## 2 Experimental Setup and Measurement  
### Testing Setup Description  
The experiment was carried out in an environmental chamber, which could maintain a relatively stable environment and meet the experimental test conditions. Air conditioning systems and heaters are used to maintain the environment. The temperature deviation of the environmental chamber was maintained at \(\pm\)0.1 \({}^{\circ}\)C, and the RH deviation was maintained at \(\pm\)3% RH.  
Figure 1 illustrates a schematic diagram of the experimental setup, which contains an air duct, a thermostatic water tank, a test sample, and data acquisition systems. The test sample was installed in the middle of the air duct. The experiment used a fan to drive moist air into the air duct system, and the speed of the fan was controlled by a transducer. A CCD camera mounted on one side of the duct was used to observe the frosting. Straighteners were installed on both sides of the sample to reduce eddy currents for uniform air distribution. A heater was installed at the air duct entrance to provide heat for MCHX defrosting. The temperature and RH sensors were inserted into the duct to measure the air temperature and RH at the inlet and outlet of the test sample. The air velocity was measured by a hot-wire anemometer. The pressure difference between the inlet and outlet of the test sample was measured by a differential pressure sensor. The pressure sensor has a measurement range of 0-200 Pa and an accuracy of 0.1% of full scale. In the refrigerator thermostat, 40% ethylene glycol aqueous solution was used as the coolant, and the lowest temperature could reach \(-\)40 \({}^{\circ}\)C without phase change. The T-type thermocouple with an accuracy of \(\pm\)0.5 \({}^{\circ}\)C was placed on the surface of the inlet and outlet tubes to measure the temperature of the fluid entering and exiting the MCHX.  
The two types of MCHXs used in this experiment are shown in Figure 2. In one MCHX, the fins on the windward side were lengthened; that is, the fins extended 4 mm beyond the flat tube, while the leeward fins were flush with the flat tube. In the other MCHX, the windward fins were flush with the flat tube, while the leeward fins were extended. This fin shape is a serpentine louver fin. The flat tubes were arranged vertically, and the fins were arranged between the flat tubes. This installation was more conducive to the downward flow of melted frost. The geometric parameters of the MCHXs are described in Table 1.  
\begin{table}
\begin{tabular}{l l l} \hline
**Component** & **Specifications** & \\ \hline \multirow{6}{*}{MCHX} & Vertical height (H): 437 mm & Horizontal length (L): 408 mm \\  & Fin height (S): 8.1 mm & Fin thickness: 0.1 mm \\  & Flat tube width (W): 20.6 mm & Louver angle (\(\theta\)): 40\({}^{\circ}\) \\  & Lower pitch (\(L_{p}\)):1.8 mm & Flat tube thickness: 1.3 mm \\  & Louver number: 10 & Fin pitch (\(F_{P}\)): 1.7 mm \\  & Number of flat tubes: 34 & Extension length (P): 4 mm \\  & Fin depth (\(F_{d}\)): 25.4 mm & Extension length (P): 4 mm \\ \hline \end{tabular}
\end{table}
Table 1: The MCHX structure parameters for two samples.  
Figure 1: Schematic diagram of the experiment setup.  
Figure 2: Experimental lower fin MCHX.  
Table 2 shows the test conditions. The experiments were performed for different inlet refrigerant temperatures (\(-7\)\({}^{\circ}\)C and \(-10\)\({}^{\circ}\)C), RH (80% and 90%), and the inlet air temperature fixed at 2 \({}^{\circ}\)C. In addition, the inlet air velocity was 0.6 m/s, 1.0 m/s, and 1.5 m/s, respectively. After MCHX worked in frosting condition for one hour, the thermostatic water tank was turned off, the heater was turned on, and the fan speed was set to the minimum for defrosting operation for 10 min. The specifications of the test instruments are shown in Table 3.