iapws.ammonia module - Calculation of density

[flork1991]

Hi all,

while comparing ammonia densities against other property modules I noticed some abnormality around Tc= 405.4K with huge changes in density.

`from iapws import ammonia as iapws

NH3_1 = iapws.NH3(T=273.15+115,P=0.1)
NH3_1.calculo()
print(f"Density at 110°C, 1 bar: {NH3_1.rho} [kg/m³]")
print(f"x = {NH3_1.x}")
#print(f"h = {NH3_1.x} kJ/kg")

NH3_2 = iapws.NH3(T=273.15+130,P=0.1)
NH3_2.calculo()
print(f"Density at 130°C, 1 bar: {NH3_2.rho} [kg/m³]")
print(f"x = {NH3_2.x}")
#print(f"h = {NH3_2.x} kJ/kg")

NH3_3 = iapws.NH3(T=273.15+140,P=0.1)
NH3_3.calculo()
print(f"Density at 140°C, 1 bar: {NH3_3.rho} [kg/m³]")
print(f"x = {NH3_3.x}")`

Best regards,
Florian

[jjgomera]

Hi, the output in terminal show the problem, the density solver iteration fail, the problem, iapws use the returned value to do the properties calculation

from iapws.ammonia import NH3

ammonia = NH3(T=273.15+130, P=0.1)
print(f"Density at 130°C, 1 bar: {ammonia.rho} [kg/m³]")
print(f"x = {ammonia.x}")

/home/jjgomera/Programacion/iapws/iapws/iapws95.py:141: RuntimeWarning: overflow encountered in exp
  fird += n*exp(-g*delta**c)*delta**(d-1)*tau**t*(d-g*c*delta**c)
/home/jjgomera/Programacion/iapws/iapws/iapws95.py:141: RuntimeWarning: invalid value encountered in add
  fird += n*exp(-g*delta**c)*delta**(d-1)*tau**t*(d-g*c*delta**c)
/usr/local/lib/python3.9/dist-packages/scipy/optimize/minpack.py:175: RuntimeWarning: The iteration is not making good progress, as measured by the 
  improvement from the last ten iterations.
  warnings.warn(msg, RuntimeWarning)
Density at 130°C, 1 bar: 156.7684068421287 [kg/m³]
x = 1

By the way, the calculo procedure is internal and not necessary call from user.

Dirty solution, define a initial value of density like that:

ammonia = NH3(T=273.15+130,P=0.1, rho0=0.5)

and you get the correct value

Density at 130°C, 1 bar: 0.5097610127632395 [kg/m³]
x = 1

Thanks for report, I have to improve density solver convergence