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Abstract: Joule-Thomson (J-T) effect is pivotal in refrigeration applications spanning a broad spectrum of temperature ranges. Miniature open-cycle J-T cryocoolers stand out for their remarkable cooling speed, capable of fast cooling from room temperature to 100 K within mere seconds. This rapid cooling capability renders them ideal for addressing immediate and transient cooling requirements, such as the cooling of infrared chips. In the cooling process of such a cryocooler, the high-pressure refrigerant undergoes pre-cooling in a heat exchanger and throttling to two-phase state before impinging on the cold plate. Subsequently, the backflow discharged into the atmosphere after passing through a cold energy recovery process in the heat exchanger. Crucially, this pre-cooling and energy recovery process depend on the performance of the heat exchanger, which serves as a critical factor governing the cooling temperature and cool-down time of the cryocooler. While previous research has extensively examined the structures, efficiency, and numerical models of heat exchangers, the equally vital jet impingement aspect affecting the cryocooler's cool-down time, has not received enough attention. It is necessary to study the energy recovery and jet impingement separately, as the heat exchanger solely influences the temperature and liquefaction rate of the jet, whereas the cryocooler's cooling efficiency is directly dictated by jet impingement. In order to study the impacts of structure and operational conditions on energy recovery and jet impingement of cryocoolers, three conical Hampson-type J-T cryocoolers were designed and tested under various working conditions. Several key factors were considered, including orifice diameter, jet height, cold plate heat transfer enhancement, backflow gap, half-cone angle, and the height of heat exchanger. Simultaneously, three crucial parameters, cold plate temperature, jet temperature, and cylinder pressure, were meticulously measured throughout the cooling process. To facilitate a comprehensive analysis, richer insights were derived by making reasonable assumptions grounded in the measured data. The experimental results demonstrated that the mass flow rate within the J-T cryocooler undergoes two distinct phases, that is, pipeline-dominant stage and the orifice-dominant stage. The temperature difference in two-phase impinging jet heat transfer is crucial for rapid cooling because it can alter the boiling mode and heat flux. Regarding the impact of structure on performance, it was found that higher jet height is beneficial for single-phase jet cooling during the pipeline-dominant stage of mass flow rate, and larger orifice diameters and backflow gap are preferable when pressure drop and heat transfer efficiency are satisfied. Moreover, larger cone angles lead to better energy recovery capacity for Hampson-type heat exchangers, and the length of the heat exchanger needs to be determined by the heat exchange efficiency of the refrigerant. The most significant improvement in cryocooler’s performance is the enhanced heat transfer treatment of cold plate.
Keywords: Miniature J-T cryocooler, Jet impingement, Energy recovery, Structural analysis
| Submitters Country | China |
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