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Title: Numerical investigation of evaporation and condensation of thin films in conjugated heat transfer systems
Authors: Sosnowski, Pawel
Keywords: conjugated heat transferthin filmevaporationcondensation
Issue Date: 23-Apr-2013
Publisher: Università degli studi di Trieste
Abstract: Evaporation and condensation of thin liquid films on solid surfaces are common elements of industrial processes. In many cases they have a significant impact on the physics of the studied case. At the same time, experimental studies can prove to be troublesome, mostly because of the amount of possible setups, complex geometries of interest, numerous materials being used and cost. For that reason it is reasonable to study this phenomena using numerical methods. Having the advantage in speed and cost of performance, computational studies become a valuable tool. For evaporation and condensation process, one has to deal with buoyancy driven fluid flows, conjugated heat transfer between gaseous and solid phases, film thickness modeling, vapor phase behavior, and phase transition of the thin fluid film into vapor phase. The strong conjunction and mutual interaction of mentioned effects is the main focus of presented work. The gas phase behavior is being calculated using incompressible Navier-Stokes equations under Boussinesq approximation. The solutions of the partial differential equations are obtained with numerical methods using Eulerian finite volume discretization (Kundu and Cohen [2002]). Time advancement is being treated with second order implicit discretization. For cases with high Reynolds number, large eddy simulation (LES) techniques are used. Due to the complexity of the geometries of interest a dynamic computation of the Smagorinsky constant is preferred, applying the lagrangian dynamic model proposed by Meneveau et al. [1996]. The liquid film present on the surface of the solids is modeled following Petronio[2010]. Since the film is thin, it is assumed that it can be represented only by its thickness. This also leads to assumption that the heat transfer through the film is instantaneous. The vapor is represented by concentration of this phase in the volume of gas. The concentration is transported by convection and diffusion. The phenomena of evaporation and condensation of the thin films are driven by the presence of concentration gradients next to the surfaces. Phase transition of vapor to fluid, or other way around, acts on the energy balance, id. est latent heat is released into the gas when condensation occurs or the solid is cooled during evaporation. The heat transport is modeled in both solid and fluid domains. The case is split into separate regions with different material properties. These regions are solved one by one in a serial way using numerical techniques consistent with domain decomposition methods described by Quarteroni and Valli [1999]. The energy transport among the regions is performed by applying a heat coupling boundary conditions. The main focus of this work is to provide a reliable model for simulation system with complex physics involving fluid motion, heat transport in multi region domains (fluid-solid), vapor transport, thin film evolution and evaporation and condensation effects on energy balance. Proposed model is validated on simple geometries and later applied to problem of evaporation in vertical channel flow. The reference to the channel case is work of Laaroussi et at. [2009]. Presented study aims in providing comprehensive insights into physical effects that appear when the solid wall is being directly modeled and when latent heat transformations are taken into account. The final test is performed on a vertical channel with forced turbulent flow, directly modeled solid walls and evaporation or condensation happening on the boundary. Having the model working within such complex frame allows for its future usage in elaborate industrial applications.
Description: 2011/2012
NBN: urn:nbn:it:units-10080
Appears in Collections:Ingegneria civile e architettura

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