Combustion simulation and the effect of the addition of water content on emission using Ansys Fluent

Objective: To Perform a combustion simulation on the combustor model and find out the effect of the addition of water content on the formation of Nox and soot. 
PART_1- Plot the variation of the mass fraction for CO2, H2O, CH4, N2, O2, NOx emissions & Soot formation.

PART_2- Observe the effect of water content in the fuel from 5% to 30% by mole. Provide line plots and contours.

Given: CAD geometry of the combustor model.

Theory:
                                                 `sf\"Combustion Simulation\"`
In ANSYS, there are different types of possible combustion simulations. One is based on MIXING and another is based on PHASES.

- Based On MIXING:
 1. Non-Premixed combustion (Direct Injection, Late injection)
 2. Premixed combustion (Carburator)
 3. Partially premixed
- Based on PHASES:
 1. Fluid phase (Volumetric reaction)
 2. Wall (Surface Reaction)
 3. Particles (Surface Reaction)
 4. Porous Region (e.g. After-treatment system)

`tt\"Non-premixed Combustion using Eddy Dissipation model\"`
In non-premixed combustion, fuel and oxidizer enter the reaction zone in distinct streams. This is in contrast to premixed systems, in which reactants are mixed at the molecular level before burning. Examples of non-premixed combustion include pulverized coal furnaces, diesel internal-combustion engines, and pool fires.

Turbulent Chemistry Interaction- Eddy Dissipation model:
It is the model that use K and epsilon to calculate the time for reaction. If k and epsilon are sufficient then reactions are going to take place even without the help of ignition source.
Reaction rates are assumed to be controlled by the turbulence, so expensive Arrhenius chemical kinetic calculations can be avoided. The model is computationally cheap, but, for realistic results, only one or two-step heat-release mechanisms should be used. Detailed Arrhenius chemical kinetics can be incorporated in turbulent flames. Note that detailed chemical kinetic calculations are computationally expensive.

`sf\"CASE SETUP\"`
STEP-1: Import Combuster geometry into Spaceclaim.

STEP-2: Use Split-Body to extract 2D geometry from 3D geometry.

Select faces and paste it into a new design to get 2D geometry.

STEP-3: Combine all the geometries to form a single geometry using Combine Option.

STEP-4: Go into Meshing to discretized the complete domain and name the boundaries.

Element size- 0.002m (Triangular Dominant)
Capture Proximity- Yes
Nos cell in gap- 3

Nodes- 19320
Elements- 37706

Boundary Naming will be as below:

`sf\"FLUENT SETUP\"`

`***`Steady-state Pressure based solver.

`***`2D space- Axisymmetric

`***`Enable Energy to capture the temperature field.

`***`Viscous Model:
Standard K-epsilon model with Standard wall function

`***`Species model:
The Species Model dialog box allows you to set parameters related to the calculation of species transport and combustion.

Species Transport model: This model enables the calculation of multi-species transport (either non-reacting or reacting, depending on the selection for Reactions).

Enable Eddy-dissipation for Turbulence-Chemistry Interaction. This will predict how the combustion will take place based upon turbulence since we are not using any ignition source.

`***` NOx model: To look at NOx formation during the process enable the NOx model.

Choose Thermal NOx & Prompt NOx.
Thermal NOx- It is formed by the oxidation of atmospheric nitrogen present in combustion air.
Prompt NOx- It is produced by high-speed reaction at the flame front.

Turbulence interaction mode:
PDF mode- temperature
PDF type- beta
Temperature variance- transported

`***` Soot model: For turbulent flows select the One-step model.

