Methane and Air Combustion Simulation

INTRODUCTION

Combustion is a high-temperature exothermic redox chemical reaction between a fuel and an oxidant, that produces oxidized, often gaseous products, in a mixture termed as smoke.

In complete combustion, the reactant burns in oxygen, and produces a limited number of products. When a hydrocarbon burns in oxygen, the reaction will primarily yield carbon dioxide and water. When elements are burned, the products are primarily the most common oxides.

Incomplete combustion will occur when there is not enough oxygen to allow the fuel to react completely to produce carbon dioxide and water. It also happens when the combustion is quenched by a heat sink, such as a solid surface or flame trap. As is the case with complete combustion, water is produced by incomplete combustion;however, Carbon, Carbon Monoxide, and/or hydroxides are produced instead of Carbon Dioxde.

Complete combustion is almost impossible to achieve since the chemical equilibrium is not necessarily reached, or may contain unburnt particles, toxins, and partially oxidized products.

Any combustion at high temperatures in atmospheric air will also create small amounts of several nitrogen oxides, commonly referred to as `NO_X`.

TYPES OF COMBUSTION BASED ON THE MIXING OF AIR AND FUEL

1. NON-PREMIXED COMBUSTION

The fuel and air are not mixed before entering the combustion chamber and enter as two different streams. Example of such combustion can be found in diesel engines, direct injection, etc.

2. PRE-MIXED COMBUSTION

The fuel and air are mixed before entering the combustion chamber. Example of such combustion is carburetor.

3. PARTIALLY PRE-MIXED COMBUSTION

The fuel and air are partially mixed before entering the combustion chamber. Example of such combustion is IC engine used in automobiles.

TYPES OF COMBUSTION BASED ON THE PHASE OF THE MIXTURE

1. COMBUSTION HAPPENING IN FLUID PHASE

Volumetric Reactions. Example: Direct Injection

2. COMBUSTION HAPPENING ON THE WALL

Surface Reactions. Example: Evaporation

3. COMBUSTION HAPPENING VIA PARTICLES

Also surface reactions and as name suggests takes place via particles. Example: Burning of Coal

4. COMBUSTION HAPPENING IN PORUS REGIONS

These reaction takes place above a high pressure drop region. Example: After treatment systems.

CHALLENGE OBJECTIVES

Performing Volumetric Combustion simulation of methane when it is injected into air at a relatively high velocity.

Using Eddy Dissipation model for source calculations for the combustion.

STEP 1: GEOMETRY PREPARATION

1. SOLID 3-D MODEL

2. 2-D AXISYMMETRIC MODEL

Assumption: Solution in the 2D axisymmetric model deos not change radially.

STEP 2: MESH GENERATION

1. Element Type: Tetrahedral
2. Element Size: 2.5 mm
3. Proximity: ON (generate at least 3 elements in gaps smaller than 0.025 mm as can be observed in methane inlet)
4. Number of Nodes: 12089
5. Number of Elements: 23416
6. Named Selections:

6.1. Air-Inlet: Left Edge as shown above

6.2. Methane-Inlet: Left Edge as shown above

Outlet: Right edge of the computation domain

6.3. Symmetry: Bottom Edge (to serve as symmetry axis for axisymmetric calculations)

6.4. Wall: Walls surrounding the methane inlet

6.5. Side-wall: Top edge of the computation domain

STEP 3 & STEP 4: FLOW PHYSICS STEUP AND SOLVING

• The simulation is a 2-D axisymmetric steady-state simulation. The energy equation is activated for this combustion simulation. For ease of the simulation, turbulence is essential for efficient mixing.
• We have used the realizable - turbulence model with scalable wall functions. The species transport equation with inlet diffusion and the diffusion energy source is also activated for the volumetric reaction of the mixture of methane and air. The chemical kinetics of the reaction are imported by default into the software.
• The turbulence-chemistry interaction is set to Eddy Dissipation which is not very computationally expensive and provides accurate results as well. The combustion reaction consists of species and occurs as follows:

`CH_4 + 2(O_2 + 3.76N_2) → 2CO_2 + 2H_2O + 7.52N_2`

• `O_2 + 3.76N_2` constitutes the air mixture.
• By fundamental stoichiometric calculations, the mass fraction of oxygen and dinitrogen gas are 0.23 and 0.77 respectively.
• As dinitrogen acts as inert in the above reaction, it is excluded from the calculations.

Boundary Conditions Summary

1. Axis

Type: axis

2. Air-inlet

Type: velocity-inlet

Velocity Magnitude: 0.5 m/s

Temperature: 300 K

Mass Fraction of Oxygen gas: 0.23

3. Methane-inlet

Type: velocity-inlet

Velocity Magnitude: 80 m/s

Temperature: 300K

Mass Fraction of methane: 1

4. Outlet

Type: pressure-outlet

Gauge Pressure: 0 Pa

5. Side-wall, Wall

Type: wall

Wall-Motion: Stationary Wall

Shear Condition: No Slip

Thermal Condition: Adiabatic

• The pressure and velocity equations are solved using the Coupled scheme.
• The spatial discretization scheme for the turbulence parameters is changed from first-order to second-order for higher accuracy.
• The solution is initialized using hybrid initialization and run for a maximum of 2000 iterations or until it reaches the convergence criteria for the variables.

STEP 5: RESULTS

1. RESIDUALS

As clearly seen above, the solution converges after about 345 iterations.

2. ANIMATIONS

2.1. TEMPERATURE ANIMATION

2.2. VELOCITY ANIMATION

3. TEMPERATURE CONTOUR (AT THE END OF SOLUTION)

4. MASS FRACTION CONTOURS

4.1. CO2

4.2. H2O

4.3. CH4

4.4. N2

4.5. O2

4.6. NO (THERMAL NOx is activated to obtain this!)

5. LOCALIZED DATA ANALYSIS USING LINE PROBES

5.1. TEMPERATURE

5.2. MASS FRACTIONS

5.2.1. CO2

5.2.2. H2O

5.2.3. CH4

5.2.4. N2

5.2.5. O2

5.2.6. NO

CONCLUSIONS

• The maximum temperature attained during this reaction is 2318.84 K.
• From the mass fraction contours, we can clearly see that the mass fractions of the reactants deplete as we move along the length of the combustion chamber.
• Correspondingly, the mass fractions of the products gradually increase along the length of the combustion chamber.
• From the reaction, we saw that dinitrogen acts as an inert compound in the reaction and from its mass fraction contour, it can be seen that is present everywhere in the domain, except at the methane inlet, without undergoing generation or depletion.