The impact of secondary air boundary conditions on CFD results in small-scale wood pellet combustion

Abstract

This work is aimed at determining the impact of Secondary Air (SA) boundary conditions on CFD combustion simulation results in a small-scale wood pellet boiler. Two sets of boundary conditions are considered: (1) The SA inflow boundary conditions (i.e., the temperature, mass flow and turbulence distribution), and (2) The SA diffuser wall boundary conditions. First, a base simulation was prepared in which the flow distribution, preheating and turbulence parameters of the SA are included in the modelling approach. From the base case results the parameters of the SA inflow and heat transfer were derived and used as boundary conditions for cases where the SA diffuser is removed from the computational domain. In a parametric study the SA inflow and wall boundary conditions are varied from those estimated (e.g., by assuming the SA has room temperature) to more realistic (i.e. identical to the calculated parameters from the base simulation). By defining a realistic SA temperature and a realistic mass flow distribution between the upper and lower SA nozzles, results were obtained with a temperature Root Mean Square Error (RMSET) of 34 ℃ and a CO emission error of 27 mg/m3. Results with a RMSET of 21 ℃ and a CO emission error of 50 mg/m3 compared to the base case, were obtained by changing the wall boundary conditions from adiabatic to average temperature. The reduced computational mesh had 23% less elements, which reduced the time to achieve convergence by 29%.