1. Introduction
In deep-sea mining projects, minerals from the seafloor, such as polymetallic manganese nodules, need to be lifted to mining platforms or vessels on the surface. In the hydraulic lifting system, centrifugal pumps are used to transport the mixture containing solid mineral particles to the surface through the lifting pipeline [
1,
2], and the performance and reliability of centrifugal pumps, which are the core power devices, are very important for safety [
3].
Pump prewhirl is a phenomenon in which a fluid acquires a certain rotational motion before it enters the impeller of the pump. Yin et al. [
4] found that the prewhirl phenomenon exists in the inlet pipe of a centrifugal pump, and CFD numerical simulations were carried out for this purpose at different flow rates. Chalghoum et al. [
5] found that the prewhirl has a greater effect on the amplitude of pressure fluctuation when studying the effect of transient pressure fluctuation on the centrifugal pump’s performance, and the stronger the prewhirl is, the greater the amplitude of the transient pressure is. Zhou et al. [
6] studied the prewhirl and found that the appropriate positive prewhirl can improve the efficiency of the centrifugal pump. Huang et al. [
7] researched the unsteady turbulent pressure fluctuations in a water jet propulsion system and found that the airflow in the conduit shows periodic prewhirl motion. Lu et al. [
8] studied the noise caused by the unsteady flow through the numerical simulation of the pump. They found the prewhirl region caused by the impeller rotation exhibits a spiraling circular motion. Yang et al. [
9] researched the effect of positive prewhirl on the hydraulic performance of the pump under non-rated flow. The results showed that, under overloaded flow conditions, the prewhirl leads to an increase in the intensity of the flow, resulting in significant energy losses. However, most of the above studies were conducted on the basis of single-phase flow. In the study of solid–liquid two-phase flow fields, it is generally assumed that the inlet is a uniform incoming flow, and the effect of the presence of the prewhirl in the inlet flow field on the motion of particles is less considered. When a pump transports particle two-phase media, its performance and internal flow field will be different from those in clear water conditions. The particles will interact with the fluid during transportation in the pump, and the flow field characteristics will be changed. The particles will collide with the over-flow components, causing wear and tear on them, leading to performance degradation as well as the failure of the centrifugal pump [
10,
11,
12]. The presence of the prewhirl phenomenon can lead to clogging and wear in the inlet pipe of centrifugal pumps with a two-phase flow of particles [
13,
14,
15,
16]. Thus, the study of two-phase flow and wear in the inlet pipe is necessary.
In terms of particle two-phase flow research methods, many scholars have used the CFD-DEM method to carry out their own studies. Li et al. [
17] researched the flow in a bend with bumps in order to investigate the wear caused by particles on bends with different wall surfaces. The results show that the effect on the wear rate is most pronounced when the position of the bumps is close to the position of the first collision of the particles. Li et al. [
18] investigated the wear of a two-phase flow with a normal distribution of the particle sizes in 90° bends using the CFD-DEM method and found that the particles form a protective layer. Tang et al. [
19] used four types of polyhedral particles and one type of spherical particle to investigate the wear in pumps. They found that the wear suffered by the pump increases gradually with an increase in the degree of sphericity. Shi et al. [
20] analyzed the flow field and particle trajectory of a pump with an optimized inlet structure. After an experimental comparison, it was found that the simulation model had good accuracy. Su et al. [
21] studied the motion of large particles in a two-stage pump, and the results showed the method can accurately predict the pump performance curve obtained from experiments and, at the same time, analyze the particles at the impeller and the worm casing information, such as position distribution and velocity. Tan et al. [
22] investigated the characteristics of non-spherical particles in a centrifugal pump. The results show that the cylindrical particles will move towards the pressure surface to form a pileup. Wang et al. [
23] investigated the spatial distribution and kinematic properties of particles with different diameters in pumps. Li et al. [
24] performed numerical simulations to study the changes in the motion of particles in a pump under different vibration conditions. They found the particle passage inside the centrifugal pump would be higher as the vibration frequency increased.
The above studies show that the CFD-DEM coupling method is reliable for a solid–liquid two-phase flow in centrifugal pump simulation. However, there is insufficient research on the prewhirl effect on particle motion characteristics in the inlet pipe. The effect of the prewhirl flow field on the particle motion will directly affect the wear position and wear degree of the flow passage wall. Therefore, based on the CFD-DEM coupling method, this paper investigates the influence of the impeller inlet prewhirl of the two-phase flow in the inlet pipe and the wear of the wall, and the results of our research can provide certain theoretical support for the design optimization of the solid–liquid mixing centrifugal pump.
4. Conclusions
The prewhirl at the impeller inlet causes changes in the fluid and particles, which affects the wear of the inlet pipe. Therefore, in this paper, based on the CFD-DEM method, a solid–liquid two-phase numerical simulation of a centrifugal pump with a flow rate of 1200 m3/h is carried out to study the particle motion and wear of the inlet pipe. The main research content and conclusions are summarized as follows:
Impeller rotation causes a change in the flow field at the inlet of the impeller, producing a pronounced helical velocity streamline. The velocity of the fluid will increase in the radial direction along the inlet pipe cross-section and will rise sharply after r/R = 0.8, reaching a maximum value at r/R = 0.98.
Prewhirl causes the particle motion at the impeller inlet to be affected, and the particles are pulled by the flow and subjected to increased fluid forces and increased velocity. Near the impeller inlet, the average fluid force on the particles is between 0.015 and 0.018 N. At the same time, the effect of rotational reverse flow can lead to an uneven distribution of particles and the existence of accumulation in the middle part of the inlet pipe.
Prewhirl leads to a change in the state of motion of the particles and thus affects the wear distribution in the inlet pipe. The high-wear regions of the inlet pipe are L4, L9, and L13 and show a triangular shape, while the low-wear regions show a band structure. The cumulative force of particles on the wall surface has a high impact on wear, while the tangential cumulative force is a more influential factor. Prewhirl leads to an increase in both the maximum and average energy loss per collision, which affects the wear of the inlet pipe.
In this paper, mainly the effect of prewhirl on the particle motion and wear of the inlet pipe is investigated. Subsequently, taking into account the coupling system of the piping and the centrifugal pumps, the effect of prewhirl on the particles, flow field, and external characteristics inside the system will be analyzed. This aspect can be explored further in the future.