Reduced Helmholtz equation of state: carbon dioxide

Water equation of state: You can see the full, state-of-the-art equation of state for water, which also uses a reduced Helmholtz approach: the IAPWS 1995 formulation Wagner & Pruß, 2002. This equation is state is available using CoolProp with the Water fluid.

One modern approach for calculating thermodynamic properties of real fluids uses a reduced Helmholtz equation of state, using the reduced Helmholtz free energy function $\alpha$ :

\alpha (\tau, \delta) = \frac{a}{RT} = \frac{u - Ts}{RT}

(1)

which is a function of reduced density $\delta$ and reduced temperature $\tau$ :

\delta = \frac{\rho}{\rho_{\text{crit}}} \quad \text{and} \quad \tau = \frac{T_{\text{crit}}}{T}

(2)

The reduced Helmhotz free energy function, $\alpha(\tau, \delta)$ , is given as the sum of ideal gas and residual components:

\alpha(\tau, \delta) = \alpha_{IG} (\tau, \delta) + \alpha_{\text{res}} (\tau, \delta) \;,

(3)

which are both given as high-order fits using many coefficients:

import matplotlib.pyplot as plt
%matplotlib inline

# these are mostly for making the saved figures nicer
import matplotlib_inline.backend_inline
matplotlib_inline.backend_inline.set_matplotlib_formats('pdf', 'png')
plt.rcParams['figure.dpi']= 300
plt.rcParams['savefig.dpi'] = 300

import numpy as np
import cantera as ct
from scipy import integrate, optimize

from pint import UnitRegistry
ureg = UnitRegistry()
Q_ = ureg.Quantity

import sympy
sympy.init_printing(use_latex='mathjax')

T, R, tau, delta = sympy.symbols('T, R, tau, delta', real=True)

a_vars = sympy.symbols('a0, a1, a2, a3, a4, a5, a6, a7', real=True)
theta_vars = sympy.symbols('theta3, theta4, theta5, theta6, theta7', real=True)
n_vars = sympy.symbols('n0, n1, n2, n3, n4, n5, n6, n7, n8, n9, n10, n11', real=True)

alpha_ideal = sympy.log(delta) + a_vars[0] + a_vars[1]*tau + a_vars[2]*sympy.log(tau)
for i in range(3, 8):
    alpha_ideal += a_vars[i] * sympy.log(1.0 - sympy.exp(-tau * theta_vars[i-3]))
display(sympy.Eq(sympy.symbols('alpha_IG'), alpha_ideal))

alpha_res = (
    n_vars[0] * delta * tau**0.25 + 
    n_vars[1] * delta * tau**1.25 + 
    n_vars[2] * delta * tau**1.50 + 
    n_vars[3] * delta**3 * tau**0.25 + 
    n_vars[4] * delta**7 * tau**0.875 + 
    n_vars[5] * delta * tau**2.375 * sympy.exp(-delta) + 
    n_vars[6] * delta**2 * tau**2 * sympy.exp(-delta) + 
    n_vars[7] * delta**5 * tau**2.125 * sympy.exp(-delta) +
    n_vars[8] * delta * tau**3.5 * sympy.exp(-delta**2) + 
    n_vars[9] * delta * tau**6.5 * sympy.exp(-delta**2) + 
    n_vars[10] * delta**4 * tau**4.75 * sympy.exp(-delta**2) + 
    n_vars[11] * delta**2 * tau**12.5 * sympy.exp(-delta**3)
    )
display(sympy.Eq(sympy.symbols('alpha_res'), alpha_res))

Carbon dioxide equation of state¶

The coefficients $a_i$ , $\theta_i$ , and $n_i$ are given for carbon dioxide:

# actual coefficients
coeffs_a = [
    8.37304456, -3.70454304, 2.500000, 1.99427042, 
    0.62105248, 0.41195293, 1.04028922, 8.327678e-2
    ]
coeffs_theta = [
    3.151630, 6.111900, 6.777080, 11.32384, 27.08792
    ]
coeffs_n = [
    0.89875108, -0.21281985e1, -0.68190320e-1, 0.76355306e-1,
    0.22053253e-3, 0.41541823, 0.71335657, 0.30354234e-3,
    -0.36643143, -0.14407781e-2, -0.89166707e-1, -0.23699887e-1
    ]

