| Liquefied natural gas(LNG)is one kind of clean energy.With annual growth rate of demand of more than 15%,it has become a rapidly developing new industry in the energy sector in China.LNG submerged pumps,as the core power transmission equipment in the LNG industry chain,the performance of it will not only effect the performance and service lifespan of the whole pump,but also will effect the safety and economy of the LNG industry.But the core technologies of the LNG submerged pump are most monopolized by foreign countries,China is short of related products with independent intellectual property rights.Therefore,to LNG submerged pump,how to design a high-efficiency LNG submerged pump,how to obtain optimization parameters of impeller and inducer to satisfying the hydraulic characteristics requirements,and how to grasp the cavitation characteristics and pressure pulsation characteristics under cavitation conditions,all of these technologies are urgent needed to be researched.These researches have great significance for China's effective use of natural gas resources,as well as for relieving the energy crisis.This study was completed under the funding of Research and Application of Key Technologies for Hydraulic Design of 10,000-ton Deepwater Floating LNG submerged Pump.This study takes the LNG submerged pump as the research object,takes the design requirements and the rationality of the structure into account to design a hydraulic model with good performance.Based on the CFD analysis technology,the hydraulic performance,internal flow field,and cavitation performance of the LNG submersible pump are studied.The performance characteristics and the pressure pulsation characteristics under cavitation conditions have laid a theoretical foundation for the optimization design of the LNG submerged pump as well as the study of the cavitation excitation characteristics.The main work and innovative results of this study are as follow:(1)The hydraulic design methods for the major flow-through components of LNG submerged pump such as impeller,guide vane and inducer were summarized and analyzed.The velocity coefficient method was used in hydraulic design of the impeller,and the minimum NPSH,maximum cavitation specific rotation speed,and optimal rim corresponding to the optimal state of the induced flow state and geometric parameters were obtained based on the Brumfield criterion.To determine the geometric parameters of the guide vanes,the design method of radial guide vanes was consulted.Based on the hydraulic design,three-dimensional modeling of the main flow-through components is completed.(2)The numerical simulation of the hydraulic performance of the LNG submerged pump was used to obtain the pressure distribution in the whole flow field,the velocity and pressure distribution in the impeller and inducer under different flow conditions.Also,the hydraulic performance of the LNG submerged pump with different inlet diameter of the impeller was studied.The research results show that the flow-head curve of LNG submerged pump decreases steadily in the range of small flow rate,and it shows a steep drop trend under large flow conditions.The optimal operating point of efficiency is biased toward large flow conditions,and the range of high efficiency area is wide,which satisfies the hydraulic performance requirements.Under small flow conditions,there is a clear flow separation phenomenon in the internal flow channel of the LNG submerged pump,which is one of the factors for the rapid decline in the hydraulic performance of the pump.As the impeller inlet diameter increases,the head and efficiency increase but the change are both less than 3%,which indicated that the inlet diameter has a positive effect on hydraulic performance but is not significant.(3)The cavitation performance of LNG submerged pump under different NPSH was simulated both constant and transient.The influence of impeller inlet diameter on the cavitation performance of LNG submerged pump was studied,and the evolution rule of cavitation bubbles over time was obtained.The results show that the critical NPSH_c of the LNG submerged pump is 4.92m,which meets the design requirements.The bubble first appeared at the leading edge of the inlet in the inducer,but in impeller it first occur in the end of the inlet suction surface,and the cavitation serious area was located near the front shroud of the impeller.With the increase of the impeller inlet diameter,the cavitation performance of the impeller undergoes a non-single change process,which indicates that there the impeller inlet diameter has an optimal value for the influence on the cavitation performance.(4)The variations of the waveforms,amplitudes and frequencies of the pressure fluctuations in the inducer,impeller and guide vanes of LNG submerged pump were studied under different flow rates and NPSH levels.And the influence of cavitation on pulsation was quantitatively analyzed,which revealed the internal pressure pulsation characteristics of the LNG submerged pump under cavitation conditions.The results of the study indicate that as the flow rate increases,the pressure pulsation within the hydraulic components becomes periodic and the amplitude decreases.With the decrease of NPSH,low-frequency signals appear in the pressure pulsation frequency domain of each over-current component,and the amplitude of the low-frequency signal increases continuously.The influence that cavitation on the pressure pulsation is more than serious in the primary impeller than in the secondary impeller.When inception occurs,the amplitude of the low frequency signal of the pressure pulsation in the inducer and primary impeller increases and exceeds the blade frequency,and becomes the dominant frequency.With the decrease of the NPSH_a,the radial force of two impellers both decreases first and then increases,and the maximum value and the maximum pulsation amplitude of radial force occur both in the case when NPSH_a=4m.When cavitation occurs,the periodicity of the radial force becomes worse in one cycle because the cavitation development process destroys the periodicity of the flow field. |
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