Understanding the effects of mesh smoothing methods in the CFD modelling of Zhundong lignite ash deposition

Slagging and fouling are the most typical causes of unscheduled solid fuel boiler shutdowns. The CFD-based prediction of the deposit growth and rates, which combine the basic mechanisms of ash particle transport, impaction, sticking, as well as the complex aerodynamics of a boiler, is helpful to optimize the design and operation of boilers. In this study, a dynamic CFD model, which contains the molten fraction based sticking model and dynamic mesh model, is developed and validated in Zhundong lignite combustion in a pilot-scale furnace. With the employment of this model, a smooth growth of deposition and an accurate simulation can be achieved without applying the mesh smoothing strategy. Further, the predicted results are in good agreement with the experimental data. The effects of two mesh smoothing methods, which are the mass spreading algorithm and the group-averaged redistribution method, have been investigated under different furnace operation conditions. Compared to the simulations without the smoothing methods, a smaller particle count is required during the simulation to obtain the accurate, efficient and stable predicted results. The smoothing methods make a small difference (within 3.0%) on the predicted heat flux and deposition rate. In contrast, the deposition thickness at the tube position α=180° after two deposition hours would be within 16.8% smaller compared to the predicted results without the smoothing methods.