Mike Dadd

High Performance Flight Cryocooler Compressor

In this paper we report on the development of a next generation flexure bearing compressor which features high efficiency, high capacity per unit mass, enhanced producibility and ease of integration into payloads. The compressor was developed for the 95K High Efficiency Cryocooler programme.

Clearance seal compressors with linear motor drives. Part 1: Background and system analysis

Linear compressors with clearance seals and flexure bearings have been used for many years for space applications, and this article describes the development of linear compressors at Oxford. A new design of moving magnet motor is introduced and the use of flexure springs, linear motors, valves and their integration are all discussed. A ring down test indicates that the ‘viscous’ damping in the linear compressor is acceptable (in terms of the decay ratio against the number of periods). The measured motor characteristics compared well with the vector fields electromagnetic finite element model. With a revised structural design it is predicted that the motor efficiency would be ∼86% for the rated power (200 W shaft power at 50 Hz) enabling the linear compressor to have a high overall efficiency. The companion paper covers the experimental evaluation of the linear compressor.

Compression Losses in Cryocoolers

Most of the loss mechanisms in Stirling and Pulse Tube cryocoolers are well documented and are relatively easy to analyze and estimate. One of these losses is related to the irreversible compression of gas in the cylinder, and the magnitude of this loss is such that it has a very significant impact on the overall system efficiency. Simple tests on a cryocooler give an estimate on the size of this loss, though the results of these tests include elements of other, known losses, such as clearance seal and pressure drop losses. The parametric variation of this ‘lumped’ loss suggests that it is not primarily due to these, or other known losses. Over a wide range of machines and conditions this ‘lumped’ loss varies with operating frequency, swept volume and pressure swing, suggesting that it is some kind of thermodynamic effect related to the area of the ‘P-V’ loop in the compression space.

Clearance seal compressors with linear motor drives. Part 2: Experimental evaluation of an oil-free compressor

Oil-free compressors are required for various applications, including cryocoolers and refrigeration systems that use compact evaporators. A novel low-cost moving magnet oil-free linear compressor is described here, and follows a companion paper on the linear motor system. The experimental apparatus to evaluate the thermodynamic performance of the linear compressor is comparatively straightforward, but the data analysis is involved, since key measurements have to be corrected for the phase response of the transducers. Harmonic fitting, using fast Fourier transform with zero-padding and minimisation algorithms, have been used for the voltage, current and displacement signals. This enables corrections to be made for the phase lag in the voltage, current and displacement measurements. The experiments using nitrogen, with a constant pressure ratio, show that the linear motor system under resonance achieves a high overall efficiency. For the design point, the motor efficiency is 74%, while for a pressure ratio of 3.0, the average overall adiabatic efficiency is 54% and the isothermal efficiency is 46%, all of which are within a reasonable range for a small compressor.

Motor and Thermodynamic Losses in Linear Cryocooler Compressors

Stirling cycle and Pulse Tube cryocoolers can be described by ideal thermodynamic cycles with discrete losses, however experiments show a systematic underestimate of power consumed. Existing correlations have been unable to explain the magnitude of this additional ‘compression loss’, but it can account for up to 50% of the power delivered to the gas. It appears that even a modest decrease could significantly improve the efficiency of future machines. One problem inherent in studying this effect is the complex geometry of cryocoolers. Experiments have therefore been performed on an existing ‘Oxford type’ moving coil compressor with the simplest compression space geometry, a flat cylinder head. Measurements were made of the intrinsic motor losses (including windage), and the power delivered to the gas. The results show that for this machine the motor losses are greater than previously thought, accounting for up to 30% of the compression loss in the original cryocooler configuration. Measurements of the heat transfer losses are also presented.

FAST RESPONSE TEMPERATURE MEASUREMENTS IN STIRLING CYCLE CRYOCOOLER COMPONENTS

One reason that heat transfer processes are not well understood is the difficulty of obtaining reliable temperature measurements when gas temperatures vary rapidly. In the work described here gas temperatures have been measured using a fine wire resistance thermometer with a 3.8 micron active sensor. The equipment represented the basic elements of a cryocooler: a clearance seal linear compressor and a wire mesh regenerator. Both were operated close to ambient temperature, with gas temperatures being measured close to the regenerator.The test rig was run at different volume ratios, frequencies (8–50 Hz), gases and filling pressures (1–26 bar). The waveforms of the gas temperature were found to vary dramatically for differing flow regimes. The results suggested that the thermometer was measuring the temperatures of two distinct volumes of gas, and that the gas must remain stratified in the compression space. A flow transition was identified from the cycle-by-cycle variations in temperature. The critical Rey...

Scaling of Cryocooler Compressors

The successful HEC cryocooler compressor has been used as the basis for two very similar compressors, one smaller and one larger than the original. The new compressors were designed largely by a direct scaling of the original.

The Linearity of Clearance Seal Suspension Systems

The classic ‘Oxford’ cryocooler has a clearance seal between the piston and the cylinder which is maintained by the use of spiral disc springs. In a typical compressor this clearance is about 12 microns, and therefore the spring suspension system must have a linearity of no more than 3 or 4 microns to avoid contact. It has always been assumed that to maintain this linearity, the surfaces between which the springs are clamped must be very flat and very parallel to each other. It has also been assumed that the flatness and parallel-ness of the clamping at the inside of the spring is more important than at the outside.

Results from field trial of a low-cost solar cooker with novel concentrator geometry

Solar cookers are generally of either box-type or make use of parabolic dishes, including approximations thereof. The former are cheap but operate at low solar concentrations and temperatures, whilst the latter often require complex mirror geometries and can be prohibitively expensive to manufacture. This paper will present the results from a field trial of a prototype solar cooker which use of a novel concentrator geometry to achieve high temperatures.