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While I am studying about Mass and Energy analysis in Control volumes in Cengel and Boles I came a across a term called flow energy. The textbook definition that was given is as follows - "The amount of energy or work that needs to be done to the fluid in order to maintain the flow in a CV (Control Volume)". I am not able to differentiate between kinetic energy of the flowing fluid and the flow energy of the flowing fluid. I tried searching for answers in various textbooks but none of them has clearly mentioned the difference instead what I found frequently was the definition of flow energy alone.

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  • $\begingroup$ What do you understand by the terms: potential energy, kinetic energy and internal energy? $\endgroup$
    – Solar Mike
    Aug 29, 2023 at 13:40
  • $\begingroup$ Potential energy - Energy possessed by the CV by virtue of it's elevation from a datum Kinetic energy - Energy possessed by the CV by virtue of its motion. Internal energy - Energy possessed by the CV by various other sources like the electromagnetic interaction in the CV. $\endgroup$
    – Prasanna B
    Aug 29, 2023 at 13:55
  • $\begingroup$ Isn't the difference in loses, e.g. due to friction? In order to maintain the flow, you need to supply enough energy to overcome the loses. So part of the flow energy would go into the fluid (kinetic, potential and internal) and the rest could be the loses. $\endgroup$ Sep 5, 2023 at 7:31

2 Answers 2

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Pressure energy, P*V, is a useful term to use when trying to determine the amount of energy in a vessel that is stagnant with no K.E. if the such a vessel where to have some flow due to some orifices perse then you can say that the fluid also has K.E. I have that helps.

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  • $\begingroup$ I require more clarification with your answer because in the same textbook it has been mentioned that the total energy of a non-flowing fluid has 3 components (Internal energy, Kinetic energy and Potential energy) and flowing fluid has 4 components (Flow energy, Internal energy, Kinetic energy and Potential energy). This has raised 2 confusions: 1. How does non flowing fluid has K.E. and 2. How is K.E. different from Flow energy. $\endgroup$
    – Prasanna B
    Aug 29, 2023 at 13:37
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Picture an automobile in which the car is in motion; the brake is applied to deaccelerate the car, but the brake fluid is not now flowing and is static but under pressure and has the velocity of the car.

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