Experimental study of controlling tip clearance flow in a pump-jet propulsor

  Objective  This paper aims to suppress the adverse effects of tip clearance flow on the hydrodynamic performance and unsteady excitation force of a pump-jet propulsor.

  Method  As for the pre-swirl stator pump-jet propulsor, an annular flexible seal structure closely matched with a rotor tip ring is used to study the validity of suppressing the clearance flow on the rotor tip. The rotor thrust and torque of the propulsor are measured by keeping the shroud approximately rigidly fixed, enabling the rotor open water efficiency to be obtained. In addition, the point of cavitation inception at each design condition is observed and recorded carefully with the help of a high-power stroboscope, and the cavitation inception curves of the propulsor with/without tip clearance are obtained through calculation. Finally, tests of shaft vibration acceleration on the pump-jet propulsor with/without tip clearance are conducted under conditions of cavitation and non-cavitation in order to evaluate the effects of diminishing tip clearance.

  Results  The results show that the thrust and torque of the rotor of the pump-jet propulsor with a flexible seal structure are significantly increased, which in turn renders open water efficiency significantly increased at low and medium advance coefficients, unchanged near the design point and slightly decreased at the high advance coefficient. Moreover, the cavitation performance of the pump-jet propulsor without clearance is better at a wide range of advance coefficient, 0.85 <J< 1.40, near the design point. In addition, compared with a pump-jet propulsor with clearance, the amplitude of axial vibration acceleration of the shaft equipped with a pump-jet propulsor without tip clearance is significantly reduced at most characteristic frequencies.

  Conclusion  The results of this study demonstrate that using a flexible seal structure to suppress tip clearance flow has a positive effect on improving open water efficiency at low and medium advance coefficients, optimizing cavitation performance at the design point and suppressing the shafting vibration of the propulsor.