In this study, a new body-fitted grid system is implemented for a specific boiler. Unlike the traditional right-angled grid, this system features radially expanding grid lines that originate from the burner nozzle and extend to the furnace wall. The twist angle of these lines gradually changes as they move upward, eventually aligning parallel with the furnace walls. Compared to conventional grids, this approach significantly reduces pseudo-diffusion errors, resulting in more accurate representations of combustion parameters. For instance, simulations of three-dimensional flow in square cavities and two-phase combustion in annular chambers demonstrate improved accuracy, as the grid adapts to complex geometries and allows flexible control over curvature and density.
When using algebraic methods to generate body-fitted meshes, the balance between smoothness and uniformity can be effectively managed. This not only enhances the accuracy of boundary condition modeling but also improves the overall versatility of the simulation program. The discrete ordinate method is employed to solve the radiative transfer equation, which is particularly well-suited for complex curved grids. The coal used in the study is bituminous with high moisture content, and the excess air coefficient plays a key role in influencing the radiation absorption properties within the flame zone.
The right-angle grid suffers from significant pseudo-diffusion errors near the nozzle, causing the jet to decay and spread rapidly. In contrast, the body-fitted grid captures the jet's deflection and merging process more accurately, maintaining a certain level of rigidity. Cold-state experiments further support this, showing that the body-fitted grid better suppresses pseudo-diffusion, leading to more realistic thermal velocity fields.
Under hot conditions, the jet width increases due to thermal expansion, and the differences between the two grid systems become more pronounced. The body-fitted grid produces a clearer jet structure, with a longer impact distance and slower contour disappearance. In contrast, the right-angle grid results in a faster jet expansion and quicker loss of structure.
These structural differences affect the distribution of combustion components. The body-fitted grid concentrates combustion products within a narrow jet width, resulting in higher localized temperatures downstream. On the other hand, the right-angle grid spreads the combustion heat more evenly, leading to lower peak temperatures. Additionally, the central high-temperature region under the body-fitted grid is smaller, while the right-angle grid creates a larger low-temperature zone near the wall due to pseudo-diffusion effects.
Oxygen concentration distribution also varies significantly. Under the body-fitted grid, oxygen levels change sharply along the jet direction, with a smaller low-concentration area at the center. The opposite trend is observed with the right-angle grid, where oxygen distribution is more uniform but less precise.
In conclusion, the use of a body-fitted grid in furnace combustion simulations significantly reduces pseudo-diffusion errors compared to conventional right-angle grids. This leads to more accurate predictions of physical parameters under hot operating conditions. The numerical model developed in this study, combined with the body-fitted grid, provides a more reliable approach for simulating combustion in four-corner tangential boilers and similar engineering problems. It serves as a robust platform for further research into furnace dynamics and combustion processes.
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