Theoretical Design And Experimental Study On Two-stage Stirling Type Pulse Tube Refrigerator Working At Liquid Hydrogen Temperature Zone For Space Application

Driven by oil-free linear compressor,Stirling type pulse tube refrigerator（SPTR）has become one of the ideal models in the fields of aerospace detection.Nowadays,China has realized the aerospace application of single-stage SPTR in liquid nitrogen temperature zone,and there is an urgent need to develop two-stage SPTR for cooling detectors in liquid hydrogen temperature zone.Based on the need of a satellite detection project of China Academy of Space Technology,this dissertation studied a two-stage SPTR driven by a gas bearing compressor（GBC）,which adopted pure inertance tubes as phase shifter and reached a refrigeration temperature below 20 K at room temperature boundary.GBC is superior in light weight,small size and little abrasion,but its working frequency should be higher than 50 Hz.At present,most of the SPTRs below 20 K work at 20-40 Hz.Since the specific heat capacity of matrix dramatically declines at low temperatures,high frequency will increase the regenerative loss.Besides,the turbulent flow at the cold end will tend to be aggravated.Thus,some new challenges have been brought out for the design of regenerative material and flow straightener at low temperatures.The detailed investigations are as follow:1.The performances of regular screen,pressed screen,HoCu2 and Er3Ni above 50Hz were theoretically calculated,and the loss changes were analyzed according to the thermodynamic principle.Experimental results proved that the pressed screen could effectively improve the regenerator performance at 20-100 K.Sage software was adopted to analyze the influences of regular screen,pressed screen,HoCu₂ and Er₃Ni at 51 Hz on conduction,heat exchange and friction losses under different combinations.Theoretical analysis indicated that since the pressed 500 mesh screen was thinner than regular 500 mesh,it had smaller porosity,deeper thermal penetration depth and higher specific heat capacity per unit volume.It could improve the performance below 30 K mainly because of the increased heat released from gas to matrix and the decreased heat exchange loss.With the increase of refrigeration temperature,the effects of pressed screen friction loss increased and the optimal length gradually decreased from 15 mm to 0 mm.The no-load refrigeration temperature of HoCu₂ and Er₃Ni instead deteriorated 1.5-2 K compared to regular stainless steel screen.Though they possessed higher specific heat capacity,the higher friction loss caused by sphere material resulted in smaller pV power at the cold end.Experimental results showed that the pressed 500 mesh screen could reduce the no-load refrigeration temperature by 1.2 K compared to the regular 500 mesh screen,and the pressed635 mesh screen could reduce 1.3 K.Pressed screen could increase cooling power by 0.1 W below 30 K,effectively increase the mid-regenerator temperature,and reduce the precooling temperature by 2 K or so.2.The performance of two types of U-shape heat exchanger were experimentally compared.It was proved that U-type SPTR could reach the same cooling performance as in-line SPTR with smoothly bent connecting tube at the cold end and proper flow straightener.For structure compactness,the transition length of the original U-shape heat exchanger was merely 2 mm.Interior right-angle slot was adopted instead of connecting tube so that the top could be a plane surface for installation.However,severe turbulent flow appeared at the cold end and the no-load refrigeration temperature was mere 83.4 K with 3 mm straightener.Turbulent flow could be suppressed when the straightener of the first and second stage increased to 7 and 6 mm,respectively,but the no-load refrigeration temperature was only 36.1K because of the introduced friction loss.New heat exchanger was then designed with attention paid to the optimization of flow conditions at the cold end.The transition lengths of the regenerator and pulse tube side increased by 2 and 3.5 times,respectively.Smoothly bentΦ4-85 mm connecting tube was adopted instead of interior right-angle slot,and the new design could reach 24.6 K with 3 mm straightener,which also proved that with smoothly bent connecting tube and proper straightener,the performance of U-type SPTR was similar to that of in-line SPTR（23.0 K）.3.After the optimization of the whole machine,the SPTR driven by 50 Hz GBC reached a no-load temperature below 20 K adopting pure inertance tube as phase shifter.Several sizes of the first and second stage pulse tubes were tested.For the first-stage pulse tube,Φ19-54 mm pulse tube designed by Sage merely reached 142 K and the hot end even frost,since the length to diameter ratio（L/D）was only 2.8 and temperature profile could not be steadily built up along the pulse tube.The multi-dimensional flowing process could not be correctly simulated by Sage,which revealed its limitation in the optimization of pulse tube size.According to present pulse tubes above 50 Hz at 80-300 K,the L/D was increased to 5.3 and the volume was increased by 12%.The new size,Φ16-85 mm pulse tube could reach a superior performance.For the second stage,the L/D was increased from 6.1 to 7.1 and the volume was decreased by 27%.Φ12-85 mm pulse tube could reduce the refrigeration temperature by 6 K compared toΦ14-85 mm.With the addition of 1 mm thick copper radiation shield,the two-stage U-type SPTR finally reached a no-load temperature of 19.2 K driven by 450 W GBC,taking pure inertance tubes as the phase shifter.20-100 K regenerative materials were regular635 mesh screen（25 mm）+pressed 635 mesh screen（15 mm）,the frequency was 50 Hz and the charging pressure was 2.2 MPa.