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

Highlights: • The buoyancy parameter proposed by Jackson shows a good single-valued relationship with HTC. • Specific heat ratio and acceleration parameter show weak independence on HTC. • A new variable Pr_t model was proposed on the basis of the analyses of the effect of Pr_t on the supercritical water heat transfer. • The new Pr_t model provided accurate heat transfer predictions, especially for the HTD cases. • The safe heat transfer characteristics of a 660 MW ultra-supercritical CFB boiler water wall were analyzed. 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 (Pr_t) 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 Pr_t 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 Pr_t on the supercritical water heat transfer. Experimental data of supercritical water were used to validate the accuracy of this Pr_t model. The results indicated that this Pr_t model provided accurate heat transfer predictions, especially for the heat transfer deterioration (HTD) cases. The variable Pr_t 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 Pr_t 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.