Comparative study of the cavitation situation and the non-cavitation situation under unsteady flow phenomena of a centrifugal pump

Numerical simulations in three dimensions, both with and without cavitation, were conducted to determine the influence of cavitation on a centrifugal pump. The reliability and accuracy of the numerical model were demonstrated through satisfactory agreement between the experimental and numerical values of the pump’s performance. An in-depth analysis of the correlation between performance characteristics and cavitation was undertaken. The research results indicate that cavity bubbles initially appear near the suction surface of the blade and gradually extend toward the middle of the blade-channel as cavitation intensifies. Moreover, based on the simulation results, it can be concluded that head characteristics are linked to the development of cavitation on the suction surface of the leading edge of the blade runner. A detailed analysis of experimental data and simulation results reveals that cavitation obstructs the flow channel and alters water flow direction, thereby diminishing operational capacity.


Introduction
Centrifugal pumps play a crucial role in industrial, irrigation, and water supply engineering.The increasing utilization of large centrifugal pumps in production lines has underscored the urgency of ensuring the stability of pump operation.The predominant sources of energy loss in these pumps stem from cavitation, flow separation, reflux, and vortex movements within different components [1,2].Cavitation, characterized by turbulent and compressible flow, significantly impacts centrifugal pumps [3,4].A decrease in the pump's inlet pressure results in the appearance and expansion of cavitation zones, leading to performance degradation and blade erosion.Studies by Ji et al. [5] and Huang et al. [6] delved into the interaction between cavitation and unstable flow, revealing that cavitation amplifies the development of eddy currents.High-precision numerical simulations unveiled the distribution of cavitation and its consequential impact on hydraulic losses.Building upon these findings, this article conducts an analysis of cavitation characteristics.

Physical configuration
In this paper, a centrifugal pump model is the subject of research, and the rated parameters of the pump are as follows:

Turbulence model and boundary conditions
The internal flow of the pump was numerically analyzed using ANSYS Fluent-2021R1.A flow solver was employed to solve incompressible turbulence, continuity equations, and three-dimensional timeaveraged Navier-Stokes equations.Figure 2 illustrates the grid distribution of the simulated pump.The ANSYS Fluent mesh 2021 R1 software was utilized to apply the Mosaic mesh type to all components of the pump, including the impeller, inlet section, and outlet section.
The SST k-ω turbulence model was used for the impeller region within a rotating coordinate system.Static coordinate systems were selected for the inlet and outlet parts, while the frozen rotor model was utilized for the rotor-stator interface.Subsequently, normal velocity inlet and pressure outlet conditions were established for the computational domain, with a convergence standard set to 10 -5 .
The ZGB (Zwart-Gerber-Belamri) model, based on the Rayleigh-Plesset equation, is categorized within the class of transport equation models in the homogeneous model framework.This model is frequently employed in cavitation simulations for estimating pump suction performance and (3) (5)

Grid independence validation
Utilizing the ANSYS Fluent Mesh 2021 R1 software, choose the Mosaic grid type for application across various pump components, such as the impeller, inlet, and outlet.Use six sets of grids with different cell counts (0.42, 0.47, 0.58, 0.96, 1.04, and 2.83 million) to compute the flow field in Table 2 .To ensure simulation accuracy, the mesh refinement process focuses on augmenting the cell count in the impeller region.Figure 2 showcases the head curves of multiphase pumps operating under diverse water flow conditions, designed with varying grid densities.The chart facilitates a comparison of results.As the grid count increases, the change in head for the centrifugal pump gradually diminishes and stabilizes, ultimately leading to the selection of a grid count of 970,000.

Experimental validation
The experiments were conducted at China Agricultural University, and all the essential data regarding the experimental setup are provided in references [8].The simulation results are match with experiment data in good agreement shown in Figure 3.

Performance curve
The characteristics in the flow are primarily influenced by changes in initial pressure and flow rate occurring during cavitation onset is shown in Figure 4.This was revealed through an analysis of simulated cavitation flow characteristics.
leading edge of the blade.As cavitation intensifies, vortices gradually move towards the outlet of the impeller within the flow passage, resulting in an increased blockage coefficient, highlighting the significant role of cavitation in inducing internal impeller vortices.Compared to non-cavitating conditions, cavitation exacerbates the instability of turbulence and vortices and reduces the pressure within the impeller.The accompanying graph illustrates a strong correlation between cavitation and vortices, commonly referred to as cavitation vortex.Additionally, the variation in pressure pulsation with cavitation is more pronounced compared to scenarios without cavitation in Figure 6.Furthermore, as cavitation progresses, the stretching term of relative vorticity in the blade tip region exhibits a more complex developmental trend.SST k-ω and ZGB were utilized to numerically simulate the flow field of a low specific speed centrifugal pump impeller and investigate impeller pressure pulsation under cavitation conditions.The conclusions drawn are as follows: (1) As the cavitation effect intensifies, the flow near the trailing edge on the suction side of the blade gradually becomes chaotic, resulting in the generation of eddy currents and increased turbulence and reduces the pressure within the impeller's flow field.
(2) The analysis of performance characteristics indicates that during cavitation occurrences, the head and efficiency of the centrifugal pump sharply decrease.The flow trajectory alters due to cavitation, leading to channel blockage and reduced pump working efficiency.
(3) The analysis of flow characteristics resulting from simulated cavitation flow indicates that the variation in head depends on the degree of cavitation.As the initial pressure decreases and the flow rate diminishes, cavitation intensifies, leading to increased hydraulic losses within the impeller.

Figure 3 .
Figure 3. Flow fields comparison of experiment and simulation.

Figure 4 .
Figure 4. Head curve between different initial pressure and rate of flow

Figure 5 .
Figure 5.The flow condition of cavitation situation and non-cavitation situation.

Figure 6 .
Figure 6.The variation of pressure pulsation over time under different working conditions.

Table 2 .
Mesh information of flow components.