A completely co-axial two-stage pulse tube cooler working at liquid hydrogen temperatures

Abstract

<p indent=0mm>Pulse tube coolers, which work at liquid hydrogen temperature, are widely used in applications such as cooling infrared sensors and superconducting devices because of the advantage of no moving parts at low temperatures. Pulse tube coolers can be divided into three types according to their shape: In-line type, U-type, and co-axial type. In-line type and U-type pulse tube coolers separate the regenerator and the pulse tube, both of which are only connected through the cold end heat exchanger. For co-axial pulse tube coolers, the regenerator normally has an annular cross section surrounding the pulse tube. They are becoming increasingly popular in practice for their compactness, but the thermal contact between the regenerator and the pulse tube may affect the system performance. Few studies have been conducted on the influence of radial thermal contact, most of which are based on a single-stage pulse tube cooler and deviate from an actual co-axial type pulse tube model. For pulse tube coolers working at liquid hydrogen or a low temperature, two-stage cold heads are commonly used. In this study, theoretical analyses are conducted on the basis of a two-stage gas-coupled pulse tube cooler to investigate the influence of radial heat transfer. Based on the simulations, a completely co-axial two-stage pulse tube cooler is set up and experimental studies are carried out. Four models (corresponding to different shapes of cold heads) are constructed to theoretically analyze the mechanism based on the Sage software. Without radial thermal contact (corresponding to a two-stage U-type cold head), the temperature distributions along the pulse tubes are nonlinear, especially the 2nd stage pulse tube due to the large temperature span. Corresponding to the position of the 1st cold end, the temperature of the 2nd stage pulse tube is high to <sc>230 K;</sc> at the same axial location, the temperature difference between the 2nd stage regenerator and the pulse tube is approximately <sc>190 K.</sc> Radial heat transfer on the 2nd stage pulse tube (corresponding to 1st stage U-type and 2nd stage co-axial type cold head or the completely co-axial two-stage cold head) can cool down the 2nd stage pulse tube effectively; at the same axial location, the temperature difference between the 2nd stage regenerator and the pulse tube decreases to <sc>60 K</sc> with the 1st stage U-type and 2nd stage co-axial type cold head; such a difference also decreases to <sc>75 K</sc> with the completely co-axial two-stage cold head. Radial heat transfer increases the enthalpy flow in the 2nd pulse tube, and the expansion efficiency of the pulse tube increases from 69% to approximately 85%. The cooling performance is also improved. Meanwhile, the radial heat transfer between the 1st stage regenerator and the pulse tube (corresponding to 1st stage co-axial type and 2nd stage U-type) has a relatively minor effect on the system performance. Exergy losses are also investigated with different models. Therefore, a well-matched geometrical arrangement of the co-axial configuration is essential for achieving the optimal performance. The completely co-axial two-stage cold head not only improves the compactness but also the cooling performance. Experiments are then performed. As a result, a completely co-axial two-stage pulse tube cooler is built. A cooling power of <sc>1.2 W</sc> at <sc>20 K</sc> is obtained with <sc>223 W</sc> PV power input.