Study On The Characteristics And Impact Mechanism Of Pressure Fluctuation In A Centrifugal Pump With Gap Drainage Blades

The centrifugal pump is widely used and plays a key role in different fields of human production and living.Besides good hydraulic and cavitation performance,its reliable dynamic characteristics is also required to maintain safety operation.Previous research results show that gap drainage blades can improve the hydraulic and cavitating performance by ameliorating the structure of the flow in impeller.This may have an impact on the pressure fluctuation,so it's meaningful to grasp the character and influcence mechanism of pressure fluctuation in the centrifugal pump with gap drainage blades,which contributes to flow controling for lower pressure fluctuation and higher stability.On the basis of developing of new numerical method,the distinction and impact mechanism of pressure fluctuation in centrifugal pump with gap drainage blades were studied by numerical,experimental and theoretical methods.By comparing the different characters of pressure fluctuation of two type centrifugal pumps,the influence mechanism of gap drainage blades were revealed and analysed.The main content can be outlined as follows:?.Experimental devices and the methodsA high-speed synchronous photo system and a pressure pulse acquisition system were build to meet the requirements for observing the cavitation evolution process and monitoring the pressure singal.And the experimental content and procedure were confirmed.?.Nonlinear hybrid RANS/LES simulation methodBased on the feature of the internal flow and pressure pulsation in centrifugal pumps,a non-linear EVM and a scalable hybrid RANS/LES strategy were constructed by introducing the Von Karman length scale Lvk to improve the capability of EVM to simulate complex flow and unsteady pressure characterisitcs.It was tested and results show its fairly effection on resolving turbulent structuresand can give more satisfactory predictions of flow fields and pressure fluctuation.?.The characteristics and Impact mechanism of pressure fluctuation in centrifugal pumps with gap drainage blades(1)The characteristics of pressure fluctuation in traditional pumpa.The amplitude of pressure fluctuation decreases with the flow rate nearly under 0.5Q0,and rises with the flow rate nearly beyond 0.5Q0,especially at overload condition.b.Pressure fluctuation decreases downstream in the volute and increases with the radius in impeller passages.The steep rising of pressure fluctuation were observed at some points of radius of 1/3 chord length of the blade in impeller passage.c.The dominant frequency is blade passing frequency in volute,and its 2nd harmonic generation in outlet diffusion section,but in impeller passages it's the rotation frequency.Low frequency signals appear under part load and over load conditions.(2)The characteristics of pressure fluctuation in gap drainage pumpThe characteristics of pressure fluctuation show similar regularity as that of traditional pump,but the pressure pulsation amplitude in gap centrifugal pump is much lower than that in the conventional pump on design and overload condition.On overload condition the extremely rising of pressure fluctuation doesn't occur at area of radius of 1/3 chord length of the blade in impeller passage.The 2nd harmonic generation enhances in volute.(3)Impact mechanism of pressure fluctuation in centrifugal pumpsa.The pressure in impeller blade passage varies with the rotation frequency for the unsymmetry of the volute.The outflow from impeller blade passage interacts with the fluid in volute,which leads to the low pressure zone at the pressure edge at the exit and strong circumferential pressure gradient on the blades.So the pressure in volute drops and rises with the passing of blades.b.The unsteady flow,such as eddy shedding from the blade at off-design condition generate low frequency pressure pulse.The flow separation in diffuser part generates pressure pulsation of blade passing frequency.c.The nonlinear convection terms account for the 2nd and higher harmonic frequency,which intensifies the pressure pulsation.d.The developing features of the unsteady flows and the low pressure zone on the circumferential blades with the deviation of the design flow rate cause that the amplitude of pressure fluctuation increases with the decreasing of flow rate nearly under 0.5Q0 and rises with the increasing of flow rate beyond 0.5Q0,especially at overload condition.(4)Improvement mechanism of pressure fluctuation by gap drainage blade The adjustment of attack angle of vise blade and it's overlapping with main blade uniform the entrance velocity distributions in each tunnel,also the jet flow from pressure side to suction side through the gap improves the flow uniformity in blade passages.Benefit from which,the pressure fluctuation amplitude is reduced by decreasing the velocity at the exit beside the pressure side of blades and perfecting the unsteady flow in the blade passage near cut water.?.The characteristic and influence mechanism of pressure fluctuation at cavitating conditionThe relation between the caviation pattern,hydraulic performance and pressure fluctuation was studied experimentally.Unsteady cavitation featured by growth/breakdown cycle of the cavity was observed experimentally in both pump,and staggered fixed cavitation were observed in pump with gap drainage blades.The cavitation with unsteady features deteriorates hydraulic performance and enhances the pressure fluctuation.The jet flow through the gap suppress the growth of cavitation on suction side of blade,and the guiding effect of vice blades restrain the occurrence of cavity on pressure side of blade,which delayed the cavitation breakdown and relieve the congestion.So the smaller outflow velocity near the pressure side at the exit and low frequency pressure pulsation amplitude are lower in gap impeller lead to lower pressure pulsation than that in conventional one before the cavitation breakdown.