Study On Fluid Flow,Heat And Mass Transfer In Curved Rectangular Micro/Nano Channels

In recent years,the problem of the fluid flow in micro/nano channels has received more and more attention due to its various applications in biological sensors,chemical sensors,thermal control system,the mixture or separation of ions,drug delivery,energy conversion,and so on.The fluid flow in micro/nano channels has many advantages such as smaller doses needed and higher reaction rate.In lab on a chip system,curved microchannels are often encountered.This is because,for a given unit chip,the effective axial length of a curved channel is usually larger than that of a straight one.In above applications about the micro-nano scale flow,accurate flow control is required.Therefore,it is necessary to establish a reasonable mathematical model of the fluid flow in curved micro/nano channels and give its corresponding solutions.In addition,the fluid flow in micro/nano channels can be applied to different aspects under different driving mechanisms.Based on these above considerations,this paper will study the pure pressure driven flows and the electromagnetic flows driven by the combination of electric and magnetic fields in curved rectangular micro/nano channels,and give the corresponding heat and mass transfer analysis.By theoretical analysis and numerical calculations,we give the advantages of the curved rectangular microchannels compared to the straightrectangular microchannels in improving energy conversion efficiency and reducing the entropy generation.Our research is divided into the following four questions.(1)The entropy generation of electromagnetic flows in curved rectangular microchannels.We study the electromagnetic flow and heat transfer of Newtonian fluids in curved rectangular microchannels.Under the assumption of a creeping flow,the analytical solution of velocity is given first.Then,considering the constant wall heat flux and thermally fully developed conditions,the numerical solutions of the temperature distribution are given.Finally,the entropy generation of this problem is calculated by using the obtained velocity and temperature distribution.According to the analysis,the entropy generation decreases with the increase of the curvature ratio and aspect ratio of this microchannel.In addition,the increase of the peclet number can lead to the decrease of the entropy generation also.(2)Study on oscillating electromagnetic flow and its mass transfer in curved rectangular microchannels.The electromagnetic flow and mass transfer of Maxwell fluids within curved rectangular microchannels are investigated.We use a linearized Maxwell fluid as the working fluid and an electrically neutral solute is injected into the fluid.Under the effect of an externally applied oscillating electric field and a constant magnetic field,the fluid begins to flow and increase the diffusion of solute.By solving the momentum equation of the fluid velocity and the diffusion equation of the solute concentration,the solutions of the fluid velocity and of the solute concentration are obtained.Then,we derive the expression of the effective diffusion coefficient of the solute.Results show that the coefficient decreases with the increaseof the curvature ratio of the curved channel.In addition,by controlling the corresponding parameters' values,the mass transfer characteristics of the electromagnetic flow in the curved nanochannel can be effectively controlled.(3)Electric energy conversion efficiency in curved rectangular nanochannels at low zeta potential.The pure pressure driven flow in nanochannels can convert the mechanical energy into the electrical energy due to the existence of the electric double layers.Under the assumption of low zeta potential and creeping flow condition,the analytical solution of electric energy conversion efficiency is obtained.By comparison,we find that the conversion efficiency in a curved rectangular nanochannel is about 17% higher than that in a straight rectangular one under a certain parameter range.(4)Steric effects on electric energy conversion efficiency in curved rectangular nanochannels at high zeta potential.At high zeta potential conditions,the influence of steric effects on the fluid velocity and energy conversion efficiency in nanochannels cannot be ignored.By using the finite difference method,numerical solutions of the electrical potential and the fluid velocity are obtained first.Then,we derive the the expression of the conversion efficiency by using the obtained electrical potential and velocity distributions.By comparison,the steric effects can improve the conversion efficiency by about 50% within a certain parameter range.