G. Walker

Miniature refrigerators for cryogenic sensors and cold electronics

Abstract A surprisingly wide diversity of sensors, instruments or devices used for military, civil, scientific and medical purposes require cooling to cryogenic temperatures. Some work better or faster at low temperatures, are more sensitive or more effective. For others cryogenic cooling is essential to utilize phenomena which exist at very low temperatures, sometimes a narrow or precise temperature range. The cooling capacities required range from fractions of a watt to as much as 10W at temperatures extending from a minimum of 1 K or less to those well above the limit of the cryogenic range (120 K). At present there is a concentration of interest for fractional watt cryocoolers at near liquid-nitrogen temperatures (80 K). These needs are currently filled by immersion in liquid or solid cryogens, miniature Linde-Hampson cryocoolers with Joule-Thomson expansion energized by compressed gas from storage bottles and miniature mechanical refrigerators. Following a long period of intensive development and evaluation of many different mechanical systems miniature Stirling engines appear to emerge as the system of choice. They are smaller, weigh less, are more efficient and can be made at a reasonable price. Early disappointment with limited life and high maintenance requirements have been alleviated as the technology matures, designs are stabilized and manufacturing and test techniques develop. The present large scale application for miniature cryocoolers in infrared night vision and missile guidance systems will probably remain and expand as ‘smart’ projectile and beam guided weapons are widely introduced. Other large scale applications are in prospect with the development of semi-conductor ‘cold electronics’ systems and the new high temperature superconducting devices.

Microcomputer simulation of Stirling cryocoolers

Abstract A design aid for Stirling cryocoolers is described. The design aid is a digital simulation program for use on microcomputers. It is written in the FORTRAN 77 language and is especially designed to be easy to use with no specialist expertise required and minimal training. The program is versatile in that it can be used for all known types and variations of Stirling cryocoolers. It is quick and, in the comparisons with practical machines accomplished to date, is as accurate in predicting cryocooler performance as the more sophisticated simulation programs. Copies of the program including a graphics package and an accompanying manual are available at nominal cost. Technical support and consulting assistance to implement the program can be provided.

Stirling cycle cooling engine with two-phase, two-component working fluid

Abstract A Stirling cycle cooling engine operates on a closed regenerative thermodynamic cycle with compression and expansion of the working fluid at different temperature levels. Conventional engines use a gaseous working fluid but a significant improvement in refrigerating capacity may be gained with a two-phase, two-component working fluid. This consists of a gaseous carrier and a component which is liquid in the cold region and gaseous in the ambient temperature spaces with the phase change occurring during the regenerative process. This causes an increase in the range of the pressure excursion of the working fluid and consequently increases the refrigerating capacity with no penalties in terms of size, weight, or cost. The degree of improvement depends on the particular design and operating conditions of the engine. The effects of variation of some of these parameters are explored.

Design guidelines for large Stirling cryocoolers

Abstract A simplified design guideline for the refrigerating capacity of Stirling cryocoolers is presented.

Excitation Of A Fluidyne Tuning Line

The fluidyne, or liquid piston Stirling engine, incorporates liquid columns as displacers and pistons in a heat engine type which can be used in conjunction with low-grade heat sources to provide useful work in the form of liquid pumped over a modest head. The liquid feedback type of fluidyne offers particular advantages including the absence of moving mechanical parts; this type of machine incorporates a liquid column to divert a small quantity of cycle work which is then used to re energise the displacer of the engine which must supply all of the pumping work of the engine. Previous research has demonstrated the import ance of correct design of the mixing of the liquid of the tuning line and the displacer. In this paper the results of an exDeriment are reported which demonstrate the response of the two liquid columns to tho varying system pressure which is both the consequence of engine operation as well as the means bv which the svstem is actuated.

