mass and heat transfer in a horizontal porous channel. The fourth chapter considers the flow of two non-mixing fluids in a channel with two horizontal insulated plates serving as walls. The plates are at constant but different temperatures. The fifth chapter considers four different models of MHD flow and mass and heat transfer. The sixth chapter investigates MHD two-fluid flow and heat transfer in a channel inclined in relation to the horizontal plane. The seventh chapter presents numerical simulations conducted using a modern software tool called Ansys CFX. The simulations are provided for two analyzed models. The first one involves a horizontal channel containing a porous medium, while the second one involves an inclined channel containing a porous medium. The distributions of velocity, temperature, and pressure were determined in all chapters, always in closed form.

U ovom radu eksperimentalno je ispitivan diskontinualni granični sloj i analogije prenosa količine kretanja, toplote i mase u fluidizovanim i pakovanim slojevima i pri hidrauličkom transportu sistema tečnost-čestice. Cilj ovog rada je upravo povezivanje karakteristika graničnog sloja sa prenosom količine kretanja, toplote i mase, sa akcentom na analogije između ovih prenosa. Fenomenološkim pristupom ovoj problematici, daje se doprinos koji bi olakšao teorijsko razmatranje višefaznih sistema tečnost-čestice (pakovani sloj, fluidizovani sloj, vertikalni transport), koji su izuzetno složeni. Za izvođenje eksperimentalnog dela istraživanja u ovoj doktorskoj disertaciji korišćeni su različiti sistemi: standardne kolone različitih prečnika sa omotačem ili bez omotača, sa pakovanim, fluidizovanim i transportnim sistemima u cilju dobijanja parametara za koeficijente prenosa količine kretanja, mase i toplote i segmentirane kolone sa fluidizovanim česticama u cilju dobijanja parametara na koje utiče debljina graničnog sloja. Sva eksperimentalna merenja su izvršena u širokom opsegu hidrodinamičkih uslova: brzina strujanja fluida, veličina čestica čvrste faze i poroznosti slojeva. Kao rezultat istraživanja razvijen je novi teorijski model za prenos mase i toplote zasnovan na analizi graničnog sloja koji se u disperznom sistemu formira od sudara do sudara čestica. Na osnovu modela dobijena je debljina difuzionog i termičkog graničnog sloja, izvedene su jednačine za korelisanje koeficijenata prelaza, Sherwood-ovog i Nusselt-ovog broja i faktora prenosa mase i toplote. Na osnovu uspostavljenog modela izvedena je jednačina za određivanje maksimalne vrednosti koeficijenata prelaza toplote i poroznosti sloja pri kojoj do nje dolazi. U ovom radu je uspostavljena analogija prenosa količine kretanja, mase i toplote u partikulativno fluidizovanim slojevima zasnovana na modelu graničnog sloja. Postojeća analogija u fluidizovanom sloju, data na osnovu normalizovanih vrednosti koeficijenta trenja čestice-fluid i faktora prenosa mase proširena je na analogiju sva tri prenosa. Dobijeni model je upotrebljen za definisanje faktora prenosa mase i toplote pri minimalnoj fluidizaciji i transportu čestica... In this thesis the discontinued boundary layer and the analogues among momentum, heat and mass transfer were examined experimentally for fluidized and packed beds and for hydraulic transport in liquid-particles systems. The aim of this paper is to connect the characteristics of the boundary layer with the momentum, heat and mass transfer, with an emphasis on analogies between these phenomena. Transport phenomena approach to this problem provides a contribution that facilitates theoretical description of multiphase liquid-particles systems (packed beds, fluidized beds, vertical transport), which are extremely complex. Different experimental systems have been used to perform the research in this dissertation: standard columns of different diameters with or without heating jacket, with packed and fluidized beds and transport systems in order to obtain parameters for the momentum, heat and mass transfer coefficients, and segmented fluidized beds columns in order to obtain parameters influenced by the thickness of the boundary layer. All experimental measurements were performed in a wide range of hydrodynamic conditions: fluid velocities, solid particles sizes and bed voidages. As a result of this research, a new theoretical model for mass and heat transfer s developed. Model is based on the analysis of the boundary layer, which in disperse systems forms between the two adjacent particles collisions. Based on the model, the thickness of the concentration and thermal boundary layer was obtained and the equations for correlating the transfer coefficients with Sherwood and Nusselt numbers and the mass and heat transfer factors were derived. On the basis of the established model, the equation for determining the maximum value of the heat transfer coefficient and the bed voidage for this value is derived. In this thesis, the analogue based on the boundary layer model for the momentum, mass and heat transfer in particulate fluidized beds is established. The existing analogy for the fluidized beds, based on the normalized values of the friction coefficient fluid-particles and the mass transfer factor, has been extended to analogy for all three phenomena. The obtained model was used to define the mass and heat transfer factors at the minimum fluidization and transport of particles...

Osnovni predmet ove disertacije je da se korišćenjem njenih rezultata omogući odvijanje proizvodnog procesa sa minimalnim troškovima održavanja uz minimalno učešće živog rada, što je inače svakodnevni zadatak savremene nauke i tehnike. Zahtevi pri reparaturi kranskih točkova su isključivo vezani za dimenzije i površinsku tvrdoću. Usled habanja točak/šina, točak gube dimenzije gazne površinne i venca, koje se repariraju navarivanjem. U cilju smanjenja zastoja na kranovima i produžetka veka kranskog točka, kao i smanjenja troškova, napravljena je multidisciplinarna analiza čiji je cilj razvoj i primena nove eksperimentalno-numeričke metode za prevcenciju pojave prslina i loma kranskih točkova. U cilju određivanja optimalne tehnologije navarivanja sprovedeno je iscrpno eksperimentalno istraživanje osobina i mikrostrukture navarenog sloja, uključujući ispitavanja svakog prolaza i optimizaciju broj prolaza i parametara navarivanja za svaki prolaz. Problem unosa toplote pri navarivanju, odnosno termomehanički problem određivanja temperature predgrevanja, je određen korišćenjem analitičkih izraza za prenos razmenu toplote, kao termovizijskom kamerom za eksperimentalno određivanje, odnosno merenje temperature. Dobijeni rezultati su upoređeni, a u dsikusiju naglašene razlike i problemi u analitičkom određivanju temperature. Numerička simulacija i procena integriteta na osnovu naponskog stanja točka u kontaktnom opterećenju sa šinom su dati korišćenjem analitičkih jednačina za Herzov pritisak (kontaktni napon), kao i odgovarajućim proračunom metodom konačnih elemenata, uključujući simulaciju uticaja prsline. Na kraju je data tehnoekonomska analiza postupka navaraivanja, odnosno procenjena njegova opravdanost (prednosti, mane) u odnosu na zamenu oštećenog točka. Pokazano je da se reparaturnim navarivanjem ostvaruju višestruke uštede, uz znatno kraće vreme poporavke. The main goal of this dissertation is for its results to enable the performing of manufacturing products with minimal maintenance costs and human labor, which represents an everyday challenge for modern science and technology. Demands for crane wheel repairs are exclusively related to dimensions and surface hardness. Due to the rail/wheel wear, the wheel loses its drift and rim surface area, which can be repaired by welding. For the purpose of reducing downtime during exploitation of cranes and extending their life, along with reduction of costs, a multidisciplinary analysis was performed in order to develop and apply a new experimental-numerical method for prevention of crack initiation and crane wheel failure. In order to determine the optimal welding technology, extensive experimental testing of the welded layer’s properties and micro-structure was performed, including the testing of every pass and the optimization of welding parameters. The problem related to the heat input, i.e. the thermo-mechanical problem of determining the preheating temperature was analyzed using analytical expressions for heat transfer, using a thermography camera for experimental measuring of temperature. Obtained results were compared and differences and issues involved with analytical determining of the temperature were emphasized in the discussion. Numerical simulation and integrity assessment based on the stress state of the wheel due to the load caused by contact with the rail are given in form of analytical equations for Hertz’s pressure (contact stress), along with the corresponding finite element method calculation, including the simulation of the effects of a crack. Finally, a techno-economical analysis of the welding procedure, i.e. its justifiability (advantages, disadvantages), compared to damaged wheel replacement, was presented. It was shown that repair surface welding achieved significant savings, while greatly reducing the time needed for repairs.

One of the most important characteristics of heat exchangers, besides thermal efficiency, is their compactness or surface area to volume ratio. The need to achieve high efficiency in a compact unit with high operating pressure has led to the development of heat exchangers with perforated plates or, as they are called in the literature, the Matrix Heat Exchangers (MHE). In this PhD thesis experimental and numerical research was undertaken to measure the thermal and flow characteristics of an assembly of perforated plates, in an effort to obtain indications for engineering calculations. The study was undertaken in the Laboratory for Thermal Engineering at the Faculty of Mechanical Engineering, Niš. In the experimental part of the study, packages of one, two and three 2 mm thick plates were used, with a porosity of 25.6% and a square arrangement of holes that were 2 mm in diameter. Water was used as the heating fluid, while the heated fluid was air. The package of plates was placed in the experimental chamber which had a fan as an inlet, with the ability to control the flow. The fluid flow rate, pressure drops and the temperatures of the fluids at the inlet and outlet of the chamber, as well as the temperature of air between the plates, were measured at the pre-defined locations in the perforated plate heat exchanger. Also, the pressure drop was measured for the perforated plate package with and without the water collector. In the numerical part of this research, the PHOENICS 2014 software package was used. In the first part of the numerical study the heat transfer from plates with the porosity from 10 to 50% was investigated. In the second part of the numerical study, a package of three plates was established and the effect of distance between the plates was also examined. Experimental results and the results of other authors were used to validate the numerical experiment. iv On the basis of experimental and numerical results, criterial equations for heat transfer and formulas for pressure drop equations were established.

The research presented in this PhD thesis is directly linked to the achievement of sustainable building development analysis of indoor environment quality. The indoor environment quality includes the achieved level of heat, air, light, noise and aesthetic comfort. In the presented PhD thesis, special attention was paid to thermal comfort. Fundamental processes of mass, momentum and heat transfer are used to describe the various mechanisms of sensible and latent heat transfer from the human being to the environment and vice versa. The investigations were conducted by experimental, numerical and statistical methods. While investigating within PhD thesis, the following scientific contributions have been made: - Concerning the two-equation turbulence models, special attention was paid to the influence of production and dissipation terms of turbulent kinetic energy due to the fluctuation of body forces. - Varying empirical constants containing modeled terms of the differential equation for ε, the modifications were proposed for the empirical constant C 3 in the way that its value is declared variable as a function of the ratio of flow velocity component in the direction of the gravity vector and lateral directions, and an expression for its determination is proposed. - Concerning the heat transfer radiation model, the new model of radiation have been proposed that treats air contaminated by carbon dioxide as a nontransparent medium, and an expression is proposed in order to determine the effective emissivity of contaminated air. The experimentally obtained values of the general thermal comfort indicators were the basis for the validation of the mathematical models proposed in the presented PhD dissertation. The proposed models are built into a numerical code of the commercial software PHOENICS, and parametric analysis was conducted through a series of six test cases, starting from the basic model, which is characterized by the fact that the turbulent model and heat transfer by radiation model are built-in, up to the case C_B which includes both modifications. In addition to the standard procedure of error analysis, statistical analysis of a complex array has been conducted, so additional statistical indicators have given the right information about the each model quality. Based on the results of the numerical experiments, it was shown that the model of radiation compared to the turbulence model in the distribution of thermal characteristics and general thermal comfort indicators has a dominant role.

