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|Low Pressure Ratio Cascaded Joule-Thomson Cryogenic Coolers|
|Keywords: Micro Cryogenic Cooler, Joule-Thomson, Low Pressure Ratio|
|This work presents the design, fabrication, and characterization of two micro/mini coldstages intended to investigate the feasibility of low pressure ratio cascaded Joule-Thomson (JT) based highly miniaturized cryogenic coolers. Single-stage single-refrigerant JT coldstages require pressures ratios greater than 8:0.1 MPa to reach cryogenic temperatures (Lerou et al., 2006). Low pressure ratio single-refrigerant coldstages have a small temperature range of cooling, limiting the ultimate temperature a single stage can reach (Ray Radebaugh, 2014). Coldstages designed to work with custom mixed refrigerants to reach cryogenic temperatures can be designed to operate with lower pressure ratios of less than 0.4:0.1 MPa (Wang, Lewis, Lin, Radebaugh, & Lee, 2013). However, mixed refrigerants have an order of magnitude lower specific refrigeration capacity when compared to single component refrigerants and can be prone to stratification of the refrigerant, further reducing refrigeration capacity (Lewis, Wang, Schneider, Lee, & Radebaugh, 2013). The gas-phase refrigerants require compression, and the pressure ratio and flow rate of micro JT coolers are currently limited by the capability of modern mini/micro-compressors (R Radebaugh, Bradley, Coolidge, Lewis, & Lee, 2014). Thus, micro JT coolers have been operated by compressors that are much larger in size, or from high- pressure gas cylinders, thus limiting overall system miniaturization. A cascaded coldstage presents a practical solution to this. The cascaded system consists several self- contained low pressure ratio single refrigerant JT cycles. Each stage has a small temperature range of cooling and is used to precool subsequent stages. Since the pressure ratios are lower, single-refrigerant JT coolers are compatible with mini/micro- compressors, thus enabling the use of such compressors to achieve overall system miniaturization. Due to the limited temperature range of each single-refrigerant cooling stage, several stages can be cascaded together to reach cryogenic temperatures. This work presents two cascaded coldstages with JT restrictions fabricated from glass capillaries with inner diameters of 50 microns and inter stage polyimide tube in tube heat exchangers with annular channels of 75 microns. The coldstages investigated use single component refrigerants operating at pressure ratios of less than 0.5:0.1 MPa. The JT restriction capillary dimensions for the coldstages developed in this work are determined by laminar flow undergoing adiabatic expansion through a circular tube. The pressure dependent properties of the two phase flow are evaluated over the length of the restriction via tabular integration for each refrigerant using properties from NIST REFPROP v. 9.1 (National Institute of Standards and Technology, Gaithersberg, 2016). A two stage coldstage is shown to cool from 293K to 228 K with a net refrigeration power of 150 mW. A three stage coldstage is shown to cool from 293 K to 193 K with net refrigeration power of 15 mW. The refrigerants for the first, second and third stages were isobutane, propane, and R-116. The coldstages presented demonstrate the feasibility of low pressure ratio cascaded JT cryogenic coolers.|
University of Colorado