Diffusion flame unstable branch#

This example uses the two-point flame control feature to march solutions down the stable and unstable burning branch for a counterflow diffusion flame. A hydrogen-oxygen diffusion flame at 1 bar is studied.

Requires: cantera >= 3.1, matplotlib >= 2.0

Tags: Python combustion 1D flow diffusion flame strained flame extinction saving output plotting

import logging
import sys
from pathlib import Path

import matplotlib.pyplot as plt
import numpy as np
import pandas as pd

import cantera as ct

logger = logging.getLogger(__name__)
logger.setLevel(logging.INFO)
logging.basicConfig(stream=sys.stdout)

Flame Initialization#

# Set up an initial hydrogen-oxygen counterflow flame at 1 bar and low strain
# rate (maximum axial velocity gradient = 2414 1/s)

reaction_mechanism = 'h2o2.yaml'
gas = ct.Solution(reaction_mechanism)
width = 18.e-3  # 18mm wide
f = ct.CounterflowDiffusionFlame(gas, width=width)

# Define the operating pressure and boundary conditions
f.P = 1.0e5  # 1 bar
f.fuel_inlet.mdot = 0.5  # kg/m^2/s
f.fuel_inlet.X = 'H2:1'
f.fuel_inlet.T = 300  # K
f.oxidizer_inlet.mdot = 3.0  # kg/m^2/s
f.oxidizer_inlet.X = 'O2:1'
f.oxidizer_inlet.T = 500  # K

# Set refinement parameters
f.set_refine_criteria(ratio=4.0, slope=0.1, curve=0.2, prune=0.05)

# Initialize and solve
logger.info('Creating the initial solution')
f.solve(loglevel=0, auto=True)

# Define output locations
output_path = Path() / "diffusion_flame_continuation_data"
output_path.mkdir(parents=True, exist_ok=True)
INFO:__main__:Creating the initial solution

Flame Continuation#

# Maximum number of steps to take
n_max = 1000

# Relative temperature defining control point locations, with 1 being the peak
# temperature and 0 being the inlet temperature. Lower values tend to avoid solver
# failures early on, while using higher values on the unstable branch tend to help
# with finding solutions where the peak temperature is very low.
initial_spacing = 0.6
unstable_spacing = 0.95

# Amount to adjust temperature at the control point each step [K]
temperature_increment = 20.0
max_increment = 100

# Try to keep T_max from changing more than this much each step [K]
target_delta_T_max = 20

# Stop after this many successive errors
max_error_count = 3
error_count = 0

# Stop after any failure if the strain rate has dropped to this fraction of the maximum
strain_rate_tol = 0.10

f.two_point_control_enabled = True

# Prevent two point control from finding solutions with negative inlet velocities
f.flame.set_bounds(spread_rate=(-1e-5, 1e20))

f.max_time_step_count = 100
T_max = max(f.T)
a_max = strain_rate = f.strain_rate('max')
data = []  # integral output quantities for each step
logger.info('Starting two-point control')

for i in range(n_max):
    if strain_rate > 0.98 * a_max:
        spacing = initial_spacing
    else:
        spacing = unstable_spacing
    control_temperature = np.min(f.T) + spacing*(np.max(f.T) - np.min(f.T))

    # Store the flame state in case the iteration fails and we need to roll back
    backup_state = f.to_array()

    logger.debug(f'Iteration {i}: Control temperature = {control_temperature:.2f} K')
    f.set_left_control_point(control_temperature)
    f.set_right_control_point(control_temperature)

    # This decrement is what drives the two-point control. If failure
    # occurs, try decreasing the decrement.
    f.left_control_point_temperature -= temperature_increment
    f.right_control_point_temperature -= temperature_increment
    f.clear_stats()

    if (f.left_control_point_temperature < f.fuel_inlet.T + 100
        or f.right_control_point_temperature < f.oxidizer_inlet.T + 100
    ):
        logger.info("SUCCESS! Stopping because control point temperature is "
                    "sufficiently close to inlet temperature.")
        break

    try:
        f.solve(loglevel=0)
        if abs(max(f.T) - T_max) < 0.8 * target_delta_T_max:
            # Max temperature is changing slowly. Try a larger increment next step
            temperature_increment = min(temperature_increment + 3, max_increment)
        elif abs(max(f.T) - T_max) > target_delta_T_max:
            # Max temperature is changing quickly. Scale down increment for next step
            temperature_increment *= 0.9 * target_delta_T_max / (abs(max(f.T) - T_max))
        error_count = 0
    except ct.CanteraError as err:
        logger.debug(err)
        if strain_rate / a_max < strain_rate_tol:
            logger.info('SUCCESS! Traversed unstable branch down to '
                        f'{100 * strain_rate / a_max:.2f}% of the maximum strain rate.')
            break