U radu „Eksploatacija niskotemperaturnih energetskih izvora korišćenjem visokotemperaturne toplotne pumpe“ autori su sintetizovali rezultate svojih sitraživanja iskorišćavanja toplote niskotemperaturnih energetskih izvora (geotermalna voda, otpadna voda procesne industrije, ...). Prikazan je način dobijanja, za okolinu, prihvatljive energije, namijenjene za grijanje stambenih jedinica upotrebom visokotemperaturnih toplotnih pumpi koje u svom radu koriste različita rashladna sredstva (radni fluid). Ove pumpe su nastale razvojem inovativne ideje 200. Godine i koriste toplotu niskotemperaturnihsekundarnih energetskih izvora (45 °C) za daljinsko grijanje stambenih jedinica sa temperaturom 90/70 °C. Obrađen je niz rashladnih fluida koj ise mogu upotrijebiti kao radni fluidi kod ovih pumpi. Izrada matematičkog modela jednostepene visokotemperaturne pumpe, koji autri koriste za određivanje radnih karakteristika pumpe, je prikazana u ovom radu, kao i simulacija rada pumpe uz korišćenje dvije različite smjese radnih fluida. Autori su u ovom radu detaljno prikazali međusobne zavisnosti ključnih radnih karakteristika visokotemperaturne pumpe i odredili tip visokotemperaturne toplotne pumpe i vrstu rashladnog fluida kao optimalno rješenje

Wall-to-bed momentum, heat and mass transfer in the vertical liquid-solids flow, as well as in the single phase flow, were studied. The aim of this investigation was to establish the analogy among those phenomena. Also, effect of particle concentration on momentum, heat and mass transfer was studied. The experiments in hydraulic transport were performed in 25.4 mm I.D. cooper tube equipped with a steam jacket, using spherical glass particles of 1.94 mm in diameter and water as a transport fluid. The segment of the transport tube used for mass transfer measurements had internal coating made of benzoic acid. In the hydraulic transport two characteristic flow regimes were observed: turbulent and parallel particle flow regime. The transition between two characteristic regimes (Y* = 0), occurs at a critical voidage ε ≈ 0.85. The vertical two-phase flow was con­sidered as the pseudofluid, and modified mixture-wall friction coefficient (fw) and modified mixture Reynolds number (Rem) were introduced for system characterization. Experimental data show that the wall-to-bed momentum, heat and mass transfer coefficients, in vertical flow of pseudofluid, for the turbulent regime are significantly higher than in parallel regime. Wall-to-bed, mass and heat transfer coefficients in hydraulic transport of particles were much higher than in single-phase flow for lower Reynolds numbers (Re lt 15000), while for high Reynolds numbers (Re gt 15000), there was not significant difference. The experimental data for wall-to-bed momentum, heat and mass transfer in vertical flow of pseudofluid in parallel particle flow regime, verify analogy among these three phenomena. U ovom radu prikazani su rezultati eksperimentalnih ispitivanja prenosa količine kretanja, toplote i mase zid-fluid u vertikalnom hidrauličkom transportu inertnih čestica i pri strujanju fluida u cevi istog prečnika. Eksperimentalna ispitivanja vršena su istovremenim merenjem potrebnih parametara za definisanje navedenih prenosa. Cilj ovih ispitivanja bio je određivanje koeficijenata prenosa pri vertikalnom hidrauličkom transportu inertnih čestica i uspostavljanje analogije tri prenosa. Eksperimentalni sistem predstavljala je vertikalna transportna cev prečnika 25,4 mm snabdevena omotačem za zagrevanje parom, kao i segmentom transportne cevi prepariranim rastopom benzoeve kiseline. Kao fluid korišćena je voda, a vršen je transport staklenih sfernih čestica prečnika 1,94 mm. U rezultatima su prikazani dobijeni koeficijenti trenja, prelaza toplote i prelaza mase. Korišćenjem koncepta pseudofluida, pokazano je postojanje analogije prenosa količine kretanja, toplote i mase u transportnoj cevi za paralelni režim strujanja.