*4.1. The Correlation between Streamline and Turbulence*

Although the problems caused by the synergistic effect of erosion–corrosion are serious, the erosion–corrosion mechanism of the elbow, as influenced by the velocity and pressure, is still not thoroughly understood because of its complexity. Numerical simulations are often used in erosion–corrosion research [20]. The velocity difference on the four featured edges near the inlet is due to high wall lift force. The slow decline of speed in the first straight section is attributed to the fact that the characteristics of the boundary layer flow field and the turbulent core area is quite different. Boundary layer has blocking effect on the fluid motion and the gradient of each flow parameter is very large leading to the formation of lift and affects the velocity of the wall.

However, as a result of the variation of pressure and turbulent intensity, the speed on four edges has different fluctuations throughout the elbow. The pressure variation, as shown in Figure 4a,b, is ascribed to the cause that constraint in geometry results in the gases to accumulate in the elbow section, and the accumulative effect exerts a relatively high pressure on the extrados and moderately relieves the pressure on the intrados. The intense turbulent intensity are due to the truth that low viscosity of gas causes vertical flow of gas: the fluid here produces an axial velocity and a radial velocity perpendicular to the tangential velocity of the main gas flow under centrifugal force. Because the very various velocity of the fluid causes different centrifugal forces in the elbow, so here the fluid exhibits complex three-dimensional motion characteristics. The research indicates that the places of pipeline including connection affect the erosion–corrosion behaviors [21]. The change in geometry causes a sharp change in the direction of flow with a large variation in velocity and friction action or retardation

on wall surface. As shown in Figure 4c,d, in the elbow section, the gases were forced to change direction with high friction velocity, which affects the fluid motion and causes low speed near the interior surface. The reduction in the amount of streamline may be due to the intense turbulence in the elbow that causes the gas movement to concentrate in some streamlines in the second straight section.

In the second straight section, the effect of turbulence intensity still affects the motion of gas, but the movement will gradually become stable along with the distance increased and the turbulence intensity dissipated.

## *4.2. The Correlation of the Electrochemical Corrosion with Material Concentration Distribution*

The model includes the electrolyte domain and the electrode surface. The concentration distribution of substances produced by carbonic acid ionization reactions is mainly concentrated at the junction of the elbow and the first straight section where turbulence intensity dramatically increased. As shown in Figure 4e,f and Figure 7, the intense turbulent intensity at the entrance of the elbow is a barrier to mass transfer, and the substance is difficult to transfer to intense turbulence intensity region and cause substance to be accumulated. Furthermore, turbulent intensity is very complex and intense at the elbow and it causes the substance to pass quickly and the remaining influence continues to the outlet boundary. The flow field characteristics in the boundary layer play a role in retarding the transport of the material concentration, thus causing the material to accumulate near the wall surface.

The variation in the electrochemical corrosion current density is a concentration-dependent process in a local region, and electrochemical reaction depends on the charge transfer reaction between the ions in the electrolyte and the electrons on the wall. The charge is conserved throughout the process. Convection and diffusion affect the concentration distribution of substance. The electrochemical corrosion current density is related to substance concentration distribution. Therefore, high current density of anode on the interior edge and low current density of anode on the exterior edge of the elbow is related to the concentration distribution of material.

## *4.3. Erosion in the Turbulence*

It is well-known that erosion has an important role in the total erosion–corrosion rates [22]. Severe erosion occurred at elbow is caused by the impact of solid particles driven by high-speed gas. At elbow, the direction and magnitude of gas was changed sharply because of the low viscosity and geometric constraint, and gas has a relatively small effect on the movement of particles. When the inertia of the solid particles is relatively large, the solid particles can pass through the streamline and hit the wall surface almost in a near linear path. For the straight sections, the random collision and erosion of the tube wall are caused by particles because of the influence of the pulsation of the flow field, but this effect is small. Therefore, the most severe erosion occurred at the elbow. In addition, the movement of the particles near the wall surface is lagged because of the boundary layer blocking effect. The geometric constraint and the effect of inertia of particles cause the particle group to form a curve near the extrados surface because of the boundary layer close to the tube wall and different turbulence characteristics.

There is an elliptically eroded area on the extrados surface of the elbow due to the presence of gravity, Brownian force, drag forces, and the turbulent intensity variations. In the first straight section, particles are gradually affected by the nature force; the trajectories of the particles gradually offset the line and gradually moved toward the extrados surface and the gravity direction. At the elbow, the particles impacted the wall and reduced their kinetic energy, which resulted in the subsequent particle group to be pushed and accumulated. In addition, the drastic changes in the flow direction, and the increase in turbulence intensity, caused the particles in the boundary layer to hit the wall surface and to be scratched along the wall surface, resulting in an elliptical erosion region from the bottom area to the top area. In addition, the formation of the cloud cluster in the second straight section is because the particles have different velocity directions and magnitudes at the entrance, which is affected by the wall constraint and high turbulence intensity at the elbow.
