To achieve lower temperatures is a subject of recent research and development activities in the field of pulse tube cryocoolers. To reach a temperature of 20 K, multi-staging is necessary in Stirling-type pulse tube cryocoolers. In the present work, Sage software is used to design a three-stage gas-coupled as well as thermal-coupled pulse tube cryocooler. A single-stage and a two-stage pulse tube cryocoolers are developed, tested and are coupled by a thermal link to build up a three-stage thermal-coupled pulse tube cryocooler. The lowest temperature of 19.61 K is obtained with a cooling capacity of 220 mW at 30 K at the third stage operating at 17 bar charge pressure and 68 Hz frequency. The phase-shifting mechanism used is a double inlet valve at the third stage while the inertance tube is used for the other stages.
Research on Stirling type Pulse Tube Cryocooler (PTC) is focused on achieving lower temperatures by cascading the stages or by multi-staging. Multi-staging can be done either by gas coupling or by thermal coupling of the stages. In the thermal coupling option, either a two stage cooler can pre-cool a single stage PTC to reach lower temperatures or a single stage PTC can cool a two stage PTC. In the present work, both these configurations are tested experimentally keeping the same two stage PTC. In case-1, the two stage PTC is used as a pre-cooling stage while in case-2, the single stage PTC is used as a pre-cooling stage. Length of the single stage is required and to be increased to match the two stages PTC for effective thermal coupling in case-1. The lowest temperature achieved in case-1 is 50.07 K where as in case-2 the lowest temperature achieved is 19.61 K at 17 bar charge pressure and 68 Hz frequency. The pressure drop in both the PTCs is compared to analyze the difference in performance.
The design of a highly efficient pulse tube cryocooler (PTC) is a subject of recent research activities. The PTC performance depends on various operating and design parameters. Regenerator is one of the very important components of the PTC which decides the low temperature that the PTC can attain. Efficiency of regenerator should be high enough, 96% or above, in order to reach very low temperature while the pressure drop in the regenerator is one of the parameters which needs to be analysed in detail. In the present work, theoretical and experimental investigations are carried out on two different single stage U type PTC. The volumes of regenerators and pulse tubes, in both the cases are kept same while the length to diameter (L/D) ratios of regenerators are changed. Investigations are carried out on these PTC with respect to pressure drop in the regenerator and net refrigeration effect obtained from the PTC at 80 K. The pressure drop increases from 0.29 bar to 2.07 bar with an increase in L/D ratio from 1.93 to 9, resulting in decrease in refrigeration effect from 6.1 W to 1.7 W at 80 K with 300 W input power. The study is further extended to understand the effect of coarse size stainless steel mesh size in the regenerator. Coarse size meshes filled up to 60% of regenerator length improved the refrigeration effect from 1.7 W to 2.8 W; however, further filling degrades the performance of the PTC. The experimental results are compared with theoretical results obtained by Sage software and Isothermal model.
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