## 2 Experimental Equipment and Data Processing  
### Frosting Visualization Test Rig with Adjustable Pressure  
The details of the frosting visualization test rig are shown in Figure 1. The test rig is composed of four parts: the humid air conditioning system, the frosting visualization cavity, the frosting image acquisition system, and the cascade refrigeration system. The target temperature of air, partial pressure of water vapor, and overall pressure in the frosting visualization cavity are mainly provided by the humid air conditioning system. The humid air conditioning system is mainly made up of compressors, freeze dryers, electric proportional valves, gate valves, buffer tanks, vacuum pumps and chillers. The goal of this research is to figure out the effect of working pressure around cooling surface on frosting. Therefore, the internal pressure of the visualized cavity needs to be adjusted steadily from low to high. With regard to high pressure, for maintaining the internal pressure of the cavity higher than atmospheric pressure, the compressor in this experiment compresses the ambient air to 0.45 MPa. Then the high pressure air is supplied into the buffer tank I to stabilize the air pressure. Two streams are directed from buffer tank I. One stream is directed into the freeze dryer and molecular sieve to remove water vapor and tiny particles in the humid air to obtain dry air. The other stream of humid air is directed into buffer tank II. By controlling the different mixing proportions of humid air and dry air, the partial pressure of water vapor can be adjusted. As for low pressure conditions, the pressure in the cavity is maintained by a vacuum pump. Buffer tank III is arranged between the vacuum pump and the visualized cavity to expand the pressure range. The temperature of air is adjusted by heat transfer between the chiller and the air at the outlet of buffer tank II.  
The frost formation is on the cooling surface inside the frosting visualization cavity, as shown in Figure 2. The test section consists of refrigerant flow, cooling surface of copper, thermocouples, and the heat insulator. The stable low temperature required for frosting on the cooling surface is provided by the injection of refrigerant flow from the cascade refrigeration system to the cooling surface. The cooling surface is a circular plane with diameter of 35 mm. In order to maintain a uniform temperature distribution, a copper material with good thermal conductivity is selected as the test surface. With the aim of reducing heat loss, a thermal insulating material is wrapped around the outside of the cooling surface. On the basis of these conditions, the temperature of the cooling surface is measured by four thermocouples that are distributed around the circumferential side of the cooling surface. Moreover, the frost crystal morphologies and frost layer characteristics on the cooling surface are obtained through the image acquisition system. In addition, the CCD camera (GaoPin GP-660V) captures frosting in the direction perpendicular to normal to the cooling surface. The air temperature inside the cavity is measured by thermocouple hanging at the entrance of the cavity.  
Figure 1: (**a**) The flowchart and (**b**) test rig of the visualized pressure adjustable frosting test, which consists of 1. compressor, 2. freeze drying equipment, 3. electropneumatic proportional valve, 4. gate valve, 5. gate valve, 6. electropneumatic proportional valve, 7. buffer tank I, 8. pressure transducer, 9. buffer tank II, 10. heat exchanger, 11. temperature sensor, 12. chiller, 13. flowmeter, 14. visualized cavity, 15. hygrometer, 16. vacuum pump, 17. atmospheric environment, 18. humid air conditioning system, and 19. data collecting system.