| The demands for cryogenic refrigeration around liquid helium temperatures are becoming urgent for increasing applications in many fields such as medical science, materials technology, aerospace exploration and other fields. Stirling type pulse tube cryocoolers (SPTCs) normally operate at 30-60 Hz, promising advantages of compact package, high reliability, long life, reduced temperature oscillation and potentially high efficiency. However, high frequency operation normally leads to large irreversible losses in the regenerator, making it difficult to operate with high efficiency. However, comparing with G-M cooler and Stirling cooler, the SPTC has still to be further improved both on efficiency and the lowest temperature. Extending SPTCs to liquid helium temperature is not a simple matter of increasing stages. There are several stubborn problems related to liquid helium temperature SPTCs, such as low regenerator efficiency at high frequency, real gas effects, insufficient acoustic power for phase shift and dramatically increased heat capacity of workin fluid compared to regenerator material, which restrict the developments of liquid helium temperature SPTCs. With the aim of improving the cooling performance of SPTC working at liquid helium temperatures, the mechanism of real gas effect on regenerator performance at liquid helium temperatures has been revealed; the methods to realize high efficiency regeneration have been investigated. Research work is carried out which focuses on the following sections:1. Thermodynamic analysis of pulse tube cryocoolers and theoretical investigation on effect of different working fluids on the loss mechanism of 4 K regeneratorBased on the thermodynamics, analysis with He-4 and He-3 as working fliud is carried out on the thermodynamic characteristics of the key components of various losses in a pulse tube cryocooler are given. Comparison of SPTC cooling performence has been made between He-4 and He-3 from views of phase shifting capability and regenerator losses with simulation model Sage and Regen3.3. The effects of the working fluid on the phase angle at the cold end are investigated in order to quantitatively reveal the coupled relationships of the lowest attainable temperature and the cooling capacity.2. Experimental investigation of the 4 K SPTC with He-4 and He-3 as working fluid respectivelyIn order to explore the effect of working fluid in a multi-stage SPTC, experiments have been carried out using He-3 and He-4 in three-stage SPTC respectively. Preliminary experimental results show that under same working conditions the SPTC replacing He-4 with He-3 can not obtain lower cooling temperature. The performance with different operating parameters is studied to identify optimization rules and guidelines to increase the performance. The analysis agrees well with experimental results on a three-stage SPTC. While running with the operating parameters optimized for He-3, the lowest temperature of the SPTC decreased from 5.4 K down to 4.03 K, which is the lowest refrigeration temperature ever achieved with a three-stage SPTC. In order to further lower the cooling temperature of 4 K SPTC, the perfomance of different type of regenerator materials have been tested.3. Theoretical and experimental investigation on the working mechanism of DC flow SPTC at liquid helium temperaturesIn this study, distribution characteristics of mass flow and energy flow in different part losses according to regenerator temperature were comprehensively concluded and summarized. Theoretical study shows that proper regenerator DC flow could enlarge the acoustic power in pulse tube cold end and decrease the effect of real gas enthalpy loss.As a result, the use of DC flow loop as assistant regulation for the 4 K Stirling type pulse tube cryocooler is proposed. Effects of mass flow rate, average pressure and operating frequency on cooling performence were studied.In order to verify the theoretical calculation and explore the possibility of enhancing performance using DC flow loop for a SPTC, different types of DC flow loops in the last stage were designed. Several different types of DC flow loops were tested and compared, the experimental results revealed that the cooling performance was significantly affected by proper DC flow loop in the last stage.The temperature distribution along regeneraor, no load temperature and precooling temperature was studied.The advantages of different types of DC flow loops were compared, then a possible solution was proposed to further suppress the negative effect of real gas enthalpy flow and lower the cooling temperature in 4 K regenerator. While running with the operating parameters optimized for DC flow, the lowest temperature of the SPTC could decrease from 7.8 K down to 6.2 K when the cold head temperature is above 7 K and decrease from 4.76 K to 4.69 K when the cold head temperature is below critical point. |
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