This study sets up an experimental platform to test the performance of a CO2 air cooler. The direct expansion system and the pump-driven system were examined by changing parameters such as evaporating temperature

storage temperature

circulation ratio

and face velocity. Experiments indicate that in the direct expansion system

the heat transfer coefficient and refrigerating capacity exhibit a decreasing trend as the evaporating temperature decreases. The heat transfer coefficient dropped from 20.2 W/(m2?K) to 16.6 W/(m2?K)

and the refrigerating capacity dropped from 7.5 kW to 6 kW as the evaporating temperature dropped from ?22 ℃ to ?47 ℃ during the experiment. A peak of the heat transfer coefficient exists for different circulation ratios in the pump-driven system. (The best circulation ratio is 3 in this paper.) For example

when the storage temperature was at ?20 ℃

the heat transfer coefficient increased by about 13.2% as the circulation ratio increased from 1 to 3. However

the heat transfer coefficient began to decrease as the circulation ratio continued to increase. The heat transfer coefficient dropped by about 2% when the circulation ratio increased to 5. When the face velocity was changed from 2.2 m/s to 2.5 m/s

the heat transfer coefficient increased by just 2.12%. However

it increased by 11.4% when the face velocity was changed from 2.5 m/s to 3.2 m/s. Next

the increase rate of the heat transfer coefficient became slow as the face velocity increased from 3.2 m/s to 3.25 m/s

which is only 0.88% in this case.