Experimental and numerical investigation on heat transfer in the vertical upward flow water wall of a 660 MW ultra-supercritical CFB boiler

Abstract

Heat transfer in the vertical upward water wall of a 660 MW ultra-supercritical circulating fluidized bed (CFB) boiler was experimentally and numerically studied. A new variable turbulent Prandtl (Prt) model was used in the numerical simulation. The effects of specific heat ratio, buoyancy parameter, and acceleration parameter on the heat transfer of supercritical water were investigated through experimental data. Results show that the independence between the heat transfer coefficient and these dimensionless parameters was weak with the majority of them, only buoyancy parameter proposed by Jackson showed a strong independence. Additional parameters are required if specific heat ratio and acceleration parameter are used to predict the supercritical water heat transfer. A new variable Prt model was proposed as a function of the turbulent viscosity ratio and the molecular Prandtl number on the basis of the analyses of the effect of Prt on the supercritical water heat transfer. Experimental data of supercritical water were used to validate the accuracy of this Prt model. The results indicated that this Prt model provided accurate heat transfer predictions, especially for the heat transfer deterioration (HTD) cases. The variable Prt model demonstrated a significant effect on the radial velocity distribution, thereby leading to an ‘‘M” shape velocity profile. The zero velocity gradients limit the turbulent mixing, thereby leading to a significant reduction in the turbulent kinetic energy and HTD. The safe heat transfer characteristics of a 660 MW ultra-supercritical CFB boiler water wall were analyzed on the basis of the experimental research and simulation with the new Prt model. The results show that the water wall adopts a smooth tube can ensure the nonexistence of film boiling and overtemperature, and the water wall heat transfer is safe.