Numerical Simulation And Experimental Research Of Unsteady Cavitation Two-phase Flow In Inducer With Thermodynamic Effect

With the rapid development of industry and increasing emphasis on the concept of green economy in our country,the liquefied natural gas(LNG)which is highly efficient and environmentally friendly gradually occupy the leading position in the clean energy.As the core equipment of the industrial chain,the performance and working life of LNG submersible pump are related to the stable development of LNG industry.Since LNG is a kind of low-temperature medium,cavitation is extremely easy to occur in the pump during the transportation of LNG.In order to improve the cavitation resistance performance of LNG submersible pump,a common method is to install an inducer in front of the first stage impeller to accomplish the aim of improving the cavitation performance.Therefore,the cavitation performance of the inducer has a critical effect on the cavitation performance of the whole pump.In this paper,with the support of the Development Project of Jiangsu Province(Key Project)(BE2015001-3),the cavitation mechanism of the inducer of LNG submersible pump in temperature-sensitive fluids was studied,and the influence of change in temperature on the development of cavitation was mainly discussed.Combined with numerical simulation and experimental research,the cavitation characteristics and pressure fluctuation characteristics of the inducer in various working conditions and mediums were systematically analyzed,which provides theoretical guidance for improving the cavitation stability of the inducer.The main work and innovation of this paper are shown as follows:(1)In view of the problem that the existing cavitation models were established based on the isothermal hypothesis which cannot meet the required simulation precision for the fluids(LNG,liquid nitrogen,etc.)whose physical properties are sensitive to the temperature,combined with B-factor theory and Antoine equation,the saturation pressure of Merkle cavitation model was corrected to make it become a function temperature,so that the heat transfer problem in simulation of the low temperature fluids can be solved.In addition,the accuracy of the correction cavitation model was verified based on the Hord's low temperature experimental data.(2)The hydraulic performance and cavitation performance of the inducer in water at room temperature and two kinds of liquid nitrogen were studied by means of the modified cavitation model with thermodynamic effect.In non-cavitation conditions,the head and efficiency of the inducer in liquid nitrogen are higher than that in water indicating that the hydraulic performance of the inducer is improved in liquid nitrogen.In cavitation condition,the quantitative analysis of the cavitation characteristic curve of three kinds of fluid were carried out and the result shows that the NPSHr of the three kinds of fluid obtained from the modified cavitation model were decreased compared with the results obtained from the original cavitation model.In addition,the declining quantity in water is obviously less than that in liquid nitrogen which means the thermodynamic effect could be ignored in room temperature water.By comparing the distribution of cavitation area,cavity shape,blade load characteristics,axial vapor volume fraction and axial pressure coefficient,the in-depth analysis of cavitation internal flow field of different fluids was done.Furthermore,the mechanism and influence factor of thermodynamic effect in inducer were revealed by comparing the temperature drop in liquid nitrogen.(3)The unsteady simulation method was used to study the evolution law of cavity shape,and the transient characteristics of the cavitating flow flied of the inducer were comprehensively analyzed from four aspects:vapor volume fraction,pressure,turbulent kinetic energy and streamline.The results show that the unsteady characteristics of the flow flied become more obvious in water at the same NPSH_a.In addition,based on the comparative analysis of cavitation volume,cavitation area and radial force,it was found that the non-uniform cavitation phenomenon in water is much more serious than that in liquid nitrogen under the same working condition indicating that stronger thermodynamic effect in liquid nitrogen can effectively inhibit the development of cavitation and thus slow down the occurrence of cavitation instability.(4)A series of experiments were carried out in water for the inducer model including external characteristic experiment,cavitation performance experiment,pressure fluctuation experiment and high-speed photography.Through the comprehensive experiments,the external characteristic curve,the cavitation performance curve,the characteristic of pressure fluctuation and the evolution law of the internal flow field were achieved,then the accuracy of the numerical simulation were verified.The experimental results show that with the increase of the flow rate,the amplitude of the pressure fluctuation at the measuring point in the middle of inducer is increased and then decreased while it at other measuring point shows the opposite change,and the strength of the low-frequency signal is decreased gradually.In cavitation condition,with the decrease of the NPSH_a,the main frequency moves to the low-frequency signal area.In the same NPSH_a,the number and corresponding amplitude of low-frequency signal are increased with the decrease of the flow rate.According to the comparison of the results of the high-speed photographic experiment,it was found that the initial cavitation position is located at the tip area of the inducer.When the cavitation deteriorates,the tip leakage vortex cavitation develops into triangular cloud cavitation with obvious increase in unsteadiness of the rear of cavitation region,and the vertical cavitation vortex appears which intensifies the development of the cavitation at the tip of inducer leading to the hydraulic instability in inducer.