[Thermo] Fix pure fluid state restoration after property calculation

After calculating properties using finite differences (e.g. cp), the fluid was
returned to its original state by setting the state in terms of pressure and
temperature. However, because setting the state using these variables is an
iterative calculation with some error tolerance, the state was not restored
exactly. By restoring the state based on the initial temperature and density,
the restored state is identical to the initial state.

interfaces/cython/cantera/test/test_purefluid.py

@@ -89,22 +89,31 @@ def test_set_minmax(self):
     def check_fd_properties(self, T1, P1, T2, P2, tol):
         # Properties which are computed as finite differences
         self.water.TP = T1, P1
+        h1a = self.water.enthalpy_mass
         cp1 = self.water.cp_mass
         cv1 = self.water.cv_mass
         k1 = self.water.isothermal_compressibility
         alpha1 = self.water.thermal_expansion_coeff
+        h1b = self.water.enthalpy_mass
 
         self.water.TP = T2, P2
+        h2a = self.water.enthalpy_mass
         cp2 = self.water.cp_mass
         cv2 = self.water.cv_mass
         k2 = self.water.isothermal_compressibility
         alpha2 = self.water.thermal_expansion_coeff
+        h2b = self.water.enthalpy_mass
 
         self.assertNear(cp1, cp2, tol)
         self.assertNear(cv1, cv2, tol)
         self.assertNear(k1, k2, tol)
         self.assertNear(alpha1, alpha2, tol)
 
+        # calculating these finite difference properties should not perturbe the
+        # state of the object
+        self.assertEqual(h1a, h1b)
+        self.assertEqual(h2a, h2b)
+
     def test_properties_near_min(self):
         self.check_fd_properties(self.water.min_temp*(1+1e-5), 101325,
                                  self.water.min_temp*(1+1e-4), 101325, 1e-2)

src/tpx/Sub.cpp

@@ -86,6 +86,7 @@ double Substance::cv()
 double Substance::cp()
 {
     double Tsave = T, dt = 1.e-4*T;
+    double RhoSave = Rho;
     double T1 = std::max(Tmin(), Tsave - dt);
     double T2 = std::min(Tmax(), Tsave + dt);
     double p0 = P();
@@ -115,13 +116,14 @@ double Substance::cp()
     }
     double s2 = s();
 
-    Set(PropertyPair::TP, Tsave, p0);
+    Set(PropertyPair::TV, Tsave, 1.0 / RhoSave);
     return T*(s2 - s1)/(T2-T1);
 }
 
 double Substance::thermalExpansionCoeff()
 {
     double Tsave = T, dt = 1.e-4*T;
+    double RhoSave = Rho;
     double T1 = std::max(Tmin(), Tsave - dt);
     double T2 = std::min(Tmax(), Tsave + dt);
     double p0 = P();
@@ -153,13 +155,14 @@ double Substance::thermalExpansionCoeff()
     }
     double v2 = v();
 
-    Set(PropertyPair::TP, Tsave, p0);
+    Set(PropertyPair::TV, Tsave, 1.0 / RhoSave);
     return 2.0*(v2 - v1)/((v2 + v1)*(T2-T1));
 }
 
 double Substance::isothermalCompressibility()
 {
     double Psave = P(), dp = 1.e-4*Psave;
+    double RhoSave = Rho;
     double x0 = x();
 
     if (TwoPhase()) {
@@ -191,7 +194,7 @@ double Substance::isothermalCompressibility()
     }
     double v2 = v();
 
-    Set(PropertyPair::TP, T, Psave);
+    Set(PropertyPair::TV, T, 1.0 / RhoSave);
     return -(v2 - v1)/(v0*(P2-P1));
 }