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Title:Heat transfer and pressure drop in the condensing superheated region with visualization and film thickness measurement
Author(s):Meyer, Melissa
Advisor(s):Hrnjak, Predrag S.
Department / Program:Mechanical Sci & Engineering
Discipline:Mechanical Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Condensing superheated
film thickness
pressure drop
flow regimes
Abstract:Traditionally, condensation has been characterized by division into three zones: desuperheating, two-phase, and subcooled regions. According to this characterization, heat transfer and pressure drop in condensers are modeled separately for the single-phase and two-phase regions. When plotted as a function of enthalpy, the correlations show a discontinuity between the single-phase and two-phase zones because the three-zone approach implicitly assumes thermodynamic equilibrium throughout the condensation process. In reality, the refrigerant is not at equilibrium, and condensation occurs outside the conventionally defined two-phase zone. Condensation actually starts when the wall temperature reaches saturation, even in the presence of superheated vapor. Similarly, condensation continues if some vapor remains even when the bulk refrigerant enthalpy is indicating a subcooled state. These two situations make a fourth and fifth zone in the condenser, classified as the condensing superheated and condensing subcooled zones. The effects of these zones on heat transfer have been described previously, but the effects on pressure drop have received less attention, and confirmation of the physical processes thought to be occurring has not yet been provided. This paper presents experimental results verifying the presence of liquid in the condensing superheated region. Flow visualization experiments with R134a revealed that condensate began to appear when the bulk refrigerant enthalpy was above saturation, as droplets and rivulets on the tube wall and then as an annular film with mist entrained in the vapor core. Liquid film thickness measurements further confirmed the growth of a condensate film at bulk enthalpies greater than saturation. The heat transfer coefficient followed the same trend seen in earlier experiments, rising sharply in the condensing superheated region for a smooth transition between the single-phase and two-phase zones. The pressure drop gradient also increased significantly above the single-phase prediction once the wall temperature fell below the saturation temperature, as shown by prior experimental results for R32. In the condensing superheated region, shear interactions between the liquid film and vapor increase the pressure drop compared to vapor alone. As condensation continues, the shear increases due to a thicker and wavier liquid layer, but friction and momentum losses decrease as the refrigerant velocity decreases. These competing factors cause a peak and subsequent decrease in the pressure drop gradient and are corroborated by the flow visualization.
Issue Date:2014-09-16
Rights Information:Copyright 2014 Melissa Meyer
Date Available in IDEALS:2014-09-16
Date Deposited:2014-08

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