Lattice Boltzmann Investigation On Battery Thermal Management Based On Natural Convection Enhanced By Nanofluid

The new energy electric vehicle(EV)has the advantages of no emission and so on,which make it significant for reducing traffic pollution and energy saving.Hence EVs have made great progress in recent years.As the one of the most significant techniques of the electric vehicles,the performance of the power battery can be easily affected by the temperature,which greatly affects the performance of the whole vehicle,thus a efficient cooling system is needed.Liquid cooling has many advantages,such as high heat transfer efficiency and strong applicability,hence it has been widely applied in battery thermal management.However,with the improvement of the mileage requirements of the electric vehicle,the battery structure is bound to be more compact and complex,which puts forward higher requirements for the liquid cooling system.To further improve the performance and reduce the consumption,a natural convection battery thermal management system enhanced by nanofluid is established and simulated by lattice Boltzmann method.Subsequently,based on the above contents,a particle motion model is established,and the natural convection heat transfer characteristics under particle migration are investigated.The main work and conclusions are as follows:(1)Based on the two dimensional interpolation method boundary condition and non-equilibrium extrapolation method,the curve boundary condition of the lattice Boltzmann model is established.To realize the heat generation of the battery under constant current,the heat generation model is simplified by using the constant heat flux boundary.On the basis of the above model,the momentum exchange boundary scheme based on LB model is used to realize the force calculation for the fluid-solid boundary,and the equilibrium distribution function is used to update the motion boundary node.(2)For the battery thermal management system based on natural convection of nanofluid,the influence of nanofluid volume fraction and Rayleigh number on heat transfer performance is studied.With the increase of nanofluid volume fraction,the maximum temperature of the battery,the average temperature and the temperature standard deviation are reduced,6% vol.Cu-water nanofluid can make the maximum temperature get a 6.5% decrease;But as time progresses,the convection fully develops,hence the viscosity of nanofluid hinders the enhanced flow,making the heat transfer enhancement effect begin to decrease.In addition,the increase of the nanofluid volume fraction relaxes the isotherm,resulting in a decrease of the Nu number on the left wall.With the full development of convection,the direction of the heat flux at the left wall changes,which leads to the decrease of the Nusselt number.Besides,Rayleigh number increases the intensity of convective heat transfer.However,when Ra is small,the increase of Ra destroys the original linear temperature field dominated by heat conduction,resulting in the heat accumulation on the battery surface,thus the temperature increases a little.Similarly,when Ra is high and the convection is strong enough,the small vortices will be strong enough to impede the heat transfer within larger range.(3)For the natural convection in a square cavity with moving particles,the influences of particle settling time,particle size and particle density on the flow and heat transfer characteristics are studied.The settlement process of particles can enhance convective heat transfer,and transport large amount heat to the wall.Because of the development of convection,the later the sedimentation of particles begins,the more heat the flow will drive,hence the heat transfer enhancement effect is more obvious when sedimentation begins later.In the process of particle sedimentation,with the increase of particle radius,the fluid driven by particles increases,and the velocity of fluid is greater,so the intensity of convective heat transfer is enhanced.The influence of density on the characteristics of flow and heat transfer in the process of settlement is more complicated.On the one hand,when the density becomes larger,the settling velocity of the particles becomes larger and the convective intensity is increased.On the other hand,when the particle density increases,the particle will descend to the bottom wall faster,so it will lose the subsequent influence on convective heat transfer,which make the original convection mode recover earlier.The particle size and density have a significant influence on the particle trajectories.The larger particle radius and higher density will make the particle easier to maintain the original state of motion;When the particle density increases,the trajectory deflect to the middle part;When particle radius increases,although it has greater inertia,it will be exposed to more fluid,causing its trajectory deflecting to the left wall.