A first principal modeling of the gasification of a char particle is performed using single step mechanism. The char particle is considered to be spherical in shape and only the physical and chemical properties can ch...A first principal modeling of the gasification of a char particle is performed using single step mechanism. The char particle is considered to be spherical in shape and only the physical and chemical properties can change in the radial direction. The carbon dioxide is used as the gasification agent that reacts with the char and form carbon monoxide. The presence of both solid and gaseous phase species makes the reaction heterogeneous. The char particle is considered with varying porosity that also allows the change in the surface area of the particle. A time invariant temperature and pressure profile is used at which the Arrhenius rate constant and diffusion is calculated. The mass conservation of model results in the form of two coupled partial differential and one ordinary differential equation. The equations are solved with a set of initial and boundary conditions using the bulk species concentration at the particle surface. A second order accurate central differencing scheme is used to discretize space while backward differencing is used to discretize time. Finally, the results are presented for the concentration distribution of CO and CO2 in radial direction with respect to time. It shows that, maximum concentration of CO is present at the center of the particle while the concentration gradient becomes higher near the particle surface. The nonlinear concentration trend due to the diffusion is effectively captured. The results show that, completed conversion of char depend upon the time provided for the reaction which can be reduced by decreasing the size of particle or increasing the reaction temperature. The sensitivity study of temperature and initial porosity also performed and showed that temperature has high impact on char conversion as compare to initial porosity.展开更多
Based on the solid-liquid two-phase mixture transportation test, the renormalization group (RNG) k-e turbulent model was utilized to simulate the solid-liquid two-phase turbulent flow in a centrifugal pump. By compari...Based on the solid-liquid two-phase mixture transportation test, the renormalization group (RNG) k-e turbulent model was utilized to simulate the solid-liquid two-phase turbulent flow in a centrifugal pump. By comparing the simulated and experimental results, inner flow features were revealed to improve the abrasion characteristic of the solid-liquid two-phase centrifugal pump. The influence of the solid phase on centrifugal pump abrasive performance is small when the particle volume fraction is less than 2.5%. The aggregation degree of the solid particles is enhanced as the particle diameter increases from 0.1 to 1 mm; however, the mixture density on the pressure side is reduced when the particle diameter increases to 1 mm for the impact of inertia. The wear on the hub is most severe for the shear stress on this position; it is also the largest. The wear characteristic is affected greatly by the parameters of the solid phase. The wear chracteristic can be optimized by decreasing the blade outlet angle. In the modified design, the blade angle is different, whereas the other geometric dimensions remain the same. The improved pump is simulated to contrast with the original pump. The results show that the values of mixture density and shear stress both decrease. The wear condition of the blade is improved to a certain extent.展开更多
文摘A first principal modeling of the gasification of a char particle is performed using single step mechanism. The char particle is considered to be spherical in shape and only the physical and chemical properties can change in the radial direction. The carbon dioxide is used as the gasification agent that reacts with the char and form carbon monoxide. The presence of both solid and gaseous phase species makes the reaction heterogeneous. The char particle is considered with varying porosity that also allows the change in the surface area of the particle. A time invariant temperature and pressure profile is used at which the Arrhenius rate constant and diffusion is calculated. The mass conservation of model results in the form of two coupled partial differential and one ordinary differential equation. The equations are solved with a set of initial and boundary conditions using the bulk species concentration at the particle surface. A second order accurate central differencing scheme is used to discretize space while backward differencing is used to discretize time. Finally, the results are presented for the concentration distribution of CO and CO2 in radial direction with respect to time. It shows that, maximum concentration of CO is present at the center of the particle while the concentration gradient becomes higher near the particle surface. The nonlinear concentration trend due to the diffusion is effectively captured. The results show that, completed conversion of char depend upon the time provided for the reaction which can be reduced by decreasing the size of particle or increasing the reaction temperature. The sensitivity study of temperature and initial porosity also performed and showed that temperature has high impact on char conversion as compare to initial porosity.
文摘Based on the solid-liquid two-phase mixture transportation test, the renormalization group (RNG) k-e turbulent model was utilized to simulate the solid-liquid two-phase turbulent flow in a centrifugal pump. By comparing the simulated and experimental results, inner flow features were revealed to improve the abrasion characteristic of the solid-liquid two-phase centrifugal pump. The influence of the solid phase on centrifugal pump abrasive performance is small when the particle volume fraction is less than 2.5%. The aggregation degree of the solid particles is enhanced as the particle diameter increases from 0.1 to 1 mm; however, the mixture density on the pressure side is reduced when the particle diameter increases to 1 mm for the impact of inertia. The wear on the hub is most severe for the shear stress on this position; it is also the largest. The wear characteristic is affected greatly by the parameters of the solid phase. The wear chracteristic can be optimized by decreasing the blade outlet angle. In the modified design, the blade angle is different, whereas the other geometric dimensions remain the same. The improved pump is simulated to contrast with the original pump. The results show that the values of mixture density and shear stress both decrease. The wear condition of the blade is improved to a certain extent.
