A novel approach to coupling the fluid and structural analysis of a boiler nozzle

Abstract

Boiler feedwater nozzles are often protected from steep thermal gradients by a fluid annulus that separates the feedwater pipe from the inside surface of the boiler. Experimental results have confirmed that hot water from the boiler is transported along the length of the annulus by a thermosyphoning flow, which is driven by natural convection effects within the annulus. This represents a highly complex flow field consisting of both laminar and turbulent flow combined with both global and local natural convection effects. This paper describes a quasi-steady thermofluid analysis of a feedwater nozzle operating at different transient conditions, which was used to predict the variation of peak stresses within the nozzle. This method represents a fast, effective approach for obtaining justifiable results for a large number of different operating transients. A steady-state FLUENT model was constructed and validated against measured data from an experiment on a similar nozzle design and the results demonstrated good agreement with the experimental data. The FLUENT model of the boiler feedwater nozzle was solved at a fixed operating condition to provide a three-dimensional profile of the heat transfer coefficients and bulk temperatures on the inner and outer surfaces of the fluid annulus. These data were imported into the finite-element analysis (FEA) code as a surface boundary condition. The FEA model was then used to predict the nozzle temperatures and stresses during the operating transient as part of a comprehensive fatigue and fracture assessment. This work has demonstrated that the use of advanced computational fluid clynamic (CFD) methods can form an integral part of the life assessment process. As a result, the boiler operator was able to significantly extend the predicted service life of the component.