Icing is a complex phase change process that is widespread in nature and industry and may have a number of negative effects. During the freezing of water into ice, air bubbles are often trapped in ice and affect the physical properties of the ice. To control the icing process, it is necessary to study these air bubbles in ice. Here, an experimental setup is built to study the growth and distribution characteristics of trapped air bubbles. The results show that the critical freezing rates for the transitions from the egg-shaped bubble region to the egg-/needle-shaped bubble region and from the egg-/needle-shaped bubble region to needle-shaped region are 22.45 ± 3.24 and 12.64 ± 1.65 μm/s, respectively. A mathematical model that can predict bubble growth is obtained by coupling the gas diffusion equation, Henry's law, and the Young–Laplace equation. The model shows that both the maximum width of the bubble and the distance between adjacent bubbles mainly depend on the freezing rate and are proportional to the inverse of the second power of the freezing rate, meaning that the maximum width and the distance gradually increase as the freezing rate decreases. These results contribute to a better understanding of icing mechanisms and inform the optimization of anti-icing and deicing methods.

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.

When an air source heat pump (ASHP) operates at low ambient temperature and high relative humidity, its outdoor coil faces frosting problem. Defrosting becomes necessary and reverse cycle defrosting (RCD) has drawn extensive attention due to its specific advantages. To fully understand the research trends of defrosting topic and further improve the RCD efficiency for ASHP, reported experimental and numerical studies around the optimization of RCD were reviewed. As summarized, RCD coupled with phase change energy storage technology is the most promising one, which resolves the fundamental problem of insufficient thermal energy during defrosting. Numerical studies mainly focused on the divisions of defrosting stages, the negative effects of downward flowing melted frost on defrosting efficiency, the energy transfer process in the metal of heat exchangers during RCD, and the validation and extension of defrosting models at different stages. For the optimization of defrosting initiation and termination control strategies, more accurate and economic measurement methods of frost layer thickness and total mass are necessary. Finally, the outlooks for future studies on RCD for ASHP are given. This work contributes to guiding the optimization and design of the ASHP, and providing insight into the investigation of frosting and defrosting mechanisms.

Cooling methods for multiple heat sources with high heat flux have rarely been reported, but such situations threaten the stable operation of electronic devices. Therefore, in this paper, the use of two microchannel heat sinks is proposed, with and without grooves, labeled Type A and Type B, respectively. Experimental investigations on the flow boiling of two microchannel heat sinks connected in parallel and in series are carried out under different mass fluxes. In addition, a high-speed camera is used to observe flow patterns in the microchannels. The cold plate wall temperature (T_w), heat transfer coefficient (HTC), and pressure drop (PD) are obtained with the use of two microchannel heat sinks. The flow patterns of the bubbly flow and elongated bubbles in the microchannels are observed. The results of the analysis indicated that the T_w, HTC, and PD of the two microchannel heat sinks connected in parallel were degraded, especially when using the Type A-B parallel connection. Compared to the use of a single heat sink, the maximum decrease in HTC was 9.44 kW/(m²K) for Type A heat sinks connected in parallel, which represents a decrease of 45.95%. The influence of the series connection on the T_w, HTC, and PD of the two heat sinks is obvious. The Type A-A series connection exerted the greatest positive effect on the performance of the two heat sinks, especially in the case of the postposition heat sink. The maximum increase in HTC was 12.77 kW/(m²K) for the postposition Type A heat sink, representing an increase of 72.88%. These results could provide a reference for a two-phase flow-cooling complex for multiple heat sources with high heat flux.

The droplet impact process on a conical tip is one of the key problems in the field of fluid mechanics and surface and interface science. This study examines the impact process of water droplets on the conical tip using experimental, numerical, and theoretical approaches. The volume of fluid method and the dynamic contact angle model are used and validated by comparing the numerical and experimental results in both the present work and literature. The effects of the Weber number, contact angle, and cone angle on the droplet behavior, especially the maximum spreading factor, are investigated. The findings indicate that the maximum spreading factor becomes larger at a larger Weber number, a smaller contact angle, and a cone angle. Based on energy conservation, two theoretical models considering the film and ring profiles are proposed to describe the droplet maximum spreading factor. The film and ring models are recommended for use when the maximum spreading factor is below and above 2.4, with the relative deviation of all calculated data less than ±18%. This study enhances the understanding of droplet impacts on complex surfaces and provides valuable guidance for engineering applications.