The performance of Stirling engines is subject to limitations resulting from power dissipation in the regenerator. The dissipation is caused by pressure gradients in the regenerator to generate flow. Without this flow the power output would by zero. Hence the dissipation is an essential element of the operation of the engine. Using linearized theory, the equation for pressure in the regenerator is solved for the case of a linear temperature distribution. The regenerator is taken to be thermally perfect. All various are taken to be sinusoidal in time. Expressions are derived for the net power output and the thermal efficiency. The net power output is optimized under various constraints. The constraint yielding the best results is fixed piston amplitude in the compression chamber. Upper bounds on the dimensionless power output are found as function of regenerator void volume and regenerator temperature. These bounds are derived in the limit of zero frequency, and ae independent of the conductance of the regenerator. Both power output and thermal efficiency decrease decreases as frequency and regenerator void volume increase.
A heart scanner based on high-T, SQUIDS is currently under development at the University of Twente. It is intended to be used in standard clinical environments without a magnetically shielded room. In order to make the application simple to use, the SQUIDS will be cooled by small cryocoolers, thus realizing a turnkey apparatus. The aimed field resolution is 50 ffRMs HZ-'/~ in a measuring band of 0.1-100 Hz. The mechanical cooler interference is reduced by incorporating two coolers and operating them in counter phase. The magnetic cooler interference is reduced by positioning the coolers and the SQUIDS in a coplanar arrangement, and by separating the SQUIDS from the cold tips with a solid conducting thermal interface. A design is presented in which a temperature of 55 K is expected with a cool-down time of less than 1 h. 0 1997 Elsevier Science Limited Keywords: high-T, d&QUID magnetometer; Stirling cryocooler; magnetocardiogrwhy SuperconductingQuantum Interference Devices (SQUIDS) are the most sensitive magnetic flux-to-voltage converting sensors. At present, their main application lies in the field of biomagnetic research where multichannel low-T, dc-SQUID based magnetometer systems are used. These systems are usually cooled by liquid helium and operated in magnetically shielded rooms to obtain an extremely lownoise measuring environment.They are expensive, require helium refills and cannot be transported in a simple manner.Because of the higher operating temperature, a much more flexible magnetometer system can be realized with high-T, SQUIDS. The workhorse of current high-T, superconducting electronics, YBa,Cu,O,, with its critical temperature (T,) of 92 K opened the possibility of operating SQUIDS in liquid nitrogen at 77 K. Also, small-scale tumkey cryocoolers, originally developed for cooling infrared sensors, are available and are very reliable'. The advantages of cryocoolers compared to a liquid-nitrogen cryostat are the lower operating temperature (which gives a potentially better SQUID performance*), the turnkey operation (no refills required), and the possibility of operating the system in all directions.In this paper the design of a heart scanner is presented that can be equipped with up to 25 high-T, SQUIDS cooled by two small Stirling-type cryocoolers. In the next section the design goals and constraints are reviewed, in which special attention is paid to the heart signal and the required sensor resolution. After that, the specific coolers that are used in this project are discussed. Because of the interference at the SQUID positions arising from the cryocoolers, a separation in space or time through a thermal interface is demanded. A separate section deals with possible thermal interfaces which can transfer heat from the SQUID unit to the coolers, and with special measures that have to be taken to reduce the magnetic and mechanical cooler interference. Then, the thermodynamic aspects of the cooling system for the heart scanner using a conductivestrip thermal interface are considered. The paper con...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.