New Approach for Multi-Rotor Mixed-Flow Pump Design and Optimization

An application of contra-rotating rotors, in which a rear rotor is in tandem with a front one and these rotors rotate in the opposite direction each other, has been proposed against a demand for developing higher specific speed axial flow pump. However, our prototype rear rotors designed with the conventional method under the same rotational speed as the front rotor have revealed several problems; the head is not satisfied even at the design flow rate, cavitation inception is observed on the rear rotor despite of the head increase of front rotor and so on. In the present study, to solve these problems, three new rear rotors are designed based on the reduced speed design with taking account of information from the previous studies. One of them, which considers the outlet flow distribution of front rotor as well as the control of secondary flow with the different maximum camber position from hub to tip and the small incidence angle for better cavitation performance, is found to show the best performance in the numerical simulations. An experimental validation is carried out for this rotor, showing that the rotor satisfies the design specifications with higher total efficiency than the existing rotor at the design flow rate. Discussions on off-design performances are also made based on the measured performance data.

Compared with conventional high-specific-speed axial flow pump, better cavitation performance and compact size have been achieved in contra-rotating axial flow pump, where the rear rotor is employed additionally to the front rotor to convert the swirling flow to the pressure rise. Meanwhile, significantly deteriorated performance has also been observed at well off-design flow rates with design rotational speed. The rotational speed control (RSC) of front and rear rotors has been experimentally proved to be effective to enhance the performance. However, thorough investigations are necessary to find the optimum rotational speeds of rotors. It may be done by computational fluid dynamics (CFD) simulations, whereas it is time-consuming to cover the wide ranges of rotational speeds. Therefore, in the present paper, a fast and effective performance prediction model is established by considering radial equilibrium condition, conservation of rothalpy and mass, empirical deviation angle, bladerows interaction and empirical losses. Experimental and CFD results are employed to validate the proposed prediction model. It is found that the proposed model shows good enough accuracy in predicting performances of contra-rotating axial flow pump under RSC in broad flow rate range. Furthermore, an energy saving application of the proposed model is also illustrated for two typical system resistances. Compared with the traditional valve control under the design rotational speed operation, the RSC method can well modify the pump head to satisfy the system resistance at wide flow rate range with the significantly improved energy performance.

Internal hydraulic analysis of impeller rounding in centrifugal pumps as turbines

Small-sized axial fan is used as air cooler for electric equipments. But there is demand for higher power of fan according to the increase of quantity of heat from electric device. Therefore, higher rotational speed design is necessary but it causes the deterioration of efficiency and increases the noise. Contra-rotating axial fan is smaller than single axial fan under the same rotational speed, and is effective against lower rotational speed design under the same diameter. In addition, contra-rotating small-sized axial fan is higher efficiency compared with single small-sized axial fan. Then the adoption of contra-rotating rotor for small-sized fan was proposed for the improvement of performance. In the present paper, experiment was conducted to investigate the effect on performance by changing the axial distance between front and rear rotors of contra-rotating small-sized axial fan. And the relationship between the performance and axial distance between front and rear rotors was clarified with the experimental results.

Mixed flow pumps driven by hydraulic motors have been widely used in drainage in recent years, especially in emergency pump trucks. Limited by the power of the truck engine, its operating efficiency is one of the key factors affecting the rescue task. In this study, an automated optimization platform was developed to improve the operating efficiency of the mixed flow pump. A three-dimensional hydraulic design, meshing, and computational fluid dynamics (CFD) were executed repeatedly by the main program. The objective function is to maximize hydraulic efficiency under design conditions. Both meridional shape and blade profiles of the impeller and diffuser were optimized at the same time. Based on the CFD results obtained by Optimal Latin Hypercube (OLH) sampling, surrogate models of the head and hydraulic efficiency were built using the Radial Basis Function (RBF) neural network. Finally, the optimal solution was obtained by the Multi- Island Genetic Algorithm (MIGA). The local energy loss was further compared with the baseline scheme using the entropy generation method. Through the regression analysis, it was found that the blade angles have the most significant influence on pump efficiency. The CFD results show that the hydraulic efficiency under design conditions increased by 5.1%. After optimization, the incidence loss and flow separation inside the pump are obviously improved. Additionally, the overall turbulent eddy dissipation and entropy generation were significantly reduced. The experimental results validate that the maximum pump efficiency increased by 4.3%. The optimization platform proposed in this study will facilitate the development of intelligent optimization of pumps.

