Aggregation And Size Effect On Flow Of Nanoparticles By MD

In this work,molecular dynamics simulation is used to investigate the flow characteristics of Cu-Ar nanofluids.Compared with traditional heat transfer fluids,nanofluids can greatly improve the heat transfer capacity of base fluids.This feature enables nanofluids to be used in various fields.However,the researches on the heat transfer characteristics of nanofluids have been greatly developed.There are less studies on the flow characteristics of nanofluids which are closely related to the preparation,use and overall energy loss.The viscosity of nanofluids is calculated based on Green-Kubo(G-K)equation considering different sizes and aggregate morphologies.The viscosity of pure liquid Ar is firstly calculated to verify the reliability of the model and the results prove that the model is reliable.Secondly,the viscosity of nanofluid is calculated considering three nanoparticle sizes.The results show that the viscosity of nanofluid increases as the decrease of size when the nanoparticle volume fraction is same.This is mainly because the reduction of size will increase the total surface area of the nanoparticles which increases the total effective volume of the nanoparticles.Finally,the viscosity of nanofluids containing different nanoparticle aggregate configuration is calculated.The results show that the aggregation of nanoparticles will increase the viscosity of nanofluid and its effect on the viscosity is more significant than the size of nanoparticles.By simulating the flow of nanofluids in parallel channels,the effects of nanoparticle size and aggregation on flow behavior are studied.We firstly simulate Couette flow to prove that the simulated fluids are viscous Newton fluids.The results show that the velocity profile of the simulated nanofluids are the same as that of pure liquid Ar.Therefore,the simulated nanofluid can be considered as viscous Newton fluid.Second,the Poiseuille flows of nanofluids containing nanoparticles of different sizes are simulated.We find that both the number and size of nanoparticles can affect the velocity distribution and volume flow of nanofluids.A decrease in the size of nanoparticles leads to an increase in the amount of nanoparticles.Excessive nanoparticle causes increase in the viscosity of the nanofluid resulting in the decrease of velocity distribution and volumetric flow rate.Then,the Poiseuille flows of nanofluids containing nanoparticle aggregates are simulated.The results show that the size and configuration of nanoparticle aggregates will affect the flow characteristics of the nanofluid,which will greatly affect the velocity distribution of the nanofluid.We use Green-Kubo principle to analyze the reasons of nanoparticles increasing viscosity and affecting flow velocity.The results show that nanoparticle aggregates increase the local viscosity of the nanofluid rather than the overall viscosity.When the nanoparticles are uniformly distributed,the viscosity of nanofluid in different directions is basically same.The viscosity of nanofluids in all directions is not exactly same when aggregation occurs.At last,we analyze the heat transfer coefficients of different nanoparticle aggregates and compare with the percentage increase in viscosity.The results show that the aggregation of nanoparticles will increase the heat transfer coefficient.Different configurations of nanoparticle aggregate have different heat transfer coefficients.In our simulated configuration,the maximum heat transfer coefficient increases by 55.1%.In addition,the percentage of nanoparticle aggregation that increases the heat transfer coefficient is greater than the percentage increase of viscosity,which means that the benefits of the presence of nanoparticles are relatively large.