        # Restore the previous solution and try a smaller temperature increment for the
        # next iteration
        f.from_array(backup_state)
        temperature_increment = 0.7 * temperature_increment
        error_count += 1
        logger.warning(f"Solver did not converge on iteration {i}. Trying again with "
                       f"dT = {temperature_increment:.2f}")

    if ct.hdf_support():
        f.save(output_path / 'flame_profiles.h5', name=f'iteration{i}', overwrite=True)

    # Collect output stats
    T_max = max(f.T)
    T_mid = 0.5 * (min(f.T) + max(f.T))
    s = np.where(f.T > T_mid)[0][[0,-1]]
    width = f.grid[s[1]] - f.grid[s[0]]
    strain_rate = f.strain_rate('max')
    a_max = max(strain_rate, a_max)

    data.append({
        'T_max': max(f.T),
        'strain_rate': strain_rate,
        'heat_release_rate': np.trapz(f.heat_release_rate, f.grid),
        'n_points': len(f.grid),
        'flame_width': width,
        'Tc_increment': temperature_increment,
        'time_steps': sum(f.time_step_stats),
        'eval_count': sum(f.eval_count_stats),
        'cpu_time': sum(f.jacobian_time_stats + f.eval_time_stats),
        'errors': error_count
    })

    if error_count >= max_error_count:
        logger.warning(f'FAILURE! Stopping after {error_count} successive solver '
                       'errors.')
        break

logger.info(f'Stopped after {i} iterations')
INFO:__main__:Starting two-point control
/home/runner/work/cantera/cantera/build/doc/samples/python/onedim/diffusion_flame_continuation.py:165: DeprecationWarning: `trapz` is deprecated. Use `trapezoid` instead, or one of the numerical integration functions in `scipy.integrate`.
  'heat_release_rate': np.trapz(f.heat_release_rate, f.grid),
INFO:__main__:SUCCESS! Traversed unstable branch down to 4.13% of the maximum strain rate.
INFO:__main__:Stopped after 134 iterations

Combine data#

T_max strain_rate heat_release_rate n_points flame_width Tc_increment time_steps eval_count cpu_time errors
0 3052.622334 2830.212041 2.443916e+06 122 0.002475 23.000000 0 10 0.031330 0
1 3052.372458 2878.030504 2.462708e+06 123 0.002363 26.000000 0 10 0.029529 0
2 3052.504262 2931.640981 2.487768e+06 125 0.002363 29.000000 0 12 0.030337 0
3 3051.760999 3001.161292 2.511980e+06 128 0.002363 32.000000 0 14 0.031602 0
4 3051.465636 3064.930233 2.539557e+06 130 0.002363 35.000000 0 12 0.031680 0
... ... ... ... ... ... ... ... ... ... ...
129 1132.415985 143040.148555 3.647836e+06 235 0.000188 11.974268 0 24 0.085215 0
130 1113.391131 112459.074362 3.045372e+06 241 0.000207 11.974268 0 26 0.088768 0
131 1091.723041 80892.273397 2.373685e+06 247 0.000236 9.947200 0 63 0.132986 0
132 1071.200031 54519.048513 1.763880e+06 256 0.000279 8.724334 0 48 0.187037 0
133 1047.379268 27374.551074 1.107357e+06 266 0.000363 6.592485 0 74 0.152036 0

134 rows × 10 columns



Plot the maximum temperature versus the maximum axial velocity gradient#

plt.figure()
plt.plot(df.strain_rate, df.T_max)
plt.xlabel('Maximum Axial Velocity Gradient [1/s]')
plt.ylabel('Maximum Temperature [K]')
plt.savefig(output_path / "figure_max_temperature_vs_max_velocity_gradient.png")
diffusion flame continuation

Plot maximum_temperature against number of iterations#

plt.figure()
plt.plot(df.T_max)
plt.xlabel('Number of Continuation Steps')
plt.ylabel('Maximum Temperature [K]')
plt.savefig(output_path / "figure_max_temperature_iterations.png")
plt.show()
diffusion flame continuation

Total running time of the script: (0 minutes 13.462 seconds)

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