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ACRC Technical Report 209PDF


Title:Modeling Condensate Drops Retained on the Air-Side of Heat Exchangers
Author(s):El Sherbini, A.I.; Jacobi, A.M.
Subject(s):air-side heat transfer
condensate retention
Abstract:Water usually condenses on the air-side surfaces of evaporators in air-conditioning and refrigeration applications. The accumulating condensate on a heat exchanger significantly affects its thermal and hydraulic performance. The goal of this research is to develop a model suitable for predicting the mass of condensate retained as drops on a heat exchanger at steady-state conditions. In order to achieve this goal, a thorough understanding of the three-dimensional shapes of drops on inclined surfaces is needed—drop shape is important to accurate volume predictions. Analysis, experiments, and computations are used to answer unresolved questions vital to predicting drop shapes for general conditions. A geometric method is developed to approximate the shape of a drop by fitting two circles to the profile taken at any azimuthal angle. The method provides an excellent tool for predicting drop volumes and for investigating variables that affect drop shapes. Experiments are used to validate the two-circle approximation. The contact line at the base of a drop is characterized as an ellipse, with the aspect ratio increasing slightly as the Bond number increases. Contact angle variations within drops are determined experimentally, and then defined in terms of the maximum and minimum angles of a drop, which are obtained for general conditions. The results show the maximum contact angle in a drop to be approximately equal to the advancing angle of the liquid-surface combination at all conditions. The minimum angle is found to decrease as the drop diameter or surface-inclination angle increases. A general relation is observed between the minimum angle of a drop and the Bond number, applicable for different liquids, surfaces, and conditions. An equation is derived relating the advancing contact angle to the receding contact angle and maximum Bond number for any liquid-surface combination. The findings, which are well-supported by data from the literature, help explain and verify some observations of earlier researchers. Size distribution functions of drops on inclined surfaces, taken from the literature, are modified to account for different geometric and surface conditions. These distribution functions along with the geometric model of drops and the contact angle results are used to develop a new model of condensate retention. The new model is successful in predicting the mass of condensate retained on coils tested by several other researchers. Preliminary analysis and experiments are presented as a step towards future extensions of the model.
Issue Date:2003-02
Publisher:Air Conditioning and Refrigeration Center. College of Engineering. University of Illinois at Urbana-Champaign.
Series/Report:Air Conditioning and Refrigeration Center TR-209
Genre:Technical Report
Sponsor:Air Conditioning and Refrigeration Project 110
Date Available in IDEALS:2009-06-12

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