Update compressibility calculation for real fluid in CanteraMixture.H (#303)

the compressibility for constant enthalpy was used

Author: wengzf20

src/dfCanteraMixture/CanteraMixture.H

@@ -119,8 +119,24 @@ public:
             const scalar& T
         ) const
         {
-            // for ideal gas, psi = CanteraGas_->density()/p = 1/RT
-            return CanteraGas_->density()/p;
+            const Cantera::AnyMap& map = CanteraGas_->input();
+            if(map["thermo"].asString()=="Peng-Robinson" || map["thermo"].asString()=="Redlich-Kwong")
+            {
+                // for non-ideal EoS, psi = \partial rho/ \partial p at constant enthalpy
+                // Currently, Peng-Robinson and Redlich-Kwong are available in Cantera
+                // The partial derivative was approximated using backward difference with a step size dx
+                double h0 = CanteraGas_->enthalpy_mass();
+                double dx = 0.0001;
+                double rho1 = CanteraGas_->density();
+                CanteraGas_->setState_HP(h0,p*(1-dx));
+                double rho2 = CanteraGas_->density();
+                return (rho1-rho2)/(dx*p);
+            }
+            else
+            {
+                // for ideal gas, psi = W/RT
+                return CanteraGas_->meanMolecularWeight()/CanteraGas_->RT();
+            }
         }
 
         scalar rho