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Title:Development and validation of a thermodynamic model for predicting mode transitions in falling film flows over horizontal tubes
Author(s):Mikhaeel, Mina Michel Kamel
Director of Research:Jacobi, Anthony
Doctoral Committee Chair(s):Jacobi, Anthony
Doctoral Committee Member(s):Glumac, Nick; Higdon, Jonathan; Hrnjak, Pega
Department / Program:Mechanical Sci & Engineering
Discipline:Mechanical Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Falling film
Thermodynamics, Availability
Droplet mode
Jet mode, Sheet mode
Mode transition
Reynolds number, Multi-effect desalination (MED)
Abstract:Horizontal-tube, falling-film heat and mass exchangers are important to many industries, with example applications in thermal desalination, vapor compression machines, absorption chillers, and natural gas liquefaction. Falling-film heat and mass exchangers are attractive for these applications because, in comparison to other designs, they provide high transport coefficients with low system charges. As a liquid falls through a bank of horizontal tubes, it forms a thin film on the surfaces of the tubes. In the inter-tube space, the flow can take three main modes, namely: droplet, jet, and sheet modes. The accurate prediction of the prevailing flow mode is very important for the design and operation of these heat and mass exchangers, because the main performance characteristics (e.g. transport coefficients, dry-out of the tube surface, and system pressure drop) are impacted by the prevailing flow mode. There are different approaches for predicting the flow mode, and while empirical correlations are most widely used in application, numerical solutions to the Navier-Stokes equations for predicting flow modes have been described in the open literature. With a numerical approach, the main challenges are related to the computational speed versus accuracy trade-off and its impact on domain selection, as well as the need to start with some assumed two-phase morphology. On the other hand, an empirical approach is obviously limited to the experimental range of the fitting data. Hence, falling-film applications call for a mode prediction method, especially for new systems, which is applicable for a wide range of operating conditions, easy to implement, and grounded in theory to help explain the physics behind the transitions. In this thesis, a falling-film mode transition criterion is developed based on the rate equation for the balance of thermodynamic availability. Expressions are developed for the main components of availability (kinetic, potential, and surface energies) of the three main falling-film modes. The thermodynamic model is then applied to predict the transitional Reynolds numbers between the falling-film modes over a wide range of operating conditions (i.e. fluid properties and tube geometries). Validation with experimental data from the literature shows higher accuracy of the thermodynamic model compared to widely used empirical correlations for most simulated conditions. As a demonstrative application, the prevailing inter-tube falling-film flow modes are studied for seawater over a wide range of operating conditions (temperature, salinity, tube diameter, and inter-tube spacing), closing a gap in the literature where there is a dearth of applicable transition data. Finally, the thermodynamic basis for this work advances our understanding of two-phase, liquid-vapor flow morphology.
Issue Date:2021-04-22
Rights Information:Copyright 2021 Mina Mikhaeel
Date Available in IDEALS:2021-09-17
Date Deposited:2021-05

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