Non-premixed combustion using Ansys Fluent

Introduction:

Combustion is a chemical process in which a substance reacts rapidly with oxygen and gives off heat. The original substance is called the fuel, and the source of oxygen is called the oxidizer.

Objective:

To perform a combustion simulation on the combustor model and to plot the mass fraction of different species at different location inside a combustor.

Types of combustion simulations available in ANSYS FLUENT:

• Non Pre-mixed combustion - This type of combustion occurs when oxidizer and fuel mixture enters the reaction zone in distinct streams or not initially mixed. eg. diesel engines
• Pre-mixed Combustion - The fuel and oxidizer are mixed at molecular level before the combustion. eg. carburettor in automobiles
• Partially Pre-mixed Combustion - Its in between non pre-mixed and pre-mixed combustion where the fuel and oxidizer are partially pre-mixed at non-uniform ratios before combustion.

In this project, we will simulating a non premixed combustion case using the eddy dissipation model in Ansys Fluent. In this model, the rate of combustion is determined the turbulent mixing of the species. They are known to generate fast combustion flames and are largely used in steady state cases where the end temperature is the parameter of interest.

Geometry:

The given 3d model is imported into spaceclaim. To save on computational resources, the case was decided to be solved as a 2d assymmetric model. So a 2d slice of the 3d geometry was extracted from the midplane.

Mesh:

A quad mesh was generated with an element size of 1mm with proximity and curvature enabled. The mesh consisted of 17121 elements.

Solver-settings:

• A steady state pressure based solver was used with energy equation turned on.
• A standard k-e turbulence model was chosen. It is important to choose a turbulent model because the combustion depends largely on eddy dissipation which occurs due to turbulence.
• As a number of elementary reactions are going to be taking place, it is important to solve for species transport.
• Volumentric reactions are enabled because the reactions are taking place in the gas phase with inlet diffusion and diffusion energy source enabled.
• The chemical kinetic mechanism chosen for combustion was methane-air.
• The air inlet is set to an inlet-velocity of 80m/s with temperature set to 300K and species mass fraction of o2 set to 0.23.
• The fuel inlet is defined with a velocity of 0.5m/s with temperature set to 300K and mass fraction of ch4 set to 1.
• The outlet is a pressure outlet and the case was solved for 1000 iterations.

Results:

Residual monitor

A good convergence can be observed for all the parameters.

Temperature animation:

The generated flame profile can be observed in the above animation.

Temperature contour:

The maximum temperature attained by the flame is 2216K and a uniform temperature distribution of 1819K was calculated at the output.

Since the case was solved as a 2d axisymmetric model, we can assume the variation of variables like temperature are going to be constant in the radial direction. We can visualise the flame profile in 3d within the combustor because of the periodic boundary condition.

The mass fractions of various species at different locations inside a combustor can be measured using line probes can be plotted using charts. 5 probes were set up at 0.02m, 0.04m, 0.06m, 0.08m, 0.1m respectively in the x-axis.

Mass fraction of CH4:

• A good diffusion rate of the primary fuel species-methane can be observed from the above plots. It is highest as it entering the combustor and is undergoes turbulent mixing with air, combustion is generated.

Mass fraction of H2O:

Mass fraction of CO2:

Mass fraction of O2:

Mass fraction of N2:

To solve for pollutants, we need to enable NOx model in the species transport solver in Fluent. A case was solved again and convergence was achieved in 60 iterations.

Mass fraction of NOx:

The highest concentration of pollutant Nox was calculated to be in order of 6e-06 which is quite small and neglible.