Study On Thermal Conductivity Enhancement Mechanism Of Charge On Nanofluids Based On MPCD-MD Method

Nanofluid was regarded as a promising working fluids developed in recent decades.Many researchers have devoted themselves into the mechanism of heat transfer enhancement of nanofluid.But there is still no well-accepted conclusion on the heat transfer mechanism in nanofluid.Various approaches such as Molecular Dynamics(MD)were employed to explore the microscopic mechanism of thermal conduct in nanofluid.At present,MD is the most common method.However,one stumbling block is MD can employed to simulate a smaller system due to the greater calculation burden.Hence,a coarsen molecular dynamics is used for reducing the computation burden.Another approach Multi-Particle Collision Dynamics(MPCD)often employed to reduce the computation cost too.In regret,the interaction between nanoparticles,especially the aggregation behavior of nanoparticles,cannot be simulated.Therefore,on the premise of ensuring accuracy requirements and improving computing efficiency,MD-MPCD hybrid method is adopted in this paper to explore the thermal conductivity mechanism of nanofluids.MD is used to simulate the aggregation due to the limited number of nanoparticles,and coarse-grained MPCD is used to simulate the flow owing to the large number of fluid particles.The momentum transfer between nanoparticles and fluid particles is realized by the collision step of MPCD.Furthermore,nanofluids are widely used in heat dissipation,lubrication,fuel and other fields.This research mainly focuses on the thermal conductivity mechanism of nanofluids.Firstly,this article have introduced the background and significance of the subject research,briefly described the domestic and foreign research status and different methods in heat transfer of nanofluid,and briefly explained the research content of this article.Secondly,the article have introduced the simulation foundations of the MPCD and MD methods,explained the coupling principle and process of the MPCD-MD method,and also elaborated the boundary conditions,ensemble selection and time step of the system simulation.Furthermore,this article have compared the calculation efficiency and accuracy of the two methods of MPCD and MD through the calculation of the Ar system.Then,in this article,the MPCD-MD method have been used to calculate the coarse-grained water molecules,and compared with the theoretical value to calculate the deviations.Meanwhile,The results were compared with the theoretical value to verify the validity of the calculation method,which have paved the way for the calculation of nanofluids.Next,the nanofluid model has been established and the calculation method of fractal dimension have been proposed,Moreover,the thermal conductivity of the empirical formul and simulation has been compared,it is found that they are relatively consistent in the law of change,the error is within 5-10%.Finally,in order to more accurately explore the impact of aggregation on the thermal conductivity of nanofluids,in this article,the relation of fractal dimension and the thermal conductivity has been verified,and the same conclusion as the predecessors has been obtained: the thermal conductivity decreases with the increase of the fractal dimension.Meanwhile,with changing the charge has been carried by the nanoparticles.The article have obtained the change law between the charge,thermal conductivity,and the fractal dimension in the three intervals of particle aggregation.In the aggregation stage,the thermal conductivity fluctuates with the increase of the charge,and the fractal dimension increases linearly with the increase of the charge.In the mixed form,the thermal conductivity first increases and then decreases with the charge increases,while the fractal dimension increases with the charge on the contrary.In the completely discrete,the thermal conductivity fluctuates with the charge increases,the value is lower than the aggregate state.Moreover,an external electric field have been added to the system,the influence of the electric field force on the heat transfer of the nanofluid has been analyzed,the changes of the fractal dimension and thermal conductivity of the nanofluid in the two cases have been explored by using constant electric field and charge,it is found that the thermal conductivity increases linearly with the increase of the electric field force,but fractal dimension in the case of a constant charge,which it is almost no change,and in the case of a constant electric field,it enhances with the increase of the amount of charge.