Narrow Results

In order to study the influence of solid particles in lead–bismuth alloy on the safety of nuclear main pump, DPM (Discrete Phase Model) is used to calculate the wear rate of the impeller of nuclear main pump under different operating conditions. By comparing the erosion rate and the most severe erosion area under different working conditions, the most unfavorable operating conditions are obtained. The results show that the erosion of impeller caused by solid particles is the most serious when the inlet velocity of particles u is 0.4m/s and particle size D is 100$\mu$m. Under the small particle size, the impeller erosion concentrated on the suction side of the blade, and under the large particle diameter, the erosion concentrated on the pressure side, and the erosion is more serious. With the increase of solid particle velocity, the erosion of the impeller increases first and then decreases. The erosion severity of impeller and front and rear cover plates caused by solid particles is not consistent. The above research can provide guidance for the anti-wear design and safe operation of the nuclear main pump impeller.

To research effects of silt particles on the performance and cavitation flow fields, silt-laden cavitation flow was simulated in the centrifugal pump. Silt mean diameters are 0.005 mm and 0.010 mm and silt concentrations are 0.5% and 1.0%. Results show that silt particles with silt mean diameter 0.005 mm and silt concentration 1.0% and silt particles with silt mean diameter 0.010 mm and silt concentration 0.5% promote the development of cavitation and distribution range of vapor is larger than pure water. Effects of silt particles and cavitation make distribution range of turbulent kinetic energy larger than pure water and streamlines are more disorderedly. Silt particles with silt mean diameter 0.010 mm and silt concentration 1.0% have little effect on cavitation and distribution range of vapor is similar with pure water. Effects of silt particles make distribution range of turbulent kinetic energy larger than pure water and streamlines are more disorderedly than pure water. With the increase of silt mean diameter and silt concentration, head and efficiency decrease gradually. For silt particles promoting the evolution of cavitation, distribution range of turbulent kinetic energy is larger and streamlines are more disorderly than silt-laden cavitation flow with silt particles inhibiting the development of cavitation.

An innovative structure of longitudinal grooves was designed on the volute casing of a centrifugal pump to improve the performance and internal flow characteristics. Comparisons were made between the model with grooved volute casing (GVC) and the model with original volute casing (OVC) operating in different working conditions. To get a better understanding of the influence of grooves on the centrifugal pump, some preliminary studies were performed with a simplified model. The results indicated that the application of GVC offers better pump performance under the large flow rate, also can stabilize the operating range of the centrifugal pump. In addition, the drag reduction effect of GVC is quite considerable compared with the pump with OVC. The pump with modified volute casing can suppress and eliminate the instability flow inside the impeller channels and volute casing. By using the GVC, the pressure and velocity distribution inside the impeller channel ameliorated especially for the 0.85 $R_i$ near the region of rotor–stator interaction between impeller and volute casing. Furthermore, the effects of GVC on flow passing ability may be recognized clearly for the flow area of the model with GVC increased by 5% at one impeller channel compared to the pump with OVC.

The performance and service life of centrifugal pumps can be influenced by the clocking effect. In this study, 3D numerical calculations based on the k-omega shear stress transport model are conducted to investigate the clocking effect in a centrifugal pump. Time-averaged behavior and transient behavior are analyzed. Results show that the optimum diffuser installation angle in the centrifugal pump is $25^{\circ }$ due to the minimum total pressure loss and radial force acting on the impeller. Total pressure loss, particularly in the volute, is considerably influenced by the clocking effect. The difference in total pressure loss in the volute at different clocking positions is 2.75 m under the design flow rate. The large total pressure loss in the volute is primarily caused by the large total pressure gradient within the vicinity of the volute tongue. The radial force acting on the impeller is also considerably affected by the clocking effect. When the diffuser installation angle is $25^{\circ }$, flow rate fluctuations in the volute and impeller passage are minimal, and flow rate distribution in the diffuser passage is more uniform than those in other diffuser installation angles. Moreover, static pressure fluctuations in the impeller midsection and the diffuser inlet section are at the minimum value. These phenomena explain the minimum radial force acting on the impeller. The findings of this study can provide a useful reference for the design of centrifugal pumps.

To reduce the calculation cost and improve the accuracy of flow field prediction, an adaptive proper orthogonal decomposition (APOD) surrogate model based on K-means clustering algorithm was proposed to reconstruct the flow field of impeller. The experiment samples were designed by introducing the perturbation of the blade control parameters such as blade wrap angle and blade angle of outlet. K-means clustering algorithm was used to classify the sample blade shapes, and find out the cluster of the objective blade. The snapshot set, which consisted of the blade shape and the flow field data of impeller, can be described as a linear combination of orthogonal basis by POD method. The radial basis function (RBF) was used to fit the orthogonal basis coefficients of the objective blade, and then the flow field of objective impeller was reconstructed. The traditional fixed sample POD (FPOD) method and the proposed APOD method were used to reconstruct the flow field in impeller, respectively, and the prediction results of the two methods were compared and analyzed. The results show that the proposed APOD method could quickly and accurately reconstruct the objective flow field. The flow field prediction accuracy of the APOD method is significantly higher than the FPOD method, and the calculation time for the flow field prediction is less than 1/360 of the CFD.

