The two-phase distribution uniformity of a refrigerant is an important factor affecting the heat exchanger performance. To address the poor uniformity of the liquid phase in the vertical header of the microchannel heat exchanger, a design scheme of a two-stage distribution header is proposed. The gas and liquid flow ratios of each branch under different dryness and total mass flow rates are compared by two-phase refrigerant distribution testing. The top branch of the empty header does not have a sufficient amount of liquid, while the bottom has excess liquid. The relative standard deviation (RSD) of the liquid phase for each branch is larger than 95%. The multiflow header uses the bottom inlet cavity to distribute the liquid, and then the liquid flows upward in the multiflow channel and enters the annular cavity through the side outlet hole. The liquid RSD of each branch is larger than 60%. When the core flow-regulated cavity and top distribution cavity are added into the multiflow header, the distribution performance is satisfactory, with RSDs of liquid and gas phases below 5% and 15%, respectively.

A closed-loop pulsating heat pipe within atmospheric temperature relies on the pulsation and phase change of the internal working fluid. It has been widely used for the heat dissipation of electronic components owing to its high heat transfer efficiency, simple structure, and low cost. In this study, the factors affecting the startup and heat transfer performance of pulsating heat pipes are divided into three categories, namely, structural parameters, operating conditions, and working fluid properties. First, the structural parameters, including factors such as diameter, cross-sectional shape, number of U-bends, and length ratio of each section, are summarized. Second, the influence of operating conditions, including the heating power, inclination angle, and liquid filling rate, is analyzed. Finally, the physical parameters of the working fluid are divided into flow and heat transfer properties. To improve the start-up and heat transfer performance of pulsating heat pipes, future research should focus on conducting in-depth investigations into the critical number of U-bends. The anti-gravity performance of heat pipes must be improved for adapting to small inclination angles and negative angle conditions. A comprehensive evaluation criterion describing the influence of physical parameters on performance should also be established.

Non-uniform frost accumulation on evaporator coils is a significant feature in air-cooled refrigerators. It is also one of the important causative factors leading to evaporator performance degradation. This paper reviews related research in three distinct areas: non-uniform frost characteristics of air-cooled refrigerator evaporators, defrost optimization based on these characteristics, and frost detection optimization. A concise introduction is provided into the formation mechanism of non-uniform frost on air-cooled refrigerator evaporators, the influencing factors, and their impact on refrigerator performance. Frost-heat matching defrosting and frost suppression strategies for the evaporators of air-cooled refrigerators are systematically summarized. The complex challenges and solutions of direct and indirect frost detection techniques under non-uniform frost conditions are analyzed. The performance degradation of air-cooled refrigerator evaporators is summarized and discussed. Finally, this study identifies shortcomings in the research field of non-uniform frost on aircooled refrigerator evaporators and proposes potential future development directions. This study aims to provide a framework for the efficient operation and energy-saving optimization of air-cooled refrigerators.