Diskontinualni granični sloj i analogije prenosa u pakovanim, fluidizovanim i transportnim sistemima tečnost-čestice

Abstract

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...