## 1 Introduction  
Under the background of carbon peaking and carbon neutrality, the application of microchannel heat exchangers (MCHXs) in refrigeration and heat pump systems has become a research hotspot. Compared with the conventional fin-and-tube heat exchanger (FTHX), the MCHX has the advantages of higher heat transfer efficiency [1, 2], less refrigerant charging [3], and lower cost [4, 5]. However, when the MCHX works in the winter, the frost will form and accumulate faster on the fin surface [6, 7], which will lead to a decrease in airflow rate and an increase in the thermal resistance, thus reducing the heating capacities of the heat pump units [8, 9, 10]. This is because the louver fins have many edges and corners, which can serve as frost nucleation points in the frosting process [11, 12]. In addition, the smaller louver fin pitch facilitates the formation of ice bridges, thus potentially promoting the frost layer growth rate [13]. The frosting characteristics of MCHXs have been studied extensively. Some studies [14] have explored the effects of surface temperature and air environmental conditions on the frost accumulation rate of MCHXs. They claimed that surface temperature and relative humidity (RH) were the main influencing parameters. The effect of fouling on the frost growth of MCHX was reported by [PERSON] et al. [15]. Results show that the light fouling (20 g) increased the heat transfer rate and the frost accumulation rate, while heavier fouling showed the opposite conclusion. In addition, some studies have shown that the poor drainage performance of the MCHX causes more residual water on the fin surface, resulting in the rapid decline of the heat exchange performance during re-frosting [16]. Moreover,the poor drainage performance of the MCHX will prolong the defrosting time, making its defrosting efficiency 22.5% lower than that of the FTHX [17]. To better inhibit frost growth and promote the removal of residual water, many studies have investigated the structural characteristics of the MCHXs. [PERSON] et al. [18] found that MCHXs with a larger louver angle (39\({}^{\circ}\)) had a more desirable behavior after several frosting periods in terms of the thermal and drainage performance. The influence of flat tube arrangement direction on the periodic frosting and defrosting cycle performance of MCHX was investigated experimentally by [PERSON] et al. [19]. Compared with the horizontal orientation, the vertical orientation had a better drainage performance and a longer frosting period. [PERSON] et al. [20] found that using a parallel fin instead of the louvered fin in MCHXs could enhance its drainage performance and extend its frosting cycle. [PERSON] et al. [21] proposed an asymmetric louver fin, which enhanced the drainage performance and the thermal performance of the MCHXs heat pump system under periodic frosting conditions. [PERSON] et al. [22] presented a modified MCHX and compared it with the conventional wavy fin MCHX. The study showed that the modified wavy fin MCHX with drainage function had the best performance under wet and frosted conditions. [PERSON] et al. [23] proposed a new louver fin with an unequal louver pitch. Results illustrated that the frost layer distribution was more uniform, and the average heat transfer coefficient of MCHX was increased by 21% when the louver pitch successively decreased by 20%. [PERSON] et al. [24] showed that the distribution of refrigerant directly affected the uniformity of frost distribution. The flat tubes of the MCHX were vertically arranged, and there was no partition in the header. The heat pump system would show higher performance and stability. [PERSON] et al. [25] compared the heating capacity of the novel plain-louvered fin MCHX and the conventional MCHX under frosting conditions. The heating time and maximum heating capacity of the new plain louver fin MCHX were 102.7% and 14.0% higher than that of the corrugated louver fin MCHX, respectively.