Challenge on Combustion
Abstract: Study of mass fraction variations of species formed during the combustion of Methane along the y-axis at four different locations using the line probes. And study the rate of pollutant NOx and soot formation by adding water in the fuel from 5% to 30%.
Combustion:
Combustion, or burning, is a high-temperature exothermic redox chemical reaction between a fuel (the reductant) and an oxidant, usually, atmospheric oxygen, that produces oxidized, often gaseous products, in a mixture termed as smoke.
Geometry:
Configuration and Dimensions of the combustion chamber used in the simulations as well as the fuel and air rates passing through the system.
This is a representation of a non-premixed combustion chamber but, due to the high level of symmetry present in the system, the calculation domain can be reduced to a 2D problem assuming no variations in the radial coordinate. If done, the computational time required for the calculations decreases drastically.
Mesh Generation:
Concerning the microscopic balances, notice that all the governing equations are differential equations with a continuous solution along the space. The system discretization is the way to assign the mathematical model to a finite element of the system, transform those differential equations in algebraic equations, and then, solve the system. That is called finite element method.
Simulation Setup:
Model definition: Species transport
Species transport The chemical reactions which take part in the gasification processes are solved by the mass, momentum, Energy Equation, and reaction of individual species.
Viscous: K-epsilon model
K-epsilon (k-ε) turbulence model is the most common Model used in Computational Fluid Dynamics (CFD) to simulate mean flow characteristics for turbulent flow conditions.
Reactions: Volumetric
Turbulence Chemistry Interaction: Eddy Dissipation
The eddy dissipation model, based on the work of Magnussen and Hjertager, is a turbulent-chemistry reaction model. Most fuels are fast burning and the overall rate of reaction is controlled by turbulence mixing. In the non-premixed flames, turbulence slowly mixes the fuel and oxidizer into the reaction zones where they burn quickly. In premixed flames, the turbulence slowly mixes cold reactants and hot products into the reaction zones where the reaction occurs rapidly. In such cases, the combustion is said to be mixing-limited, and the complex and often unknown chemical kinetics can be safely neglected. In this model, the chemical reaction is governed by a large-eddy mixing time scale. Combustion initiates whenever there is turbulence present in the flow. It does not need an ignition source to initiate the combustion. This type of model is valid for the non-premixed combustion, but for the premixed flames the reactant is assumed to burn at the moment it enters the computation model, which is a shortcoming of this model as in practice the reactant needs some time to get to the ignition temperature to initiate the combustion
Mixture Material: Methane-air
Analysis:
Case 1: Variation of species mass fraction along the y-axis formed during the combustion process is analyzed using line probes placed at 0.1m, 0.2m, 0.3m, 0.4m.
Case 2: Observing the effect of NOx formed by adding water in fuel by 5%, 15%, and 30%.
The below table gives the mass fraction of species formed under the different composition of water in fuel(methane).
Conclusion:
It is clear that the addition of water has a significant effect on NOx formation. Independently of the contact mode, as more water is added to the system, less NOx is formed. This is supposed to be because the addition of water has a direct effect on the temperature profile of the system. It reduces the peak temperature which implies a reduction of the thermal NOx.