Negative values for Cp

[pklad...@gmail.com]

Greetings.

I require the values of the specific heat capacity under constant pressure, Cp, within the two-phase liquid-vapor region. The values for the saturated liquid and vapor are included. Unfortunately i receive negative values near the saturated liquid region, usually around steam quality of 0.10. Cp was called using pressure, quality and pressure, enthalpy input pairs:

    Cpx=CP.PropsSI('C','P',plow,'Q',qx,working_fluid)

    Cpx=CP.PropsSI('C','P',plow,'H',hx,working_fluid)

The pressure corresponds to the saturation pressure for 30 oC. The working fluids tested were R124, Isobutane, R245fa, Isopentane and Cyclopentane.

Any ideas?

Thank you in advance.

P.S. 

Software used:

CoolProp v6.4.1

Python 2.7.16

Debian 10

[pklad...@gmail.com]

I forgot to mention that i am using the Python wrapper for NBP reference state.

[Matthis Thorade]

cp is not defined in the two-phase region of pure fluids.

cp would be (dh/dT)_p but for pure fluids in the two-phase region dT is zero at constant p, evaporation temperature and pressure are coupled, so if pressure is constant evaporation happens at a constant temperature. cp is therefore infinite in the two-phase region.

Not sure whether CoolProp should return any value here, probably it shoudl return an error instead.

[pklad...@gmail.com]

Thank you for quick response.

I see what you mean. My objective is to calculate the overall heat transfer coefficient of a working fluid undergoing phase change. There are correlations that take, among other properties, Cp into account. 

I must admit that this is rather perplexing. I will look into it further and post an answer, should i find one.