A thermoacoustically driven cooler capable of reaching temperature below 77 K with no moving part

Abstract

The pulse tube cooler has no cryogenic displacer and has attracted lots of attention in the field of cryocooler research. On the other hand, the thermoacoustic engine can generate self-oscillation and output work without moving components. Combining both technologies leads to a cryogenic cooler system with no moving components at all, which has great advantages of high reliability, low manufacturing cost, etc. Limited by largest available pressure ratio of thermoacoustic engines, up to now the best results on such a combined system are 88.6K when standing-wave thermoacoustic engine is used and 80.9K when traveling-wave thermoacoustic engine is used. Both the traveling-wave thermoacoustic engine and pulse tube cooler have oscillating flow inside. The phase relationship between pressure wave and volume flow rate is very important for conversion between heat and acoustic work. To achieve high efficiency, both systems require that traveling-wave acoustic field (which actually means pressure wave and volume flow rate is in phase) be realized somewhere inside the regenerator. To realize the above-mentioned phase relationship in a pulse tube cooler, pressure wave should lead volume flow rate by a certain degree (<90°) at pulse tube hot end due to compliance effect of the pulse tube. The orifice type pulse tube cooler cannot achieve this. Although the double-inlet type pulse tube cooler can realize this, but it has an intrinsic serious DC flow problem. The inertance tube turns out to be a better solution. It makes use of the acoustic characteristic of small diameter tube to generate the phase leading effect. In literature, the inertance tube is always connected with a large volume buffer (namely reservoir). According to our analysis, inertance tube with one end closed could as well provide the phase relationship needed at the pulse tube hot end. The reservoir is no longer necessary. For a traveling-wave thermoacoustic engine, which consists of a feedback loop and a resonance tube connected with the loop, arrangement of the loop components’ impedance is the key to realizing travelingwave acoustic field inside its regenerator. This impedance is related to the frequency of the thermoacoustic selfoscillation. In turn, the frequency is mainly determined by the dimension of the resonance tube. Besides, geometrical shape of the resonance tube is also very important. A suitable shape can eliminate higher harmonics and reduce viscous dissipation, which could lead to higher pressure ratio and efficiency. The inertance tube and resonance tube are generally cylindrical tubes with lengths being several meters. Their proper functioning in respective systems relies on the length and the ratio of radius to viscous penetration depth. Because of this, the inertance tube at pulse tube hot end is chosen among copper tubes with a diameter of several millimeters and length of several meters. The resonance tube of the engine consists of two sections: tapered tube and straight tube, whose details are introduced below. Configuration of the pulse tube cooler is shown in Fig. 1(a). The main dimensions are: regenerator i.d.15 mm, length 50 mm; pulse tube i.d.6mm, length 75 mm. It is designed to work at about 80Hz and needs an input work of about 30 W. We did a series of experiments before coupling the cooler with the thermoacoustic engine to verify the effect of inertance tube with one end closed. When the regenerator is filled with 300# stainless steel mesh, average pressure 2.8 MPa, frequency 84 Hz, and inlet pressure ratio 1.11, comparison results are listed in Table 1. It is apparent that the inertance tube much improves the cooler’s performance. When we finally coupled the cooler with the thermoacoustic engine, some changes were made: the regenerator is re-filled with 400# meshes; the inertance tube is changed into a 1.5 m long i.d.2 mm copper tube connected with a 1.6 m long i.d.4.8 mm copper tube. The tube with smaller diameter has the connection with pulse tube hot end. Configuration of the traveling-wave thermoacoustic engine is shown in Fig. 1(b). The loop is mainly made of i.d.80 mm tube and has a circumference of about 2 m. An