Combustion analysis using ANSYS Fluent

Aim: To learn combustion analysis in ANSYS Fluent.

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. 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 Dioxide.

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.

The Fluent solver has capabilities of solving different types of combustions. Some them are:

Based on initial mixing state of air and fuel:

Pre-mixed combustion: When air and fuel are already mixed with each other in definite ratio, before entering combustion chamber, the reaction type is called premixed combustion. E.g. Carburettor

Partially pre-mixed combustion: Fuel and air are mixed with each other just before entering combustion chamber. The mixture is not perfectly mixed with each other.

Non pre-mixed combustion: The fuel and air are injected separately in combustion chamber. E.g. Diesel engines.

Based on phase of mixture:

Fluid phase: Fuel is in fluid phase before combustion. E.g. Volumetric reactions

Wall: Combustion takes place on wall.

Particle: Combustion takes place on surface of fuel particles.

Porous: Combustion takes place inside pores of material: E.g. Charcoal, wood.

Analysis:

Geometry:

The real combustion chamber is usually cylindrical but simulating it would require high computational power. Therefore, axisymmetric study is done to reduce number of elements required. The dimensions used are:

The fuel used is methane (CH₄). The width of fuel inlet is calculated based on required air fuel ratio. The inlet velocity for air inlet is 0.5 m/s and for fuel inlet is 80 m/s. The methane–air mixture requires 17.2 as air fuel ratio.(Reference 1)

Meshing:

Method = All triangles

Global Sizing = 5 mm

Sizing: Sphere of influence (For having fine mesh near fuel inlet)

Sphere centre: (30, 25) mm

Sphere radius: 30 mm

Element size: 2 mm

Number of elements: 41628

The element quality is very good as lowest quality is 0.74

Setup:

1) Steady state analysis

2) Axisymmetric

3) Energy equation model enabled

4) Viscous model: Standard k-e model with standard wall functions

5) Species model:

Species transport

Methane-air mixture, Volumetric reactions

Inlet diffusion and diffusion energy source enabled

Turbulence chemistry interaction: Eddy dissipation

NOx model enabled:

Thermal and prompt NOx are enabled

Fuel species: CH4

Interaction: Turbulence/species

6) Boundaries:

Air inlet:

Velocity: 0.5 m/s

Temperature: 300K

Species mass fraction: O₂ = 0.232

Fuel inlet:

Velocity: 80 m/s

Temperature: 300K

Species mass fraction: CH₄ = 1

7) Standard initialization from air inlet

8) Total iterations = 500

(NOx model is enabled to calculate the NOx emissions. Otherwise, Fluent would leave any combustible oxygen remaining as it is till it leaves the outlet.)

Results:

(Various plots shown below have 2 images. The upper image shows the zoomed contour of fuel inlet. The graphs are plotted 150 mm equidistant from each other starting from 150 mm.)

1) Residuals:

2) Temperature:

The maximum temperature reaches at 4000 Deg C at the inlet where combustion just begins.

3) Velocity:

4) CH4 mass fraction:

5) O2 mass fraction:

6) CO2 mass fraction:

7) H2O Mass fraction:

8) N2 Mass fraction:

9) NOx mass fraction:

10) Density & Temperature:

Comments on results:

1) CO2 and H2O mass fractions look similar because they are generated at same point during combustion.

2) By comparing contours of O2, N2 and NOx, we can say that some oxygen moles were remaining and they got burnt up with N2 to form NOx in right side regions.

3) From comparison of density and temperature contour, we can see the region where temperature rises and because of what species or reactions it rises.

Conclusion:

Combustion simulation in ANSYS Fluent is learnt.

References:

1) Finding the right air-fuel ratio:

https://www.brighthubengineering.com/machine-design/15235-the-stoichiometric-air-fuel-ratio/

2) https://aviation.stackexchange.com/questions/33089/how-is-the-correct-air-speed-for-fuel-combustion-obtained-at-the-inlet-of-the-co

3) https://en.wikipedia.org/wiki/Combustor