Study On The Flow And Heat Transfer Characteristic Of Electroosmosis Under The Action Of Magnetic Field

Along with the development of microfluidics and nanofluids,the micro scale transport and separation technology are widely applied in the fields of MEMS,biology,chemistry and medicine,as well as the thermal control of microelectronic devices.Initially,the conventional and simplistic pressure gradient,which commonly used in the macro-scale flow,were employed to drive the flow of micro-fluidics.However,researchers have gradually realized the defects of single pressure driving mechanism,such as energy loss caused by friction,inability to achieve accurate operation in micro devices and so on.Therefore,more efforts have been made to find more ideal actuation mechanisms.With the rapid development of chip laboratory technology,electroosmotic driving has been widely used in micro-fluidic devices because of its simple design requirements,lack of moving elements,small sample dispersion and effect circuit reconfigurability.Because the buffers and solutions in many applications are conductive,more and more attention has been paid to the combined electric and magnetic fields driven mechanism.The magnetohydrodynamic(MHD)microfludic devices has the advantages of simple fabrication,low working voltage and bidirectional flow,which is widely used in chemical and biomedical processes.However,in most of the research of magnetohydrodynamic flow,it isusually assumed that the velocity vanishes at the wall.Actually,with the development of research on micro-scale fluids,it is increasingly realized that the classical non-slip boundary conditions are not always applicable.Despite many disagreements about the magnitude of induced slip,however,through direct measurement technology and molecular dynamics simulation,it can be seen that the slip length varies in the range of tens of nanometers to several microns.This range is similar to the actual size of micro-fluidics and nano-fluid devices,which indicates that there may be considerable slip effect in the micro-fluidic flows.Therefore,it is necessary to discuss micro-fluidic flows considering the wall slip effect under the action of electromagnetic field,and further make use of such effect to induce transverse or circulating flows in channel.This paper mainly discusses the following three aspects:(1)The flow and heat transfer characteristics of magnetohydrodynamic electroosmotic flow in rectangular microchannel are studied in this paper.The Newtonian fluid in rectangular microchannel is driven by the mixture of pressure gradient,electroosmotic force and electromagnetic force.First,we obtain the analytical expression of velocity distribution in two-dimensional case under the Debye-Hückel linearization approximation.Then,under the boundary assumption of uniform heat flux on the wall and considering the effects of viscous dissipation,Joule heating and electromagnetic coupling heat,temperature distribution and Nusselt number of the fluid are derived by use of the finite difference method.The results show that the effect of transverse electric field on the control of the flow is obvious.The new transverse electric field results in a trend that the velocity and temperaturedistribution increase firstly increase and then decrease with the increase of Hartman number,while there is only a single decreasing trend without the transverse electric field for the velocity and temperature.(2)The study of flow?heat transfer and entropy generation of non-Newtonian fluid(mainly the third grade fluid)through the parallel plates is carried out.The conducting third grade fluid flows are driven by the Lorentz force applied by the electric field and magnetic field.First,under the assumption of unidirectional flow,we obtain numerical solutions for velocity-distributions of the third grade fluid by Chebyshev spectrum method.Based on the obtained velocity distribution,we further obtain the temperature?Nusselt number and entropy generation of the third grade fluid.The influences of dimensionless governing parameters,including non-Newtonian parameter,magnetic field and viscous dissipation on the above obtained physical quantities are systematically investigated.The results show that the temperature and Nusselt number decrease with the increase of non-Newtonian physical parameters,but the trend of local entropy and total entropy is opposite,which shows that the third grade fluid parameters can promote the generation of local entropy.Moreover,we also find that the increase of non-Newtonian parameters will lead to the increase of critical Hartmann number.(3)A study of fully developed electromagnetohydrodynamic(EMHD)flow through a microchannel with patterned hydrodynamic slippage on the channel walls is presented.Under the assumptions of unidirectional flow and small Reynolds number,the governing equations for the velocity with patterned slip boundary conditions aresolved analytically by perturbation techniques.In addition,numerical solution for the velocity is obtained by using the finite-difference method,which is found to be in good agreement with the analytical solutions within admissible parameter range.The effects of different parameters on the velocity and volume flow rate due to patterned hydrodynamic slippage are discussed in details,including wave-number,Hartmann number,the amplitude of the patterned slip length and normalized electric field strength.The results show that patterned slippage over microchannel walls can induce transverse flows,which can lead to the increase of mixing rates in microfluidic devices.In addition,we also find that precisely flow control can be achieved by controlling the magnetic flux,wave-number and by a good choice of the electric field intensity.Our analysis can be used for the design of efficient micro-fluidic mixers.