Study On The Gas Flow Field In The Reactor For Fabricating Carbon Nanotube Fibers

Carbon nanotubes are allotropes of carbon with cylindrical tubular nanostructures.It is a novel material with a unique molecular structure and strong covalent bonds between carbon atoms.Nowadays there is a worldwide upsurge in making carbon nanotube fibers,which can be obtained either by wet-processing,array-drawing or chemical vapor deposition.In the CVD method,carbon sources and catalysts are carried by courier gas through the tube where chemical reactions occur rapidly at high temperatures.The equipment required is simple and the spinning speed is rather fast.In recent years,literatures about carbon nanotube fibers have put emphasis on the characterization of the properties and many articles on the reaction process of CVD tend to focus on the chemical mechanisms.There is a lack of detailed description and in-depth study of the gas flow field in the furnace which has important influence on fiber quality.The traditional experimental methods are expensive and time-consuming,but the simulation of the flow field in the carbon nanotube fiber reactor by computer can not only shorten the research period,but also greatly reduce the cost.This thesis take a vertical carbon nanotube fiber reactor as the research object and mainly consists of three parts.The first part is the preparatory work of numerical simulation using the Fluent software.Firstly,two two-dimensional simplified models(i.e.right-angle model and trumpet-like model)of the vertical carbon nanotube fiber reactor are constructed.Different portioning methods are used to acquire grids of good quality in the Gambit software.Then a mathematical model of flow field numerical simulation is built,identifying the flow as laminar flow at first.The core is the governing equations composed of continuity equation,momentum equation and energy equation,along with the boundary layer equation which can simplify the operation.All the work above is to lay the foundation for CFD calculation.The second part is to use the Fluent software to conduct numerical simulation of the flow field of two kinds of carbon nanotube fiber reactors.As detailed as possible,the related parameter settings are described.By solving the control equations iteratively,the distribution of velocity field and temperature field is obtained,and the differential influence of various inlet structures on the flow field is analyzed.The results have shown that the velocity fields of the two models are obviously different,while the temperature fields are similar.Meanwhile,reflux zone is found near the outlet of both flow fields,which will to some extent support the "airflow forming theory" of the "sock" structure of carbon nanotubes.The third part is to modify and optimize the flow field model,and try exploring the influence of geometric parameters on the flow field.A cylindrical inner sleeve is added in the reactor,and the change of gas properties with temperature is also taken into account.The new results show that there is a reflux zone near the outlet of the inner sleeve,which is consistent with the observed rising smoke,and can possibly explain the phenomenon that carbon nanotubes tend to accumulate in the center of the reactor.To sum up,modeling and numerical simulation of the gas flow field in the vertical carbon nanotube fiber reactor have been carried out in this thesis,which will provide theoretical support for some phenomena occurring in the process of fiber formation and lay a certain foundation for future studies on the CVD spinning of carbon nanotube fibers.