Research On The Dynamic Behavior Of Nanoparticles And The Characteristics Of Convective Heat Transfer And Resistance In Nanoparticle Two-phase Flow

Nanoparticle two-phase flow is common in nature,and has a very wide range of applications in engineering thermophysics,material science,medical technology and other fields.Aerosols,which are the most typical gas-solid/gas-liquid nanoparticle two-phase systems,are suspensions of fine particles in gases,such as smoke,dust,fog and haze.In recent years,the haze phenomenon has occurred frequently in China,and nanoparticles whose size under 100 nanometers in the air can easily be inhaled by the human respiratory system and deposition in the alveoli,which can seriously damage the health of people.In the engineering field,the suspensions prepared by dispersing the different shapes of nanoparticles into the traditional heat transfer medium can improve the efficiency of heat transfer process with a slight increase of the pumping power,so as to saving energy.The two-phase flow system with nanoparticles usually contains a large number of particles.Therefore,we need to focus on the statistical laws of particle population and solve the corresponding general dynamic equation in order to study the dynamic evolution of nanoparticles.The equation describes the evolution of nanoparticle size distribution by considering different dynamic events of particles such as coagulation,fragmentation,nucleation,phase change,chemical reaction,etc.,as well as combining the external influence of convection,diffusion and external force on particle transport process.Although the equation has the feature of multiscale,non-linearity and complexity,it can be numerically solved by the method of moments or other methods quickly and accurately,the results are available for analyzing and predicting the flow characteristics of aerosols or nanofluids.All the research in the thesis branched out based on the nanoparticle two-phase flow.It expands the spatial and temporal evolution of particles in the two-phase flow from the zero-dimensional system to the three-dimensional flow field,from the internal flow field to the external flow field,from laminar flow to turbulent flow,from spherical nanoparticles to non-spherical nanoparticles,from one-way coupling to two-way coupling.The transport phenomenon is taken into consideration including the mass transfer,momentum transfer and energy transfer process.Combined with the theoretical analysis and experiments,the numerical simulation is the main method used to study the particle dynamics.The Research focuses on the dynamic behavior of nanoparticles and convective heat transfer and resistance characteristics of nanoparticle two-phase flow for revealing the mechanism of the flow phenomenon,summing up some conclusions,looking forward to future research challenges and applications.Firstly,a new type of direct expansion method of moments with(n/3)th order of moments for nanoparticle Brownian coagulation was deduced.The model has high computational accuracy and efficiency,and the moments have clear interpretations on physical quantities of particles undergoing coagulation in the flow field.Then,coupled with large eddy simulation method,the direct expansion method of moments was applied to the calculation of the two-phase flow in the plane jet flow for the first time,and the validity of the method was verified.The moments and parameters of particles were analyzed and compared.By discharging the aerosol through a nozzle from a high-pressure chamber,the nanoparticle aggregates were dispersed to nearly primary particle size in experiments.Then the dispersion process of aerosols by high pressure discharge was simulated numerically using the method of moments.After comparing the numerical results with the experimental results,the mechanism of the dispersion was discussed,and concluded that the effect of dispersion depends on the pressure difference on both sides of the nozzle.In addition,a coupled numerical model for the two-phase flow containing non-spherical nanoparticles was constructed,and the convective heat transfer and resistance characteristics of the two-phase flow field with cylindrical nanoparticles in the channel were investigated.The local Nusselt number calculation formulas for laminar nanofluids flow containing cylindrical particles were given.Finally,the complex turbulent nanofluids flow were studied and simulated in a circular tube.And the mechanisms of convective heat transfer and resistance characteristics of the turbulent flow of cylindrical particles were also analyzed.Compared to increase the aspect ratio of cylindrical nanoparticles,increase the particle volume concentration would be more effective on enhancing the convective heat transfer intensity in industrial applications although it will cause a slight increase of resistance.It is of great theoretical and practical significance for improving the heat transfer performance of nanofluid containing non-spherical particles and preparation of nanometer devices using nanofibers.