Research On Unsteady Characteristics Of Silt-laden Cavitating Flow In A Centrifugal Dredging Pump

Centrifugal dredging pumps are widely used in dredging,constrction,mining and other industries.The work environment is always adverse and the pump often runs in off-design conditions.Sometimes cavitation occurs and it leads to unsteady silt-laden cavitating flow,affecting the safe operation of the unit and even resulting in failure of the pump.It is of great theoretical and engineering significance to study the mechanism of silt-laden cavitating flow and its unsteady characteristics.Aiming at studying the silt-laden cavitating flow in dredging pumps,the theoretical analysis and modeling of the mass exchange between water and vapor and the momentum transfer between water,sand and vapor were carried out.The forces acting between the three phases were analyzed.Then governing equations of the silt-laden cavitating flow were established.The turbulent vorticity coefficient in the SAS-SST model was modified by considering the effect of cavitation compressibility and the solid-phase turbulent diffusion.Based on the cavitation test results,sensitivity analysis of the vaporization coefficient in ZGB cavation model was performed and its value was amended.Numerical simulations were carried out to analyze the unsteady head,flow separation characteristics,pressure fluctuation characteristics and unsteady cavitation characteristics in different conditions when the pump was transporting clean water.The results show that the flow separation characteristics inside the impeller is determined by the joint effet of the finite blade number effect,the inverse pressure gradient in the blade channel and the asymmetric volute outside the impeller exit.The flow separation inside the impeller is weakened by the cavitation.In the design condition and part-load conditions,cavitation also helps to improve the symmetry of separation vortex distribution in the impeller.The fluctuation characteristics of vapor volume directly affects the frequency and amplitude of the head fluctuation and pressure fluctuation.When the pump was transporting silt-lade flow,numerical simulations were conducted to study the pump performance,head fluctuation characteristics,two-phase flow structure in the impeller and pressure fluctuation with different sand concentrations and different particle sizes.The results show that due to the existence of sand particles,the head curve obtains a "positive slope" feature when the flow rate is below 50% of the design flow rate.Flow rate,sand concentration and particle size jointly affect the position,scale and circumferential propagation speed of the separation vortexes as well as the pressure fluctuation characteristics within the impeller.In the "positive slope" region of the head curve,the symmetry of the impeller flow field deteriorates with the increase of the sand concentration and the particle size.Numerical simulations were performed to predict the unsteady silt-laden cavitating flow in the dredging pump with different flow rates.It indicates that the existence of sand particles accelerates the occurrence and development of cavitation.Affected by the asymmetry of the sand distribution in the impeller and its unsteady characteristics,the symmetry of the vapor distribution and the fluctuation of the vopaor volume are clearly different from those within clean water.At the same time,cavitation bubbles and sand particles jointly change the pressure fluctuation characteristics and increase the flow instability in the pump in the design condition and part-load conditions.