PROPANE COMBUSTION (THREE-STEP EDBR)

I)Pre-processing

    I-1)Ignition Region
    I-2)Combustion Model
    I-3)Material and Scalar Properties:
    I-4)Boundary Conditions
    I-5)Control Parameters

We use a three-step reaction of the following form :

C3H8 + 1.5 O2 -> 3CO + 4H2
CO + 0.5O2 -> CO2
H2+0.5O2 -> H2O

We used the same mesh set up before with some boundary condition modificationsto allow for combustion modelling
The physical properties of air(23.2% oxygen and 76.8% nitrogen, by mass), leading rectants (C3H8,CO,H2) and products (CO2, H2O) are assumed to be as follows :

Air C3H8 CO H2 CO2 H2O

Molecular Weights 28.96 44.0 28.0 2.0 44.0 18.0

Density Ideal gas

Molecular viscosity 1.81e-05 Pa s(constant)

Specific heat Polynomial function of temperature

Thermal conductivity 2.637e-02

It is assumed that both air and fuel enter the combustion chamber at a pressure of 1 bar and temperature of 293 K.

I)Pre-processing

We use the mesh set up begore so we copy the file tut.mdl in a new directory

I-1)Ignition Region

For this model, an ignition region needs to be specified. To do this, we collect together the cells to be designated as ignition cells as follows
In the I/O window, we type the command
CSET NEWS GRAN 0.049 0.0751,,, -0.1 0.51 2
A total of 90 cells should be selected. We assign a separete cell type of a different colour to these cells so as to be able to distinguich them easily from the rest of the cells in the model, the procedure is :
- Check that cell plot type is Hidden Surface
-View>Isometric>1,1-1
-Click Cell plot
-Select Tools>Cell Tool
-Highlight Table#4 in the cell Table scroll list
-Click Edit Types to display the Cell Table Editor
-Enter 5 in the Color Table Index box
-Enter Ignition_cells in the name box
-Click apply and Close
-In the Cell Tool, select Modify Type>Cell Set
-Click Replot and notice the change of cell colour in the ignition region
-Click All Cells and then Cell Plot
-Close the Cell Tool

I-2)Combustion Model

To set up the combustion model parameters, first we activate the chemical module after we chhose a reaction scheme of type Local Source. The defaut setting for the flame type pop-up menu is Diff.Flame, which the desired option
We supply the combustion reaction equation, we type the three equqtions in the reaction text box and we accept the default settings.
We signal to prostar that special treatment is to be applied to the ordinary rectant (oxygen)
-In Reactant Parameters list, highlight reactant no 1
-check that option Fixed-Fraction is selected
-Check that the fixed oxygen mass fraction in the incoming air stream is 0.232.

We specify the ignition parameters as follows:
-Type 4 in the Cell Type text box
-Type 0.05 in the Fraction txt box
-Select option Iteration
-Type 50 and 150 in the start and finish text boxes
-Click Apply

This complete the specification of leadind reactants and combustion products.

I-3)Material and Scalar Properties:

We check the material and scalar properties and we make the necessary adjustemets as follows:

1) Specify a polynomial variation for the background fluid specific heat
-Select Polynomial in the Specific Heat pop-up menu
-Click apply
-Click Define Polynomials to display the Polynomial Function Definition dialog
-Highlight N2 in the scroll list, click Apply Data Base Substance button and then click Graph Data to see graphical display of the polynomial variation
-Click Close

2) Turn on the chemico-thermal enthalpy equation solver:
-Open the Thermal Models panel and click Show Options
-Select option Chemico-Thermal in the Enthalpy menu
-Click Apply
It is good to practice to define one of the ignition cells as the monitoring point

3) Assign the chemical reaction scheme defined earlier to that stream
-Open chemical Scheme panel
-Click option button Chemical Scheme Number and then type 1 in the adjacent text box
-Click Apply
-Select the Additional Scalars folder and then open the Molecular Properties window
-Click Define Polynomials to display the Polynomial Function Definition dialog
-Click option button Scalar
-Select scalar no 1 via the Scalar Number slider
-Click Close to exit from the dialog

I-4)Boundary Conditions

We modify boundary values for the fuel stream as follows :
-Select folder Define Boundary Conditions and then open panel Define Boundary regions
-Select region no.3 in the scroll list and type 293.0 for Temperature and 1.8 for density
-Click apply
-Open the Scalar Boundaries panel
-Highlight region no.3 in the Boundary region scroll list, type 1.0 in the Concentration text-box and click apply
-Select C3H8 in the Scalar scroll list, type 1.0 in the concentration text box and click Apply

We also modify values for the air streams (region 4,5 and6) as follows:
-In the Scalar Boundaries dialog, highlight region 4
-Select O2,type 0.232 in the Concentration box and click Apply
-Select N2 , type 0.768 and click apply
we repeat the above sequence of steps for regions 5 and 6

I-5)Control Parameters

We check the solution control parameter settings and we specify new values for under-relaxation factors and tolerance:
-Select folder Analysis Controls
-sub folder Solution Controls and then Equation Behavior
-Open the Primary Variables panel and then select the Solver Parameters tab
-Type 0.7 in the Relaxation Factor text box for Temperature
-Type 0.001 in the Residual Tolerance text box for Temperature
-Type 0.8 in the Relaxation Factor text box for Density
-Click Apply

II) Results

The model converges after 912 iterations, we obtained the results shown in the following:

# Velocity magnitude:

we can note that the mixture is put in rotation moving due to the cyclic boundary condition . We also see that the flluid is accelerating due to the combustion
and the maximum of velocity is observed near the output and the axis of symmetry. We can also note a little zone of recirculation in the entree of the combustor.

# Temperature

We can note that the temperature in the entree of the combustor is not important because the is not yet beeing mixed to the oxygen. Temperature at the top zone has not changed because of the adiabatic separation we have set.

Now, we define a section whose plane bisects the mesh and passes through the axis of symmetry :
-View>Axix>+X
-Click Section Slice and then use the screen cursor to draw a vertical line that bissects the visible mesh.
-View>Snormal
-Select cell plot type Section(Surface)
and after, using the Pgetv panel we get this plots:

Initial and boundry conditions are simply viewd and verified in this frames.

Maximum temperature seems to be of a good value compared with given temperature of propane gases in bibliographe.

Gasous density is also found to corrolate to theoritical values found in bibliographic books. In fact, unburned gas density is greeter then burned gaz ones.

This figure shoes that fuel is burned in quasitotalite fare away combustor wall. It is due to air velocity arriving from the top part of the combustor.