Study On Unsteady Flow Characteristics In Centrifugal Pump Based On The Omega Vortex Identification Method

Centrifugal pumps are widely used in various industrial process systems,and the internal flow is very complicated.Fluid flow experiences the occurrence of unsteady flow phenomenon.,which leads to the complex hydraulic excitation,instability of the pump's operating state,exacerbated energy dissipation,and insufficient performance.So it is of great significance to study the unsteady flow excitation mechanism in the centrifugal pump.Fluid flow experiences the formation and collapse of vortices in nature.The internal flow of a centrifugal pump is also a vortex motion and the unsteady flow excitation mechanism is closely related to the vortex generation and evolution in the flow channel.In order to explore the unsteady flow characteristics in the pump,excitation mechanism and transient flow characteristics,this paper takes a centrifugal pump as the object and the following research is done:Firstly,according to the basic design parameters,the hydraulic design of the centrifugal pump was performed using the velocity coefficient method,and the 3D modeling software Proe was used to establish the centrifugal pump calculation domain model.The software ICEM was used to divide the calculation domain into a high-quality hexahedral mesh and performs grid independence.The software Fluent wsa used to numerically calculate the flow field in the pump,and the external characteristic test of the centrifugal pump was also performed.The results show that the numerical calculation is consistent with the experimentally obtained external characteristic curve.At low flow conditions,there is a wake structure in the area where the suction surface of the impeller meets the front cover.and in this area,the pressure and velocity distribution is uneven,the turbulent kinetic energy is high,and the turbulent pulsation is strong.Under large flow condition,the uniformity of the flow field pressure and velocity distribution is improved when the flow rate is small,but a low velocity zone appears at the exit of the diffusion section to the side close to the tongue,where a flow shedding phenomenon occurs and the turbulent kinetic energy value is high.Secondly,it was found that compared with the traditional vortex recognition method,the Omega method has obvious advantages,it was insensitive to the change of threshold and it had clear physical definition.Compared with the SST and DDES turbulence models,the LES turbulence model captures more vortex information andthe results were more accurately.The LES model and Omega method were used to analyze the unsteady flow and vortex structure in the centrifugal pump.The results showed that the time-domain signals of the unsteady pressure pulsation had periodic characteristics,and the peak values of the pressure pulsation frequency-domain signals corresponded to the leaf frequency,indicating that the leaf frequency was the main excitation of the pressure pulsations.At low flow conditions,the pressure pulsation intensified,and the value of the vortex area is large,indicating that the number of vortices was closely related to the unsteady pulsation intensity of the flow field.The identification of the flow field vortex structure can more vividly show the flow characteristics in the pump.Compared with the traditional flow field analysis method,the main advantage was that it can show the flow from the perspective of the fluid vortex motion.The vortex evolution process of tongue was mainly characterized by the periodic shedding of the wake vortex structure and the impact of the tongue.The shedding frequency of the vortex was the leaf frequency.The rotor/stator interaction at the tongue was mainly stimulated by the periodic shedding of the vortex at the exit of the impeller and impacting the tongue.Finally,the transient rotor stagnation process of the centrifugal pump closed piping system was studied.The results showed that when the rotor stagnation occurs,the pump inlet and outlet pressures and radial forces change.In the rotor linear stagnation mode,the shorter the stagnation time caused the various values to change dramatically.In the rotor exponential stagnation mode,the various abrupt changes of the values are more dramatic than the linear stagnation.During the linear stagnation process of the rotor(t=0.010s),the force change at the elbow is more complicated.The larger the radius of the cross-section of the elbow,the greater the pressure.When analyzing the safety of the system,we should focus on the force at the elbow.With the stagnation of the rotor,the vortex structure in the pump gradually decreases.When the rotor speed drops to zero,the fluid transport basically stops,the continuity of the vortex structure in the impeller is poor,and the distribution is messy.