Computation of Localized Erosion-Corrosion in Disturbed Two-Phase Flow
Erosion-corrosion is accelerated corrosion of a metal as a result of the flow removing the protective film from the surface that otherwise slows down the rate of corrosion. In some cases of solid/liquid flows particle-wall impacts can additionally cause erosion of the underlying metal. Study of erosion-corrosion in disturbed two-phase flow requires knowledge from several engineering disciplines including: turbulent fluid flow, particle motion, erosion and corrosion. Very few studies exist that cover this problem adequately especially from a modelling point of view. This thesis describes the development of a general predictive numerical model for localized erosion-corrosion of metals in disturbed two-phase liquid/solid flow. The flow structure is determined by the application of two phase flow version o fa k-e eddy viscosity model (EVM) of turbulence. The Eulerian approach for the fluid flow is coupled with a Lagrangian approach for particle motion. Local values of fluid velocity and turbulent and molecular transport coefficients are determined along with particle-wall interactions in terms of impact velocity, angle and frequency. The corrosion component of the model assumes mass transfer control. The mass transfer rates are determined by the solution of the mass transport equation simultaneously with the fluid flow equations. The erosion is determined on the basis of the computed particle-wall interactions and cutting wear erosion equations. Unlike the erosion-corrosion in flow in a straight pipe,or other simple geometries,in disturbed flow there is no obvious relation between the bulk flow parameters and the local rates of erosion-corrosion, so they must be measured or obtained from numerical simulation studies. We have chosen the numerical path with the objective of developing predictive models for erosion-corrosion. Before the developed overall model was verified, each component (e.g. flow, erosion, corrosion) was separately validated. The hydrodynamic model including two approaches for the near wall region, the wall function approach (WF) and the low Reynolds number approach (LRN),was tested by comparing the predictions with laser Doppler velocimetry (LDV) measurements of liquid flow through a sudden pipe expansion. Satisfactory agreement for the mean flow parameters has been achieved with both approaches. To validate the model of mass-transfer-controlled corrosion,mass transfer in aqueous, turbulent, recirculating, aqueous flow was simulated with a LRN k-e EVM model. The predictions were tested against experimental data for flow through a sudden pipe expansion, Good agreement was obtained over a wide range of Reynolds numbers. The predictions of particle motion made with the two-phase flow model, which includes a EVM k-e model of turbulence (WF), accompanied with via Lagrangian particle motion model, have shown an overall good agreement with the LDV measurements, for flow of 0.1% water/sand slurry through a sudden pipe expansion. Lagrangian models of particle motion provide details of individual particle trajectories and particle-wall impacts. Predictions of erosion rates made with a cutting wear model and based on predicted particle-wall interactions were in good agreement with the erosion measurements, for a water/sand slurry flow througha sudden expansion ina stainless steel pipe. In the erosion-corrosion process it was assumed that the protective scale was completely removed by the eroding particles. Corrosion rates were calculated on the bases of predicted local oxygen-mass transfer rates. Predictions of metal loss by erosion-corrosion (erosion plus corrosion in our model) were compared with measurements for flow of oxygen saturated 2% water/sand slurry through a mild steel sudden pipe expansion. It was found that the dominant mode of metal loss is corrosion, with the sand removing the protective rust film. The rate of erosion of the base metal was an order of magnitude smaller than the rate of corrosion. The model of erosion-corrosion used was successful in predicting the profile and rates of metal loss in the geometries studied. After components and the overall model have been successfully verified for flow through a sudden pipe expansion,erosion-corrosion in flow over a groove in a pipe has been predicted to demonstrate the generality of the model.