Flow characteristics and loss mechanism inside the helical pipe with large-caliber and large-scale Dean number were analyzed in this study. Numerical simulation was carried out for exploring velocity distribution, pressure field, and secondary flow by varying coil parameters such as Dean number, curvature radius, and coil pitch. The velocity gradient in the cross-section increases along the pipe and causes unsteady flow in the pipe. Large pressure differences in the 180°and 315°cross-section generate centrifugal forces on the pipe. The secondary flow is the major factor resulting in flow loss, presented obviously by the streamlines to analyze the effects of pipe parameters on the vortices. The vortex center shifts toward the upper wall with the increase in Dean number and takes a slight deflection with the increase in coil pitch. Meanwhile, a correlation of the flow loss extent inside the pipe as a function of friction factor was presented. The increases in curvature radius and coil pitch can diminish the friction factor to reduce flow losses. The accuracy of the numerical methodology was also validated by conducting corresponding experiments and empirical mathematical analysis. The maximum deviation between the experimental values and the simulated results of the pressure drop is just 2.9%.

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Double-volute is an effective technique to reduce radial hydraulic force on the centrifugal pump and thereby mitigate the pump-casing vibration induced by unsteady flow characteristics. The mechanism of the double-volute structure balancing radial force on the impeller and volute was investigated on the basis of volute cross-sections by using Computational Fluid Dynamics (CFD) method. The tested performances and simulated inner-flow characteristics of two pumps with single-volute and double-volute were compared in this paper. The performance-test results verify the veracity of CFD method and illustrate that double-volute pump has some losses in terms of pump head and operation efficiency. The numerical simulations reveal that double-volute pump has smaller radial-force magnitude than single-volute pump on the abnormal conditions. Steady pressure field and transient pressure variations of pumps were explored to account for radial-force characteristics of double-volute pump. Compared with the single-volute structure, obvious pressure increases were found in the upper chamber (single part) of the double-volute, while the static pressure decreased in the lower chamber (double chambers). This situation reduces the pressure difference between two volute cross-sections in the collinear radial direction, resulting in smaller radial hydraulic force. Moreover, the transient simulations present the same phenomenon. The radial-forces distribute more uniformly in the double-volute pump, which can alleviate some vibrations.

In this article, to improve the vibration characteristics of a high-speed centrifugal pump, two new types of complex impeller were designed for replacing the original long-blade impeller. The experimental apparatus of pump vibration was designed and used to test the vibration signals of the model pump with the original impeller in frequency field. The vibration amplitude of 0.12 mm/s2 is observed on the shaft rotating frequency, while the amplitudes of 0.04 mm/s2, 0.14 mm/s2, and 0.2 mm/s2 respectively on the conditions of 0.8 QN, QN, and 1.2 QN appear on the impeller blade passing frequency. The computational fluid dynamics method was used to study internal flow characteristics in the pumps with three impellers for mitigating the pump vibration. Steady pressure nephogram shows big pressure difference existed in the complex-impeller pump, which means that the complex impeller can provide the high-lift water. Addition of the splitter blade can make the pressure-fluctuation amplitude of model pump to reduce obviously by the unsteady time–frequency analyses. Furthermore, two complex impellers were machined and used to conduct the contrast experiment on the vibration characteristics between the pumps with the original impeller and complex impeller. The results reveal that the water head of pump with the complex impeller is higher than that of the original pump, which verifies the simulated results. Meanwhile, the complex impeller has a positive effect on the decrease of pump vibration on the frequency domain, especially on the blade passing frequency the vibration amplitudes decrease by 0.08 mm/s2.