Research And Application Of The Thermophoresis In Aqueous Media Under High Temperature And High Pressure

Thermophoresis of charged colloidal particles in aqueous solution is a new research direction,which has a wide prospect in nuclear electricity generation,aerospace,biological medicine and other fields.For example,the thermophoresis slipping mechanism in nanofluids is one of the important factors of enhancing the convection heat transfer.Compared with other convection heat transfer mechanism,natural convection has the advantages of low-energy,high reliability and low noise.Therefore,it is important to study the thermophoresis behavior and mechanism of charged colloidal particles.Based on the fixed particle reference system,the thermophoresis model of a single charged particle is established.The numerical model consists of the coupling process of the temperature field,the ion concentration field,the thermoelectric field and the velocity field.The governing equations are the energy conservation equation,the Nernst-Planck equation,the Poisson equation,the continuity equation and Stokes equation respectively.On the basis of this study,the effect of thermophoresis on natural convection heat transfer of nanofluids in a two-dimensional square cavity is considered.The main research work is as follows:The thermophoresis of nanoparticles has important application value in the nuclear reactor cooling system and the thermal management of fuels cells.Most existing studies of thermophoresis focused on nonmetallic particle under normal temperature and pressure.To solve this problem,we performed a numerical analysis on the thermophoresis phenomenon of the alumina particle in aqueous solution under high temperature and high pressure.The nonlinear temperature field,thermoelectric effect and high temperature and high pressure conditions are considered.Numerical simulations revealed that the thermophoresis of particle in aqueous solution is accelerated under high temperature and high pressure.The temperature field is linear when the thermal conductivity of particles equals to that of aqueous solution.When the electric double layer(EDL)thickness is thick compared to the particle radius,the EDL deformation effect is dominant,and the high temperature increases the thermodiffusion coefficient.When the EDL thickness is thin,the thermoelectric effect becomes dominant,and the high temperature decreases the thermodiffusion coefficient.If the thermal conductivity of particles is much higher than that of aqueous solution,the resulting nonlinear temperature field has negligible effect on the thermophoresis when the EDL thickness is thick,but it seriously weakens the thermophoresis when the EDL is thin.The thermophoresis mechanism between particles and base fluid is an important mechanism of enhanced heat transfer of nanofluids.Most existing studies often regard nanofluids as single-phase flow and ignore the influence of the thermophoresis mechanism on particle distribution and convective heat transfer.Therefore,the theoretical value obtained is quite different from the experimental measured value.To solve this problem,based on the previous numerical results,we performed a nanofluids natural convection model under high temperature and high pressure in a closed square cavity,which considered the influence of the Rayleigh number,the particle volume fraction and the EDL thickness on heat transfer.Numerical simulations revealed that the convective heat transfer performance is significantly enhanced under high temperature and high pressure conditions.When the Rayleigh number increases,the convective heat transfer in the cavity increases,and the influence of thermophoresis migration on the convective heat transfer becomes stronger.However,the convective heat transfer performance increases first and then decreases with the increase of the particle volume fraction,which is related to the increase of fluid viscosity resistance with the increase of the particle volume fraction.When the EDL is thick,the particle thermophoresis velocity is high,the fluid fluidity is reduced,and the heat transfer performance is poor.When the EDL is thin,the particle thermophoresis velocity is low,the fluid motion resistance is reduced,and the heat transfer performance is significantly improved.