| With the vigorous development of the quantum information technology and deep space exploration,there is an increasing demand for the cryocoolers which can work effectively in the extremely-low temperature range around 2 K and below.Among all kinds of cryocoolers,the multi-stage precooling JT hybrid cooling cycle with helium as the working fluid is an effective approach to realize the temperatures around 2 K and below,especially the hybrid cryocooler composed of a regenerative multi-stage highfrequency pulse tube cryocooler and a recuperative Joule-Thomson(JT)cryocooler,which adopts Oxford-type linear compressor with high reliability and long lifetime as the driving sources at the warm end while has compact structure and no moving parts at the cold end.These features endow this type of hybrid cryocooler with the prominent advantages such as high reliability,long lifetime,small size,light weight and high refrigeration efficiency.Therefore,it has become one of the most stable and reliable cryogenic technology with broad space application prospects among the multi-stage precooling JT hybrid cooling cycle systems.A three-stage pulse tube precooling JT hybrid cryocooler developed by the Northrop Grumman Aerospace Systems was successfully loaded on the launched James Webb Space Telescope in 2021 to work for the astronomical exploration mission,which further verified this kind of hybrid cryogenic cycle technology in space.This paper carries out a systematic theoretical and experimental study on the multi-stage precooling JT hybrid cryogenic technology at temperatures around 2 K and below,and the main investigations and results are summarized as follows:(1)An enthalpy flow model of the multi-stage precooling JT hybrid cooling cycle is established to clarify the operation mechanism of the hybrid cryocooler under different precooling modes,which lays a theoretical foundation for the cycle analysis of the whole system.In order to clarify the operation mechanism of multi-stage precooling JT hybrid cryocooler,an enthalpy flow model of the JT cooling cycle with the unit mass flow rate is established,and the effects of variables such as multi-stage precooling temperatures and cycle pressures on the last stage cooling performance are investigated.Based on the ideal enthalpy flow model,the general variation law of the JT last stage cooling performance is revealed.Considering the actual thermophysical properties of He-4 and He-3 working fluids from 300 K to 1 K,the relationship between the optimized high pressures and precooling temperatures of the two working fluids under different cooling temperatures is analyzed,which lays a theoretical foundation for the overall cooling performance analysis of the multi-stage precooling JT hybrid cryocooler with different working media.(2)A mass flow rate model of the JT cooling cycle at extremely-low temperature around 2 K and below is established,and the influence of the working fluid deviating from the ideal gas effect on cycle parameters is studied.For the orifice valve,which is the core device producing JT effect,the inlet and outlet boundary conditions of it in the steady-state stage of the cooling cycle are analyzed,and the choked flow conditions determining the mass flow rate through the orifice are discussed.According to the variation characteristics of fluid enthalpy,entropy,velocity and density in the throttling process,four governing equations are introduced to solve the choked state parameters,and a mass flow rate model suitable for any boundary conditions is established.The deviation factor and real mass flux density at different stagnation temperatures are quantitatively studied in detail.The variation of the mass flux with operating parameters of the cooling cycle is calculated.The He-4 and He-3 mass flux calculation equations are given by numerical fitting method,which provides the theoretical basis for the optimizations of gross cooling capacity and precooling capacity distribution of the hybrid cryogenic cycle.(3)Thermodynamic optimization analysis of the multi-stage precooling JT hybrid cooling cycle at around 2 K and below.Based on the enthalpy flow model and the mass flow rate model of the orifice,the flow impedance of the orifice is analyzed with the gross cooling capacity as the optimization object,and the restriction conditions of the orifice diameter suitable for around 2 K and below are proposed.Combined with the real gas mass flux,the design equation of the ideal orifice diameter is given.The variation law of the gross cooling capacity is systematically studied from two aspects: upstream pressure and precooling temperature.Based on the optimization of the gross cooling capacity,the precooling capacity at different stages is quantitatively calculated in the low-pressure region and high-pressure one,respectively.The distribution of the three stage precooling capacities is discussed in detail.(4)The heat loss of each component of multi-stage precooling JT hybrid cryocooler is analyzed,and the optimization methods to reduce the heat loss are put forward.To improve the overall cooling performance of the multi-stage precooling JT hybrid cryocooler,the physical model and structural configuration of the cryocooler components are described in detail.Using heat transfer research methods,the effects of structural design parameters,low temperature material properties and surface emissivity on the leakage heat loss are discussed from the aspects of heat conduction and heat radiation.The conduction heat loss and radiation one of the multi-stage cold shields and the counter-flow heat exchangers are compared and studied.Seven heat loss optimization methods are summarized,which provide guidance for the practical design of the multi-stage precooling JT hybrid cryocooler.(5)A four-stage high-frequency pulse tube precooling JT hybrid cryocooler is developed and experimentally studied.The practical application was verified by coupling with an actual superconducting nanowire single photon detector.Based on the above theoretical research,a four-stage high-frequency pulse tube cryocooler(PTC)is used to precool the JT cryocooler,and thus the hybrid cryocooler is developed.The experimental platform is built,and the experimental studies are systematically carried out,which verifies the accuracy and rationality of the above theoretical analyses,and furthermore suggests a bypass accelerated cooling approach to improve the cooling process.By the use of He-4 in the PTC subsystem while He-3in the JT one,a no-load temperature of 1.36 K is experimentally obtained with the total input power of 366 W,which was the lowest temperature ever reported actually achieved based on the hybrid cycle composed of the multi-stage high frequency PTC and the JT cycle.By comparison,when He-4 is used as the only working fluid in the entire system,a no-load cooling temperature of 1.8 K can still be achieved with an input power of 458 W.The latter hybrid cryocooler is further loaded with an actual superconducting nanowire single photon detector(SNSPD),and the experimental measures indicates that the cryocooler can provide effective cooling at 1.84 K and the favourable electrical environment as well,which ensured the SNSPD to work stably and efficiently. |
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