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ABSTRACT
This work presents a Computational Fluid Dynamics (CFD) study of the non-premixed combustion of natural gas with air in an axisymmetric cylindrical chamber, focusing on the contribution of the chemical reaction modeling on the temperature and the chemical species concentration fields. Simulations are based on the solution of mass, momentum, energy and chemical species conservation equations. Thermal radiation heat transfer in the combustion chamber is computed through the Discrete Transfer Radiation Method, and the Weighted-Sum-of-Gray-Gases model solves the dependence of gas absorption coefficient on the wavelength. Turbulence is modeled by the standard k-ε model. Regarding the combustion modeling, it is performed a comparison of solutions obtained with the combined Eddy Break-Up/Arrhenius (EBU/Arrhenius) and the Steady Laminar Diffusion Flamelet (SLDF) models. The finite volume method is employed to treat the differential equations. Among other results, the solution of the governing equations allows for the determination of the region where combustion takes place, the distribution of the chemical species and the velocity fields. The numerical results are compared to experimental measurements, showing varied agreements. Results indicate that, in this case, the EBU/Arrhenius model can predict the flame temperature and the concentration of the most important species with better accuracy than the more sophisticated SLDF model.
Latin symbols
| = | weighting factor for the gth gray gas | |
| = | flamelet characteristic strain rate | |
| = | von Kármán’s constant | |
| = | pre-exponential coefficient | |
| = | molar concentration of the kth species | |
| = | specific heat of the mixture | |
| = | specific heat of the kth species | |
| = | constants of the SLDF model | |
| = | constants of the standard k-ε model | |
| = | distance between fuel and oxidizer jets in a counterflow flame configuration | |
| = | mixture mass diffusivity | |
| = | activation energy | |
| = | term that controls the generation of | |
| = | mean total enthalpy of the mixture | |
| = | mean total enthalpy of the kth species | |
| = | molar enthalpy of formationturbulence intensity | |
| = | unity identity tensor | |
| = | radiative blackbody intensity | |
| = | radiative intensity of gth gray gas | |
| = | mean turbulent kinetic energy | |
| = | kth reactant that has the least value of | |
| = | turbulence characteristic length scale | |
| = | total number of chemical species | |
| = | total pressure of the gaseous mixture | |
| = | operational pressure | |
| = | partial pressure of carbon dioxide | |
| = | partial pressure of water vapor | |
| = | Prandtl number | |
| = | turbulent Prandtl number | |
| = | radiative heat flux | |
| qw | = | heat flux at the wall |
| = | mean massic rate of formation or destruction of the kth species | |
| = | mean volumetric rate of formation or destruction in all the cth equations where the kth species is present | |
| = | universal gas constant | |
| = | radiative path ray | |
| = | module of the tensor of the average strain rate | |
| = | mean source term of thermal radiation | |
| = | molecular and turbulent Schmidt numbers | |
| = | mean temperature of the mixture | |
| = | reference temperature | |
| T+ | = | nondimensional temperature |
| Tw | = | temperature at the wall |
| Tf | = | near-wall fluid temperature |
| = | vector of mean velocities | |
| = | dimensionless velocity | |
| = | friction velocity | |
| = | inlet axial velocity | |
| = | relative velocity between fuel and oxidizer jets in a counterflow flame configuration | |
| = | mixture molecular mass | |
| = | molecular mass of the kth species | |
| = | distance from the wall | |
| = | dimensionless distance from the wall | |
| = | mean mass fraction of the kth species | |
| = | mixture fraction | |
| = | mean mixture fraction | |
| = | mixture fraction variance |
Greek symbols
| = | constant | |
| = | temperature exponent in each cth reaction | |
| = | mean dissipation rate of | |
| = | solid angle | |
| = | radiative absorption coefficient | |
| = | radiative absorption coefficient of the gth gray gas | |
| = | concentration exponent in each cth reaction | |
| = | thermal conductivity | |
| = | gaseous mixture dynamic viscosity | |
| = | eddy viscosity | |
| = | Prandtl numbers for | |
| = | Stefan–Boltzmann’s constant | |
| Ψ | = | constant of the logarithmic wall function |
| = | density of the gaseous mixture | |
| = | density of the pure oxidizer stream in a counterflow flame configuration | |
| = | shear stress on the wall | |
| = | stoichiometric coefficient of kth species in the cth reaction | |
| = | scalar dissipation rate | |
| = | stoichiometric scalar dissipation rate | |
| = | mean stoichiometric scalar dissipation rate |