## 3 Results and Discussion
### Analysis of Frost Growth Rate  
The frost accumulation rate and the total amount of frost of the two MCHXs are shown in Figures 7 and 8, respectively. It can be noticed from Figure 7 that the RH had a significant impact on the frost rate. This is because a higher RH will carry more moisture, increasing the mass transfer coefficient and promoting the accumulation of frost. Comparing Figure 7a and Figure 7b, for all inlet air velocity (0.6 m/s, 1 m/s, 1.5 m/s), the mass of frost at RH = 90%is 33.0-50.2% greater than that of the RH = 80% at 10 min. Similarly, compared with Figure 7c and Figure 7d, at RH = 90%, an increase in the mass of frost of 18.8-61.2% was observed as compared to RH = 80% at 10 min. However, in the case of a larger early frost rate, the airflow passage will be blocked faster, and the frost accumulation rate will enter into a slow growth earlier. Furthermore, the inlet temperature of the coolant also had a significant influence on the frost accumulation rate. For all inlet air velocity (0.6 m/s, 1 m/s, 1.5 m/s) and the air RH (80% and 90%), the frost accumulation at T\({}_{\rm c,in}\) = \(-\)10 \({}^{\circ}\)C was 7.0-19.1% higher than that of the T\({}_{\rm c,in}\) = \(-\)7 \({}^{\circ}\)C at 10 min. Besides, with the increase of u\({}_{\rm a}\), the amount of frost on the fin surface will increase significantly. Irrespective of the inlet coolant temperature and RH, the impact of wind speed on the mass of frost was more significant. For all the inlet coolant temperatures (\(-\)7 \({}^{\circ}\)C and \(-\)10 \({}^{\circ}\)C) and the inlet relative humidity (RH = 80% and RH = 90%), the mass of frost at u\({}_{\rm a}\) = 1.0 m/s was 21.9-64.7% higher than that of the u\({}_{\rm a}\) = 0.6 m/s. Similarly, at u\({}_{\rm a}\) = 1.5 m/s, an increase in mass of frost of 74.8-94.2% and 17.9-46.7% was observed as compared to u\({}_{\rm a}\) = 0.6 m/s and u\({}_{\rm a}\) = 1.5 m/s, respectively. As discussed earlier, the increase in air velocity can make the frost distribution more uniform, so more frost can be attached to the fin surface. On the other hand, the increase of u\({}_{\rm a}\) can make the frost density larger, thus increasing the mass of the frost.  
Figure 7: The frost accumulation variation with time for different MCHXs under frosting conditions [(**a**) T\({}_{\rm c,in}\) = \(-\)10 \({}^{\circ}\)C, RH = 80%, **(b**) T\({}_{\rm c,in}\) = \(-\)10 \({}^{\circ}\)C, RH = 90%, **(c)** T\({}_{\rm c,in}\) = \(-\)7 \({}^{\circ}\)C, RH = 80%, **(d)** T\({}_{\rm c,in}\) = \(-\)7 \({}^{\circ}\)C, RH = 90%].  
It can be seen in Figure 8 that the frost accumulation of MCHX-A was always larger than that of MCHX-B under different frosting conditions. This is mainly influenced by: (1) The extended fins on the windward side catch a lot of frost. (2) The fins extending on the windward side make the heat exchanger frosting more uniform. The overall frost accumulation of MCHX-A is 10.3-46.9% higher than that of MCHX-B under different frosting conditions. In general, the frost accumulation of MCHX-A at lower air velocity (0.6 m/s) increased the most compared with that of MCHXH-B at four frosting conditions. With the increase of u\({}_{\text{a}}\), the proportion of improvement gradually decreased. This trend was similar to that of the heat transfer capacity. This is because, with the increase of u\({}_{\text{a}}\), the frost uniformity increased, so the extended fins on the windward side promoted the frost uniformity potential to be relatively reduced. In addition, under the condition that the RH and inlet coolant temperature changed to promote frost growth, compared with MCHX-B, the MCHX-A had a greater increase in frosting mass. In general, due to the higher heat transfer capacity of MCHX-A, the accumulation of frost on MCHX-A was bound to be greater, but the increased frost was more evenly distributed on the fin, so its frosting performance was better.