False diffusion in numerical simulation of combustion processes in tangential-fired furnace

Abstract

Numerical simulation serves as one of the most important tools for analyzing coal combustion in Tangentially Fired Fumaces (TFF) with NUMERICAL FALSE DIFFUSION as one key problem that degrades the simulation accuracy, especially for complex flow patterns. False diffusion often completely compromises the accuracy, leading to erroneous predictions. This paper reviews various methods to reduce the numerical diffusion. In computational fluid dynamics (CFD), false diffusion originates from a truncation error of the Taylor series approximation of the derivative and multidimensional discretization effects. Higher-order upwind convective schemes were designed to reduce truncation errors, while grid line adjusting methods were developed to reduce crossflow diffusion. This paper compares numerical and experimental results for isothermal flows to evaluate these methods. Results with the standard upwind scheme in a rectangular Cartesian mesh are compared with results in body-fitted meshes for comprehensive combustion processes in a TFF. Analysis of the false ditfusion effect in the x, y, z directions and the artificial viscosity distribution in a rectangular mesh shows where the false diffusion overtakes the real physical diffusion and where the mesh must be refined or grid line must be adjusted to improve TFF combustion simulations.

Numerical study of a tangentially fired boiler for reducing steam tube overheating

“…Tangential fired boiler is also called corner-fired furnace [1], which is determined by its furnace shape and the position of the fuel/oxidizer inlet. It is normally with four sides and four corners where the fuel and air are supplied.…”