Linlin Cao

Low Speed Design of Rear Rotor in Contra-Rotating Axial Flow Pump

Abstract The application of contra-rotating rotors for higher specific speed pump has been proposed in our studies, which is in principle effective for reducing the rotational speed and/or the pump size under the same specification of conventional axial flow pump. In the previous experiments of our prototype, the cavitation inception at the tip region of the rear rotor rather than that of the front rotor and the strong potential interaction from the suction surface of the rear rotor blade to the pressure surface of the front one were observed, indicating the possibility to further improve the pump performance by optimizing rotational speed combination between the two rotors. The present research aims at the design of rear rotor with lower rotational speed. Considering the fact that the incoming flow velocity defects at the tip region of the rear rotor, an integrated inflow model of ‘forced vortex’ and ‘free vortex’ is employed. The variation of maximum camber location from hub to tip as well as other related considerations are also taken into account for further performance improvement. The ideas cited above are separately or comprehensively applied in the design of three types of rear rotor, which are subsequently simulated in ANSYS CFX to evaluate the related pump performance and therefore the whole low speed design idea. Finally, the experimental validation is carried out on one type to offer further proofs for the availability of the whole design method.

Experimental investigation of blade rows interactions in contrarotating axial flow pump

As a high specific speed pump, the contra-rotating axial flow pump with two rotors rotating reversely has been proved with higher hydraulic and cavitation performance, while in our previous researches, our prototype rotors designed with equal rotational speeds for both the front and the rear rotors was also confirmed with the strong potential interaction between two blade rows. In the present study, the experimental investigations were focused on the rotor-rotor interactions in the contra-rotating rotors under two rotational speed combinations, an equal speed and a different speed ones with the lower speed of rear rotor; the latter is determined aiming at relieved rotor-rotor interaction. As the major experimental approach, casing wall static pressure measurements were conducted at pressure taps covering from upstream to downstream of the both rotors, and the pressure fluctuation modes were investigated by the FFT analyses. By series of pressure taps with different peripheral locations prepared at several axial locations, the pressure fluctuation modes with frequencies non-synchronous to the BPF (blade passing frequency) components were recognized, and confirmed to be related to the rotor-rotor interaction on the basis of theoretical analyses on the rotor-stator interaction in conventional rotor-stator types.Copyright

Experimental investigation of blade-to-blade pressure distribution in contra-rotating axial flow pump

Abstract As a high specific speed pump, the contra-rotating axial flow pump with two rotors rotating reversely has been proved with higher hydraulic and cavitation performance, while in our previous researches, the potential interaction between two blade rows was distinctly observed for our prototype rotors designed with equal rotational speed for both front and rear rotors. Based on the theoretical and experimental evidences, a rotational speed optimization methodology was proposed and applied in the design of a new combination of contra-rotating rotors, primarily in expectation of the optimized blade pressure distributions as well as pertinently improved hydraulic performances including cavitation performance. In the present study, given one stationary and two rotating frames in the contra-rotating rotors case, a pressure measurement concept taking account of the revolutions of both front and rear rotors simultaneously was adopted. The casing wall pressure data sampled in time domain was successfully transferred into space domain, by which the ensemble averaged blade-to-blade pressure distributions at the blade tip of two contra-rotating rotors under different operation conditions were studied. It could be seen that the rotor pair with the optimized rotational speed combination as well as work division, shows more reasonable blade-to-blade pressure distribution and well weakened potential interaction. Moreover, combining the loading curves estimated by the measured casing wall pressure, the cavitation performance of the rotor pairs with new rotational speed combination were proved to be superior to those of the prototype pairs.

Blade rows interaction in contra-rotating axial flow pump designed with different rotational speed concept

The contra-rotating axial flow pump consisiting of counter-rotating tandem rotors has been expermentally confirmed with better performances than the conventional axial flow pump, but it is known to suffer from the significant potential interaction between the counter-rotating blade rows, which is responsible for the repetitive stresses and unfavourable to the reliable operation. Consequently, to improve the realiability of contra-rotating axial flow pump including the reduction of the blade rows interaction, a new type of rear rotor was designed in the previous study by the rotational speed optimization methodology with some additional considerations. In the present study, to understand the effectiveness of the new design method, instantaneous static pressure fluctuations on the casing wall under the design and off design conditions are investigated by means of experimental and numerical simulation methods. The Fourier analysis is employed to process the data obtained from experiments and numerical simulations, and the axial distribution of the Blade Passing Frequency (BPF) amplitude is obtained. The new rear rotor shows weakened BPF amplitude both upstream and downstream especially at the positions between the two blade rows in both CFD and EFD analyses, implying the reduced blade rows interaction with the new rear rotor.

On high efficiency operation of contra-rotating axial flow pump with rotational speed control toward effective energy saving

For high specific speed pump with high performance operation, a contra-rotating axial flow pump has been proposed and the effectiveness of rotational speed control of front and rear rotors has been demonstrated experimentally. In the present paper, a prediction method of pump characteristics under the condition of optimized rotational speed control is proposed, based on consideration of flow mechanism in both front and rear rotors. The accuracy of prediction is examined for two combinations of front and rear rotors. Then, the amount of energy saving by application of rotational speed control method for the contra-rotating axial flow pump is demonstrated and an advantage of usage of contra-rotating pump is discussed.