Molecular Dynamics Study Of Nanoparticle Shape On Heat Transfer Behavior Of Nanofluids

As a new type of fluid working medium,nanofluids have been widely researched by many scholars because of their good heat transfer and flow characteristics,but so far there is still no unified conclusion on the internal mechanism of nanofluids to enhance heat transfer.Most researchers reflect the heat transfer characteristics of nanofluids by calculating the thermal conductivity,but they ignored the specific heat capacity and thermal diffusion coefficient,and the shape of nanoparticles is basically concentrated on spherical particles.Some studies have found that nano-particles with other shapes have better effect on heat transfer enhancement of nano-fluids,but the studies are relatively few in this direction and lack certain theoretical support.Therefore,it is very important to explore the influence of particle shape on the heat transfer characteristics of nanofluids.In this paper,the molecular dynamics simulation method is used to calculate and analyze the thermal conductivity of nanofluids with different shapes,such as spherical,columnar and flaky,and to study their specific heat capacity and thermal diffusion coefficient respectively,to explore the mechanism of the influence of particle shape on the heat transfer of nanofluids.Firstly,the relevant theories of molecular dynamics simulation are expounded.The thermal conductivity and thermal conductivity mechanism of nano-fluids are preliminarily calculated and analyzed from two aspects of temperature and volume fraction,which provides certain basic support for the follow-up work.The results show that both temperature and particle volume fraction strengthen the thermal conductivity of nanofluid.With the increase of temperature,the internal energy of the nano-fluid increases and the diffusion ability of the nano-fluid is improved.The increase of the volume fraction of the nanoparticles changes the structure of the nanofluid and makes it show more similar thermal conductivity characteristics to solid particles.Then,the effects of particle shape and length-diameter ratio on the thermal conductivity of nanofluids were studied.The results show that the thermal conductivity of lamellar nanofluids is the highest and spherical nanofluids is the lowest.The thermal conductivity of nanofluids decreases first and then increases with the increase of length-diameter ratio.When the length-diameter ratio is 1,the thermal conductivity effect is the worst.The strengthening mechanism of nano-fluids was analyzed from three aspects of the energy distribution,the proportion of active atoms and the motion characteristics of particles.It is found that the thermal conductivity of the nano-fluid is consistent with the variation trend of potential energy ratio,kinetic energy ratio and energy ratio.When the proportion of active atoms of nanoparticles is higher,the heat conduction enhancement effect of nano-fluid is better.When the nanoparticles have higher average rotational angular velocity,the diffusion ability of the surrounding particles can be improved effectively,the heat exchange between particles can be intensified,and the thermal conductivity of the nano-fluid can be improved.The diffusion coefficient modified by the average rotational angular velocity can better reflect the thermal conductivity of nanofluids.Finally,the heat transfer characteristics of nano-fluids are further studied from the aspects of specific heat capacity and thermal diffusion coefficient.In this chapter,the effects of particle volume fraction,particle shape,particle length-diameter ratio and particle number on specific heat capacity and thermal diffusion coefficient of nanofluids were simulated.The results show that the specific heat capacity of the nano-fluid is inversely proportional to its density,and the specific heat capacities of nanofluids with different shapes are positively correlated with the specific heat capacities of nanoparticles.The thermal conductivity of nanofluids increases with the increase of thermal diffusivity.