Thermodynamic Study On Mechanism Of A Precooled JT Cryocooler Working At Liquid Helium Temperature

The JT cryocoolers(JTCs)precooled by Stirling or Stirling-type pulse tube cryocoolers have become the main cooling technology at liquid helium temperature(LHT)for space applications.However,the best relative Carnot efficiency achieved by these precooled JTCs is only 2%,which is still possible to be further improved.Most of the published researches on this kind of cryocoolers are about experimental results.The detailed theoretical studies on the intrinsic mechanism or the methods to optimize the overall performance are rare.To solve these problems is crucial for improving the efficiency of the cryocoolers.Aiming at the shortages in this research field,we carry out the following work in this study.1)The steady-condition cooling characteristics of a LHT-JTC are revealedBased on thermodynamics,the steady working states of a LHT-JTC are analysed systematically.The intrinsic mechanism for the difference between specific cooling capacity,q,and specific latent cooling capacity,qL,is revealed.This difference only exists when the working fluid is helium.The characteristics are verified with the experimental results.It is pointed out that qL is the important parameter to evaluate the performance of a LHT-JTC.The value of qL is affected by precooling temperature and high pressure of the JTC.2)The overloaded condition and its destabilization mechanism are discoveredWith thermodynamic analysis,the overloaded destabilization,which is caused by the heat load exceeding the cooling capacity of a JTC,is discovered.The temperature behaviors under typical overloaded conditions are illustrated theoretically and verified with experiment.The phenomenon that mass flow changes under the overloaded condition is explained quantitatively with the theory of two-phase choking.The theoretical calculation results match the experimental data well.3)A method to optimize the COP of the complete cryocooler considering the real performance of the compressor and the precooler is developedWith fitting the processed measurement results,the models of suction volume flow and exergetic efficiency of a scroll-type compressor without oil lubrication are obtained.They are applied to complete the steady-state thermodynamic model of a precooled JTC together with the empirical performance correlation of Stirling-type cryocoolers.Based on this model,a method to optimize the high pressure and precooling temperatures for optimal COP of the complete cryocooler is proposed.