This study investigates trapezoidal (scheme A) and triangular (scheme B) bionic structures, inspired by the leading edge pectoral fin of a humpback whale, arranged at the impeller blade leading edge of a guide vane mixed-flow pump. The effects of them on the cavitation performance and internal flow were compared through numerical simulation, and the accuracy of the numerical simulation was verified by experimental test. Entropy production analysis quantified and visualized flow losses induced by cavitation and bionic structures. Results show both schemes suppress cavitation development, with scheme B exhibiting slightly better hydraulic performance: 0.78% head increase at critical cavitation (NPSHa = 9.46 m) and 15.35% at severe cavitation (NPSHa = 8.27 m). Entropy production analysis indicates that after adding the bionic structures, at the severe cavitation stage turbulence dissipation is reduced by 21.9%–22.8% and wall entropy production is decreased by 10.6%–11.2% by suppressing large-scale vortexes and inducing secondary flows. The bionic structures optimize leading-edge pressure distribution, delay flow separation, and disperse cavitation bubble collapse, enhancing cavitation resistance and stability. The findings offer theoretical and engineering insights for bionic optimization in hydraulic machinery.

Cavitation phenomenon has strong transient characteristics and is highly influenced by geometric structure. In this study, the cavitation performance with influence of blade tip clearance for a mixed-flow pump is studied using the renormalization group (RNG) k–ε turbulence model and the Zwart–Gerber–Belamri cavitation model. The dominant frequency and maximum amplitude values at non-cavitation condition and different cavitation conditions are compared and associated with flow field features. The results show that the dominant frequency value under incipient cavitation and critical cavitation is 3.2 f i and when the cavitation is severe, the frequency value changes to 3.5 f i . Then, the influence of tip clearance width on cavitation performance of the mixed-flow pump is also discussed. The results show that the increase in tip clearance will significantly aggravate the performance drop of the pump under cavitation conditions. The critical net positive suction head value increases in 4.79% of the value under no-tip clearance condition. At the same time, by the inner flow field observation and analysis, the morphological of cavitation bubbles is also changed, and the cavitation bubbles tend to attach to the blade suction side and the attachment length increases as the tip clearance increases.

The present paper describes the parametric design of a mixed-flow water-jet pump. The pump impeller and diffuser geometries were parameterized by means of an inverse design method, while CFD analyses were performed to assess the hydrodynamic and suction performance of the different design configurations that were investigated. An initial pump design was first generated and used as baseline for the parametric study. The effect of several design parameters was then analyzed in order to determine their effect on the pump performance. The use of a blade parameterization, based on inverse design, led to a major advantage in this study, because the three-dimensional blade shape is described by means of hydrodynamic parameters, such as blade loading, which has a direct impact on the hydrodynamic flow field. On the basis of this study, an optimal configuration was designed with the aim of maximizing the pump suction performance, while at the same time, guaranteeing a high level of hydrodynamic efficiency, together with the required mechanical and vibrational constraints. The final design was experimentally tested, and the good agreement between numerical predictions and experimental results validated the design process. This paper highlights the contrasting requirements in the pump design in order to achieve high hydrodynamic efficiency or good cavitation performance. The parametric study allowed us to determine design guidelines in order to find the optimal compromise in the pump design, in cases where both a high level of efficiency and suction performance must simultaneously be achieved. The design know-how developed in this study is based on flow field analyses and on hydrodynamic design parameters. It has therefore a general validity and can be used for similar design applications.

Experiments and theory relating to fan noise sources are reviewed with emphasis on axial flow machines. At supersonic rotor speeds, the steady shock pattern attached to a rotor is an efficient radiator of sound. In most practical cases of subsonic rotor operation, however, direct radiation from the rotor-locked pressure field is negligible compared with the indirect radiation, or scattering, caused by circumferential distortions in the steady flow field surrounding the rotor. Random timewise modulation of the distortion changes the scattered spectrum from discrete to continuous with a gradual progression from narrow-band tones to broad-band noise as the modulation bandwidth is increased. Similar scattering occurs when a nonuniform unsteady flow impinges on stator vanes, but here the radiated frequency is that of the impinging flow. Finally, for blades operating in flows free from circumferential distortions, self-generated turbulence becomes an important source of noise. The paper describes the physical mechanisms involved in each of these processes, including the generation of unsteady lift by turbulence. Order-of-magnitude sound-power estimates are compared where possible with experiment.

