Flow modifying screens in turbulent flows: An application to pulverized coal-fired boilers

Boiler tube failure occurs in pulverized coal-fired boilers due to particle impact erosion resulting in costly outages. Erosion is accelerated by localized high velocity flow conditions. Expanded metal screens are installed to control erosion by modifying the flow velocity, reducing excessive gas velocities and redistributing the ash load. The objective of this study was to develop a fundamental understanding of the turbulent flow through expanded metal screens by an experimental and numerical parametric study of the flow through screens in a wind tunnel, develop analytical models describing the screen interaction with the turbulent flow, and incorporate the developed screen models into numerical simulations of boiler flow.; Expanded metal screens turn the flow with a complex array of vaned elements. Measurements of the turbulent flow through three screen types with varying strand thickness and widths using cross-wire anemometry were carried out in a specially constructed low-turbulence wind tunnel. The mean velocity, pressure drop and turbulence distributions of expanded metal screens are presented as a database for Computational Fluid Dynamics (CFD) validation. The flow turning was found to vary with the dimensions of the strands that make up the screen. The turbulence generated by the screens was found to approach isotropy in the far-field and decays at a rate proportional to distance downstream to the power {dollar}-{dollar}5/7. The turbulence was found to scale with the thickness of the screen strands.; A screen deflection model was developed based on the inertial losses due to the presence of the screen and the flow turning characteristics of the screen due to the vane structure of the screen. This model, incorporated into a CFD solver, was used to approximate the influence of the screens on the flow in full boiler simulations where it would be computationally expensive, due to large differences in scale, to grid the geometry of the expanded metal screens. The screen model successfully predicted the flow fields patterns in the wind tunnel for the various screen configurations and was successfully implemented in the simulation of the flow modification by expanded metal screens in the economizer section of a utility boiler.; The turbulent flow through a single screen element was modelled numerically. The governing equations are described through a control-volume finite element method on an unstructured tetrahedral mesh. Two turbulence models were compared: the standard {dollar}kappa{dollar}-{dollar}varepsilon{dollar} and the Renormalized Group (RNG) method {dollar}kappa{dollar}-{dollar}varepsilon{dollar}. The RNG {dollar}kappa{dollar}-{dollar}varepsilon{dollar} gave a better prediction of the velocity and turbulence distributions behind the screen element. The numerical simulations of the flow field behind the screen elucidated important characteristics of the flow for future modelling of particulate flow through expanded metal screens.