Theoretical And Experimental Investigation On Precooling Methods Of A Stirling Pulse Tube Cryocooler Working At Liquid Hydrogen Temperatures

Stirling pulse tube cryocoolers(SPTCs)operate at frequencies above 20 Hz with high energy flow density,low maintenance requirements for linear compressors,and no moving parts at low temperatures.It possesses abundant advantages containing high reliability,long lasting,simple structure,compactness and light weight,making it ideal for aerospace and other fields and attractive for a lot of institutes and researchers worldwide in recent years.However,in order to achieve cooling at the temperatures of 20 K or below,it is necessary to include precooling from a higher-temperature cold source to compensate for the losses.As key components of the SPTC,the regenerator and pulse tube are the main components causing losses and the main objects to be precooled.In the company of the precooled multi-stage regenerator with significant losses,only a small part of the limited acoustic power from the linear compressor reaches the cold end making it difficult to improve the efficiency of the SPTC.In order to improve the cooling performance of the SPTC,this paper studied the method of precooling the transmission tube by using an external cold source and that of precooling the pulse tube with direct current flow(DC flow)from inside.Main contents of the paper are shown below:1)The precooling effects of the cryocoolers were analyzed thermodynamically,and the precooling methods of the SPTC were systematically summarized and classified.The effects of precooling in improving the theoretical efficiency of the cryocooler system were examined based on thermodynamics.The effects of precooling on the regenerator and the pulse tube were studied from the perspective of the alternating energy flow in the pulse tube cryocooler.The precooling methods of the SPTC,including heat exchange method,precooling source,precooling objects,etc.,were summarized.A map of precooling methods classification was compiled.For the purpose of fully precooling the working fluid,a method of precooling the transmission tube and the hot end of the regenerator was proposed,using a cold source with a wide source and plenty cooling capacity.Besides,a method of directly precooling the working fluid in the pulse tube from the inside by using DC flow to transport cold end refrigeration capacity was developed.2)The working mechanisms of the precooled transmission tube were revealed through numerical simulations to specify the optimal insulation structure for high efficiency acoustic power transmission,and the performance of the precooled transmission tube were verified through experiments.A one-dimensional model of the SPTC was established.Calculation results show that replacing the high-temperature part regenerator with a precooled transmission tube can reduce the required input acoustic power,but it will increase the precooling load.A two-dimensional numerical model of the precooled transmission tube was extablished.the work and heat transfer processes in the precooled transmission tube were calculated and analyzed from the perspective of multi-dimensional and micro-scale.The calculation results show that the heat loss of the precooled transmission tube is mainly caused by the heat transfer of the working fluid near the wall.The optimal volume of the precooled transmission tube is 15 – 17 times the gas swept volume at the cold end and the optimal aspect ratio is 8 – 10.A liquid nitrogen precooled SPTC experimental system was built to compare the cooling performance of the precooled transmission tube with that of the traditional hightemperature part regenerator.The SPTC with the two structures obtained 0.66 W and 0.83 W cooling capacity at 25 K,respectively.The input p V power was 26.8 W and 142.0 W,respectively.Considering the precooling liquid nitrogen consumption based on the efficiency of an air separation system,the efficiency of the SPTC with the two structures is 0.51% and 0.47%,respectively.It proves the feasibility of replacing the high-temperature part regenerator with the precooled transmission tube to drive the SPTC at the liquid hydrogen temperatures with low acoustic power.By further optimization,the SPTC with precooled transmission tube has reached the lowest cooling temperature of 17.7 K and is able to obtain 1.01 W of cooling capacity at 22 K with 42.6 W input p V power.3)The mechanisms of the DC flow precooling the pulse tube from the inside and its different effects on the SPTC performance at 20 K temperatures and 80 K temperatures were revealed.The simulation calculation of the SPTC reveals that the DC flow from the regenerator to the pulse tube in the SPTC can reduce the temperature of the pulse tube,increase the enthalpy flow of the pulse tube and its expansion efficiency.In the SPTC at 20 K temperatures,the DC flow can improve the linearity of the temperature distribution in the regenerator and reduce the regenerator loss as well,thereby improving the cooling performance.However,in the SPTC at 80 K temperatures,the DC flow increases the temperature gradient at the cold end of the regenerator as well as the regenerator loss,thus deteriorating the performance of the SPTC.The performances of the SPTC at 20 K temperatures with the effect of naturally introducing the DC flow with the double-inlet structure,restricting the DC flow with the doubleinlet structure,and only normally inducing the DC flow were compared.The results reveal that the double-inlet structure can assist phase shifting to improve the performance of the regenerator,but it will also increase the pulse tube temperature gradient and the pulse tube loss.The DC flow can always improve the performance of the SPTC by precooling the pulse tube whether the phase shifting is good or not.The experimental results verify that the DC flow will deteriorate the performance of the SPTC regardless of the direction in the 80 K SPTC.However,in the 20 K SPTC,the DC flow from the regenerator to the pulse tube and the double-inlet structure can both decrease the temperature at the middle of the pulse tube by about 130 K,the noload refrigeration temperature reduced by about 6 K,and the cooling capacity at 22 K increased by about 1 W,which verifies the effect of DC flow of improving the performance of the SPTC by precooling the pulse tube.