Numerical analysis of axial force on dredging pump considering influence of axial clearance

Abstract: It is an important issue that the axial forces exerting on rotors greatly affect the operation stability of centrifugal pumps. The axial clearance between the impeller shroud and the volute casing cover is a key factor that influences the magnitude and direction of the axial force. The axial force is mainly derived by pressure difference on the outer surface of front and back shrouds and hydraulic pressure on the inner surface of the impeller. The dredging pump studied in this paper was a shrouded centrifugal pump with 9 backward curved back blades on the front shroud. Adjusting the axial position of the impeller could simultaneously change the front clearance and back clearance. Numerical simulations were carried out for 3 axial clearances. Based on RANS numerical method, SST k-ω turbulence model was adopted to calculate the 3D turbulence flow in the whole flow passage of the pump. The impeller and the back-blade domain were set as rotating domains with the same rotational speed in the model test. The front and back shroud were set as rotating walls with the same rotational speed. For each operating condition, average values of the three were taken as the final results. The predicted efficiency, head and shaft power agreed well with the experimental results, which indicated the reliability of the numerical method. The relationship between axial clearance and characteristics of pump performance and axial force was analyzed within a wide range of flow rates. The numerical results showed that as the front clearance increased, the efficiency of the pump dramatically declined and the head decreased by different amplitudes, while the shaft power changed little. The component axial force on the back shroud was much larger than others, and thus it determined the direction of the gross axial force. By contrast, the component axial force on impeller inner surface was the smallest. The 3 components of axial force changed with flow rate and axial clearance on different levels. The total axial force gradually decreased as the flow rate increased, and it was the larger when the front clearance was wider. Affected by the rotating back blades, the pressure distribution on the front shroud was in disorder and the pressure gradient was large, while on back shroud, the pressure distribution presented smooth circles and the pressure changed uniformly in radial. As the front clearance increased, the pressure distribution became more homogeneous on both the front shroud and the back shroud. The pressure was larger on the back shroud than that on the front shroud, the area of the back shroud was larger, and hence the characteristics of the total axial force was determined by the component on the back shroud. Therefore, in the preliminary hydraulic design of dredging pumps, pump performance and axial force characteristics should be comprehensively considered and set suitable axial clearance sizes.