Transientflow Effects

Difficulties in the design of heal exchangers for Stirling engines arise from the cyclic transient-flow elTects. Most industrial heat exchangers are subject to steady constant-flow conditions with relatively slow rates of change in the flow conditions. This is by no means the case for the heat exchangers used in Stirling engines where the flow conditions change continuously and experience wide variations in pressure, density, and velocity, to the extent of reversing the flow direction twice per cycle. All this complicates the situation considerably and makes the design of the regenerator and other heat exchangers a difficult art.

Initial contemplation of Stirling engines leads one to believe that when the engine is operating, the working fluid flows from the expansion space through the heater, regenera.or, and cooler to the compression space, and then retraces its step in returning to the expansion space.

Such a view is oversimplified and applicable only to the ideal Stirling engine. In practice none of the fluid ever moves all the way from the expansion to the compression space. Instead, a given hypothetical particle of fluid simply oscillates cyclically in a limited region of the engine in similar fashion to ocean driftwood repeatedly washed up on the shore and swept away once more by the receding waves.

This is illustrated in Fig. 7.4 which shows the cyclic trajectory as a function of crank angle of particular particles of the working fluid in a Stirling cycle cooling engine. The cyclic trajectories were calculated by Walker (1960) using Schmidt isothermal theory. For the particular case considered no particle passed into more titan two regions of the engine per cycle i.e., expansion space/freezer, freezer/regenerator, regenerator/ cooler and cooler/compression space. As can be seen from Fig. 7.4 no particle ever passed right through the regenerator.

Walker (1960) appears to have initially recognized the phenomenon of

Expansion space

I- rcezcr

Regenerator -

Cooler

Compression space

Expansion space

Regenerator -

Compression space

0 40 80 120 160 200 240 280 320 360 Crankanglc (Datum al displacer T.D.C.I

FlO. 7.4. Fluid particle displacement. The figure shows the cyclic trajectory of Individual particles of the working fluid in a Stirling-cycle cooling engine, calculated using the Schmidt isothermal theory The significant point of interest is that no particle of the working fluid ever passes from the expansion space to the compression space, or indeed ever passes right through the regenerator.

However the topic has received considerable recent attention (Organ 1976) and the development of sophisticated computer simulation programs has elevated the display of particle trajectories almost to a nouvcau art form (Schock 1978b).

In the course of calculating the particle trajectories mentioned above. Walker (1960) determined the mass rates of flow of working fluid into ami out of the compression and expansion spaces of the subject cooling engine. I he characteristics obtained thereby are reproduced in Fig. 7.5. Examples are given for two different mean pressures of the hydrogen working fluid.

40 80 120 160 200 240 280 320 360 (a> Flow diagram for mean pressure o: 26 at m

Crank-angle <b) Flow diagram for mean pressure of I i at m

Flow into expansion spacc

Flow out of t&v¿j dead space

Flow into ■Zul dead space

Row out of compression space

Flow into compression space

Mow out or expansion space

40 80 120 160 200 240 280 320 360 (a> Flow diagram for mean pressure o: 26 at m

Crank-angle <b) Flow diagram for mean pressure of I i at m

FiO. 7.5. Mass How in a Stirling-cycle cooling engine The figure shows the cyclic mass-flow rate for a Stirling-cycle cooling engine,•calculated using the Schmidt isothermal theory, for mean pressures of (a) 26 and <l>) 11 atm.

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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