Water usually condenses on the air-side surfaces of evaporators in air-conditioning and refrigerationapplications. The accumulating condensate on a heat exchanger significantly affects its thermal and hydraulicperformance. The goal of this research is to develop a model suitable for predicting the mass of condensateretained as drops on a heat exchanger at steady-state conditions. In order to achieve this goal, a thoroughunderstanding of the three-dimensional shapes of drops on inclined surfaces is needed???drop shape isimportant to accurate volume predictions. Analysis, experiments, and computations are used to answerunresolved 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 theprofile taken at any azimuthal angle. The method provides an excellent tool for predicting drop volumes andfor investigating variables that affect drop shapes. Experiments are used to validate the two-circleapproximation. The contact line at the base of a drop is characterized as an ellipse, with the aspect ratioincreasing slightly as the Bond number increases. Contact angle variations within drops are determinedexperimentally, and then defined in terms of the maximum and minimum angles of a drop, which areobtained for general conditions. The results show the maximum contact angle in a drop to be approximatelyequal to the advancing angle of the liquid-surface combination at all conditions. The minimum angle is foundto decrease as the drop diameter or surface-inclination angle increases. A general relation is observedbetween the minimum angle of a drop and the Bond number, applicable for different liquids, surfaces, andconditions. An equation is derived relating the advancing contact angle to the receding contact angle andmaximum Bond number for any liquid-surface combination. The findings, which are well-supported by datafrom 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 toaccount for different geometric and surface conditions. These distribution functions along with the geometricmodel of drops and the contact angle results are used to develop a new model of condensate retention. Thenew model is successful in predicting the mass of condensate retained on coils tested by several otherresearchers. Preliminary analysis and experiments are presented as a step towards future extensions of themodel.
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