Gas & Petrochemical: Erosion in a 90-degree Knee

Description

This project simulates erosion in a 90-degree pipe elbow (knee) using ANSYS Fluent, investigated through CFD analysis. Erosion in bends is a critical concern in the gas and petrochemical industry, where pipelines routinely transport fluids carrying entrained solid particles over long distances.

In a straight run of pipe, fluid impurities pose little problem. The difficulty arises when the flow changes direction: the suspended solid particles, owing to their inertia, cannot follow the fluid streamlines through the turn. This causes the particles to decouple locally from the carrier fluid and strike the pipe wall, gradually wearing away the material — the phenomenon known as erosion.

In practice, industrial fluids are almost never pure, so erosion is an unavoidable challenge in pipeline transport. Flow turbulence intensifies the effect: the more turbulent the flow, the greater the momentum carried by the particles, and the harsher their impact on the wall. These impacts are most severe wherever the flow changes direction, which is exactly why erosion is concentrated at bends and elbows. Beyond turbulence, several other factors govern the extent and pattern of erosion, including particle size, particle mass flow rate, the redirection of the particle path, the number of wall impacts, and the overall flow rate.

Because erosion tends to occur precisely where the flow redirects, fittings and joints — particularly elbows — are the primary locations examined when assessing erosion in a pipeline network.

Methodology

Every simulation begins by defining the computational domain. Although the geometry is a single elbow joint, it was subdivided into separate sections to enable a structured mesh, with each segment meshed individually so that boundary-layer settings could be applied precisely.

The 3D geometry was created in ANSYS Design Modeler, and meshing was performed in ANSYS Meshing. The domain was split into four parts, each meshed with a structured grid. A structured mesh offers faster and more accurate CFD solutions — an advantage that matters greatly for erosion studies, since the boundary layer, path lines, and particle tracking must all be resolved cleanly as the flow travels through the bend. The final mesh contains 4,319,695 elements.

Because the mesh was sufficiently fine, the Enhanced Wall Treatment method was used in place of standard Wall Functions for near-wall modeling, providing higher accuracy at the boundaries. The Discrete Phase Model (DPM) was applied to represent the solid particles carried within the pipeline.

Conclusion

An important consideration is that the upstream pipe length must be long enough for the flow to fully develop before reaching the elbow; otherwise, the particle distribution entering the bend may yield unrealistic results. The velocity magnitudes of both the fluid and the particles can be readily examined from the corresponding contours.

In addition, the particle concentration and — most importantly — the erosion contour are presented, offering a comprehensive picture of erosion behavior in pipeline bends.