An Investigation of Certain Thermodynamic Losses in Minature Cryocoolers
Abstract
Stirling cycle cryocoolers developed at Oxford have typically been designed using a second order methods whereby the ideal Stirling efficiency is degraded by a number of discrete loss mechanisms. In all cases the eventual machines perform less well than expected, and it always appears as if an additional thermodynamic loss is acting. This empirically calibrated loss is therefore included as part of the normal design procedure and there is anecdotal evidence that this is an approach taken by other manufactures. Although this loss might be caused by imperfect heat transfer, existing theories do not agree with its magnitude. A project was therefore started to measure the losses in the simplest possible geometry, a linear compressor with a plain 'top-hat' cylinder head. It was hoped that by characterizing the losses in this geometry and applying them to full machines these called "compression loss" could be explained. Since the loss is quite large it could allow significant improvements to be made for future machines. A well calibrated measurement system was developed and a linear compressor commissioned. To enable a sufficiently good energy balance to be produced electromagnetic motor losses and windage were measured. It immediately became clear that these were more significant than had been assumed previous studies. In fact it appeared as if a significant proportion of the "compression loss" might be explained by these new measurements. With the losses expected from analytic analyses. Agreement was not perfect, however, and this is thought to be due to the incompleteness of the heat transfer theory, particularly with regard to the flow through the clearance seal. Future possibilities for work are suggested and it is hoped that these measurements can be used as a baseline for testing theoretical work which will enable efficiencies to be increased not just in Stirling type coolers, but also in pulse tubes and linear alternators.
Document Details
- Document Type
- Technical Report
- Publication Date
- Jan 17, 2005
- Accession Number
- ADA432813
Entities
People
- Jaime Reed
Organizations
- University of Oxford