Numerical Study On Heat Transfer Of Nanoaluid Flowing In A Tube

The process of energy transfer has penetrated into people's daily life and industrial production.With the deepening of energy conservation and emission reduction,people are constantly studying new heat exchange systems to meet various energy conversion needs.Nanofluids,as an important technology to enhance heat exchange,have been widely concerned in recent years.In this paper,the numerical simulation method is used to study the convective heat transfer of different kinds of nanofluids in horizontal circular tube and special-shaped cross tube.The influence of the thermophysical parameters of nanofluids and the structure of cross tube on the convective heat transfer of nanofluids in the tube is studied.In this paper,starting from the basic thermophysical parameters of nanofluids,the thermal conductivity calculation model of copper water nanofluids is obtained by sorting out and fitting the experimental data of the thermal conductivity of nanofluids,and the accuracy and applicability are verified.On this basis,the accuracy and applicability of single homogeneous flow model,two-phase mixed model and DPM model(discrete model)are analyzed.It is found that the operation of single homogeneous flow model is simple,but the deviation of calculation results is too large;the analysis results of DPM model are very consistent with the experimental results,but the requirements of calculation conditions and calculation time are very strict Therefore,considering the velocity difference and interaction between the particle phase and the base liquid phase,this paper chooses the two-phase mixing model as the mathematical model of numerical simulation,and the calculated results are within the allowable error range.The mathematical models of convective heat transfer of Cu-H2O Nanofluids and Al2O3-H2O nanofluids in horizontal tubes were established and three-dimensional numerical simulation was carried out.The influences of three factors on the convective heat transfer of nanofluids,such as the type of nanofluids,the diameter of nanoparticles and the volume fraction of nanoparticles,were studied.The results show that the convective heat transfer coefficient of Cu-H2O nanofluids is higher than that of Al2O3-H2O nanofluids under the same volume fraction and the same particle diameter.The main reason is that the thermal conductivity of Cu is higher than that of Al2O3 particles.For the same nanofluid,the convection heat transfer coefficient of nanofluid increases with the decrease of nanoparticle diameter in the same volume fraction.This is because the smaller the nanoparticle diameter is,the more the nanoparticle number is,the larger the specific surface area is,and the stronger the interaction between the nanoparticle and the base fluid enhances the heat transfer;similarly,with the nanoparticles With the increase of the sub volume fraction,the convective heat transfer coefficient increases and the heat transfer is strengthened.Moreover,it is found that the friction coefficient of nanofluids does not increase significantly with the addition of nanoparticles,which is consistent with the experimental results.Finally,the heat transfer criterion of nanofluid convection in the tube is obtained by sorting out and fitting the simulation data in this chapter.The criterion takes three factors into account:the diameter,Reynolds number and volume fraction of nanoparticle,which can fully reflect the impact of each factor on the convection heat transfer.The convective heat transfer and flow characteristics of Cu-H2O nanofluid in a cross-section tube were simulated.The comprehensive effects of the parameters of nanofluid and cross-section tube on the flow and heat transfer were discussed,and compared with the convective heat transfer of pure water in the cross-section tube.The results show that the convective heat transfer coefficient of nanofluid increases with the increase of nanoparticle volume fraction and the decrease of particle diameter.In this simulation,the convective heat transfer coefficients of Cu-H2O nanofluids with 25 nm particle diameter and 2.0%particle volume fraction are 80.2%higher than that of pure water.For Cu-H2O nanofluids with particle volume fraction of 0.5%and particle diameter of 25 nm,the convective heat transfer system in 1,2 and 3 is 49.5%,40.0%and 34.4%larger than that in the tube.With the decrease of pitch,the formation of boundary layer can be interrupted continuously.At the same time,the higher the rib is,the stronger the jet and vortex are,and the better the heat transfer enhancement effect is.Considering the comprehensive heat transfer performance of the composite heat transfer of nanofluid and cross tube,through the calculation of FTEF,it is found that the comprehensive heat transfer factors of 1#?3#?5#heat transfer tube selected in this paper are greater than 1,and the maximum can reach 1.30,so the composite heat transfer has higher comprehensive heat transfer performance,which can meet the requirements of the increasingly strong heat transfer equipment.