[Python/1D] Generalize initial profile generation for diffusion flames

The calculation of the stoichiometric mixture composition now works for
arbitrary fuel and oxidizer mixtures, including multi-component fuels and fuels
containing oxygen. In addition, it is not necessary to specify any additional
arguments when generating the initial profile.

interfaces/cython/cantera/examples/onedim/diffusion_flame.py

@@ -49,12 +49,6 @@ f.oxidizer_inlet.T = tin_o
 f.flame.set_steady_tolerances(default=tol_ss)
 f.flame.set_transient_tolerances(default=tol_ts)
 
-# construct the initial solution estimate. To do so, it is necessary
-# to specify the fuel species. If a fuel mixture is being used,
-# specify a representative species here for the purpose of
-# constructing an initial guess.
-f.set_initial_guess(fuel='C2H6')
-
 # Set the boundary emissivities
 f.set_boundary_emissivities(0.0, 0.0)
 # Turn radiation off

interfaces/cython/cantera/examples/onedim/diffusion_flame_batch.py

@@ -70,7 +70,6 @@ def interrupt_extinction(t):
 f.set_interrupt(interrupt_extinction)
 
 # Initialize and solve
-f.set_initial_guess(fuel='H2')
 print('Creating the initial solution')
 f.solve(loglevel=0, refine_grid=refine)
 

interfaces/cython/cantera/examples/onedim/diffusion_flame_extinction.py

@@ -59,7 +59,6 @@ f.set_grid_min(1e-20)
 temperature_limit_extinction = 500  # K
 
 # Initialize and solve
-f.set_initial_guess(fuel='H2')
 print('Creating the initial solution')
 f.solve(loglevel=0, refine_grid=refine)
 

interfaces/cython/cantera/onedim.py

@@ -494,59 +494,49 @@ class CounterflowDiffusionFlame(FlameBase):
         super(CounterflowDiffusionFlame, self).__init__(
                 (self.fuel_inlet, self.flame, self.oxidizer_inlet), gas, grid)
 
-    def set_initial_guess(self, fuel, oxidizer=None, stoich=None):
+    def set_initial_guess(self, fuel=None, oxidizer=None, stoich=None):
         """
-        Set the initial guess for the solution. The fuel species must be
-        specified:
-
-        >>> f.set_initial_guess(fuel='CH4')
-
-        The oxidizer and corresponding stoichiometry must be specified if it
-        is not 'O2'. The initial guess is generated by assuming infinitely-
-        fast chemistry.
+        Set the initial guess for the solution. The initial guess is generated
+        by assuming infinitely-fast chemistry.
         """
+        if fuel is not None or oxidizer is not None or stoich is not None:
+            warnings.warn(
+                'Arguments to CounterflowDiffusionFlame.set_initial_guess are '
+                'unused and deprecated and will be removed after Cantera 2.3.')
 
         super(CounterflowDiffusionFlame, self).set_initial_guess()
 
-        if stoich is None:
-            if oxidizer is None:
-                if 'O2' in self.gas.species_names:
-                    oxidizer = 'O2'
-                elif 'o2' in self.gas.species_names:
-                    oxidizer = 'o2'
-                else:
-                    raise Exception("Can't identify default oxidizer species.")
-            if oxidizer.upper() == 'O2':
-                stoich = 0.0
-                if 'H' in self.gas.element_names:
-                    stoich += 0.25 * self.gas.n_atoms(fuel, 'H')
-                if 'C' in self.gas.element_names:
-                    stoich += self.gas.n_atoms(fuel, 'C')
-            else:
-                raise Exception('oxidizer/fuel stoichiometric ratio must be '
-                                'specified since the oxidizer is not O2')
-
-        kFuel = self.gas.species_index(fuel)
-        kOx = self.gas.species_index(oxidizer)
-
-        s = stoich * self.gas.molecular_weights[kOx] / self.gas.molecular_weights[kFuel]
-        phi = s * self.fuel_inlet.Y[kFuel] / self.oxidizer_inlet.Y[kOx]
-        zst = 1.0 / (1.0 + phi)
+        moles = lambda el: (self.gas.elemental_mass_fraction(el) /
+                            self.gas.atomic_weight(el))
 
+        # Compute stoichiometric mixture composition
         Yin_f = self.fuel_inlet.Y
-        Yin_o = self.oxidizer_inlet.Y
-        Yst = zst * Yin_f + (1.0 - zst) * Yin_o
-
         self.gas.TPY = self.fuel_inlet.T, self.P, Yin_f
         mdotf = self.fuel_inlet.mdot
         u0f = mdotf / self.gas.density
         T0f = self.fuel_inlet.T
 
+        sFuel = moles('O')
+        if 'C' in self.gas.element_names:
+            sFuel -= 2 * moles('C')
+        if 'H' in self.gas.element_names:
+            sFuel -= 0.5 * moles('H')
+
+        Yin_o = self.oxidizer_inlet.Y
         self.gas.TPY = self.oxidizer_inlet.T, self.P, Yin_o
         mdoto = self.oxidizer_inlet.mdot
-        u0o = mdoto/self.gas.density
+        u0o = mdoto / self.gas.density
         T0o = self.oxidizer_inlet.T
 
+        sOx = moles('O')
+        if 'C' in self.gas.element_names:
+            sOx -= 2 * moles('C')
+        if 'H' in self.gas.element_names:
+            sOx -= 0.5 * moles('H')
+
+        zst = 1.0 / (1 - sFuel / sOx)
+        Yst = zst * Yin_f + (1.0 - zst) * Yin_o
+
         # get adiabatic flame temperature and composition
         Tbar = 0.5 * (T0f + T0o)
         self.gas.TPY = Tbar, self.P, Yst
@@ -558,6 +548,8 @@ class CounterflowDiffusionFlame(FlameBase):
         zz = self.flame.grid
         dz = zz[-1] - zz[0]
         a = (u0o + u0f)/dz
+        kOx = (self.gas.species_index('O2') if 'O2' in self.gas.species_names else
+               self.gas.species_index('o2'))
         f = np.sqrt(a / (2.0 * self.gas.mix_diff_coeffs[kOx]))
 
         x0 = mdotf * dz / (mdotf + mdoto)

interfaces/cython/cantera/test/test_onedim.py

@@ -412,7 +412,7 @@ class TestDiffusionFlame(utilities.CanteraTest):
         self.sim.oxidizer_inlet.X = oxidizer
         self.sim.oxidizer_inlet.T = T_ox
 
-        self.sim.set_initial_guess(fuel='H2')
+        self.sim.set_initial_guess()
 
     def solve_fixed_T(self):
         # Solve with the energy equation disabled