Given the following definition of flow work:
Reading and following the derivations I understand that in theory, this is only accurate when the fluid accelerates from rest. However, if we consider that a fluid is already in a state of steady flow we are still expected to use enthalpy instead of instead of internal energy. Essentially, this flow work is compensation work (between the inlet and outlet) required to keep a continuous flow. But, if we have fluid in a frictionless pipe already moving steadily (at constant velocity), I understand that we should still expect flow work. Therefore, my question is, what is this flow work compensating for?
To explain the compensation aspect I would suggest an FBD of each particle in a steady flow. There would be no net force and equal and opposite Pressure/Area forces from every direction. Neglect Gravity. Now this flow work (Force*distance) is defined as maintaining flow, so what is the force that is resisting flow? We would assume surface friction but I believe that this still exists in frictionless pipes.
Basically, my question is what is causing this change in energy (IN THEORETICAL SENSE) and why can we represent it as Pv?
Intuitively, the only explanation I can come up with is that this has something to do with the friction within the fluid particle itself due to intermolecular bonds, or that in the frictionless theoretical example flow work does not exist and that flow work is defined to be an approximation to the friction in reality.
In the case of intermolecular bonds, would we not expect there to be specific constants due to the strength of bonds? Also couldn't this friction become negligible when the particles are at the same speed? It seems odd that it would make such a difference.
If it is the friction between the pipe, shouldn't the calculating be concerned with surface area and coefficient of friction?
Please let me know the faults in my logic and the intuition behind what this term actually is. In addition, why Pv is a valid representation. Thanks in advance