Viscous dissipation and isenthalpic expansion

[arkie87]

This question pertains to "viscous dissipation" term in the energy equation in flows.

I am a bit confused about viscous dissipation. It seems to me that viscous dissipation is essentially the same thing as pumping power, such that if I pump 1 m3/s of air at 1 kPa, it takes 1 kW. To get 1 kPa pressure drop, I need a lot of friction, and that friction will cause viscous dissipation, which will result in 1 kW of heating i.e. ~0.8 K temperature rise in the air.

For instance, for the simple case of steady state, fully developed laminar flow between parallel plates, one case show that the viscous dissipation term exactly equals the pumping power.

However, for an ideal gas, enthalpy is only a function of temperature. If I expand the air through a valve instead, thermodynamics tells me the temperature wont have changed if the process is isenthalpic.

How do i reconcile these two? Why wont the air going through the valve heat up if it is dissipating the same amount of power?