Effect of incidence angle on cavity blockage and flow stability in the net positive suction head drop of mixed-flow pumps

The cavitation is an inevitable factor in pumps used in the whole industry, which is a major cause of energy loss and mechanical breakdown. In this study, the cavitation phenomena at the design flow rate were numerically analyzed for two pumps with different incidence angles. The design flow rate for both models was located near the best efficiency point (BEP). The incidence angle was determined with the impeller inlet diameter and the blade angle. A pump with a smaller incidence angle consistently showed a stable flow pattern as the inlet pressure decreased, whereas a pump with a larger incidence angle contained non-uniform flow streamlines despite a very small amount of the generated cavities. The flow pattern at the impeller inlet was handled by the shape and thickness of the generated cavities which could act as an additional blockage in the pumps. The inception and growth of the cavity with a decrease of inlet pressure were also inferred, which was specifically quantified as the blockage ratio. A pump with a larger incidence angle performed poor cavitation characteristics and obtained the pressure fluctuation and cavity oscillation. The magnitude of pressure fluctuation was indicated using the fast Fourier transform (FFT) analysis. The experimental tests were performed on both pumps to validate the numerical results.

Compressible Simulation of Flow and Sound Around a Small Axial-Flow Fan With Flow Through Casing Slits

Small axial fans are used for cooling electronic equipment and are often installed in a casing with various slits. Direct aeroacoustic simulations and experiments were performed with different casing opening ratios to clarify the effects of the flow through the casing slits on the flow field and acoustic radiation around a small axial fan. Both the predicted and measured results show that aerodynamic performance deteriorates at and near the design flow rate and is higher at low flow rates by completely closing the casing slits compared with the fan in the casing with slits. The predicted flow field shows that the vortical structures in the tip vortices are spread by the suppression of flow through the slits at the design flow rate, leading to the intensification of turbulence in the blade wake. Moreover, the pressure fluctuations on the blade surface are intensified, which increases the aerodynamic sound pressure level. The suppression of the outflow of pressurized air through the downstream part of the slits enhances the aerodynamic performance at low flow rates. Also, the predicted surface streamline at the design flow rate shows that air flows along the blade tip for the fan with slits, whereas the flow toward the blade tip appears for the fan without slits. As a result, the pressure distributions on the blade and the torque exerted on the fan blade are affected by the opening ratio of slits.

Experimental Analysis on Pressure Fluctuation Characteristics of a Centrifugal Pump with Vaned-Diffuser

Experimental measurements to analyze the pressure fluctuation performance of a centrifugal pump with a vaned-diffuser, which its specific speed is 190. Results indicate that the main cause of pressure fluctuation is the rotor-stator interference at the impeller outlet. The head of the pump with vaned-diffuser at the design flow rate is 15.03 m, and the efficiency of the pump with a vaned-diffuser at the design flow rate reaches 71.47%. Pressure fluctuation decreases gradually with increasing distance from the impeller outlet. Along with the increase of the flow rate, amplitude of pressure fluctuation decreases. The amplitude of pressure fluctuation at the measuring points near the diffusion section of the pump body is larger than other measuring points. The variation tendency of pressure fluctuation at P1–P10 is the same, while there are wide frequency bands with different frequencies. The dominant frequency of pressure fluctuation is the blade passing frequency. The rotor-stator interference between the impeller and the vaned-diffuser gives rise to the main signal source of pressure fluctuation.

Modeling Study of Design Flow Rates for Cascade Water Supply Systems in Residential Skyscrapers

Skyscrapers are common nowadays around the world, especially in cities with limited development area. In order to pump water up to the higher level of a skyscraper, a cascade water supply system has to be installed. Currently, cascade water supply systems are mainly designed based on practical experiences or requirements of existing standards/guidelines that, in fact, are not specifically for skyscrapers. However, thorough studies on cascade water supply system designs are still limited in the literature. This study proposes mathematical models and uses Monte Carlo simulations to evaluate the design flow rate of a typical cascade water supply system that feeds various appliances in a residential skyscraper in Hong Kong. Graphs that showed the correlations between the inflow rate in the supply pipe and water volume in the tank are obtained. While tank storage volume is confirmed, the design flow rate of the cascade water supply system can be determined from these graphs. The proposed mathematical models can also be applied to evaluate the design flow rate of cascade water supply systems in other types of skyscrapers (e.g., office, commercial building) as well as with the changes in water demand patterns in the models.

Influences of the Flow Cut and Axial Lift of the Impeller on the Aerodynamic Performance of a Transonic Centrifugal Compressor

In this study, the influences of the flow cut and axial lift of the impeller on the aerodynamic performance of a transonic centrifugal compressor were analyzed. The flow cut is a method to reduce the flow rate by decreasing the impeller passage height. The axial lift is a method of increasing the impeller passage height in the axial direction, which increases the impeller exit width (B2) and increases the total pressure. A NASA CC3 transonic centrifugal compressor with a backswept angle was used as a base compressor. After applying the flow cut, the total pressure at the target flow rate was lower than the total pressure at the design point due to the increase in the relative velocity at the impeller exit. After applying the axial lift, the total pressure at the design flow rate was increased, which was caused by the reduction in the relative velocity as the passage area at the impeller exit was increased. By applying the flow cut and axial lift methods, it was shown that the variation in relative velocity at the impeller exit has a significant effect on the variation in total pressure. In addition, it was found that the relative velocity at the impeller exit of the target flow rate is maintained similar to the base impeller when the flow cut and the axial lift are combined. Therefore, by combining the flow cut and the axial lift, three transonic centrifugal impellers with flow fractions of 0.7, 0.8, and 0.9 compared to the design flow rate were newly designed.

