Miniature Joule-Thomson (JT) cryocoolers are attractive for many applications due to their small size and resulting fast cool-down time. Finned-tube heat exchangers are the most widely used heat exchanger for miniature JT cryocoolers. The basic configuration, known as a Giauque-Hampson (GH) or coiled tube heat exchanger, involves the high-pressure stream flowing through a finned-tube that is helically coiled upon a cylindrical core while the low-pressure stream flows over the fins in the annular space created by the core and the inner diameter of a shell. Recent advances in technology have increased interest in JT cryocoolers that can provide cooling potential in the temperature ranges of 125 to 150 K. To achieve high efficiency and use a low-cost compressor, the JT cryocooler must provide cooling at low values of pressure ratios and operating pressure. To provide cooling under these conditions, a proper gas mixture must be selected as the working fluid. While it has been suggested that the heat transfer coefficient (htc) of the return stream is a key parameter affecting the behavior of the entire heat exchanger of a mixed-gas Joule-Thomson (MGJT) cryocooler, there is still no data or theory in open literature that characterizes the heat transfer and pressure drop characteristics of two-phase multi-component mixtures on the shell side in these heat exchangers. Beyond the broad goal of investigating gas mixture selection for MGJT cryocoolers, the experimental work in this study aimed to gain insight into these thermal characteristics by developing a test facility capable of measuring the two-phase htc for this geometry at operating conditions of interest to MGJT cryocooling. The capabilities of the test facility were demonstrated with a semi-flammable mixture. The size of the GH heat exchanger prototype and operating parameters of the test facility were consistent with those of interest for MGJT cryocoolers. Measurements of the two-phase htc of the mixed gas on the shell-side of the GH heat exchanger prototype were collected. For the mixture examined, the two-phase htc was found to be between 12 to 19 W/m2-K with uncertainties of approximately 12% for qualities in the range of 0.31 to 0.62. This data reveals that the shell side is the dominant thermal resistance for these operating conditions, even though the fins provide a larger surface area. Therefore, the htc of the mixed gas on the shell-side is crucial for cryocooler design and predicting the overall performance. While only a small amount of data was collected in this study, the data collected clearly demonstrates the need for and importance of developing accurate correlations for two-phase multi-component mixtures on the shell-side of GH heat exchangers for operating conditions consistent with MGJT cryocoolers. A large data collection campaign is proposed and enabled by the test facility developed in this work. Only with these correlations can the effects of the mixture selection on the pressure drop and the effectiveness of the heat exchanger be considered in the design of a MGJT cryocooler for optimal performance.

Cryocoolers 10 is the premier archival publication of the latest advances and performance of small cryogenic refrigerators designed to provide localized cooling for military, space, semi-conductor, medical, computing, and high-temperature superconductor cryogenic applications in the 2-200 K temperature range. Composed of papers written by leading engineers and scientists in the field, Cryocoolers 10 reports the most recent advances in cryocooler development, contains extensive performance test results and comparisons, and relates the latest experience in integrating cryocoolers into advanced applications.