Numerical Study On Heat Transfer Characteristics Of Large Fuel Particles

As people's demanding on the reduction of pollutant emissions and efficient and clean fuels are getting higher and higher,it is important to conduct the research on emission reduction and efficient combustion.The pollutant emission of domestic large coal particles and formed biomass particles are larger.Few studies were reported on large fuel particles,there is a large temperature gradient inside the large fuel particles,which shows a great effect on water evaporation,devolatilization and char combustion process,etc.Therefore,it is necessary to have a deep understanding of the temperature distribution inside large fuel particles.It is of great significance to study the heat transfer characteristics of large fuel particles by the numerical simulation method to analyze the internal heat conduction inside the large particles and the convective heat transfer between the particles and fluids.The validation of the numerical simulation on the convective heat transfer between a single spherical large particle and its surrounding air is first introduced.The effects of boundary conditions including the constant temperature and the uniform heat flux as well as the fluid-solid coupling boundary conditions on particle–fluid convection heat transfer is discussed.It is found that the time-and surface-averaged Nusselt numbers obtained by the simulations with fluid–solid coupling boundary and the constant temperature boundary agreed well with the empirical formula of Ranz-Marshall,while the simulation results with the uniform heat flux boundary condition is larger than those of the other two boundary conditions.The distribution of local Nusselt number is closely related to the fluid flow state.The distribution of the time-and surface-averaged local Nusselt number indicated that when the flow was in the steady symmetric regime,the Nusselt number decreased from the front stagnation point to the rear stagnation point.While the flow is in the unsteady vortex regime,the time-and surface-averaged local Nusselt number firstly decreased from its maximum at the front stagnation point to a minimum value near the separation point,and then gradually increased to the rear stagnation point.Meanwhile,the fluctuation frequency of Nusselt number is consistent with that of drag coefficient.The convection heat transfer between a single spherical particle and its surrounding water is then simulated to further verify the fluid-solid coupling model.It is found that the distribution trend of local Nusselt number is consistent with that of air-particle heat transfer,but the fluctuation frequency is not consistent with that of the drag coefficient,as the Prandtl number and the heat capacity of water are larger than those of air,the heat transfer does not follow the flow fluctuation completely.A thermal inversion phenomenon is found by the fluid-solid coupling simulation.The influence of the computational domain,boundary conditions and Prandtl number on the thermal inversion phenomenon is further discussed.The results show that the computational domain has some influence on the time-and surface-averaged Nusselt numbers,but the thermal inversion phenomenon occurs in all the computational domains simualted.The fluid-solid coupling simulations can capture the thermal inversion phenomenon,while the isothermal simulations cannot.Thermal inversion phenomenon occurs in the simulated Prandtl number ranges at 7 ? 700,and this phenomenon can occur at the maximum Reynolds number of higher than 2630.The fluid-solid coupling model is used to simulate the heat transfer of a large granular bituminous coal and its surrounding air.The simulation results show that the time-and surface-averaged Nusselt number matched well with the empirical formula of Ranz-Marshall.Due to the large internal thermal resistance of the particle,there is a strong non-uniformity of the temperature distribution inside the particle,thus affecting the characteristics of water evaporation,devolatilization and char combustion process of large fuel particles.