# I Use of Entropy for a Control Volume in Energy Balance

1. Mar 12, 2016

### Acnhduy

Hi all,

I'm having some trouble figuring out why entropy is used instead of enthalpy for an open system.

From what I understand, an open system uses entropy to calculate internal energy. Since the control volume is constant (i.e. Δv = 0), wouldn't using : h = u + PΔv effectively be h = u? So in this case, wouldn't you use enthalpy instead of entropy since volume isn't changing?

Just to confirm, in an open system like a turbine or something, with the assumption of no heat transfer and negligible potential energy, the energy balance becomes:

W = ΔU
W = m2(h2 + ke2 + pe2) - m1(h1 + ke1 + pe1)

... and with steady flow ...

w = (h2 + ke2) - (h1 + ke1)

Please let me know if I am misunderstanding something, thanks!

Last edited: Mar 12, 2016
2. Mar 12, 2016