Challenge on Combustion

Combustion :

• Combustion, a chemical reaction between substances, usually including oxygen and usually accompanied by the generation of heat and light in the form of flame.
• The rate or speed at which the reactants combine is high, in part because of the nature of the chemical reaction itself and in part because more energy is generated than can escape into the surrounding medium, with the result that the temperature of the reactants is raised to accelerate the reaction even more.
• A combustion reaction is a reaction in which a substance reacts with oxygen gas, releasing energy in the form of light and heat. Combustion reactions must involve O2 as one reactant.
• Combustion of an organic fuel in air is always exothermic because the double bond in O₂ is much weaker than other double bonds or pairs of single bonds, and therefore the formation of the stronger bonds in the combustion products CO2 and H2O results in the release of energy.
• The bond energies in the fuel play only a minor role, since they are similar to those in the combustion products; e.g., the sum of the bond energies of CH₄ is nearly the same as that of CO2.

Part-1 Objective :

To Perform a combustion simulation on the combustor model and plot the variation of the mass fraction of the different species’ in the simulation using line probes at different locations of the combustor.

Geometry used for simulation :

• We have been given a 3D geometry of a cylinder having a small groove around te central axis of the cylinder.
• It is considered that the smal area inside the groove is the inlet port for fuel & the larger area outside the groove is the inlet port for air.
• To simplify our simulation we can consider the combustion to be axisymmetric and we can perform the simulation on a 2D plane along the central axis.
• Let us extract the 2D geometry from the given 3D geometry.

Given 3D Geometry:

Inlet Groove :

• Now by using the split body command we can split the whole cylinder into quater cylinder as shown in the figure below.

• To extract the 2D geometry select the faces on the required plane and copy the faces to the clipboard. in the figure the 2D faces selected are highlighted in orange colour.
• Now open a new space claim window and paste the 2D geometry in the XY plane.

2D Geometry :

Computational Meshing :

• Here it is best to use the triangular meshing.
• The mesh refinement is done.
• The element size is 5mm and the proximity along the edges is turned on so that we can get finner meshes near the groove.

Setup & Simulation :

• As we are simplifying the geometry to 2D, we have to specify the axisymmetry condition for the simulation. This can be given in the setting up physics options.
• Then we can choose the viscous model as k-epsilon which is suitable for this simulation.

• Then to introduce the presence of the substances required for simulation, let us specify the requirements in the species dialogue box.
• Then switch on the formation of NOx & soot, as we are conserned about their presence.
• Choose the one-step option for soot & give the species formula for the fuel & Oxidant.

• Then we can specify the boundary conditions for air & Fuel inlets.
• Here we have to specify both inlet velocity & inlet species mass fraction as we need initial conditions for the species.
• By calculating the mass fractions of the species by balancing the chemical equation of the combustion reaction, we can enter the values in the respective boundaries.
• In air inlet, we specify the velocity to be 0.5 m/s & the O2 species mass fraction as 0.23.
• In fuel inlet the velocity is taken as 80 m/s so that with the high velocity the fuel can be sprayed to a larger spread inside. The CH4 species mass fraction is given as 1.

• Then initialise the simulation using Hybrid initialisation and calculate the results for several itrations until it attains convergence.

Part - 1 Results :

Residual :

Temperature Animation:

Temperature Contour :

Line probes and plots :

• To see the mass fractions of different species after combution, we can use CFD post.
• The contours of mass fraction of Different species can be taken and line probes can be setup between the length of the contours to find the average mass fraction along the line.
• Let us create many lines at specific intervels along the plane as shown in the figure below & plot chart using the lines.

Mass Fraction of CO2 :

Contour:

Plot :

Mass Fraction of H2O :

Contour:

Plot :

Mass Fraction of CH4 :

Contour:

Plot :

Mass Fraction of N2 :

Contour:

Plot :

Mass Fraction of O2 :

Contour:

Plot :

Mass Fraction of NOx :

Contour:

Plot :

Mass Fraction of soot :

Contour:

Plot :

Part-2 Objective :

To add the water content in the fuel from 5% to 30% by mole and observe the effect of it on the results and to provide line plots and contours to prove our claim.

Why NOx & soot formation should be Reduced :