Through some math, we can find an expression for pressure:

P = R T \rho \left[ 1 + \delta \left(\frac{\partial \alpha_{\text{res}}}{\partial \delta} \right)_{\tau} \right]

(4)

Use this expression to estimate the pressure at $T$ = 350 K and $v$ = 0.01 m $^3$ /kg, and compare against that obtained from Cantera. We can use our symbolic expression for $\alpha_{\text{res}} (\tau, \delta)$ and take the partial derivative:

# use Cantera fluid to get specific gas constant and critical properties
f = ct.CarbonDioxide()
gas_constant = ct.gas_constant / f.mean_molecular_weight
temp_crit = f.critical_temperature
density_crit = f.critical_density

# conditions of interest
temp = 350
specific_volume = 0.01
density = 1.0 / specific_volume

# take the partial derivative of alpha_res with respect to delta
derivative_alpha_delta = sympy.diff(alpha_res, delta)

# substitute all coefficients
derivative_alpha_delta = derivative_alpha_delta.subs(
    [(n, n_val) for n, n_val in zip(n_vars, coeffs_n)]
    )

def get_pressure(
    temp, specific_vol, fluid, derivative_alpha_delta, tau, delta
    ):
    '''Calculates pressure for reduced Helmholtz equation of state'''
    red_density = (1.0 / specific_vol) / fluid.critical_density
    red_temp_inv = fluid.critical_temperature / temp

    gas_constant = ct.gas_constant / fluid.mean_molecular_weight
    
    dalpha_ddelta = derivative_alpha_delta.subs(
        [(delta, red_density), (tau, red_temp_inv)]
        )
    
    pres = (
        gas_constant * temp * (1.0 / specific_vol) * 
        (1.0 + red_density * dalpha_ddelta)
        )
    return pres

pres = get_pressure(temp, specific_volume, f, derivative_alpha_delta, tau, delta)

print(f'Pressure: {pres / 1e6: .3f} MPa')

f.TV = temp, specific_volume
print(f'Cantera pressure: {f.P / 1e6: .3f} MPa')

Pressure:  5.464 MPa
Cantera pressure:  5.475 MPa

Our calculation and that from Cantera agree fairly well! They are not exactly the same because Cantera uses a slightly different formulation for the equation of state.

Let’s compare the calculations now for a range of specific volumes and multiple temperatures:

fig, ax = plt.subplots(figsize=(8, 4))

specific_volumes = np.geomspace(0.001, 0.01, num=20)
temperatures = [300, 400, 500]

for temp in temperatures:
    pressures = np.zeros(len(specific_volumes))
    pressures_cantera = np.zeros(len(specific_volumes))
    
    for idx, spec_vol in enumerate(specific_volumes):
        pressures[idx] = get_pressure(
            temp, spec_vol, f, 
            derivative_alpha_delta, tau, delta
            )
        
        f.TV = temp, spec_vol
        pressures_cantera[idx] = f.P
    
    ax.loglog(specific_volumes, pressures/1000., 'o', color='blue')
    ax.loglog(specific_volumes, pressures_cantera/1000., color='blue')

bbox_props = dict(boxstyle='round', fc='w', ec='0.3', alpha=0.9)
ax.text(2e-3, 4e3, '300 K', ha='center', va='center', bbox=bbox_props)
ax.text(2e-3, 1.6e4, '400 K', ha='center', va='center', bbox=bbox_props)
ax.text(2e-3, 7e4, '500 K', ha='center', va='center', bbox=bbox_props)

ax.legend(['Reduced Helmholtz', 'Cantera'])
plt.grid(True, which='both')
plt.xlabel('Specific volume (m^3/kg)')
plt.ylabel('Pressure (kPa)')
fig.tight_layout()
plt.show()

We can see that the pressure calculated using the reduced Helmholtz equation of state matches closely with that from Cantera, which uses a different but similarly advanced equation of state.

References¶

Wagner, W., & Pruß, A. (2002). The IAPWS Formulation 1995 for the Thermodynamic Properties of Ordinary Water Substance for General and Scientific Use. Journal of Physical and Chemical Reference Data, 31(2), 387–535. 10.1063/1.1461829