Numerical characterization of the aerodynamics in fixed-grate biomass burners

This paper investigates the numerical simulation of the aerodynamics of biomass burners operating in small-scale, fixed-grate technologies. The efficiency of these boilers is largely determined by the fluid patterns originated in the combustion chamber, as a consequence of the interaction of primary and secondary inlets. A set of CFD computations have been carried out for a case-study burner, seeking the comparison for the isothermal-flow solutions given by Reynolds Averaged Navier-Stokes equations (RANS) and by Unsteady RANS equations (URANS). The influence of both spatial and temporal discretization is discussed, using the Grid Convergence Index (GCI) based on Richardson extrapolation. The results indicate that RANS solutions are slightly more sensitive to grid parameters, while URANS solutions show a better convergence behavior. Validation has been reasonably achieved by comparing the URANS velocity profiles against experimental measurements. As a consequence, a mathematical tool is now available to support design modifications of the biomass burner, combining simplicity, reliability and economy. (C) 2012 Elsevier Ltd. All rights reserved.