Closes #86, add Pr_pT and tests.

Author: raldridge11

Reference/EnglishUnits.xlsm

@@ -1273,6 +1273,23 @@ def my_ps(pressure, entropy):
 #Rem   tc = toSIunit_tc(tc_pT(p, T))
 #Rem   Pr_pT = Cp * 1000 * my / tc
 #Rem End Function
+def Pr_pT(pressure, temperature):
+    heatCapacity = cp_pT(pressure, temperature)
+    viscosity = my_pT(pressure, temperature)
+    thermalConductivity = tc_pT(pressure, temperature)
+
+    if Constants._errorValue in (heatCapacity, viscosity, thermalConductivity):
+        return Constants._errorValue
+
+    if englishUnits:
+        heatCapacity = Convert.toSIUnit(heatCapacity, 'entropy')
+        viscosity = Convert.toSIUnit(viscosity, 'viscosity')
+        thermalConductivity = Convert.toSIUnit(thermalConductivity, 'thermal conductivity')
+
+    
+    return heatCapacity*1000.0*viscosity/thermalConductivity
+
+
 #Rem Function Pr_ph(ByVal p As Double, ByVal h As Double) As Double
 #Rem   Dim Cp As Double
 #Rem   Dim my As Double

Reference/SIUnits.xlsm

@@ -0,0 +1,42 @@
+# -*- coding: utf-8 -*-
+'''
+* Water and steam properties according to IAPWS IF-97
+* By Magnus Holmgren, www.x-eng.com
+* The steam tables are free and provided as is.
+* We take no responsibilities for any errors in the code or damage thereby.
+* You are free to use, modify and distribute the code as long as authorship is properly acknowledged.
+* Please notify me at magnus@x-eng.com if the code is used in commercial applications
+'''
+import os
+import sys
+import unittest
+
+import numpy as np
+
+file_directory = os.path.dirname(__file__)
+src_path = os.path.join(os.path.abspath(file_directory), "..", "XSteamPython")
+sys.path.append(src_path)
+import Data
+import XSteamPython as stm
+
+class Test_Pr_pT(unittest.TestCase):
+
+    def tearDown(self):
+        stm.englishUnits = False
+
+    def test_Pr_pT(self):
+        pressure, temperature, prandtlCompare = Data.getTwoDimensionalTestData('SIUnits_Pr_pT.npz')
+        prandtl = Data.calculatePropertyFromTwoDimensions(stm.Pr_pT, pressure, temperature)
+        np.testing.assert_array_almost_equal(prandtl, prandtlCompare, decimal=1)
+
+    def test_Pr_pT_English(self):
+        stm.englishUnits = True
+        pressure, temperature, prandtlCompare = Data.getTwoDimensionalTestData('EnglishUnits_Pr_pT.npz')
+        prandtl = Data.calculatePropertyFromTwoDimensions(stm.Pr_pT, pressure, temperature)
+        np.testing.assert_array_almost_equal(prandtl, prandtlCompare, decimal=1)
+
+    def test_Pr_pT_error(self):
+        self.assertAlmostEqual(stm.Pr_pT(-1.0, -1.0), 2015.0, places=2)
+
+if __name__ == '__main__':
+    unittest.main()