Bluff-Body Mild Burner

Bluff-Body Mild Burner CFD Simulation, ANSYS Fluent Training

Description

This project simulates combustion within a bluff-body mild burner using ANSYS Fluent. A burner is a device that combines a controlled amount of air with fuel within a safe enclosed space, converting fuel energy into heat energy while producing combustion gases as a byproduct. Since the resulting flame transfers heat into the chamber interior through both convection and radiation, the Discrete Ordinates (DO) radiation model is applied, alongside the Species Transport model to capture the combustion process occurring within the chamber.

The burner operates by spraying fuel through a dedicated jet inlet into the chamber, while air enters symmetrically from four directions, combining with the fuel to sustain the flame. The chamber's internal flow path is cyclic — part of the gas exits through the exhaust section, while the remainder recirculates back into the enclosure along the same circular path.

Several assumptions were applied to the simulation: it was run under steady-state conditions using a pressure-based solver, with gravitational effects excluded.

Geometry & Mesh

The 3D geometry was designed in Design Modeler. Given the model's symmetrical structure, only a 90-degree section was modeled, with the two lateral surfaces defined as symmetry boundaries. The geometry consists of three small-diameter inlet ducts (two air inlets and one fuel inlet) and one small-diameter exhaust outlet pipe.

The domain was meshed in ANSYS Meshing using an unstructured grid totaling 1,107,286 elements, with boundary layer mesh applied at the inlet and outlet sections to improve the accuracy of near-wall flow behavior.

Methodology

Key simulation settings included:

• Viscous model: Realizable k-epsilon with enhanced wall treatment

• Species model: Non-premixed combustion

• Radiation model: Discrete Ordinates (DO), with the energy equation enabled

• Boundary conditions: Velocity inlets for air (2 m/s, 300 K) and fuel (1 m/s, 300 K), each with internal emissivity of 1, zero NO pollutant mass fraction, and mixture fraction settings appropriate to each stream (fuel set to a mean mixture fraction of 1); pressure outlet at the exhaust (0 Pa gauge, internal emissivity of 1); outer walls set to zero heat flux with opaque boundary type and internal emissivity of 1

• Solution methods: Coupled pressure-velocity coupling, PRESTO! for pressure discretization, and second-order upwind schemes applied across momentum, energy, turbulent kinetic energy, turbulent dissipation rate, pollutant NO, discrete ordinates, mean mixture fraction, and mixture fraction variance

• Initialization: Hybrid method

Conclusion

The simulation captures the combustion behavior within the bluff-body mild burner, characterizing how the cyclic recirculating flow pattern sustains flame stability while combining radiation and convective heat transfer mechanisms to distribute thermal energy throughout the chamber. The resulting flow, temperature, and species distribution fields reflect the coupled effects of the non-premixed combustion process and the recirculating exhaust pathway central to this burner's mild combustion design.