Narrow Results

The existence of pressure pulsations greatly increases the vibration and noise of pumps and harms their service life. In this paper, a casing treatment was employed to explore its impact on the pressure pulsations. A U-tube type groove was created at the inlet end-wall of a centrifugal pump and front cover of the impeller to connect the impeller with the inlet pipe by passing impeller leading edge. An unsteady numerical investigation was launched of the pump with and without this casing treatment, to study its influence on the pressure pulsations inside the pump and the mechanisms behind. The numerical results of the pump without casing treatment was first compared with the test performance of the pump to validate the numerical method, and gave excellent agreements with the test results. The CFD results also showed that the casing treatment increases the head coefficient and efficiency of the pump. Pressure pulsations at a reduced mass flow condition were studied by monitoring unsteady pressure signals generated by the CFD at various locations inside the pump. A Fast Fourier transform (FFT) was performed on the signals. The pump employs a double tongues volute with each tongue covering 180${}^{\circ }$ circumference. However, the two tongues are not identical with regard to the discharge of the pump. These geometric features of the volute and the pump’s operating condition generate several pressure pulsations in the frequencies of $f_n$, $2f_n$, $3f_n$ in the original pump. Due to the circumferential unifying capability of the casing treatment and its improvement to the impeller flow, these pulsations at impeller inlet are weakened or disappear when the U-tube is present. The pressure pulsation inside the impeller is less affected by the treatment. The $3f_n$ pulsation at volute tongues also decreases or disappears for the same reasons, but $f_n$ pulsation increases slightly and this is due to the improved pressure recovery in the volute by the treatment which increases the pressure difference across one of the volute tongues. The unsteady radial force of the impeller exerting on journal bearings becomes more uniform and smaller when the casing treatment is employed.

Instantaneous cavitating turbulent flow in a two-stage centrifugal pump with diffuser was simulated using a hybrid RANS/LES model and rotating corrected-based cavitation model in this paper. The predicted results of numerical simulation were in good agreement with the experimental results. The mechanism of pressure pulsation in the two-stage centrifugal pump was discussed. Some representative main frequencies of pressure pulsation such as main blade passing frequency, sub-blade passing frequency and intersection frequency of impeller blade and diffuser blade were analyzed systematically. Uncertainty estimation was used to ensure the accuracy of experimental results and it was also used to analyze the variation of pressure pulsation and vibration signals at different positions with the intensification of cavitation degree in the centrifugal pump. According to the results of uncertainty estimation, the center frequency of 1/3 octave band and the root mean square method were used to evaluate the energy change of the pressure pulsation signals and vibration signals at different frequency bands as the cavitation number decreases. The characteristics of pressure pulsation and vibration signals at different positions were analyzed in different frequency bands.

Previous work has shown that performance and internal flow characteristics of a centrifugal pump can be significantly improved with grooved volute casing (GVC). However, it has been found that the selection of the design parameters of the groove structure also has a direct impact on the performance output, internal flow pressure pulsation and erosion wear characteristics of the overflow components of centrifugal pump, so it is necessary to further analyze the design rules of the groove structure parameters. In this study, we first investigated the influence of the number of grooves on the head, efficiency and unsteady pressure pulsation characteristics of the internal flow field of the centrifugal pump, and on this basis, the correlation between different particle parameters and the erosion wear of key overflow components under the conditions of solid–liquid two-phase flow were also studied, and the erosion wear characteristics of the inner wall of the volute casing of centrifugal pump with GVC and original volute casing (OVC) structures were compared. This research leads to the conclusion that when the number of grooves is 3, the groove structure has the least influence on the performance of the centrifugal pump, and the overall change of the performance curve is more stable. Additionally, the pressure pulsation at each monitoring point of the GVC under the same flow condition is smaller, and when the number of grooves increases, the pressure pulsation amplitude also decreases. When the number of grooves is 3, the GVC shows a more significant flow improvement effect under all flow conditions. Based on the improvement of the groove structure on the flow stability, the particle motion behavior can be affected at the same time, so that the pump with GVC can mitigate the erosion wear of the inner wall of the volute casing under the solid–liquid two-phase flow conditions, which improves the critical performance and service life of the key overflow components of the pump.

