Investigation of Certain Thermodynamic Losses in Miniature Cryocoolers

摘要

Measurements were made of gas temperatures and power losses for a compression volume that included a regenerative heat exchanger. A fine wire resistance thermometer with a rapid response was used to measure the gas temperature for flows between the piston/cylinder volume and the regenerator. The temperature profiles exhibited features that varied with different gases and operating conditions. These results were interpreted in terms of a stratified tidal gas volume that is subject to thermal diffusion and flow turbulence effects. A few nitrogen results for high pressure and low frequency were very unstable with significant high frequency components. A tentative interpretation was that this behaviour was associated with the transition from laminar to turbulent flow conditions. At the other end of the regenerator in a separate fixed volume, a twin thermocouple probe was used to investigate the performance of a compensation technique designed to improve the time response of the bare thermocouples. The technique involves arranging two thermocouples with different time constants such that they experience the same gas temperature variation. The gas temperature is reconstructed by using various approaches for estimating the response times. It is shown that this technique can be considered, provided that the noise levels are low enough. For the measurements described here the noise could only be sufficiently reduced by ensemble averaging. This would be a limitation in circumstances where the waveform is not very repeatable. The power loss measurements showed that a loss existed that could be correlated by a combination of Reynolds number and the experimental parameters pressure, frequency and stroke. Attempts to formulate this in a non-dimensional form were not successful suggesting that key parameters have been missed. One aspect in particular that needs closer attention is the effect of phase angle between mass flow and pressure variation.