Experiments and theory relating to fan noise sources are reviewed with emphasis on axial flow machines. At supersonic rotor speeds, the steady shock pattern attached to a rotor is an efficient radiator of sound. In most practical cases of subsonic rotor operation, however, direct radiation from the rotor-locked pressure field is negligible compared with the indirect radiation, or scattering, caused by circumferential distortions in the steady flow field surrounding the rotor. Random timewise modulation of the distortion changes the scattered spectrum from discrete to continuous, with a gradual progression from narrow-band tones to broad-band noise as the modulation bandwidth is increased. Similar scattering occurs when a non-uniform unsteady flow impinges on stator vanes, but here the radiated frequency is that of the impinging flow. Finally, for blades operating in flows free from circumferential distortions, self-generated turbulence becomes an important source of noise. The paper describes the physical mechanisms involved in each of these processes, including the generating of unsteady lift by turbulence. Order-of-magnitude sound power estimates are compared, where possible, with experiment.

This paper explores the windmilling operational boundary and aerodynamic characteristics of an axial wide-chord fan rotor according to the zero-torque characteristic. The interesting results show the radial profiles of the aerodynamic parameters at the rotor exit nearly collapse to the same curve for different windmilling speeds, which indicate that windmilling flow has a strong similarity. Meanwhile, there are three different operating modes along the blade span. The formation mechanism and evolution process of the tip leakage flow at the windmill are also analyzed. The reversal of the pressure difference will generate two kinds of tip leakage flows in opposite directions. A clear blockage zone appears near the blade tip under the joint action of the two kinds of tip leakage flows. Finally, this study proposes a mathematical model to predict the windmilling speed of a high-bypass-ratio turbofan based on the aerodynamic similarity of windmilling flow and the one-dimensional semiempirical windmilling mass flow function. This can be applied to the early stages of high-bypass-ratio turbofan development and can evaluate safety characteristics during engine windmilling operations.

The hydrodynamic design optimization and performance analysis of a mixed-flow waterjet pump have been achieved using the validated commercial computational fluid dynamics (CFD) code. The detailed flow field in the waterjet pump is investigated by the CFD code adopted in the present study. Predicted characteristic curves, including the cavitation performance, agree very well with the test data for a designed mixed-flow waterjet pump over the entire operating conditions. A hydraulically detailed performance analysis with a net positive suction head can be used efficiently as a tool for the practical design optimization and assist in the understanding of the operational characteristics of general purpose mixed-flow pumps.

As one of the core components of the fire-fighting water-supply system, the performance of a fire pump directly determines the extent of damage caused by the fire. Compared with conventional pumps, the design requirements of fire pumps not only need to ensure that the head of the pump is at 0Qd, 1.0Qd, and 1.5Qd and its efficiency is at 1.0Qd but also consider the cavitation performance at each flow rate, which presents a greater challenge for the design of high-performance fire pumps. By optimizing the design of a centrifugal fire pump with a specific speed of 24.7, numerical calculations were performed to obtain the best optimized scheme Y4. The results show that at the design flow rate the best optimized scheme improves the efficiency by 9.17% compared with the original scheme, and the head meets the design requirements of the fire pump while avoiding the hump phenomenon. Through a comparative analysis, it was found that the optimized scheme Y4 can reduce the pressure pulsations at the outlet of the pump and improve the cavitation performance at each flow rate. The experiment verifies that the head of the best optimized scheme at the design flow rate is 74.43m, the pump efficiency is 40.22%, and there is no hump in the head curve, which can meet the design and use requirements of the fire pump. The maximum reduction in the outlet pressure pulsations coefficient in the best optimized scheme was 47.12% on average. Compared with the original scheme, the critical net positive suction head (NPSHr) of the best optimized scheme was reduced by 21.5%, 17.6%, 15.7%, and 16.8%, respectively.

Mechanism analyses of the unsteady vortical cavitation behaviors for a waterjet pump in a non-uniform inflow

A mixed-flow blood pump for long-term applications has been developed at the Helmholtz-Institute in Aachen, Germany. Central features of this implantable pump are a centrally integrated motor, a blood-immersed mechanical bearing, magnetic coupling of the impeller, and a shrouded impeller, which allows a relatively wide clearance. The aim of the study was a numerical analysis of hydraulic and hemolytic properties of different impeller design configurations. In vitro testing and numerical simulation techniques (computational fluid dynamics [CFD]) were applied to achieve a comprehensive overview. Pressure-flow charts were experimentally measured in a mock loop in order to validate the CFD data. In vitro hemolysis tests were performed at the main operating point of each impeller design. General flow patterns, pressure-flow charts, secondary flow rates, torque, and axial forces on the impeller were calculated by means of CFD. Furthermore, based on streak line techniques, shear stress (stress loading), exposure times, and volume percentage with critical stress loading have been determined. Comparison of CFD data with pressure head measurements showed excel-lent agreement. Also, impressive trend conformity was observed between in-vitro hemolysis results and numerical data. Comparison of design variations yielded clear trends and results. Design C revealed the best hydraulic and hemolytic properties and was chosen as the final design for the mixed-flow rotary blood pump.