Miniature Stirling cryocoolers: trends in development☆

Abstract Following a lengthy period of development, Stirling refrigerators have emerged as the preferred system for the miniature cryocoolers used in infrared night-vision, missile guidance systems and other low capacity cryogenic sensors. Single stage expansion integral and split-Sterling refrigerators having capacities of 0.25-1 W at 80 K are in series volume production. They are characterized by increasing reliability (multithousand hours operation). Future preference is anticipated for split-Stirling systems with close tolerance seals replacing rubber contact seals and linear electric motors increasingly preferred as the compressor drive. Present difficulties with the cooler—sensor interface and of fluid leakage will be overcome by manufacture of integrated cooler—sensor units welded leak-proof and having no provision for field servicing. Eventual production is anticipated of throw-away, radio tube like, cryocooler-sensor units capable of plugging in to ambient temperature circuits. Control of compression speed in accordance with load demand will be routine. The use of multistage expansion Stirling cryocoolers for superconducting electronics is anticipated with the development of the high temperature superconducting materials having critical temperatures ≈ 20 K and operating temperatures ≈ 10 K. Availability of a reliable, compact, relatively low cost, 10 K refrigerator would eliminate the need for liquid helium cooling and open possibilities for application of superconducting electronics on a broad front for diverse military and civil purposes.

Small-scale liquefaction of hydrogen

Abstract Hydrogen may be stored compactly as a cryogenic liquid at low pressure and temperature, i.e. 20 K at atmospheric pressure. Gaseous hydrogen at normal temperature may be liquefied on a small scale using a cryogenic refrigerator (cryocooler). Stirling refrigerators are well suited to this duty. These machines operate on a closed thermodynamic regenerative cycle with compression and expansion of the working fluid (helium) at different temperature levels. Very low temperature Stirling refrigerators have several stages of expansion, typically two or three for a hydrogen liquefier. A concept for an electrically driven Stirling hydrogen liquefier of low capacity is described. Stirling machines can also be used as power systems converting heat to work. A second concept is described for a combustion heated Stirling — Stirling hydrogen liquefier. Conceptual design studies for both units have been carried out including computer simulation of the power system and cryocooler systems. These indicate the net energy flow and principal dimensional parameters permitting first order estimates of costs for prototype manufacture and the unit costs in series production.

Generalized ideal reference cycle for regenerative refrigeration: part 1 isothermal systems

Abstract A generalized ideal reference cycle for regenerative refrigeration systems is discussed. The cycle has isothermal processes of compression and expansion for heat addition to and heat rejection from the cycle. The regenerative processes are assumed to be partly constant volume and partly constant pressure processes. Equations are developed for cycle work, heat lifted, coefficient of performance and the pressure ratio: mean effective/maximum cycle pressures. Various special cases are deduced from the generalized analysis including the Stirling and Ericsson cycles.

International Contributions To Stirling Cycle Machine Technology

The IECEC has been since 1966 the major forum for the dissemination of Stirling Cycle machine technology. Only once in the twenty-four year history of the IECEC has a paper not been presented which dealt with the Stirling. In total between 1966 and 1989 over 500 hundred papers have been published involving more than 1000 authors from 18 different countries. Interest in the Stirling has increased dramatically over the years, and shows no sign of abating. The IECECs must take much credit for this continued interest since the papers presented over the years represent the bulk of the modern literature dealing with Stirling technology. This paper is a review of this literature.

Generalized ideal reference cycle for regenerative refrigeration: Part 2. Adiabatic systems

Abstract A generalized ideal reference cycle for regenerative refrigeration systems is discussed. The cycle has adiabatic processes of compression and expansion. Cyclic heat addition to and rejection from the cycle occur at constant pressure or constant volume immediately preceding the regenerative thermal processes. These are assumed to occur partly at constant volume, partly at constant pressure. Equations are developed for cycle work, heat lifted, coefficient of performance and the pressure ratio: mean effective/maximum cycle pressures. Various special cases are deduced from the generalized analysis including the pseudo-Stirling and pseudo-Ericsson cycles. Results are presented for the Stirling and pseudo-Stirling cycles to illustrate the effect of volume compression ratio and regenerative effectiveness on the cycle Coefficient of Performance. The pressure ratio p mean /p max for the same two cycles are compared to a function of the volume compression ratio.