| As the key equipment of a small modular reactor（SMR）,the charging pump is an important part of chemical and volume control system.It affects the safety and stability of SMR significantly,but mainly depends on import currently.The pump has the characteristics of ultra-high head,ultra-low flow and ultra-high speed.The structure of multi-stage vertical centrifugal pump is adopted.And the specific speed is low,so it is difficult to design and match the impeller and diffuser.The internal flow structure is complicated and the rotor/stator interaction is intense.The hydraulic excitation force caused by unsteady flow is an important factor to induce the pump vibration.Therefore,it is necessary to develop the high efficiency hydraulic model of the charging pump and reveal the unsteady flow field evolution process and its excitation characteristics in the pump,which can provide reference for the nationalization of small reactor charging pump.In this paper,the charging pump for the CAP10 small modular reactor is chosen as the research object.The hydraulic model of the charging pump is developed.The model pump and its visual test platform are designed to verify the accuracy of numerical simulation.The unsteady flow structures are captured and analyzed to reveal the flow evolution process in model pump.Combined with the extracted pressure pulsation signals,the relationship between the unsteady flow structure and the pressure pulsation is established.The unsteady numerical simulation of the intermediate stage of the charging pump is carried out to reveal the flow field and excitation characteristics of the pump at ultra-high speed.The main work and conclusions are as follows:1.According to the actual operation requirements of the charging pump,the impeller and diffuser are designed and optimized,the main geometric parameters are determined,and the hydraulic model of the charging pump meeting the performance indexes is obtained.2.The model pump is designed by model-for-algorithm,and the three-dimensional unsteady flow structure in the pump is calculated based on the DDES numerical method.The relationship between the rotor/stator interaction（RSI）,transient vortex evolution process and pressure pulsation is mainly explored.The results show that the main frequency of the pressure pulsation in the impeller is the diffuser passing frequency(f DPF),and in the diffuser and casing,they are 5 times shaft frequency（5f R）and the blade passing frequency(f BPF)caused by the rotor/stator interaction.The Karman vortex shedding from the trailing edge and suction side periodically with f BPF is captured.The characteristic frequency of the vortex spectrum in the diffuser is the blade passing frequency(f BPF).The low-frequency peak signals in the casing is prominent,and the amplitudes(f BPF)are distributed unevenly in the circumferential direction of casing.There are through flow and circulating flow near the outlet pipe,and the flow field is disordered,which indicates that the unsteady flow structure in the casing is an important source of pressure pulsation besides the rotor/stator interaction.3.High frequency pressure pulsation sensor is used to measure the pressure pulsation signal in the model pump,and the unsteady pressure pulsation characteristics in the pump are obtained.The pressure pulsation energy peaks in the diffusers appear at f BPFand high harmonics,and the effect of rotor/stator interaction is significant.The pressure amplitude(f BPF)and RMS values of different points distribute unevenly in the circumferential direction of casing.Near the outlet pipe region,the pressure amplitudes(f BPF)are small and the RMS values are large.At the casing left side,the pressure amplitudes(f BPF)and RMS values are large.With the flow rate increase,the pressure pulsation energy of f BPF at each measuring point increases first and then decreases,the RMS value decreases,and the flow field becomes more uniform in the pump.4.The particle image velocimetry（PIV）is used to measure the flow field in the model pump to obtain the time-averaged velocity field and vortex structure distribution at different flow rates.The flow field evolution process in the pump is analyzed,and it is found that the jet-wake flow pattern at the impeller outlet and flow separation which occurs at the leading edge and suction side of the diffuser are captured.The large-scale flow separation and backflow are generated at the left side of the outlet pipe caused by the through flow,resulting in a high vortex area.The flow field at the casing left side is more uniform,the backflow at the diffuser suction side due to flow separation is affected by the upstream flow,and the low speed area decreases.5.Based on DDES numerical method,the unsteady flow field in the intermediate stage of the charging pump is calculated.The results show that the main frequency of pressure pulsation in the diffuser of the charging pump is 5 times shaft frequency（5f R）and blade passing frequency(f BPF),which is the same as that in the model pump.The rotor/stator interaction between the return vane and the secondary impeller causes the peak of pressure pulsation energy at 4 f BPF.Combined with the captured unsteady flow field evolution process,the pressure characteristics in the guide vane and return guide vane are correlated,and the excitation mechanism of the charging pump has been mastered. |
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