1    Introduction

Due to energy shortage and environmental pollution, heat pump units are widely used in building heating, industrial heating, mate-rial drying, ventilation, and dehumidi?cation because of their ef?- cient,  environment-friendly,  and  safe  features  [].  In  2021, China’s  National  Energy  Administration  proposed  to  further reduce the proportion of coal consumption and vigorously pro- mote the use of heat pump technology for cooling and heating in the hot summer and cold winter areas [].

When the climate is cold in winter and the heat pump unit is heating, frost will form on the ?nned surface of the outdoor ?nned coil evaporator when the temperature is below 0 O C and below the dew-point temperature of the outdoor air []. The frost will have an obvious impact on the operation of the heat pump unit [], including: ‹ blocking the channels between the ?ns, increasing the air?ow resistance and making the energy consumption of the fan  increase;  ›  increasing the thermal  resistance  of the  heat exchanger, making the heat transmission performance  of heat exchanger decline, and reducing heat transfer capacity; ? reduc- ing evaporating temperature, reducing energy ef?ciency ratio, and deteriorating  the  operating  performance  of  heat  pump  units. Therefore, the frost problem has become a big obstacle to the pop- ularization and application of heat pump units, and the defrosting method is usually adopted to solve the problem in the operation of heat pump units []. However, there are still some problems in the operation of heat pump units, such as short defrosting cycles, heat supply interruption caused by frequent defrosting, and reduction of indoor comfort [].

Delaying  the  frost  formation  can  effectively  prolong  the defrosting cycle and improve the heating performance of the heat pump units. Research on delaying frost can be broadly divided into  three  aspects  []:  ‹  change  the  outdoor  environmental parameters []. Changing the temperature, humidity, airspeed and other  natural  factors  of the  outdoor  environment  needs  high requirements for equipment, space, and other conditions, which is dif?cult to ?t in a large area; › change the ?nned characteristics [–], although in the early stage this way has a good effect of frost  suppression,  its  ability  of  frost  suppression  gradually decreases with the increase of time and the thickness of frost layer, and there are certain limitations because the process of material preparation is complicated and it is dif?cult to mass- produce;  ?  use  the  external  force  [],  for  example,  the application of external magnetic ?eld, electric ?eld, ultrasonic, mechanical vibration, and other methods to delay the formation of frost, but the principle of its effect on frost is not clear at present, pending further research.

Delaying frosting is dif?cult to achieve, so increasing ?n spac- ing to prolong the time for the frost layer to block the air channels, prolong the defrosting cycle, reduce the defrosting frequency, and improve heat pump heating or cooling capacity is an effective measure to take. People usually take different ?n spacing of the air cooler according to the different temperatures of the cold stor- age: the ?n spacing of 8mm is selected for the cold storage tem- perature from —25 O C to — 16 O C, 12mm is selected from —25 O C to —35 O C, and the ?n spacing can even reach 16mm in the freez- ing room with high humidity. The increase in the ?n spacing of the air cooler has a signi?cant effect on prolonging the defrosting cycle of the cold storage, but it will also change the degree of strengthening of the heat transfer area of the  ?nned coils  air cooler, and the heat transfer performance of the whole air cooler will also be changed.

In 2018, Zhang et al. [] found that the use of a larger ?n pitch (3.2mm) is advantageous in ensuring a relatively steady heating1Corresponding author.

Contributed  by  the  Heat  Transfer  Division  of ASME  for  publication  in  the JOURNAL    OF    HEAT    TRANSFER .  Manuscript  received  September  22,  2021;  ?nal manuscript received April 30, 2022; published online June 7, 2022. Assoc. Editor: Milind A. Jog.

output, as a frosted ?nned tube heat exchanger with a smaller ?n pitch (2mm) would experience a quicker drop in its heat transfer rate when its windward side was blocked by frost. In 2020, Liu et al. [] found through numerical simulation that when the ?n spacing is small, the heat transfer capacity and pressure drop of the heat exchanger are in?uenced by the spacing; as the ?n spac- ing increases, the in?uence of the spacing on the heat transfer capacity and pressure drop decreases gradually, but too  small (1mm) or too large (9mm) ?n spacing will limit the comprehen- sive performance. In the  actual  operation process  of the heat pump, frost