Research On Low Temperature Cavitation Characteristics Of LNG Submerged Pump

As the core power transmission equipment in the process of LNG production,transfer and transportation,the performance of LNG submerged pump has a great impact on the safety and economy of LNG system.The temperature of liquefied natural gas is about-162?,and the low temperature condition has a very high demand on the material and performance of the submerged pump,especially the cavitation phenomenon.This study was completed under the funding of Research and Application of Key Technologies for Hydraulic Design of10,000-ton Deepwater Floating LNG submerged Pump.In this paper,a LNG submerged pump is taken as the research object.Based on the airfoil experimental data in the literature,the turbulence model and cavitation model in the variable temperature environment are modified.Based on the modified turbulence model and cavitation model,the internal flow pattern and unsteady cavitation flow characteristics of LNG submerged pump are studied.The main research work of this paper is as follows:?1?Based on the test results of hydrofoil airfoil and NACA0015 airfoil,the applicability of turbulence model?k-? model,k-?model,RNG k-? model,SST k-?model?at different temperatures is studied,and the turbulence model is modified and verified by density correction method and filter correction method.It is found that when the medium temperature is 25?,the length,thickness and shape of the cavity calculated by k-? model and k-?model are close to each other,and the distribution of the surface pressure coefficient calculated by k-? model is the most consistent with the experimental value.The results of NACA0015 airfoil are improved by two modified turbulence models.When the medium temperature is 50?,the k-?FBM model and k-? FBM model have high prediction accuracy in the middle and downstream of the airfoil.When the medium temperature is 70?,the simulation results of each turbulence model are close to each other.By comparison,k-? FBM model has better applicability.?2?Taking the liquid nitrogen at different temperatures as the working medium,considering the effect of thermodynamics on the fluid temperature in the cavitation process,the Zwart-Gerber-belamri cavitation model is modified based on the hord hydrofoil,and the numerical verification of the low-temperature cavitation is carried out by the hord hydrofoil and ogive blunt body hydrofoil respectively.Compared with schnerr Sauer model,Merkle model,Kubota model and Zwart-Gerber-belamri model,the modified Zwart-Gerber-belamri cavitation model can accurately predict the cavitation area in low temperature liquid nitrogen.At the same time,the core area of cavitation obtained by the modified model is more sensitive to temperature,and the pressure drop,temperature and cavitation intensity in the cavitation area change rapidly with the change of inlet temperature,which verifies the accuracy and effectiveness of the modified Zwart-Gerber-belamri cavitation model for low-temperature cavitation simulation.?3?In this paper,the influence of the axial distance between the inducer and the impeller on the performance of the submerged pump is studied.The external characteristics and the internal flow field distribution of the submerged pump are analyzed.The results show that the numerical calculation is in good agreement with the experimental results,and the high efficiency region is 0.8Q_des?1.4Q_des.With the increase of axial distance,the head of submerged pump decreases under the condition of small flow.Under the L_z=0mm scheme,there are less separation vortices in the first stage impeller,and the velocity streamline distribution is more uniform.The overall flow state of the submerged pump is the best of the four schemes.Under the condition of small flow,the flow separation in the submerged pump is obvious,the flow is disordered and the energy dissipation is serious.The flow state in the secondary impeller is more stable than that in the primary impeller.There is an obvious low pressure area at the inlet flange of inducer and the inlet hub of the first stage impeller,which is prone to cavitation.?4?Based on the modified turbulence model and cavitation model,the unsteady numerical calculation of the LNG submerged pump is carried out.The cavitation performance of the LNG submerged pump is mainly analyzed.The cavitation volume distribution,pressure distribution and cavitation development law in the impeller and inducer are studied.The influence of cavitation on the pressure pulsation of the submerged pump is explored.It is found that the critical NPSH_a calculated by numerical method is 4.8m.When the NPSH_a continues to decrease,the head drops sharply,which indicates that serious cavitation occurs in the pump.With the decrease of NPSH_a,the size and strength of cavitation in inducer and first stage impeller increase gradually,and the area of low pressure area increases gradually.The cavitation phenomenon in inducer is more serious than that in impeller.Most of the flow channels have been blocked near the critical cavitation condition,which is the root cause of the drop of lift.The influence of cavitation on the primary impeller is greater than that on the secondary impeller.The low-frequency signal increases with the decrease of NPSH_a.The pressure fluctuation of each stage impeller is mainly affected by the low-frequency cavitation and the dynamic static interference.