A Note on Isentropic Flow ‘Perpetual Motion’
Since isentropic flow is so efficient – that is to say it can be made almost 100% ‘loss free” - it naturally invites speculation as to the possibility of a ‘perpetual motion’ or ‘quasi-perpetual motion’ flow model. However, the long standing prohibition against heat engine or thermodynamic perpetual motion as a violation of the second law of thermodynamics, naturally creates a well founded caution – or even resistance to- such possibility in other areas of physics.
However, one other example of quasi -perpetual motion occurs in outer space, where loss-free
motion of a space satellite through a frictionless space in the absence of any
substantial gravitational or other force field,
results in quasi-perpetual orbital motion under
It seems well worthwhile, therefore, to examine the motion of isentropic gas flows in a bit more detail from this standpoint.
Frictionless Isentropic Flow
Ideal isentropic flow is frictionless, so that its accelerations and decelerations are ideally lossless. In practice isentropic accelerations are nearly ideal with very small pressure- head losses due to friction, turbulence or viscous effects ; however, low loss decelerations are more difficult to achieve [12,3].
Invention No. 1 in a Perpetual Motion Mode.
In Section 2 an isentropic air motor is described with an extremely large amplification of flow power in a De Laval converging nozzle.The resulting available power is so many times greater than the necessary mass flow air power from a conventional low efficiency vacuum pump that perpetual motion from a feed back of amplified flow power is obvious as a self- sustaining motion. However, if flow losses are considered, the operation becomes one of a decline in power towards an eventual cessation of the flow. This is best seen from an analysis of mass flow rate rather than from that of flow power.
For example, let us take a vacuum source sustaining a mass flow rate of 0.074 kg/s for a power input of 1690 watts. This flow rate corresponds to an air flow power of flow of only about 100 watts (or a vacuum source efficiency of 100/1690 = 5.9%).
If we pass this mass flow through a De Laval Nozzle ( Fig. 1) of throat diameter of 1.98 cm the flow will then be sonic at the nozzle throat. The throat power will be P= ˝ x 0.074 x 3132 = 3625 watts.
Since it only requires 100 watts of flow power to sustain 0.074 kg/s inflow to the nozzle, we then, by inserting a bypass feed-back duct connecting the throat to the system flow exit, bleed off this amount of power from the throat, and ( Figure 2) we can then shut down the inefficient vacuum source which requires 1690 watts to deliver 100 watts of flow power and the system will be self sustaining - if there are no frictional or other losses. Under this assumption the motion would be self-sustaining indefinitely.
If losses are present, the mass flow will have to decrease. This mass flow decrease is usually observed as a pressure rise or a flow velocity decrease. For frictional losses, the pressure is related to the velocity squared so that the observed velocity will drop rapidly at first and then much more slowly. Low losses will therefore exhibit a long slow decline in any self-sustaining flow velocity.
In the absence of any flow losses perpetual flow motion is possible. The practical limits and usefulness of the possibility remain to be fully investigated.
1. Munson, Bruce, R., Donald F. Young, and Theodore H. Okiishi, Fundamentals of Fluid Mechanics, Wiley and Sons, New York, 1990.
2. Shapiro, Ascher, H., The Dynamics and Thermodynamics of Compressible Fluid Flow, 2 Vols., John Wiley & Sons, New York.1954.
3. R. Courant and K.O.
Friedrichs, Supersonic Flow and Shock Waves. Interscience,
Copyright Bernard A. Power , May 2011
Section 3 To be posted in near future