Numerical Research On Heat Transfer Characteristics Of Nanofluids In Inclined Heated Surface Cavities

Natural convection widely exists in natural and industrial environment,so the study of natural convection has important environmental and industrial significance.Natural convection in a side heated enclosure is a very classical heat transfer model,which can be widely used in all kinds of heat exchangers.In practical engineering application,this kind of side heated exchanger may be not placed vertically due to the limitation of conditions,resulting in a certain angle between the heated wall and the horizontal line.Therefore,the research on the inclined heated surface cavity also has an important significance.Furthermore,as a new type of heat transfer medium with high thermal conductivity,nanofluids can be used in the side heated cavity to enhance the heat transfer.The lattice Boltzmann method is used to investigate the thermal convection characteristics of nanofluids in inclined heated wall cavities in this paper.The main research contents and conclusions are as follows.(1)Based on Boussinesq hypothesis,combined the coupled double distribution function model and the non-equilibrium extrapolation boundary scheme,a two-phase lattice Boltzmann model for nanofluids was developed.This LBM model analyzed the forces acting on the particles in the base fluid and added the force system to the binary mixture model.The model revealed the enhancement mechanism of energy transport in nanofluids.And different thermal boundary conditions were set by using non-equilibrium extrapolation scheme.In this paper,the model was used to simulate the natural convection of nanofluids in inclined heated wall cavities.(2)The influence of Rayleigh number of nanofluids on heat transfer efficiency was simulated by establishing lattice Boltzmann models of inclined heated wall cavities with different structures.The results show that the flow velocity of nanofluids increases with the increase of Ra number,and the heat transfer mode of nanofluids in the cavity gradually changes from heat conduction to heat convection,which enhances the effect of heat transfer.When the Ra=10~6,the average Nu number of the inverted parallelogram cavity reaches the maximum of 3.9669,and the average Nu number of trapezoid cavity was 2.4457,with a difference of 62.2%.The total entropy change of the system with the inverted parallelogram cavity is also the largest,which is 17.267.In addition,it was found that there is a flow dead zone in the inclined heated wall cavity,which weakens the convective heat transfer.(3)The influence of different thermal boundary conditions on heat transfer effect was simulated by setting wall temperature boundary condition of different cavities.By comparing the heat transfer effect of cavities with five kinds of boundary conditions,it was found that increasing the temperature of the upper heated wall can better enhance the heat transfer effect.Except the uniform and the gradients temperature type II distribution,the other three kinds of thermal boundary conditions weaken the heat transfer effect of the whole system due to the gradual decrease of wall temperature in the upper part of the cavity and the negative local Nu number.The average Nu number of the inverted parallelogram cavity with gradients temperature type II distribution is5.23,which is about 31.74%higher than that of uniform distribution and 76.69%higher than that of gradients temperature type I distribution.In the inverted parallelogram cavity,the total entropy change of the system with gradients temperature type II distribution is 17.39,and the total entropy change of the system with gradients temperature type I distribution is 16.73.(4)The natural convection characteristics of nanofluids in four nanofluids with different particle sizes were simulated by setting the lengths of different particle diameters.The results show that the heat transfer effect of nanofluids with particle radius between 10nm and 50nm has little difference.Comparing the average Nu number of different fluids,it can be found that the nanofluids with particle radius R_p=20nm has the best heat transfer effect,and the average Nu number in four cavities is 3.173.The average Nu number of nanofluid with R_p=50nm is the lowest,and the average Nu number in four cavities is 3.120.Through the analysis of the numerical simulation results,the influence of different factors on the convection and heat transfer characteristics of nanofluids in the cavities are revealed,which has a certain guiding role in the enhancement of natural convection heat transfer in the side heated cavity.In the follow-up work,the lattice Boltzmann model can be further improved to study the influence of multiple physical fields and the precipitation process of nanoparticles.In this paper,there are 5 tables,39 figures and 114 references.