Fuel- CH4
Oxidant- O2

`***` Boundary Conditions:

Air_Inlet:
Type- Velocity inlet
Velocity magnitude- 0.5 m/sec
Temperature- 300K
Species: O2- 0.23 (mass fraction basis)

Fuel_Inlet:
Type- Velocity inlet
Velocity magnitude- 80 m/sec
Temperature- 300K
Species:
PART-1- CH4- 1 (mass fraction basis)
PART-2- CH4- Input parameter (mole fraction basis)
            H2O- Input parameter (mole fraction basis)   

Axis:
Type- Axis

Outlet:
Type- Pressure based
Gauge pressure- 0 Pa

Wall and Wall_side:
Type- Wall
Wall motion- Stationary
Shear condition- No slip
Thermal- 300 K

`***`Solution will be initialized using the Hybrid Initialization method.

`sf\"RESULTS\"`

`sf\"PART-1\"`

In this part, we are going to see the variation of mass fraction of different species using the line probe at different locations. Species that we will be looking for CO2, CH4, H2O, N2, O2, NOx emission, and Soot Formation.

The line probe has placed equidistant in the entire domain to capture the data as below:

Contour Plots:

Temperature distribution inside computational domain-

From the simulation, the maximum value for the temperature inside the domain is found out to be 2309.3601 K.

In such type of simulation, we majorly focus on average outlet temperature that is found out to be 1354.82 K

Mass fraction of all species and Variation of the Mass fraction at a different location is shown in below plots:

Species- CO2

Species- H2O

Species- CH4

Species- N2

Species- O2

Species- NO pollutant

Species- Soot formation

`sf\"PART-2\"`

In this part of the challenge, we majorly focus on emissions i.e. NO and Soot coming out after combustion takes place inside the combustor. As we know the formation of such harmful combustion products affects the environment and humans.
So, we employ a method of adding water content in the fuel from 5% to 30% by mole to observe the effect on emissions.

Here, we have completed the parametric study. The CH4 content and H2O content have been set as Input parameter whereas NO emission, Soot formation and Average Temperature at the outlet is set as an output parameter.

Temperature Variation in all case of design points:

As we can from above contour plots and the simulation results we got a range of temperatures is decreasing as we are increasing water content in fuel CH4.

NO emission:
NOx emission is dependent on the temperature inside the combustor after the combustion of fuel such as hydrocarbons.

Here, from the simulation results, we can see how the adding of water during intake causing a reduction in NO emission from Outlet of a combustor.

Variation of NO throughout domain has been captured using a line probe.

SOOT formation: 
It is a mass of impure carbon particles resulting from the incomplete combustion of a hydrocarbon. It is more properly restricted to the product of the gas-phase combustion process but is commonly extended to include the residual pyrolyzed fuel particles such as coal, cenospheres, charred wood, and petroleum coke that may become airborne during pyrolysis and that are more properly identified as cokes or char

Soot contour plot:

As we can see from the results obtained from simulation, there is a reduction in soot emission from the outlet after adding water content into the fuel.

Variation of Soot formation throughout the domain has been captured using a line probe.

`sf\"CONCLUSION:\"`
In this way, we have successfully simulated 2D combustion inside the burner by Eddy dissipation- TCI model using CH4 as fuel and air as an oxidizer.
We have completed this study in two parts.
In the first part, we have seen a variation on the mass fraction of participating species such as CH4, CO2, H2O, CO2, N2, O2, NO emission, and Soot formation throughout the domain using line probe in CFD-POST.

In the second part, we majorly focus our study to limit or reduce the NOx emission and soot formation by adding water content from 5% to 30% with fuel content by mole fraction.

`***`Instead of simulating whole 3D domain we have restricted our study to axisymmetric 2D study as it requires less computational power and takes less computational time to compare to a 3D analysis provided geometry needs to be axisymmetric.

`***` In the non-premixed combustion i.e. in a combustor, combustion of fuel depends on the turbulent kinetic energy (K) and turbulent dissipation rate (epsilon) rather than the presence of an ignition source, unlike other combustion models.

`***` From the plots and results we got from the simulation we can say that adding water content during the inlet reduces the adverse impact of NOx, Soot emissions on the environment and humans by limiting its value.

`***`Increase in water content also leads to a reduction in temperature at the outlet of a combustor and CO2 as well.

Animation: