Performance Study Of The CO₂ Air-cooler

CO2, as a naturally exist substance and an environmentally friendly refrigeration, contributes little to the greenhouse effect and the ozone depletion. CO2 have broad prospects as refrigerant since it can be used in refrigeration status above-50℃ and has relatively large latent heat of vaporization due to its unique physical property. Air-cooler(cooling fans), as one of the most important parts and the major energy consuming equipment in the refrigeration storage system, has a direct impact on the system cooling effects and energy consumption relative to its heat transfer performance. Therefore, there is great significance in studying CO2 air-cooler.With the development of computer science, simulation method has been used in optimization design and performance prediction of air-cooler. In this paper, a simulation model is built with steady state distributed parameter method. A model machine is studied on the air-cooler performance test bed. The heat exchange performance of the machine is analyzed with both the result of the experiment and simulation. The main contents and conclusions are as follows:1. Comparing the simulation result with the experiment result, the average error of the total heat transfer coefficient of the air cooler is approximately 16% when the refrigerant is NH3, and 8% when the refrigerant is CO2. Proving the correctness and accuracy of the simulation model built in this paper, which can be used in heat transfer performance prediction and optimization design of the air cooler.2.The air cooler performance test result shows that at temperature range of 0℃~-25℃ inside the calibrating tank, the refrigerating capacity turns out to be 35% higher when the refrigerant is CO2 rather than NH3.3. Comparing the simulation result with the experiment result under the condition of multiple liquid feed and the temperature range of the calibrating tank within 0℃~-25℃, when refrigerant is NH3 and CO2. The result shows: 1) the total heat transfer coefficient of the air-cooler increases with the increase of the calibrating tank temperature under the temperature range of the calibrating tank within 0℃~-25℃; 2) with the increase of the circulating ratio, the heat transfer coefficient increases before it decreases, and reaches maximum when the circulating ratio is about 3.4. Simulation calculation of the CO2 flow boiling heat transfer and pressure drop inside the tube is carried. The variation tendency of the CO2 side heat transfer coefficient with the dryness is analyzed. The CO2 flow heat transfer coefficient shows irregular change with the increase of the dryness, and is mainly associated with the flow pattern of CO2. The pressure drop gradient increases before it decreases with the increase of the dryness, and reaches maximum when CO2 dries out.5. Numerical simulations are carried by changing the refrigerant side CO2 working condition and by changing the structure size of air-cooler. The effects of various parameters on the performance of air-cooler are calculated and analyzed, providing a reference and suggestion for optimization design of air cooler.