## 3 Results and Discussion
### Heat Transfer Analysis  
The heat transfer rate and overall heat transfer capacity of the two MCHXs are shown in Figures 5 and 6, respectively. As expected, the higher RH caused a rapid decrease in the heat transfer rate. This is because with the increase of RH, the moisture content of the inlet air increases, thus improving the mass transfer potential. With the increase of mass transfer potential, the frosting rate is accelerated, and the heat transfer rate is rapidly reduced. On the whole, RH had a significant effect on the heat transfer rate. The comparison between Figure 5a and Figure 5b shows that when the inlet coolant temperature was \(-10\)\({}^{\circ}\)C, at RH = 90%, the heat transfer rate dropped rapidly after 10 min, while at RH = 80%, the heat transfer rate dropped slowly after 20 min. In addition, the comparison between Figure 5c and Figure 5d shows that when the inlet coolant temperature was \(-7\)\({}^{\circ}\)C, at RH = 90%, the heat transfer rate dropped rapidly after 15 min, while at RH = 80%, the heat transfer rate dropped slowly after 25 min. From the above analysis, it can be concluded that when RH increases, the growth of the frost layer is significantly promoted, thus worsening the heat transfer rate. Furthermore, the coolant inlet temperature has a significant effect on the heat transfer rate. By comparing Figure 5a and Figure 5c, Figure 5b and Figure 5d, it can be concluded that at lower inlet coolant temperature, the heat transfer rate of the heat exchanger declines earlier, and the decline trend is more obvious. This is because the temperature difference between the fin surface and the air increases, leading to the rapid growth of the frost layer, which deteriorates the heat transfer of the heat exchanger. Figure 4: Schematic diagram of frost layer distribution for two heat exchangers at different frosting stages. As expected, the heat transfer capacity of MCHX-A is greater than that of MCHX-B. As discussed in Section 3.1, the extended fins delay frost blocking and maintain a better heat transfer rate throughout the frost cycle. The overall heat transfer capacity of MCHX-A was 9.6-49.7% higher than that of MCHX-B under different frosting conditions. Moreover, the heat transfer capacity of MCHX-A at lower air velocity (0.6 m/s) increased the most compared with that of MCHX-B at four frosting conditions, as shown in Figure 6. With the increase of u\({}_{\text{a}}\), the proportion of improvement gradually decreased. This is because when u\({}_{\text{a}}\) decreased, the air momentum also decreased, and the frost growth rate on the rear side slowed, while the frost growth on the front side had an insignificant influence, resulting in a more uneven distribution of the frost layer [30]. The MCHX with fins extended on the windward side can effectively improve the frosting uniformity, and the worse the frosting uniformity, the greater the potential for improvement. With the increase of u\({}_{\text{a}}\), frost uniformity increased, and the potential of MCHX-A to improve heat transfer performance decreased. Moreover, when the inlet coolant temperature and RH changed to promote frost growth, the growth of the frost on the front side would be more active, resulting in a larger decrease in air humidity passing through the rear side; the driving force of the frost growth on the rear side was reduced. Therefore, the distribution uniformity of the frost layer was worse. As shown in Figure 6b, the more serious the frost condition, the more uneven the  
Figure 5: The variation of heat transfer rate with time for different MCHXs under frosting conditions [(**a**) T\({}_{\text{c,in}}=-10\)\({}^{\circ}\)C, RH = 80%, (**b**) T\({}_{\text{c,in}}=-10\)\({}^{\circ}\)C, RH = 90%, (**c**) T\({}_{\text{c,in}}=-7\)\({}^{\circ}\)C, RH = 80%, (**d**) T\({}_{\text{c,in}}=-7\)\({}^{\circ}\)C, RH = 90%]. first layer distribution. Therefore, MCHX-A has the greatest potential to improve thermal performance, and the overall thermal performance is improved by more than 27%.