[samples] Avoid import errors for graphviz draw

samples/python/reactors/mix1.py

@@ -62,12 +62,12 @@
 # Create two mass flow controllers connecting the upstream reservoirs to the
 # mixer, and set their mass flow rates to values corresponding to
 # stoichiometric combustion.
-mfc1 = ct.MassFlowController(res_a, mixer, mdot=rho_a*2.5/0.21)
-mfc2 = ct.MassFlowController(res_b, mixer, mdot=rho_b*1.0)
+mfc1 = ct.MassFlowController(res_a, mixer, mdot=rho_a*2.5/0.21, name="Air inlet")
+mfc2 = ct.MassFlowController(res_b, mixer, mdot=rho_b*1.0, name="Fuel inlet")
 
 # %%
 # Connect the mixer to the downstream reservoir with a valve.
-outlet = ct.Valve(mixer, downstream, K=10.0)
+outlet = ct.Valve(mixer, downstream, K=10.0, name="Valve")
 
 sim = ct.ReactorNet([mixer])
 
@@ -84,4 +84,8 @@
 # %%
 # Show the network structure
 # --------------------------
-diagram = sim.draw(print_state=True, species="X")
+try:
+    diagram = sim.draw(print_state=True, species="X")
+    diagram.view("mix1.gv")
+except ImportError as err:
+    print(f"Unable to show network structure:\n{err}")

samples/python/reactors/reactor2.py

@@ -57,12 +57,12 @@
 # conduct heat
 
 # add a flexible wall (a piston) between r2 and r1
-w = ct.Wall(r2, r1, A=1.0, K=0.5e-4, U=100.0)
+w = ct.Wall(r2, r1, A=1.0, K=0.5e-4, U=100.0, name="Piston")
 
 # heat loss to the environment. Heat loss always occur through walls, so we
 # create a wall separating r2 from the environment, give it a non-zero area,
 # and specify the overall heat transfer coefficient through the wall.
-w2 = ct.Wall(r2, env, A=1.0, U=500.0)
+w2 = ct.Wall(r2, env, A=1.0, U=500.0, name="External Wall")
 
 sim = ct.ReactorNet([r1, r2])
 
@@ -118,4 +118,8 @@
 # %%
 # Show the final state
 # --------------------
-sim.draw(print_state=True, species="X")
+try:
+    diagram = sim.draw(print_state=True, species="X")
+    diagram.view("reactor2.gv")
+except ImportError as err:
+    print(f"Unable to show network structure:\n{err}")