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.

This paper proposes a new method that obstacles are attached to both the suction and pressure surfaces of the blades to suppress cavitation development. A centrifugal pump with a specific speed of 32 is selected as the physical model to perform the external characteristic and cavitation performance experiments. SST $k-\omega$ turbulence model and Zwart cavitation model were employed to simulate the unsteady cavitation flow in the pump. The results indicate that the numerical simulation results are in good agreement with the experimental counterparts. After the obstacles are arranged, the maximum head decrease is only 1.37%, and the relative maximum drop of efficiency is 1.12%. Obstacles have minimal impacts on the variations of head and efficiency under all flow rate conditions. The distribution of vapor volume in the centrifugal pump is significantly reduced after obstacles are arranged and the maximum fraction reduction is 53.6%. The amplitude of blade passing frequency decreases significantly. While obstacles decrease the intensity of turbulent kinetic energy near the wall in the impeller passages to effectively reduce the distribution of cavitation bubbles, and control the development of cavitation. After the obstacles are set, the strength of the vortex in the impeller passages is weakened significantly, the shedding of the vortex is suppressed, flow in the impeller becomes more stable.

To overcome the problems of large calculation cost and high dependence on designers’ experience, an optimization design method based on multi-output Gaussian process regression (MOGPR) was proposed. The hydraulic design method of centrifugal pump based on the MOGPR model was constructed under Bayesian framework. Based on the available excellent hydraulic model, the complex relationship between the performance parameters such as head, flow rate and the geometric parameters of centrifugal pump impeller was trained. The hydraulic design of the impeller for M125-100 centrifugal pump was performed by the proposed MOGPR surrogate model design method. The initial MOGPR design was further optimized by using the proposed MOGPR and NSGA-II hybrid model. The initial sample set for NSGA-II was designed by Latin hypercube design based on the MOGPR initial design. The relationship between the impeller geometry and the CFD numerical results of the sample set was trained to construct the surrogate model for pump hydraulic performance prediction. The MOGPR surrogate model was used to evaluate the objective function value of the offspring samples in NSGA-II multi-objective optimization. The comparison of the pump hydraulic performance between the optimized designs and the initial design shows that the efficiency and the head of the tradeoff optimal design are increased by 2.5% and 2.6%, respectively. The efficiency of the optimal head constraint design is increased by 3.2%. The comparison of the inner flow field shows that turbulent kinetic energy decreases significantly and flow separation is effectively suppressed for the optimal head constraint design.

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.

In this study, the flow structure and effect of different pump rotational speeds on a centrifugal pump’s performance are experimentally and numerically investigated. The internal flow field pattern within the pump has been analyzed and discussed using the CFD technique. The numerical results are compared with experimental data under a wide range of operating conditions. The comparison results between them have indicated a considerable agreement. The pressure variations are gradually increasing from inlet to outlet impeller of the pump. The results note that when the impeller rotates near the tongue region, the pressure in this region was higher than in other parts. Also, the interaction between the impeller and volute tongue region is actually according to the impeller blades’ relative position concerning the tongue region. Furthermore, the pressure and velocity variations within a centrifugal pump increase with rotational impeller speed.

OpenFOAM is an object-oriented C++ library of classes and routines of use for writing CFD codes. It has a set of basic features similar to any commercial CFD solver, such as turbulence models and discretization schemes. The paper presents the numerical studies of sediment wear in a centrifugal pump impeller using OpenFOAM code, which is an Open Source CFD Package. The 3-D turbulent particulate-liquid two-phase flow equations are employed in this study. Hashish erosion model was implemented in this code. The sand volume fraction distribution, sand erosion rate distribution, wall shear strain rate distribution and wall stress distribution in the impeller were analyzed. Simulation results have shown that the main sediment wear of impeller is at the suction side of the inlet and the pressure side of the outlet.

With the other geometric parameters of a centrifugal pump fixed, five kinds of impellers with different blade thickness were designed under different conditions of the impeller blade thickness. Based on standard k-ε equation and SIMPLE algorithm, numerical simulation of pumps was carried out by using Fluent under six different operational conditions, respectively. The relationship between the blade thickness and the hydraulic performance of pump was analyzed from the external characteristics and internal flow field. The results have showed that when the blade thickness increased in a certain range of blade thickness, the best efficiency point of pump moved to the small flow direction and a little bigger efficiency was gained. Additionally, internal turbulence energy losses of pump were increasing gradually under the designed operational condition.

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.

In this study, the flow structure and effect of different pump rotational speeds on a centrifugal pump’s performance are experimentally and numerically investigated. The internal flow field pattern within the pump has been analyzed and discussed using the CFD technique. The numerical results are compared with experimental data under a wide range of operating conditions. The comparison results between them have indicated a considerable agreement. The pressure variations are gradually increasing from inlet to outlet impeller of the pump. The results note that when the impeller rotates near the tongue region, the pressure in this region was higher than in other parts. Also, the interaction between the impeller and volute tongue region is actually according to the impeller blades’ relative position concerning the tongue region. Furthermore, the pressure and velocity variations within a centrifugal pump increase with rotational impeller speed.