[Python/Examples] Updated premixed counterflow twin flame example

Corrected method of calculating strain-rate, and added functionality to compute
consumption speed

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

@@ -9,9 +9,50 @@
 import cantera as ct
 import numpy as np
 
+# Differentiation function for data that has variable grid spacing Used here to
+# compute normal strain-rate
+def derivative(x, y):
+    dydx = np.zeros(y.shape, y.dtype.type)
+
+    dx = np.diff(x)
+    dy = np.diff(y)
+    dydx[0:-1] = dy/dx
+
+    dydx[-1] = (y[-1] - y[-2])/(x[-1] - x[-2])
+
+    return dydx
+
+def computeStrainRates(oppFlame):
+    # Compute the derivative of axial velocity to obtain normal strain rate
+    strainRates = derivative(oppFlame.grid, oppFlame.u)
+
+    # Obtain the location of the max. strain rate upstream of the pre-heat zone.
+    # This is the characteristic strain rate
+    maxStrLocation = abs(strainRates).argmax()
+    minVelocityPoint = oppFlame.u[:maxStrLocation].argmin()
+
+    # Characteristic Strain Rate = K
+    strainRatePoint = abs(strainRates[:minVelocityPoint]).argmax()
+    K = abs(strainRates[strainRatePoint])
+
+    return strainRates, strainRatePoint, K
+
+def computeConsumptionSpeed(oppFlame):
+
+    Tb = max(oppFlame.T)
+    Tu = min(oppFlame.T)
+    rho_u = max(oppFlame.density)
+
+    integrand = oppFlame.heat_release_rate/oppFlame.cp
+
+    I = np.trapz(integrand, oppFlame.grid)
+    Sc = I/(Tb - Tu)/rho_u
+
+    return Sc
+
 # This function is called to run the solver
 def solveOpposedFlame(oppFlame, massFlux=0.12, loglevel=1,
-                      ratio=3, slope=0.15, curve=0.25, prune=0.05):
+                      ratio=2, slope=0.3, curve=0.3, prune=0.05):
     """
     Execute this function to run the Oppposed Flow Simulation This function
     takes a CounterFlowTwinPremixedFlame object as the first argument
@@ -23,14 +64,15 @@ def solveOpposedFlame(oppFlame, massFlux=0.12, loglevel=1,
     oppFlame.show_solution()
     oppFlame.solve(loglevel, auto=True)
 
-    # Compute the strain rate, just before the flame. It also turns out to the
-    # maximum. This is the strain rate that computations comprare against, like
-    # when plotting Su vs. K
-    peakStrain = np.max(np.gradient(oppFlame.u, np.gradient(oppFlame.grid)))
-    return np.max(oppFlame.T), peakStrain
+    # Compute the strain rate, just before the flame. This is not necessarily
+    # the maximum We use the max. strain rate just upstream of the pre-heat zone
+    # as this is the strain rate that computations comprare against, like when
+    # plotting Su vs. K
+    strainRates, strainRatePoint, K = computeStrainRates(oppFlame)
 
+    return np.max(oppFlame.T), K, strainRatePoint
 
-# Use the standard GRI3.0 Mechanism for CH4
+# Select the reaction mechanism
 gas = ct.Solution('gri30.cti')
 
 # Create a CH4/Air premixed mixture with equivalence ratio=0.75, and at room
@@ -39,26 +81,34 @@ def solveOpposedFlame(oppFlame, massFlux=0.12, loglevel=1,
 gas.TP = 300, ct.one_atm
 
 # Set the velocity
-axial_velocity = 0.25 # in m/s
+axial_velocity = 2.0 # in m/s
 
 # Domain half-width of 2.5 cm, meaning the whole domain is 5 cm wide
 width = 0.025
 
 # Done with initial conditions
-
 # Compute the mass flux, as this is what the Flame object requires
 massFlux = gas.density * axial_velocity # units kg/m2/s
+
 # Create the flame object
 oppFlame = ct.CounterflowTwinPremixedFlame(gas, width=width)
 
-# Now run the solver
+# Uncomment the following line to use a Multi-component formulation. Default is
+# mixture-averaged
+#oppFlame.transport_model = 'Multi'
+
+# Now run the solver. The solver returns the peak temperature, strain rate and
+# the point which we ascribe to the characteristic strain rate.
+
+(T, K, strainRatePoint) = solveOpposedFlame(oppFlame, massFlux, loglevel=1)
+
+# You can plot/see all state space variables by calling oppFlame.foo where foo
+# is T, Y[i], etc. The spatial variable (distance in meters) is in oppFlame.grid
+# Thus to plot temperature vs distance, use oppFlame.grid and oppFlame.T
 
-# The solver returns the peak temperature and strain rate. You can plot/see all
-# state space variables by calling oppFlame.foo where foo is T, Y[i] or whatever
-# The spatial variable (distance in meters) is in oppFlame.grid Thus to plot
-# temperature vs distance, use oppFlame.grid and oppFlame.T
-(T, K) = solveOpposedFlame(oppFlame, massFlux)
+Sc = computeConsumptionSpeed(oppFlame)
 
-print("Peak temperature: {0}".format(T))
-print("Strain Rate: {0}".format(K))
+print("Peak temperature: {0:.1f} K".format(T))
+print("Strain Rate: {0:.1f} 1/s".format(K))
+print("Consumption Speed: {0:.2f} cm/s".format(Sc*100))
 oppFlame.write_csv("premixed_twin_flame.csv", quiet=False)