[1D] Use fixed point temperature closer to unburned gas temperature

This improves convergence in cases where the domain is too short to achieve full
oxidation and the maximum temperature within the domain is significantly lower
than the flame temperature computed from equilibrium.

interfaces/cython/cantera/onedim.py

@@ -422,14 +422,14 @@ def set_initial_guess(self):
         # Pick the location of the fixed temperature point, using an existing
         # point if a reasonable choice exists
         T = self.T
-        Tmid = 0.5 * (T0 + Teq)
+        Tmid = 0.75 * T0 + 0.25 * Teq
         i = np.flatnonzero(T < Tmid)[-1] # last point less than Tmid
-        if Tmid - T[i] < 0.5 * (Tmid - T0):
+        if Tmid - T[i] < 0.2 * (Tmid - T0):
             self.set_fixed_temperature(T[i])
-        elif T[i+1] - Tmid < 0.5 * (Teq - Tmid):
+        elif T[i+1] - Tmid < 0.2 * (Teq - Tmid):
             self.set_fixed_temperature(T[i+1])
         else:
-            self.set_fixed_temperature(0.5 * (T[i] + T[i+1]))
+            self.set_fixed_temperature(Tmid)
 
         for n in range(self.gas.n_species):
             self.set_profile(self.gas.species_name(n),