Experimental Study on Solidification Characteristics of Sessile Urine Droplets on a Horizontal Cold Plate Surface under Natural Convection

Ultrafast Axial Freezing in a Liquid-Filled Capillary Tube

Liquid-filled capillary tubes are a kind of standard component in life science (e.g., blood vessels, interstitial pores, and plant vessels) and engineering (e.g., MEMS microchannel resonators, heat pipe wicks, and water-saturated soils). Under sufficiently low temperatures, the liquid in a capillary tube undergoes phase transition, forming an ice nucleus randomly on its inner wall. However, how an ice layer forms from the nucleus and then expands, either axially or radially to the tube inner wall, remains obscure. We demonstrated, both experimentally and theoretically, that axial freezing along the inner wall of a water-filled capillary tube occurs way ahead of radial freezing, at a nearly constant velocity 3 orders in magnitude faster than the latter. Rapid release of latent heat during axial freezing was identified as the determining factor for the short duration of recalescence, resulting in an exponential rise of the supercooling temperature from ice nucleation via axial freezing to radial freezing. The profile of the ice-water interface is strongly dependent upon the length-to-radius ratio of the capillary tube and the supercooling degree at ice nucleation. The results obtained in this study bridge the knowledge gap between the classical nucleation theory and the Stefan solution of phase transition.

Localized Characteristics of the First Three Typical Condensation Frosting Stages in the Edge Region of a Horizontal Cold Plate

Condensation frosting usually causes a negative influence on heat exchangers employed in engineering fields. As the relationships among the first three typical condensation frosting stages in the edge regions of cold plates are still unclear, an experimental study on the localized condensation frosting characteristics in the edge region of a cold plate was conducted. The edge effects on the water droplet condensation (WDC), water droplet frozen (WDF) and frost layer growth characteristics were quantitatively investigated. The results showed that the number of droplets coalescing in the edge-affected regions was around 50% greater than in the unaffected regions. At the end of the WDC stages, the area-average equivalent contact diameter and coverage area ratio of water droplets in the edge-affected regions were 2.69 times and 11.6% greater than those in the unaffected regions under natural convection, and the corresponding values were 2.24 times and 9.9% under forced convection. Compared with the unaffected regions, the WDF stage duration in the edge-affected regions decreased by 63.6% and 95.3% under natural and forced convection, respectively. Additionally, plate-type and feather-type frost crystals were, respectively, observed in natural and forced convection. The results of this study can help in the better understanding of the condensation frosting mechanism on a cold plate, which provides guidelines for optimizing the design of heat exchanger structures and system control strategies facing frosting problems.