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fix(cubics): replace rounded SRK/PR Omega coefficients with exact values (#2745)

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Replaces truncated literature values (e.g. 0.42747, 0.07780) with the
full-precision analytical constants from Bell & Deiters, IECR 2021
(https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=931868).

Closes #2742

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>
This commit is contained in:
Ian Bell
2026-04-19 20:12:57 -04:00
committed by GitHub
parent 497c044d5b
commit df712bfa26
2 changed files with 12 additions and 10 deletions
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14
src/Backends/Cubics/GeneralizedCubic.cpp
View File

@@ -634,13 +634,13 @@ double AbstractCubic::d3_alphar_dxidxjdxk(double tau, double delta, const std::v
}
double SRK::a0_ii(std::size_t i) {
// Values from Soave, 1972 (Equilibrium constants from a ..)
double a = 0.42747 * R_u * R_u * Tc[i] * Tc[i] / pc[i];
// Exact value: 1/(9*(2^(1/3)-1)); see Bell and Deiters, IECR, 2021
double a = 0.42748023335403414043900347952220 * R_u * R_u * Tc[i] * Tc[i] / pc[i];
return a;
}
double SRK::b0_ii(std::size_t i) {
// Values from Soave, 1972 (Equilibrium constants from a ..)
double b = 0.08664 * R_u * Tc[i] / pc[i];
// Exact value: (2^(1/3)-1)/3; see Bell and Deiters, IECR, 2021
double b = 0.08664034999649577215890158147700 * R_u * Tc[i] / pc[i];
return b;
}
double SRK::m_ii(std::size_t i) {
@@ -651,11 +651,13 @@ double SRK::m_ii(std::size_t i) {
}
double PengRobinson::a0_ii(std::size_t i) {
double a = 0.45724 * R_u * R_u * Tc[i] * Tc[i] / pc[i];
// Exact value; see Bell and Deiters, IECR, 2021
double a = 0.45723552892138218938000849856422 * R_u * R_u * Tc[i] * Tc[i] / pc[i];
return a;
}
double PengRobinson::b0_ii(std::size_t i) {
double b = 0.07780 * R_u * Tc[i] / pc[i];
// Exact value; see Bell and Deiters, IECR, 2021
double b = 0.07779607390388455972148597969400 * R_u * Tc[i] / pc[i];
return b;
}
double PengRobinson::m_ii(std::size_t i) {

8
src/Backends/Helmholtz/HelmholtzEOSMixtureBackend.cpp
View File

@@ -2650,7 +2650,7 @@ CoolPropDbl HelmholtzEOSMixtureBackend::SRK_covolume() {
// Get the parameters for the cubic EOS
CoolPropDbl Tc = get_fluid_constant(i, iT_critical), pc = get_fluid_constant(i, iP_critical);
CoolPropDbl R = 8.3144598;
b += mole_fractions[i] * 0.08664 * R * Tc / pc;
b += mole_fractions[i] * 0.08664034999649577215890158147700 * R * Tc / pc;
}
return b;
}
@@ -2843,16 +2843,16 @@ CoolPropDbl HelmholtzEOSMixtureBackend::solver_rho_Tp_SRK(CoolPropDbl T, CoolPro
for (std::size_t i = 0; i < components.size(); ++i) {
CoolPropDbl Tci = components[i].EOS().reduce.T, pci = components[i].EOS().reduce.p, acentric_i = components[i].EOS().acentric;
CoolPropDbl m_i = 0.480 + 1.574 * acentric_i - 0.176 * pow(acentric_i, 2);
CoolPropDbl b_i = 0.08664 * R_u * Tci / pci;
CoolPropDbl b_i = 0.08664034999649577215890158147700 * R_u * Tci / pci;
b += mole_fractions[i] * b_i;
CoolPropDbl a_i = 0.42747 * pow(R_u * Tci, 2) / pci * pow(1 + m_i * (1 - sqrt(T / Tci)), 2);
CoolPropDbl a_i = 0.42748023335403414043900347952220 * pow(R_u * Tci, 2) / pci * pow(1 + m_i * (1 - sqrt(T / Tci)), 2);
for (std::size_t j = 0; j < components.size(); ++j) {
CoolPropDbl Tcj = components[j].EOS().reduce.T, pcj = components[j].EOS().reduce.p, acentric_j = components[j].EOS().acentric;
CoolPropDbl m_j = 0.480 + 1.574 * acentric_j - 0.176 * pow(acentric_j, 2);
CoolPropDbl a_j = 0.42747 * pow(R_u * Tcj, 2) / pcj * pow(1 + m_j * (1 - sqrt(T / Tcj)), 2);
CoolPropDbl a_j = 0.42748023335403414043900347952220 * pow(R_u * Tcj, 2) / pcj * pow(1 + m_j * (1 - sqrt(T / Tcj)), 2);
k_ij = 0;
//if (i == j){