Characteristics Of Vortex And Pressure Fluctuation In Tip Clearance Of A Multiphase Pump

With the new round of energy structure adjustments,the global oil and gas development trend towards the ocean is more obvious.Meanwhile,as an important technical equipment for offshore oil and gas transportation,the application of multiphase pumps is also extremely wide.The multiphase pump is a rotor-powered turbomachinery,and it has a compact structure,wide operating points,and can operate efficiently at high gas void fraction.However,there is a certain clearance between the impeller blade tip(blade tip)and shroud.The complicated flow patterns emerge in the tip clearance,such as vortex,secondary flow and flow separation.At the same time,the tip leakage vortex(TLV)is formed when the tip leakage flow(TLF)flows out the clearance.The TLV and TFL phenomenon not only deteriorates the flow pattern and reduces the efficiency of the unit,but also induces noise and vibration,and even appears mechanical failures,which seriously threaten the stability and safety of the unit’s operation.Therefore,the research not only shows great scientific value,but also has perfect engineering application value.To reveal the flow characteristics in the tip clearance and the correlation mechanism of TLV and pressure fluctuations in the multiphase pump,and further improve the level of oil and gas exploitation equipment,the inner flow,TLF,TLV,and the correlation characteristics of TLV and pressure fluctuation in multiphase pump are investigated.First,the flow characteristics in impeller and diffuser of the multiphase pump with different tip clearances under water and gas-liquid two-phase cases are analyzed,and the phase distribution,the variation law of gas-liquid velocity in tip clearance and pump and so on are mainly studied.Results reveal that the pressure contours near the TLV are deflected.At the same time,it is found that when the tip clearance increases,the gas accumulation in the impeller and diffuser is improved,and the vortex in the diffuser gradually decreases,and the flow state gradually stabilizes.In addition,the tip clearance mainly affects the velocity difference of gas-liquid two-phase in the impeller,however,it has relatively less effect on the velocity difference of gas-liquid two-phase in the diffuser.Secondly,based on the swirling strength method,the trajectory of the TLV is predicted,and the flow characteristics in the tip clearance,such as vorticity,velocity and turbulent kinetic energy,are qualitatively and quantitatively analyzed.At the same time,the vortex characteristics in the tip clearance and blade tip region are investigated,and the effect of different IGVFs on flow characteristics is emphatically analyzed.Results show that when the TLF enters the tip clearance,a tip separation vortex is formed.When the TLF flows out the tip clearance,the jet effect forms a shear layer,which entrains to form a TLV.In addition,it is found that the axial velocity in the tip clearance from radial direction of 0.8 to 0.9 reaches the maximum,and the radial velocity satisfies the cylinder independence hypothesis.Furthermore,the distribution of vorticity in the tip clearance is related to the leakage and the strength of the jet effect.At the same time,it is found that the IGVF has a greater effect on the location and range of the TLV.Finally,the pressure coefficient and pressure fluctuation intensity are defined,and the temporal-spatial evolution of the TLV is revealed.Meanwhile,the relationship between the TLV and the pressure fluctuation intensity under different tip clearances is analyzed.In addition,the characteristics of the vortex in the tip clearance,the pressure fluctuation intensity,and the frequency characteristics of the pressure fluctuation in the rotor-stator interface region under the variation of IGVF cases are contrasted and analyzed.Results point out the greater pressure fluctuation intensity in the impeller flow passage appeared near the TLV,and its temporal-spatial evolution is mainly divided into the following three stages: splitting stage,contraction stage and recurrence stage.Meanwhile,the initial point of TLV formed by gas gradually moves to the trailing edge(TE)with the increase of the tip clearance.Moreover,the existence of gas lets the location of the maximum pressure fluctuation intensity in the tip clearance closer to the TLF outlet when the tip leakage vortex is firstly formed,and the flow separation in the tip clearance penetrates the entire tip clearance in advance along the streamwise direction.Besides,it is also found that the larger pressure fluctuation amplitudes in the impeller and diffuser are concentrated at 7 and 3 times fn and low frequency under water case.However,the larger pressure fluctuation amplitudes are concentrated at low frequency under gas-liquid cases,at the same time,the pressure fluctuation at other multiples blade passing frequency(BPF)is disappeared,and the dominant frequency amplitude of the pressure fluctuation under the gas-liquid case is at least one order of magnitude smaller than that under the water case.The research results in this paper lay the foundation for the investigation of TLF,TLV dynamics and pressure fluctuation prediction in the multiphase pump.Meanwhile,it also has a guiding significance for the operation stability research in this pump and the improvement of efficiency in oil and gas transportation system.