Effect of Stall Cells on Pressure Fluctuations Characteristics in a Centrifugal Pump

Rotating stall is an unsteady flow phenomenon, which always leads to instability and efficiency degradation. In order to reveal pressure fluctuations in the impeller of centrifugal pump induced by stall cells, the flow structures in a volute-type centrifugal pump were calculated using Large Eddy Simulation (LES) method. The predicted results of the numerical model were compared with experimental flow-head curve. The simulation results were in good agreement with the experimental results. The stall phenomenon occurred when the flow rate dropped to 70% of design flow rate. Three stall cells located at the entrance of passages could be observed, which remained stationary relative to the rotating impeller. With the decrease of flow rate, the area occupied by stall cells gradually increased. The peak value of pressure fluctuation at 25% of design flow rate is obviously larger than that at 50% of design flow rate. For the unstalled or stalled passage, the impeller-volute interaction played a leading role in the pressure fluctuations of the impeller. For the stalled passage, the amplitude of the low frequency induced by stall cell is relatively insignificant.

Efficiency improvement and evaluation of a centrifugal pump with vaned diffuser

In order to enhance the efficiency of centrifugal pump, the structure of a centrifugal pump with vaned diffuser, whose specific speed is 190, was numerically improved by trimming back-blades of impeller and smoothing sharp corner in annular chamber. The energy performance, the internal flow field, the axial force, the radial force, and the pressure pulsation of the pump were analyzed. Results show that efficiency of the improving scheme 1 under the design flow rate is 77.47%, which can balance 69.82% of the axial force, while efficiency of the improving scheme 2 under the design flow rate is the maximum, which could still balance 62.74% of the axial force. The pressure pulsations of the improving scheme 2 at the typical monitoring points are less than that of the improving scheme 1 and the original scheme. The difference of the radial force peak between the improving scheme 1 and the improving scheme 2 is very small. The vector distributions of the radial force of the improving scheme 1 and the improving scheme 2 are more uniform than that of the original scheme. Considering the efficiency, pressure pulsation, and axial force, experiment measurements on the improving scheme 2 were carried out to verify the effectiveness of the improvement result. Results of energy performance experiment show that efficiency of the improving scheme 2 under the design flow rate is 76.48%, which is 5.26 percentage points higher than that of the original scheme.

Evaluation of Design Flow Rate of Water Supply Systems with Low Flow Showering Appliances

The installation of water efficient appliances is an efficient demand-side water management measure favored by policymakers and water providers nowadays. The adoption of low flow showerheads in large and complex plumbing systems will not only contribute to a great reduction of shower water use in the whole water system, but also further influence the water supply system design. Hence, it is necessary to justify the redesign of existing water supply systems (such as the pipe size, storage tank volume, pumping arrangement etc.) in terms of the use of low flow showerheads. This study uses Monte Carlo simulations to evaluate the design flow rate for a typical high-rise roof tank water supply system in Hong Kong with the installation of low flow showerheads. The simulation results indicate that a full installation of low flow showerheads can decrease the design flow rate by 15%, corresponding to an energy efficiency improvement of 1.5%. The potential for water savings and associated energy savings can be significantly higher when all installed appliances in homes are water efficient (e.g., showerheads, water taps, washing machines). Further work is required to evaluate the redesign of existing water supply systems for a sustainable future.

Three-Objective Optimization of a Centrifugal Pump to Reduce Flow Recirculation and Cavitation

This work presents a three-objective design optimization of a centrifugal pump impeller to reduce flow recirculation and cavitation using three-dimensional (3D) Reynolds-averaged Navier–Stokes equations. A cavitation model was used to simulate the multiphase cavitating flow inside the centrifugal pump. The numerical results were validated by comparing them with experimental data for the total head coefficient and critical cavitation number. To achieve the optimization goals, blockage at 50% of the design flow rate, hydraulic efficiency at the design flow rate, and critical cavitation number for a head-drop of 3% at 125% of the design flow rate were selected as the objective functions. Based on the results of the elementary effect (EE) method, the design variables selected were the axial length of the blade, the control point for the meridional profile of the shroud, the inlet radius of the blade hub, and the incidence angle of tip of the blade. Kriging models were constructed to approximate the objective functions in the design space using the objective function values calculated at the design points selected by Latin hypercube sampling (LHS). Pareto-optimal solutions were obtained using a multi-objective genetic algorithm (MOGA). Six representative Pareto-optimal designs (POD) were analyzed to evaluate the optimization results. The PODs showed large improvements in the objective functions compared to the baseline design. Thus, both the hydraulic performance and the reliability of the centrifugal pump were improved by the optimization.