Research On The Influence Of Additives On CO₂ Transcritical Cycle System Performance

In this paper,based on CO₂ transcritical cycle system with throttle valve,in order to improve the efficiency of the system and save the cost of operation,a concept adding additives into CO₂ system is put forward.CO₂ system has low efficiency,especially under low evaporation temperature conditions.In order to improve the system performance,alternative optimization methods of refrigeration system are presented in this paper.Six types of CO₂ refrigeration cycle are investigated based on thermodynamic analysis.The main differences among these cycles are the arrangement of throttling devices,expanders or economizer.The influence of two-stage compression,expanders and economizer on system performance was analyzed.Comparisons among these cycles are made from COP,discharge temperature of compressor and thermodynamic perfectibility perspective.This paper took four matters?N₂,Ar,R32 and DME?as additives.The influence of additives on thermo-physical properties of CO₂ and the performance of CO₂ transcritical cycle system is compared.The additives include two non-condensable gases?N2 and Ar?and the other two matters?DME and R32?that their molecular weights are similar to CO₂.The action that additives are added result in the more or less change of the thermo-physical parameters of CO₂.By theoretical calculation and analysis,the changes of the thermo-physical parameters reduce the refrigerating capacity of evaporator,specific power and discharge temperature of compressor,and improve refrigeration efficiency using N2 as additives.The more the addition quantity of non-condensable gases is used in the system,the more the system refrigeration performance increases.The refrigerating capacity of evaporator,specific power and discharge temperature of compressor using R32 as additives are increase,and refrigeration efficiency of cycle also is improved.Evaporator is an important component of the CO₂ transcritical refrigeration cycle,which heat transfer performance directly impacts performances and compactedness of system.By mechanism analysis non-condensable gases can accelerate nucleation boiling as nucleation site.The lower nucleation barrier reduces superheat degree.Through the theoretical calculation,lower nucleation temperature can increase the heat transfer coefficient and refrigerating capacity.The evaporation pressure is lower,the pressure drop is higher.Through establishing CO₂ transcritical cycle with throttling valve/expander experiment device that can be switched,the tests of CO₂ with and without additives are carried out.System parameters with different types of additives in different concentration are obtained.The results show low dose additives?N2 and Ar?can improve evaporator heat transfer performance under the low evaporating temperature,and enhance performances with increasing in concentration of solution.But in high concentration?4%?and change greatly of physical parameters,the evaporator performance degradated.Moreover,compressor power increases with the presence of N2 and Ar.When the evaporation temperature is less than 0? and the concentration is less than or equal to 3%,its refrigerating performance is superior to pure CO₂.When the evaporation temperature is 0?,COP reduces.When the concentration is 1% and 2%,the effect on the thermal properties is small.But the increases of discharge pressure can improve heating capacity and performance of cooler due to the increase of flow velocity.When R32 and DME are considered,the effects are caused by change of CO₂ physical parameter.Aside from the higher evaporating temperature,Evaporator heat transfer performance coefficient increases and compressor power reduces with the presence of R32 and DME.