Volume 2 No.3        May 1999

STIRLING CRYOGENICS

Cryogenic technology is concerned with producing very low temperatures. The cryogenic limit starts from the range 80 - 120 K in which nitrogen becomes a liquid, all the way down to 2 - 4 K. The device that produces these low temperatures is called a cryocooler. The principle of operation of a cryocooler is similar to a refrigerator, except that the gas-liquid phase change essential for a commercial refrigerator is not permitted here, since most refrigerants solidify at the low temperatures of interest. Cryocoolers normally utilize pressurized helium as the working fluid, which does not undergo a phase change down to 4 K. The isotope HeII condenses at a lower temperature of 2.2 K, displays superfluidity and can also be used as a working fluid for cryogenic applications.

The Stirling cycle is thermodynamically appropriate for producing low temperatures without requiring a phase change in the working fluid. Stirling cycles are increasingly being used in specialized applications related to cooling, heating and power generation. Stirling devices are structurally simple, easier to maintain and hence have a high reliability. Therefore, rapid advances are presently seen in cycles applied to cooling devices for Infrared sensors and mirrors used in space, liquefaction and separation of gases and electronic devicesthat exploit superconductivity.

Consider a Stirling cycle running as a refrigerator, drawing work input from an electric motor and in turn producing a cold space by pumping thermal energy out of it into the ambient. The working fluid is a gas that is filled in a portion of a cylindrical tube bounded on both ends by two pistons. In practice, the pistons are kinematically coupled in the sense that one is a driver and the other, a follower. The driven piston is called a displacer. In the central portion of the tube, a densely-packed collection of mesh screens called a regenerator is placed. On each side of the regenerator, two heat exchangers are mounted, serving as heat sources and sinks of the thermodynamic cycle. One of the heat exchangers pumps heat into the gas from a lower temperature ambient (the cold space) and the other rejects beat from the gas to the normal environment. The piston demarcates the point on the system boundary where work is given to the gas; in practice, the displacer can be replaced by a gas column piston. The regenerator acts as a thermal sponge, absorbing energy into its solid phase when exposed to the hot gas and releasing it to the cold gas at a later stage in the cycle. Inside the regenerator, there is a distribution from the ambient to the cold space temperature. The motion of the gas at any point in the tube is close to sinusoidal. However, a steady temperature distribution is obtained in the regenerator during continuous operation of the cryocooler. The cooling effect is produced when high pressure cold gas undergoes further expansion in the device.The Stirling cycle described above is quite similar to the Joule-Brayton cycle that is employed in gas turbines, except that the regenerator is a necessary component in the former. Regenerators are also used in gas turbine cycles but they are used to extract waste heat from exhaust gases in order to improve the cycle efficiency. In contrast, the regenerator enables the Stirling cycle to be completed and forms a primary part of the system (Figure 1).

The importance of cryocoolers in low-temperature electronics has been sensed by the space research organization of our country. The liquid nitrogen bottles previously employed are being phased out and the satellites are supplied with Stirling coolers. This has led to a significant increase in the life of the satellites. The liquid nitrogen plant at IIT Kanpur is also based on the Stirling principle.

Research is currently in progress to improve the performance-to-weight ratio of the Stirling cooler primarily by reducing its size and weight. Increasing the piston speeds leads to a mismatch between the cycle operation and the regenerator time constant. Large temperature differentials in the cooler lead to the production of thermoacoustic waves and hence to a loss of performance (Figure 2). In contrast, cryocoolers having all the components of a Stirling cycle but based on principles of thermoacoustics have also been reported (Figure 3). One of the most significant proposals has been the elimination of the displacer itself, in turn replaced by a gas pulse that undergoes oscillatory motion in the tube (Figure 4). Pulse- tube cryocoolers of this type have been tested on the laboratory scale. Figure 5 is a conceptual cryocooler with no pistons, and hence no moving parts, the process of compression and expansion being completed by two gas pulses on each side of the regenerator. The gas pulses are to he produced by impulsive beating and cooling of the tube at each end. This cooler when developed will represent the ultimate miniaturization that is possible in the field of Stirling cryogenics.

For more details contact:

Professor K Muralidhar

Department of Mechanical Engineering

Indian Institute of Technology Kanpur

Kanpur 208016

Telephone: (0512) 59 7182

e-mail: kmurli@iitk.ac.in

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