Theoretical And Experimental Study On Heat Transfer Characteristics Of Magnetic Fluid Based On High-speed Seal

To enhance the comprehensive national strength competition and maintain national defense security,military and aerospace are constantly developing,resulting in more and more high-speed sealing occasions.Magnetic fluid seal shows more and more unique application advantages due to zero leakage,long life and high reliability.Not only can they meet the stringent sealing requirements in the high-end sealing fields,but also guarantee the good working performance of equipment and weapons.However,high-speed(linear velocity>20m/s)magnetic fluid seal has not achieved breakthroughs in China.The two main factors that affect the performance of magnetic fluid high-speed seal are centrifugal force and temperature rise.Among them,temperature rise is the most critical reason directly causing the failure of magnetic fluid high-speed seal.Therefore,in order to broaden the application of magnetic fluid seal and solve the heat transfer problem of magnetic fluid high-speed seal,this thesis has conducted theoretical and experimental research on heat transfer characteristics of magnetic fluids in sealing channel based on high-speed sealing conditions.The main work is as follows:(1)The formulars of viscous resistance torque and friction dissipation of magnetic fluid high-speed seal are derived.The influence of the rheological properties of magnetic fluid on the viscous resistance torque are experimentally studied.Then,substitute the viscosity-temperature calculation formula into the friction dissipation formula for correction.(2)By characterizing the basic properties of the prepared water-based,engine oil-based,diester-based and perfluoropolyether-based magnetic fluids,suitable magnetic fluids are selected to be applied in the high-speed seal.And suitable methods of calculating the thermophysical properties of magnetic fluids are selected,for example,Brinkman model is used for viscosity calculation and Maxwell classic model is used for thermal conductivity calculation.Finally,the thermophysical properties of four different magnetic fluids are obtained.(3)By simplifying the calculation model of the temperature field in the high-speed magnetic fluid sealing gap,the calculation formula of the maximum temperature of the magnetic fluid in the gap is derived and modified.The revised calculation results are verified to be accurate,and then the maximum temperatures of the diester-based magnetic fluid in the sealing gap without cooling are calculated at linear velocities of 22.6 m/s ～ 50.3 m/s.The results illustrate that temperature rise is the prominent problem of high-speed seal.Although this method of temperature calculation simplifies and ignores some conditions and deviates from the actual value,it is suitable for engineering applications to quickly and easily estimat the temperature in the seal,and it is accurate and effective within an error range.(4)Based on the conventional structure of magnetic fluid high-speed seal with cooling measures,reasonable assumptions and simplifications were made to obtain a model for studying the heat transfer characteristics of magnetic fluid under the conditions of high-speed seal.An experimental platform for convection heat transfer of magnetic fluid in a horizontal channel under a non-uniform permanent magnetic field is designed and built.The main structure of the test section of the test bench is basically the same as the structure of magnetic fluid high-speed seal,and the materials used are exactly the same to ensure the authenticity of the physical model.The heating section uses a DC power supply to heat the resistance wire which connected to the bottom surface of the test channel,keeping a constant heating power.The main equipment of the cooling system is a low-temperature chiller,which can provide a stable and constant cooling temperature for the pole pieces.The temperatures are measured by K-type thermocouples,and the measurement results are displayed and recorded by the temperature inspection instrument and data acquisition computer.Then dimensionless numbers such as the Rayleigh number Ra,magnetic Rayleigh number Ram,and Nusser number Nu of magnetic fluids in the horizontal channel under a non-uniform magnetic field are obtained by performing dimensionless processing on the experimental data.By analyzing the error of measurement and calculation,the uncertainty of experiment is obtained.The uncertainty of Rayleigh number,magnetic Rayleigh number,Nusselt number,and heat flux density are 2.46%,1.41%,4.51% and 7.51%.The water-based magnetic fluid was used as a comparative working medium,and the Nusselt number was measured,which verified the validity and reliability of the experimental data.(5)The effects of magnetic field,cooling temperature,heating power,and gap size on the convection heat transfer behavior of magnetic fluids were measured.The results show that the convection heat transfer performance of magnetic fluids under the action of a non-uniform magnetic field is significantly enhanced compared to the absence of a magnetic field.Increasing the strength of magnetic field can improve the natural convection heat transfer.Reducing the cooling temperature of the channel upper wall can significantly improve the heat transfer efficiency and heat transfer rate,and it can reduce the temperature of the magnetic fluid and the channel wall as well as increase the temperature difference.However,continuously reducing the cooling temperature does not always improve heat transfer.There is a cooling temperature limit that reaches the maximum thermal equilibrium state for a constant heating power.Increasing the heating power will increase the temperature of the magnetic fluid and the channel bottom wall.But at the same time,the temperature difference will increase,which will cause the instability of convection heat transfer.Under the same heating power and cooling temperature,increasing the size of gap will increase the temperature of the magnetic fluid and the channel bottom wall as well as decrease the heat transfer efficiency.Reducing the gap size will reduce the temperature of the magnetic fluid and the channel bottom wall,and increase the heat transfer efficiency.In addition,natural convecetion heat transfer of different magnetic fluids are measured.The results compared the heat transfer efficiency and heat transfer performance of them under the same operating conditions.The natural convection dimensionless correlations of water-based,engine oil-based,diester-based and perfluoropolyether-based magnetic fluids are fitted by the experimental data based on the natural convection criterion equation.The results are expected to predict the natural convection heat transfer performance of magnetic fluids in a specific range.Finally,the natural convection heat transfer mechanism of magnetic fluid under the non-uniform magnetic field and temperature gradient are discussed by measuring the distribution of the magnetic field and temperature in the test channel.It is considered that the Kelvin force and buoyancy caused by the gradient of magnetic field and temperature induce the magnetic fluid to generate a flow roller in the horizontal channel,which leads to the disturbance of thermal boundary layer and the migration of particles,thereby enhancing the natural convection heat transfer of magnetic fluid.The research results mainly fill the deficiency of the research on the heat transfer related problems of existing magnetic fluid high-speed seal in China.To some extent,they provide theoretical and experimental reference for solving the temperature rise problem in magnetic fluid high-speed seal.It is expected to bring hope of solving magnetic fluid high-speed seal problem,and there is a great potential for future applications in the field of high-speed seal.There are 71 figures,18 tables,159 references in this thesis.