| Description |
Large-eddy simulation (LES) is becoming a more accurate tool than Reynolds-Averaged Navier-Stokes (RANS) for predicting the characteristics of coal combustion. LES coupled with Direct Quadrature Method of Moments (DQMOM) was carried out to simulate pulverized-coal combustion in the large laboratory-scale furnace: 100 kW down-fired oxy-fuel combustor (OFC) furnace and industrial-scale boiler simulator facility (BSF). This work analyzes the time and spatial scales in the aforementioned combustion systems to validate some underlining assumptions of the LES/DQMOM method. Firstly, the quality of LES work indicates how much the turbulent kinetic energy is resolved for BSF and OFC systems. Secondly, the effects of unresolved turbulent scales on the particle motions were analyzed using the Stokes number approach. To thoroughly understand these effects, the homogeneous-isotropic particle-laden flows are investigated under different Stokes number regimes. This work uses Langevin stochastic models and multifractal theory to reconstruct SGS velocity and apply it to the turbulent particle-laden flows. To improve the near-wall turbulence prediction, this work explored the sigma SGS model and tested it via homogeneous-isotropic turbulent flows. The performance of two subgrid-scale models (Sigma model, dynamic Smagorsinky) and two wall-stress models (linear model, log-law) for wall-bounded flows is investigated by simulating the turbulent channel flow. Ash deposition to walls in coal boilers can have a severe impact on the heat-transfer efficiency of the system. In this work, a new ash-deposition model implemented within the LES/DQMOM framework, coupled with a one-dimensional conjugate wall heat-transfer model, was presented. The model integrates three sticking mechanisms: melt fraction, viscosity, and energy conservation upon collision by defining a deposit mechanics regime map. The new model fully considers the parameters of fly ash and predeposited medium. First, the experimental data for deposition in the OFC are used for validation. The multiscale, multiphysics approach considered here incorporates several physical mechanisms that impact the heat-transfer prediction in the industrial pulverized-coal boiler predictions. This coupled model has been applied to simulating the pilot-scale boiler (BSF). The simulations reveal the deposition pattern in industrial-scale boilers. This work can be directly applied to simulating industrial pulverized-coal boilers, with the hopes that it will aid in troubleshooting of current operations and help design more efficient systems for the future. iv |
