3.11. Hydrogen oxidation in a batch reactor using CHEMKIN mechanism

This tutorial shows how to import thermodynamic information of species and kinetic mechanism from CHEMKIN into MFiX. A batch reactor in a single cell is simulated in this tutorial. The mechanism for the oxidiation of hydrogen from Conaire et al. [1] is applied.

3.11.1. Load the project into MFiX

The basic model has been set up in the input file, “hydrogen_oxidation.mfx”.

  • On the main menu select New project.

  • Open the project hydrogen oxidation with CHEMKIN mechanism, step by step.

open project

  • Choose the location for your working directory and click OK.

create project

Note

The main purpose of this tutorial is to show how to use CHEMKIN mechanism in MFiX. For more details about the model setup, please refer to other tutorials.

3.11.2. Generate mesh

On the Mesher pane:

  • Click Generate to generate the mesh

  • Click Accept to accept the mesh, and switch to the Modeler tab (bottom left of GUI).

generate mesh

3.11.3. Import species for fluid phase

On the Fluid pane:

  • Click Add under Species. This will open the Fluid species popup window.

add species

  • Select Load from file (CHEMKIN format) in Source of the popup window

import thermo file

  • Click Choose file, choose “thermodynamicInfo.txt” in the popup window, and click Open.

select thermo file

  • After importing the file, the species defined in the file will show in the window.

  • Select all the species (click on first species “AR” and drag down) and click import from database to import all species. Click Done.

Note

Users have the flexibility to select the required species for their case from the provided list. If a species is not defined in the CHEMKIN file, users can import it from the built-in Burcat database or an external Burcat file as well.

select species

  • For reacting cases, constant specific heat capacity is not allowed. Change it to “Mixture” in “Properties”.

setup cp

3.11.4. Create Initial and Boundary conditions

On the Initial conditions pane, set the following conditions as the initial conditions.

Property

Initial Value

Volume Fraction

1.0

Temperature

800 K

Pressure

101325 Ps

U-Velocity

0.0 m/s

V-Velocity

0.0 m/s

W-Velocity

0.0 m/s

Composition

H2: 0.1, O2: 0.8, N2: 0.1, OH: 0.0, TestSpecies: 0.0

On the Boundary conditions pane, No-Slip and adiabatic walls have been setup.

3.11.5. Import kinetic mechanism

On the Chemistry pane, Options sub-pane:

  • Check the Enable Arrhenius model for fluid phase reactions checkbox

  • Change the Minimum mass fraction for reactions for Fluid reactions to 0 to consider all species with mass fraction larger than 0 in the reactions

  • Check the Enable stiff chemistry solver checkbox

  • Change the Absolute tolerance, Relative tolerance and Minimum reaction rate in stiff solver to be 1e-09, 1e-08 and 1e-20.

Note

Users can change the minimum mass fraction for reactions and the parameters for stiff solver based on the accuracy requirement of their cases.

setup arrhenius rate

On the Chemistry pane, Reactions sub-pane:

  • Click Load Load reaction definitions from text file and select Chemkin format

  • Select the mechanism file “chemkin_mechanism.CKI” in the popup window. Click Open.

import mechanism
select mechanism

  • Then all the reactions with all the information will be imported

reactions

3.11.6. Setup output, monitor and run parameters

The output, monitor and run parameters have been setup. Users can modify them based on their own requirement.

To check the change of species, we need to save the species into the output/monitor files.

On the Output pane:

  • Select the VTK sub-pane. An output has been setup.

  • In the Fluid of Select cell data to write at the bottom of GUI, select the checkboxes for species in Mass fraction.

On the Monitors pane:

  • An monitor has been setup with the gas temperature saved every 0.001s.

  • In the Fluid of Select cell data to write at the bottom of GUI, select the checkboxes for species H2 and O2 in Mass fraction.

3.11.7. Run the project

  • Save the project by clicking the Save button

  • Run the project by clicking the Play button

  • On the Run Solver dialog, select the default solver from the combo-box

  • Click the Run button to start the simulation

3.11.8. View results

This tutorial simulates the oxidation of H2 in a batch reactor. Users should expect to see consumption of H2 and O2 and a temperature increase from the exothermic reaction.

Results can be viewed, and plotted, while the simulation is running.

  • Create a new visualization tab by pressing the Add next to the Model tab

  • Select Monitors to view the results.

  • On the Monitors results tab, the visibility of the variables can be controlled with the menu under the plot.

monitor control

  • Hide the gas temperature by unclick the Visibility for variable t_g. The following plot with the transient changes of the mass fraction of H2 and O2 will be shown.

monitor species

  • Hide the gas species by unclick the Visibility for variables x_g_H2 and x_g_O2. The following plot with the transient change of the gas temperature will be shown.

monitor temperature

3.11.9. References