Investigation of improving cool-down speed of Stirling type pulse tube cryocooler with ambient displacers

For a cryocooler, besides high efficiency at the working temperature, the cool-down speed is also important for some applications. Because of the difference in structure and operating mechanism, different cryocoolers have different characteristics for cool-down speed. This work introduces a Stirling type pulse tube cryocooler with ambient displacers which works in the liquid nitrogen temperature region. Strategies for improving the cool-down speed from room temperature to liquid nitrogen temperature have been investigated through simulation. The pulse tube cryocooler is designed to provide a nominal cooling power of 17.8 W at 77 K with 232.5 W of input electric power at 70 Hz. By fixing the maximum displacer movement in the simulation, the characteristics of how tuning the frequency around 70 Hz can change the cool-down speed without exceeding the allowable displacement of the piston and the current draw of the actual compressor are investigated. The strategy for tuning both the frequency and voltage turns out be effective in the beginning stage of the cool-down process with an improvement of the cooling power by about 15%-20%. The gain becomes less obvious as the cold head temperature drops. In conclusion, tuning both the frequency and voltage during the cool-down process helps the cold-head to reach its final temperature faster but the average gain is not as big as expected.