The Analysis Of Electrokinetic Flow And Heat Transfer Characteristic Through A Two-layer Microfluidic System

Due to the rapid development of microfluidic devices such as micro-electro-mechanical systems(MEMS),biochemical and biomedical instruments,drug delivery biochip,chemical separation devices and thermal management of microelectronic systems,microfluidics transport processes have received considerable attention over the last decade.Comparing with traditional single-layer fluid systems,two-layer or multi-layer fluid systems in micro/nanoscale fluid devices can be defined as such flow control devices containing two or more incompatible fluids.These fluid systems are widely used in multidisciplinary fields such as biology,medicine,chemistry.Based on such two or multi layer flow conversion devices,T-sensor ? and H-filter ? devices have been designed and widely applied in the analysis of multilayer fluid flow and heat transfer.Under different fluid driven mechanisms,there are rather great differences in the analysis of fluid motion characteristics,especially for dealing with the similar problem in micro/nanoscale devices.Based on this consideration,this paper will focus on the flow,heat transfer and energy conversion of two-layer fluid system in micro/nanofluidic devices withconsideration of the driving mechanism of pressure,electric field,magnetic field,or their combination.We will investigate the electrokinetic flow through a two-layer microfluidic system under the uniform electromagnetic field,deeply understand the coupling mechanism and rule of electromagnetic field,flow field and temperature field and and reveal the effect of external magnetic field on reducing the joule heat generated by the imposed electric field.Based on the theoretical analysis,we also understand the advantages of two-layer fluid system in reducing joule heat effect and improving energy conversion efficiency compared with single-layer fluid system.The specific problems include the following three aspects:(1)The entropy generation analysis of two-layer magnetohydrodynamic electroosmotic flow through a microparallel channel is performed in this study.The two immiscible fluid flows are both driven by a combination of electroosmotic force,pressure gradient and electromagnetic force.Under the framework of Debye-Hückel linearization approximation as well as the assumption of thermally fully developed and the condition of constant wall heat flux,the distributions of velocity and temperature are analytically derived and they are utilized to compute the entropy generation rate.The effects of fluid physical parameter ratios on the distributions of two-layer fluid velocity and temperature are firstly discussed.Then the local and total entropy generation rates are investigated for different magnetic field parameter(Ha)and the viscous dissipation parameter(Br)under the appropriate fluid physical parameter ratios.The results show that the entropy generation rate strongly depends on the velocity and temperature fields and the local entropy generation reveals adecreasing trend form the microchannel wall towards the fluid interface for both bottom and upper layer fluid.(2)The entropy generation analysis is investigated in two-fluid dragging systems.The bottom layer fluid is considered as electrolyte solution affected by the applied magnetic field and the upper layer fluid is viewed as non-conducting viscoelastic Phan-Thien-Tanner(PTT)fluid.Under the combined influences of electric and magnetic fields,the upper layer non-conducting PTT fluid can be dragged by the bottom layer fluid due to the interfacial shear stress.Firstly,we obtain the analytical velocity expressions for both bottom layer and upper layer fluids under the unidirectional flow assumption.The bottom layer fluid velocity distribution shows a classical M-type velocity profile.The upper layer fluid flow can be viewed as the plate Couette flow or Couette-Poiseuille flow.Subsequently,the thermal transport characteristic and entropy generation analysis are discussed in the present two-fluid dragging system.The results show that magnetic field can enhance the local entropy generation rate,but viscoelastic physical parameter can restrain the local entropy generation rate.The present theoretical research can be used in the design of thermofluidic device.By manipulating the electric and magnetic fields strength and the ratio of fluid rheological properties,the fluid motion and heat transfer characteristics can be manoeuvred efficiently.(3)In order to conduct extensive investigation of electrokinetic energy conversion in nanofluidic devices,the analysis of streaming potential of pressure driven flow in the two-layer fluidic system through a nanochannel is investigated theoretically.Underthe influences of the interfacial electric potential difference and the interface charge density jump,we first obtain the analytical electric potential distribution in the present two-layer fluidic system with consideration of Debye–Hückel linearization assumption.Then the analytical expressions of streaming potential field and flow velocity are derived.Based on the obtained the streaming potential,we finally give the analytical electrokinetic energy conversion efficiency in two-layer fluidic system and discuss the influences of related physical parameters on it.The theoretical result shows that the streaming potential can be viewed as a criterion to estimate the electrokinetic energy conversion efficiency.The electrokinetic energy conversion efficiency can be enhanced by the viscosity ratio and the interfacial slip length,but be restrained by the permittivity ratio and ion friction coefficient.Comparing to the single-layer fluidic system,the electrokinetic energy conversion efficiency can be augmented obviously in the two-layer fluidic system by selecting optimized flow parameters.