# custom.py (Source)

```"""
Solve a constant pressure ignition problem where the governing equations are
implemented in Python.

This demonstrates an approach for solving problems where Cantera's reactor
network model cannot be configured to describe the system in question. Here,
Cantera is used for evaluating thermodynamic properties and kinetic rates while
an external ODE solver is used to integrate the resulting equations. In this
case, the SciPy wrapper for VODE is used, which uses the same variable-order BDF
methods as the Sundials CVODES solver used by Cantera.

Requires: cantera >= 2.5.0, scipy >= 0.19, matplotlib >= 2.0
Keywords: combustion, reactor network, ignition delay, user-defined model, plotting
"""

import cantera as ct
import numpy as np
import scipy.integrate

class ReactorOde:
def __init__(self, gas):
# Parameters of the ODE system and auxiliary data are stored in the
# ReactorOde object.
self.gas = gas
self.P = gas.P

def __call__(self, t, y):
"""the ODE function, y' = f(t,y) """

# State vector is [T, Y_1, Y_2, ... Y_K]
self.gas.set_unnormalized_mass_fractions(y[1:])
self.gas.TP = y, self.P
rho = self.gas.density

wdot = self.gas.net_production_rates
dTdt = - (np.dot(self.gas.partial_molar_enthalpies, wdot) /
(rho * self.gas.cp))
dYdt = wdot * self.gas.molecular_weights / rho

return np.hstack((dTdt, dYdt))

gas = ct.Solution('h2o2.yaml')

# Initial condition
P = ct.one_atm
gas.TPX = 1001, P, 'H2:2,O2:1,N2:4'
y0 = np.hstack((gas.T, gas.Y))

# Set up objects representing the ODE and the solver
ode = ReactorOde(gas)
solver = scipy.integrate.ode(ode)
solver.set_integrator('vode', method='bdf', with_jacobian=True)
solver.set_initial_value(y0, 0.0)

# Integrate the equations, keeping T(t) and Y(k,t)
t_end = 1e-3
states = ct.SolutionArray(gas, 1, extra={'t': [0.0]})
dt = 1e-5
while solver.successful() and solver.t < t_end:
solver.integrate(solver.t + dt)
gas.TPY = solver.y, P, solver.y[1:]
states.append(gas.state, t=solver.t)

# Plot the results
try:
import matplotlib.pyplot as plt
L1 = plt.plot(states.t, states.T, color='r', label='T', lw=2)
plt.xlabel('time (s)')
plt.ylabel('Temperature (K)')
plt.twinx()
L2 = plt.plot(states.t, states('OH').Y, label='OH', lw=2)
plt.ylabel('Mass Fraction')
plt.legend(L1+L2, [line.get_label() for line in L1+L2], loc='lower right')
plt.show()
except ImportError: