Counterflow flame initial condition

doc/sphinx/python/onedim.rst

@@ -23,6 +23,11 @@ CounterflowDiffusionFlame
 ^^^^^^^^^^^^^^^^^^^^^^^^^
 .. autoclass:: CounterflowDiffusionFlame
 
+   .. note::
+      ``CounterflowDiffusionFlame.set_initial_guess`` accepts ``mode="linear"``
+      to initialize profiles by linearly interpolating inlet temperature and
+      composition.
+
 CounterflowPremixedFlame
 ^^^^^^^^^^^^^^^^^^^^^^^^
 .. autoclass:: CounterflowPremixedFlame

interfaces/cython/cantera/onedim.py

@@ -925,16 +925,24 @@ def __init__(self, gas, grid=None, width=None):
 
         super().__init__((self.fuel_inlet, self.flame, self.oxidizer_inlet), gas, grid)
 
-    def set_initial_guess(self, data=None, group=None):
+    def set_initial_guess(self, data=None, group=None, mode="stoich"):
         """
         Set the initial guess for the solution. By default, the initial guess
         is generated by assuming infinitely-fast chemistry. Alternatively, a
         previously calculated result can be supplied as an initial guess via
         'data' and 'key' inputs (see `FlameBase.set_initial_guess`).
+
+        :param mode:
+            Initial guess mode. ``'stoich'`` (default) uses the stoichiometric,
+            infinitely-fast chemistry guess. ``'linear'`` linearly interpolates
+            inlet temperature and composition profiles.
+            If ``data`` is provided, this option is ignored.
         """
         super().set_initial_guess(data=data, group=group)
         if data:
             return
+        if mode not in ("stoich", "linear"):
+            raise ValueError("mode must be 'stoich' or 'linear'")
 
         # Compute stoichiometric mixture composition
         Yin_f = self.fuel_inlet.Y
@@ -955,7 +963,37 @@ def set_initial_guess(self, data=None, group=None):
             raise CanteraError("Mass flow for fuel and/or oxidizer "
                                "must be positive")
 
-        zst = 1 / (1 + self.gas.stoich_air_fuel_ratio(Yin_f, Yin_o, 'mass'))
+        zz = self.flame.grid
+        dz = zz[-1] - zz[0]
+        a = (u0o + u0f)/dz
+        L = - 0.5 * (rho0o + rho0f) * a**2
+
+        x0 = np.sqrt(mdotf*u0f) * dz / (np.sqrt(mdotf*u0f) + np.sqrt(mdoto*u0o))
+        nz = len(zz)
+
+        if mode == "linear":
+            zrel = (zz - zz[0])/dz
+            T = (1.0 - zrel) * T0f + zrel * T0o
+            Y = np.outer((1.0 - zrel), Yin_f) + np.outer(zrel, Yin_o)
+            T[0] = T0f
+            T[-1] = T0o
+            Y[0] = Yin_f
+            Y[-1] = Yin_o
+
+            self.flame.set_profile('velocity', [0.0, 1.0], [u0f, -u0o])
+            self.flame.set_profile('spreadRate', [0.0, x0/dz, 1.0], [0.0, a, 0.0])
+            self.flame.set_profile("Lambda", [0.0, 1.0], [L, L])
+            self.flame.set_profile('T', zrel, T)
+            for k, spec in enumerate(self.gas.species_names):
+                self.flame.set_profile(spec, zrel, Y[:,k])
+            return
+
+        afr = self.gas.stoich_air_fuel_ratio(Yin_f, Yin_o, 'mass')
+        zst = 1 / (1 + afr)
+        if not np.isfinite(zst) or zst <= 0.0 or zst >= 1.0:
+            raise CanteraError(
+                "Stoichiometric initial guess requires a reactive fuel/oxidizer "
+                "pair; use mode='linear'.")
         Yst = zst * Yin_f + (1.0 - zst) * Yin_o
 
         # get adiabatic flame temperature and composition
@@ -966,16 +1004,9 @@ def set_initial_guess(self, data=None, group=None):
         Yeq = self.gas.Y
 
         # estimate strain rate
-        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]))
-        L = - 0.5 * (rho0o + rho0f) * a**2
-
-        x0 = np.sqrt(mdotf*u0f) * dz / (np.sqrt(mdotf*u0f) + np.sqrt(mdoto*u0o))
-        nz = len(zz)
 
         Y = np.zeros((nz, self.gas.n_species))
         T = np.zeros(nz)

interfaces/cython/cantera/onedim.pyi

@@ -346,6 +346,7 @@ class BurnerFlame(FlameBase):
         self,
         data: SolutionArray[Solution] | DataFrame | str | Path | None = None,
         group: str | None = None,
+        mode: Literal["stoich", "linear"] = "stoich",
     ) -> None: ...
     def solve(
         self,

test/python/test_onedim.py

@@ -1295,6 +1295,41 @@ def test_bad_boundary_conditions(self):
         sim.fuel_inlet.X = "H2: 1.0"
         sim.set_initial_guess()
 
+    def test_linear_initial_guess_inert(self):
+        gas = ct.Solution("h2o2.yaml")
+        gas.TP = 300, ct.one_atm
+        sim = ct.CounterflowDiffusionFlame(gas, width=0.02)
+        sim.fuel_inlet.mdot = 0.2
+        sim.oxidizer_inlet.mdot = 0.2
+        sim.fuel_inlet.X = "AR:1.0"
+        sim.oxidizer_inlet.X = "N2:1.0"
+        sim.fuel_inlet.T = 300
+        sim.oxidizer_inlet.T = 600
+
+        with pytest.raises(ValueError, match="mode must be 'stoich' or 'linear'"):
+            sim.set_initial_guess(mode="bad-mode")
+
+        with pytest.raises(ct.CanteraError, match="Stoichiometric initial guess"):
+            sim.set_initial_guess()
+
+        sim.set_initial_guess(mode="linear")
+
+        zrel = (sim.grid - sim.grid[0]) / (sim.grid[-1] - sim.grid[0])
+        k_ar = gas.species_index("AR")
+        k_n2 = gas.species_index("N2")
+
+        assert sim.T[0] == approx(sim.fuel_inlet.T)
+        assert sim.T[-1] == approx(sim.oxidizer_inlet.T)
+        assert sim.Y[k_ar, 0] == approx(1.0)
+        assert sim.Y[k_n2, 0] == approx(0.0)
+        assert sim.Y[k_ar, -1] == approx(0.0)
+        assert sim.Y[k_n2, -1] == approx(1.0)
+        assert np.allclose(sim.Y[k_ar], 1.0 - zrel)
+        assert np.allclose(sim.Y[k_n2], zrel)
+
+        sim.energy_enabled = False
+        sim.solve(loglevel=0, refine_grid=False)
+
     def run_restore_diffusionflame(self, fname):
         gas = ct.Solution("h2o2.yaml")
         sim = ct.CounterflowDiffusionFlame(gas)