Simulation And Experimental Study On The Performance Of Air Cycle Refrigeration System

In the air-cycle refrigeration system, air is used as refrigerant, which is the naturalrefrigerant, and has none pollution to the environment, but the coefficient of performance ofair cycle refrigeration system is relative lower compared with that of the compressionrefrigeration system. So, for the air cooling system, it is important to improve its coolingperformance. And more importantly, because of the water vapor in the compressed air iscondensed or freezes in the turbo-expander, which is the great harm to the safety of thesystem. Therefore, it is of great theoretical and practical importance to study on theproblems of improving the efficiency of the air cooling system and reducing the system airhumidity ratio.In this paper, in order to reduce the air humidity ratio at the turbo-expander inleteffectively, a high-efficiency water separator was added in the original system to separatethe condensate water and to drain off the free water from the system. The low temperatureelectric heater was also improved to enhance the performance of the system. A series ofsimulations and experiments for the air cycle refrigeration system were performed underdifferent working conditions on the rebuilt experimental set-up, including three kinds ofregenerative cycle systems. The experimental results were analyzed and compared with thesimulation results. The following conclusion can be obtained from the simulation andexperimental results:(1) The simulation and experimental results show that the expansion ratio ofturbo-expander increases with the increase in the compressor inlet pressure, so thetemperature at the turbine outlet is reduced, the COP and refrigerating capacity is improved.The simulation results indicate that in the two stage recuperator system, when thecompressor inlet pressure rises from170kPa to200kPa, the increment of system COP isabout33%. The increase of experimental value under the same conditions is about30%.The rise of the inlet pressure of the centrifugal compressor almost have no effect on the airhumidity ratio of the turbo-expander inlet, however, the amount of the vapor condensed inthe turbo-expander is increased with the inlet pressure, and the experimental results showthat the rate of increment is highest in the none recuperator system.(2) The simulation and experimental results show that as the recuperator and thecorresponding water separator are added in the system, the efficiency of the refrigerationsystem and dehumidification efficiency are improved significantly. The simulation resultsindicate that compared with the none recuperator system, the humidity ratio of theturbo-expander inlet drop by44%in the two stage recuperator system. The decline of theexperimental humidity ratio under the same conditions is about40%. Simultaneously, the recuperator can also improve the refrigerating capacity and COP of the system. Theexperimental results show that when the compressor inlet pressure is200kPa, comparedwith none recuperator process, the COP of the two stage recuperator system increases by40%. The increment of simulation value under the same conditions is about48%.(3) The simulation and experimental results show that the COP and refrigeratingcapacity of the system increase with the rise of the refrigeration temperature. Theexperimental results show that the COP of the two stage recuperator system increases from0.23to0.38as the refrigeration temperature rises from-20℃to-10℃under the conditionof the compressor inlet pressure200kPa, the increment of COP is about65%. But theincrement of simulation COP under the same conditions is only about36%. The simulationresults show that with the rise of the system refrigeration temperature, both the air humidityratio of the turbo-expander inlet and the amount of vapor condensed in the turbo expanderincrease. The air humidity ratio of the turbo-expander inlet increases from0.927g/kg to1.24g/kg as the refrigeration temperature rises from-25℃to-5℃under the condition ofthe compressor inlet pressure200kPa, the increment of the air humidity ratio of theturbo-expander inlet is about34%.(4) The experimental results show that the expansion ratio of turbo-expander increasewith the increase in the compressor pressure ratio, and the rate of increment become morequickly. The equivalent flow of the turbo-expander increases with the increase in theexpansion ratio of the turbo-expander. And when the expansion ratio of the turbo-expanderis about3, the equivalent flow achieves the maximum value.