Simulation of a reacting agitated bed of straw pellets by a resolved coupled DEM/CFD method using a blocked-off approach

Abstract

A resolved coupled DEM/CFD approach including a so called “blocked-off” method is presented which allows a continuous transition from the dense reacting pellet bed, with a CFD mesh resolving the voids between the pellets, to the freeboard above the pellet bed with a volatile gas flame present, where much coarser meshes are sufficient. The approach is tested and evaluated simulating the processes in a static bed and in a mechanically agitated bed of straw pellets. The pellets are approximated as spherocyclinders and treated as thermally thick objects. Momentum, radiation, heat and mass exchange at the interfaces to the pellets are calculated using wall functions, eliminating the requirement of shape specific correlations. Thermochemical source terms are determined by modelling conduction, drying, devolatilization and char burn-out for each fuel object. Local adaption of the effective turbulent viscosity, based on a Very Large Eddy model, accounts for different flow regimes and mesh densities in the pellet bed and the freeboard. Converted pellet mass is transferred to the passing fluid. Flame temperature and species distribution are determined from resulting mass fractions in the CFD domain and provide boundary conditions for thermochemical calculations of the pellets. First results show advantages of the approach, especially for the simulation of agitated fuel beds, by resolving the dynamic interaction between the gas phase and the pellets. This allows to interpret the difference in the reaction progress of static and agitated fuel beds which is difficult to assess by experiments.