Optimization Of Cryogenic Submerged Pump With An Inducer And Experiment Research Of Cavitation Performance

Nowadays,Liquefied Natural Gas(LNG)has accounted for more and more in the world's energy consumption,becoming one of the three major fossil fuels.Since the 12 th Five-Year Plan,much importance has been placed by the government on the development of deep-sea natural gas fields.LNG is recognized as a kind of clean energy around the world and has the advantages of energy conservation,environmental protection,reliability and high economic profit.LNG submerged pump is the most important power equipment in marine engineering equipment,which determines the stable production and safe operation of LNG.In the 1960 s,submerged pump technology begins to be used in the field of LNG and the design of the submerged pump is safer than that of the pump with motor externally installed.In addition,since all the components are immersed in the liquid,a good sound insulation effect can be achieved and the working noise is very small.Due to the above structural characters,the submerged pump has been applied to almost all LNG receiving terminals.When transporting LNG,LNG submerged pump usually delivers LNG with temperatures between-163 and-120 °C.Since the pump unit is usually operating in cryogenic condition,cavitation is easy to occur in the submerged pump,which asks for better anticavitation performance of the submerged pump.To achieve this goal,it is one of the main methods to add the inducer in front of the first stage impeller of the submerged pump.The paper focuses on the three-dimensional hydrofoil and LNG cryogenic submerged pump with inducer.Through theoretical analysis,numerical simulation,external characteristic experiment and high speed photography experiment,the cavitation flow mechanism is systematically studied.The main contents are as follows:(1)Three common turbulence models(k-?,RNG k-?,and SST k-?)are modified based on filter based model and density-corrected model.Three turbulence model before and after the modification are used to calculate single phase and gas-liquid phase flowing around a NACA0015 hydrofoil with water as working fluid at different temperatures and conduct a contrastive analysis based on experimental results.Through analysis,under the condition of cavitation,the modified k-? model eliminates the influence of turbulence scale.Under the three temperatures,the root mean square error and maximum deviation of modified RNG k-? are small,the range of low pressure zone,the adverse pressure gradient of the closure zone at cavity tail and the law of cavitation development are consistent with the experimental results.At the same time,the modified RNG k-? reveals the law of cavitation with temperature variation: as the temperature increases,vapor volume fraction decreases,cavitation intensity decreases,cavity area decreases,adverse pressure gradient becomes smaller and the gasliquid interface becomes blurred,which shows a good modification effect.(2)Three cavitation models with different evaporation and condensation coefficients(Schnerr-Sauer,Merkle and Kubota)are applied to calculate cavitation flowing around a Hord hydrofoil with liquid nitrogen as working fluid at different temperatures.Through analysis,the calculation results of the root mean square error,the maximum deviation and the distribution of the upper surface pressure coefficient of the airfoil simulated by the Merkle model are consistent with the experimental results and the error is relatively small,which shows the strong adaptability of the model in a cryogenic environment.In order to reduce the calculating error,taking the root mean square error between simulated results of Merkle model and the experimental data about the upper surface pressure and temperature of the hydrofoil as the objective function,evaporation and condensation coefficient as design variables,the optimal Latin hypercube for sampling,the evaporation and condensation coefficients are optimized using radial basis function and multi-island genetic algorithm.At the same time,the saturated steam pressure term of the optimized Merkle model is modified with considering thermodynamic effects in a cryogenic environment.Compared with the Merkle model,the simulation results of modified Merkle model show that the pressure in cavitation core region decreases,cavity length decreases,and vapor volume fraction decreases.The higher the liquid nitrogen temperature is,the greater the pressure drop in the cavitation core region is,the greater the shortening of cavity length is,the greater the decreasing degree of vapor volume fraction in cavitation core region is,the more obvious thermodynamic effects is.(3)The cavitation flowing of inducer is numerically calculated with water at normal temperature and liquefied natural gas at-163? as working fluid.The results show that the law of inducer cavitation development is the same in different working fluid.The cavitation first appears at the outer edge on the suction surface of the inducer,forming a recirculating vortex cavitation and breaking down at the tube wall.As the cavitation number decreases,the cavitation develops further,and diffusing into the inducer with the main flow,the sheet cavitation is formed,and the cavitation breaks down inside the inducer.As cavitation number is lower than the critical number of cavitation,cavity fills the whole flow field and shows an asymmetrical pattern,which causes vibration and noise,seriously affects the operation of the inducer,and the blade cannot do work.Because the low pressure area of the inducer is located at the outer edge of the suction surface and the high pressure area is located at the outer edge of the pressure surface,the leakage flow caused by the pressure difference and the mainstream action form a backflow,and the backflow forms a leakage vortex cavitation in the low pressure area.Compared with the Merkle model,vapour volume fraction,cavitation area and cavitation intensity in the core region of the cavitation decrease simulated by the modified Merkle model at the same cavitation number with liquefied natural gas as working fluid.The more the cavitation intensity decreases with the decrease of the cavitation number,the more obvious thermodynamic effects is.In order to improve the anti-cavitation performance of inducer,based on Isight multi-objective optimization platform,combined radial basis function with multi-island genetic algorithm to systematically optimize the blade inlet angle and outlet angle of inducer,with the critical cavitation number of inducer as objective function and head coefficient as a constraint.(4)The reliability of numerical calculation is verified by the external characteristic experiment with water as working fluid,and the effects of non-inducer,original inducer and optimized inducer on cavitation performance of submerged pump are analyzed.The law of cavitation development of submerged pump with inducer,the cavitation shedding process of inducer at different time and the cavitation degree of inducer under different working conditions are revealed by high-speed photography experiment.The thermodynamic effects on cavitation performance of submerged pump with liquefied natural gas as working fluid is predicted.