15 Psia

To aoilu

Blood pump heat exchanger

I n. 17.3. Schematic diagram of McDonoel-Douglas nuclear-isotope or electric-heating Stirling engine and hydraulic blood pump (after Johnston e: al. 19751.

connection between the displacer cylinder and the converter.

In Ibis engine the 'piston' consisted of a flexing metal diaphragm with helium gas on one side and hydraulic fluid (a petroleum base fluid) on the other. The regenerator was the annular gap between the displacer and the cylinder wall.

Motion of the displacer in the cylinder caused helium to pass through the regeneratoi from the hot space above to the cold space below the displacer resulting in variation of the pressure level of the helium gas between 0.9 and 1.3 MN/nr (.135 and 1851b per sq in). The pressure change acting on the 'piston' diaphragm caused il to flex and pump the hydraulic fluid at the same pressure range. A differential area hydraulic pump created hydraulic pressure (1.4 MN/nr' or 200 lb per sq in). This was the delivery pressure to the hydraulic actuator of the blood pump.

33-W capsule

Radioisotope helium vent tube


Vacuum _ insulation

Displacer Engine cylinder Bellows seal Displacer drive piston

Heat exchanger Equalization line

Drive spring'

High pressure accumuhiloi Depletion valve

Make-up valve

Converter piston

—-Buffer chambcr

Make-up valve Back-up plate

Metal diaphragm Transfer plate

Hydraulic lines to pump actuator

Ftc. 17 4. Cross-section of McDonnel-Douglas radioisotope, Stirling engine heated module

The high pressure hydraulic lluid acted as ihe coolant for the engine and carried the waste heat for final dissipation to the hlood in a heat exchanger adjacent to the blood pump.

An interesting feature of the McDonnel-Douglas engine was the flcx-ural element used to locate the upper end of the displaccr in the cylinder. This was adopted in preference to a sliding guide ring at the hot end so as to eliminate the generation of wear debris. The original version of the flexing element is shown in Fig. 17.5. The present, improved flexing element has a spiral configuration to provide a long flexure length and so reduce the stress levels as shown in Fig. 17.6, for long life and reduced heat transfer.

Four flexing metal bellows were used in the engine module to separate the hydraulic fluid and helium working gas, the diaphragm, the displaccr drive bellows, and the two buffer bellows. All these flexors do not sustain a pressure difference and may therefore have a long life. Nevertheless

Tabic 17.1 Power source design and performance parameters for McDonnel-Douglas artificial heart (after Johnston e: ai 1977),

System 5 System 6 System 7 Prototype designation (implantable.) (implantable) (implantable)

Overall system characteristics

Solar Stirling Engine Basics Explained

Solar Stirling Engine Basics Explained

The solar Stirling engine is progressively becoming a viable alternative to solar panels for its higher efficiency. Stirling engines might be the best way to harvest the power provided by the sun. This is an easy-to-understand explanation of how Stirling engines work, the different types, and why they are more efficient than steam engines.

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