| Cryocoolers working at liquid helium temperatures have many important applications in the fields of space exploration, military, medicine, transportion, and low-temperature physics. The Stirling pulse tube cryocooler (SPTC) is a potential candidate in space applications because of the advantages of high reliability, high efficiency, compactness, and low mass, and has become a hot topic these years. However, the working mechanism of a4K SPTC that operates at relatively high frequency is more complicated than that of the Gifford McMahon (GM) PTC that operates at the relatively low frequency of1-2Hz, and has not yet been well understood. In this study, theoretical and experimental investigation has been carried out to explore the refrigeration mechanism of the multistage SPTC working at liquid helium temperatures. The following content are included:1. Structure investigation of multistage SPTCsThe characteristics of coupling methods and arrangement of the hot end of the pulse tube (short as pulse tube arrangement) are systematically investigated at the first time. The influence of coupling methods on regenerator efficiency and mass flow distribution and the influence of pulse tube arrangement on phase shift capability, pulse tube efficiency and precooling heat load are mainly investigated. The investigation reveals that the regenerator efficiency of gas-coupling method is higher that of thermal-coupling method, that the gas distribution of gas-coupling method is more apt to be influenced by working parameters and geometry parameters than that of thermal-coupling method. The investigation also reveals that phase shifting capability is enhanced by placing the pulse tube hot end at cold, meanwhile the expansion efficiency of the pulse tube is improved and the pressure ratio also improved. However, the precooling heat load increases. The structure investigation of multistage SPTC is the first step for the design of multistage SPTCs working at liquid helium temperatures.2. Design method investigation of multistage SPTCs working at liquid helium temperaturesA new method of designing multistage SPTCs working at liquid helium temperatures is proposed in this paper, a series of steps including determining the structure, working parameters, temperature profile, geometry parameters are carried out and matching between stages is finally checked. A4K multistage SPTC is designed based on this method. The thermal coupling structure is fixed first, with the third-stage pulse tube hot end placed at cold. Secondly, the basic working parameters of frequency and charge pressure of each stage are determined based on the analysis of gas and material properties and also between-stage matching. The temperature profile is determined based on the summary of the performance of multistage SPTCs. Regenerator material is selected based on the temperature profile. Geometry parameters are determined based on the numerical calculation of Sage, REGEN and some other code. The length and diameter of regenerators are mainly fixed concerning regenerator efficiency and cooling power, the volume and aspect ratio of the pulse tubes are mainly fixed concerning the expansion efficiency, the phase span and the requirement of gas strengthening, the geometry of the inertance tubes are fixed based on the calculation of simplified thermoacoustic theory. Finally, the impedance matching between stages is checked. This method provides a reliable design method for multi-stage SPTCs.3. Experimental investigation of the three stage SPTC A three stage SPTC is designed and constructed. The experimental Investigation of this cooler has verified the design method of multistage SPTCs. The optimum frequency and charge pressure of three stages are close to the designed value. The second stage working at designed condition reached a no load temperature which is among the best results of inertance-phasing two stage SPTCs. The optimum frequency and charge pressure of the third stage are less than0.1Hz and0.1MPa of differences from the design values. The temperature distribution of the three stages is close to the design value, the difference is only0.5K,0.7K, and0.07K of three stages. The efficiency of the three stages are close to the designed value. The first stage and second stage reaches a no-load temperature of35.1K and14.9K, which is close to the lowest no-load temperature of single stage and two stage SPTCs. The three-stage SPTC reaches4.97K, which is the first time a three-stage SPTC reaches liquid helium temperatures. The influence of working parameters at liquid helium temperatures is investigated theoretically and experimentally. The investigation reveals that the influence of frequency, charge pressure, input power and precooling temperature is different on regenerators, pulse tubes and inertance tubes, and a new method for improving the cooling efficiency is proposed. This method is verified experimentally, and a no-load temperature of4.76K has been achieved, which is lower than that reached by a four-stage SPTC working with He-4. The experimental and theoritical investigation improves the refrigeration mechanism investigation of SPTCs working at liquid helium temperatures. |
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