Numerical Investigation of Three-Dimensional Hydrodynamics and Heat Transfer in a Hot-Water Boiler During the Combustionof Heterogeneous Fuel Pellets

Global trends toward energy decarbonization and energy efficiency enhancement drive the large-scale transition of small-scale municipal thermal power engineering to renewable energy sources, particularly biomass. The rapid development of the fuel pellet market has expanded the raw material range from traditional wood pellets to heterogeneous agro-pellets and municipal waste. However, switching to alternative fuels poses risks of slagging, efficiency degradation, and local thermal overloading of equipment. Traditional empirical calculation methods fail to account for the complex three-dimensional aerodynamics and diffusion-kinetic combustion characteristics of alternative biomass. For instance, flame instability and uneven heat absorption by waterwalls can reduce boiler reliability by 15…20 %. This highlights the urgency of implementing computational fluid dynamics (CFD) methods to evaluate operational regime stability. This study aims to investigate the influence of the physicochemical properties of six fuel pellet types on three-dimensional hydrodynamics, turbulent structure, temperature fields, and heat transfer in a 50 kW Feniks hot-water boiler. This is achieved through numerical CFD modeling in ANSYS Fluent using the RANS approach, the Realizable k- turbulence model, the Lagrangian Discrete Phase Model (DPM), and the Discrete Ordinates radiation model. As a result, the study demonstrates that the geometric configuration of the boiler provides high aerodynamic versatility (flame velocity variation does not exceed 3,3 %). The model validity is confirmed by the agreement between the calculated flame temperatures and adiabatic values (with a deviation of less than 8 %). The obtained results and calculated spatial fields can be utilized to optimize pellet boiler designs and evaluate their environmental performance.