Waterjet propulsion has many advantages when operating at high-speed conditions. As a special way of navigation, it is mostly used in high-speed ships and shallow draft ships. In this paper, a mixed-flow waterjet pump was taken as the research object. For the two cases of non-uniform inflow and uniform inflow, a modified RANS/LES method was adopted for unsteady calculation of the whole channel, aiming at investigating the influence mechanism of the non-uniform inflow on the energy performance and pressure pulsation characteristics of the waterjet pump. The hydrodynamic characteristics of the waterjet pump were comprehensively analyzed such as head, efficiency, axial-force, internal flow and pressure pulsation. It is found that the non-uniform inflow will reduce the external characteristics of the waterjet pump and lead to the huge fluctuation of energy performance with time. Low-speed swirls occur locally in the intake duct for non-uniform inflow, in which condition the vorticity is much higher than that for uniform inflow. In terms of the low-speed area, $P_{\mathrm{\text{Rey}}}$ and $P_{\mathrm{\text{tur}}}$, the values under non-uniform inflow condition are generally larger than those under uniform flow condition when in the impeller and guide vane zone. The dominant frequencies of pressure pulsation are, respectively, $f_n$, 7$f_n$ and 4$f_n$ in the intake duct, impeller and diffuser, which are almost consitent for the two cases. However, the frequency features are more diverse, and the amplitudes corresponding to the same frequencies are more intense for non-uniform inflow.

To improve the performance of a low-specific-speed centrifugal pump, the bionic structure extracted from the dung beetle was distributed on the blade surface to reduce the drag. The flow fields in the centrifugal pumps with and without the bionic structure blade were calculated based on the RNG $k$–$𝜀$ turbulence model. The results showed that the bionic structure can reduce the energy loss and improve the hydraulic efficiency of the centrifugal pump, with a maximum increase of 5%. The bionic structure can affect the turbulence structure of the boundary layer near the wall and reduce the blade outlet velocity gradient and wall shear, thereby improving the pressure pulsation characteristics of the centrifugal pump. With the increase in flow, the drag reduction gradually becomes evident for the impeller with a bionic structure.

In this study, the hub cutting of the impeller was presented to suppress the rotor–stator interference between the blade and volute tongue in a high-speed centrifugal pump with double volutes. The flow fields in the centrifugal pumps with four hub-cutting angles 0, 15^∘, 30^∘ and 45^∘ were numerically simulated to investigate the influence on the internal flow and pressure pulsation using SST $k-\omega$ turbulent model. The predicted results of numerical simulation were in good agreement with the experimental ones. The results show that the relative velocity streamline distribution is more uniform and smoother in hub-cutting impeller compared with that in the original one. Also, the head and efficiency of the pumps with hub-cutting impeller are higher than that of the prototype pump, with the maximum increase of 1.97% of the head for the hub-cutting angle 45^∘ under the design flow rate. The pressure pulsation intensity at volute 1 and the unsteady radial force of the hub-cutting pumps are smaller than that of the prototype pump due to the flow improvement. In addition, the maximum and average values of the unsteady radial force decrease with the increase of the cutting angle.

In this study, the anti-cavitation performance and cavitation flow characteristics in a hydrodynamic levitated micropump were investigated based on numerical simulation and experiment. The cavitation characteristic curves and the development process of cavitation in the levitated micropump was firstly analyzed. Special emphasis was put on the effects of eccentricity on the anti-cavitation performance. The results show that as the eccentricity increases, the critical cavitation number gradually decreases, indicating that the eccentric rotation is beneficial to improve the anti-cavitation ability of the levitated micropump. The coupling effects between the radial force on the impeller and cavitation were also numerically studied. With the decrease of cavitation number, the radial force on the impeller gradually declines at first, then has a sudden increase and finally reduces with fluctuation. The drop of the radial force will lead to the decrease of eccentricity, resulting in the deterioration of cavitation further. In addition, the unsteady pressure pulsation was analyzed. The predominant frequencies of pressure pulsation are the blade passing frequency (BPF) and the harmonic frequency of BPF under both noncavitation and critical cavitation. Under critical cavitation, the amplitude of BPF has a drop, while the amplitude of low frequency less than BPF becomes larger.

In order to optimize the matching of impeller with extra-thick blades and volute in centrifugal pump, the structure displacement and velocity of different volutes were simulated by two-way coupling Fuild-Structure Interaction method. The numerical results show that volute is effected by alternately exciting force due to the flow field interaction between the impeller and the tongue, and vibration displacement and vibration velocity distribution changes cyclically at different time steps. The ratio of volute base circle diameter and impeller diameter D₃/D₂ has a significant impact on volute vibration. When D₃/D₂ is less than 1.03, the un-uniform velocity distribution in blade outlet leads to the strong pressure pulsation, especially near tongue of volute. The peak-to-peak value of the pressure pulsation near tongue region is about 3-5 times of the other region in volute. However, the peak-to-peak value of the pressure pulsation decreases sharply when the ratio D₃/D₂ gradually increase. When D₃/D₂ is more than 1.03, the pressure pulsation amplitude is reduced by about 50%, and about 80% near tongue region. Pressure pulsation amplitude of the case B is reduced sharply by increasing the ratio D₃/D₂ without reducing the performance of pump, which will provide basis for the low-noise opertation.