Wetting and frosting/defrosting study on microgrooved surfaces.

摘要

Surfaces with parallel microgrooves have been studied widely, especially for their potential to promote water drainage. There is increasing interest in understanding and manipulating the effects of surface wettability on the condensation and frosting processes, to minimize condensate retention and frosting penalties, and to promote effective defrosting. In the present study, the effects of microgrooved surface topography on metal substrates, fabricated without any chemical modification of the surface, on the wettability, condensation, frost formation, and frost melt-water drainage characteristics are studied. Different metal surfaces (brass, copper, and aluminum) are studied because of the technical importance of these metals as working materials in a wide range of heat transfer applications. Through a systematic study of microgrooved brass surfaces, fabricated by a micro end-milling process, the effect of parallel, periodic microgroove geometry on the wettability and droplet mobility is examined experimentally and compared to that of flat surfaces. The substrates have groove depths in the range of 26 to 122 microm, groove widths of 27 to 187 microm, and are about 45 mm x 45 mm in size, with a thickness about 3 mm. Wetting anisotropy, contact angle hysteresis, and drainage behavior of water droplets on the microgrooved surfaces are found to be significantly affected by the variation in groove geometry parameters. Deposited water droplets of a wide range of volumes slide much more readily on microgrooved surfaces than on the flat baseline surfaces, and a significant reduction in the critical sliding angle is obtained for the microgrooved samples. The sliding angles exhibit a significant groove geometry dependence and are found to increase with pillar width and decrease with groove depth. Frost/defrost/refrost experiments are conducted inside a chamber maintaining a controlled environment, under a wide range operating conditions and over multiple frost/refrost cycles. The size, shape, and growth patterns of the condensed water droplets are found to vary considerably between the microgrooved and flat brass surfaces. The groove geometry not only affects the condensation and initial stages of frost formation, rather the effects are profound in long frost cycles as well. The presence of microgrooves is found to alter the frost properties in the frost/refrost cycles and in general, increase the frost thickness and decrease the frost density compared to those on the flat surface. Variations of frost properties with microgroove topography are found to be repeatable and periodic in behavior after the 3rd frost cycle. Microgrooved surfaces manifest a significant improvement in frost melt-water drainage and a reduction in the frost melt water retention of up to 70% over that on the flat baseline surfaces is achieved. These samples consistently exhibit lower frost melt water retention compared to flat surfaces for a broad range operating conditions. Drainage of the frost melt is influenced strongly by the groove geometry and is promoted by an increase in the pillar width, but drainage is relatively insensitive to changes in the groove depth. The relationship between frost structure, frost properties, and frost melt water drainage with groove dimensions is discussed, emphasizing the importance of the morphological features. The consistent improvements in condensate and frost melt water drainage from the microgrooved samples as compared to that on the flat baseline in a wide range operating conditions is very encouraging for reducing the condensate retention and frosting penalty in practical applications. The findings of this study can be used in designing microgrooved metal surfaces with desired wetting and liquid drainage properties for air-conditioning, refrigeration and heat pumping applications. The work might be useful in a broad range of applications where water retention, frosting and defrosting play important roles.