| As the crucial heat-exchange equipment in industrial production,shell and tube heat exchangers are widely applied in fields of petroleum refinery,power generation,metallurgy,and so on,and their performance directly affects the efficiency of energy utilization.However,in the process of practical application the heat transfer intensity is lower and the heat transfer surface is seriously fouled,which has resulted in large amounts of energy waste and brought great losses to the related enterprises.In this regard,a variety of heat transfer enhancement technologies,such as finned tube,corrugated tube,wire coil,vortex generator,twisted tape and assembled rotors,have been developed by domestic and foreign scholars in order to improve the heat transfer capability and restrain the deposition of fouling.Because of the dual characteristics of heat transfer enhancement and online self cleaning,assembled rotors possess a broad application prospect.However,due to the lack of understanding of the basic characteristics like rotation and disturbance,and the lack of research on heat transfer enhancement and anti-fouling performance,the application has been greatly limited.The research progress of the various heat transfer enhancement technologies at home and abroad was summarized in this thesis,and the shortage of research on their anti-fouling performance was pointed out;then the assembled rotors was taken as the research object,and the basic characteristics of rotation and disturbance and the heat transfer enhancement and anti-fouling performance were deeply studied by means of experiments and numerical simulations,in order to lay a theoretical and technical foundation for the industrial application of assembled rotors.In addition,by combining the unique advantages of heat transfer enhancement and self cleaning of assembled rotors in shell and tube heat exchangers,a scheme that applying it to cultivate microalgae in tubular photobioreactor was creatively proposed,and a preliminary study on the performance optimization of tubular photobioreactor with assembled rotors was carried out in order to improve the inherent poor mixing performance and eliminate the attached growth of microalgae on the tube wall,and eventually improve the cultivating efficiency of microalgae.The research work in this thesis is as follows:(1)Research on the basic characteristics of assembled rotorsThe principles and methods of rotational speed measurement were introduced,then a device of rotational speed measurement for assembled rotors was developed based on the principle of period method,and an experimental table was built.The effects of the axial position,the flow rate,the lead of the rotor,the diameter of the rotor and the blade number of the rotor on rotational speed were investigated,and experimental correlations of the rotational speeds of various rotors were acquired through nonlinear surface fitting.The PIV(Particle Image Velocimetry)technology and its working principle were introduced next,and a PIV experimental apparatus was built.The flow fields in circular tube with assembled rotors were experimentally studied by the PIV technology,and the velocity fields on both transverse and longitudinal sections were obtained.The changes of turbulence intensity and radial velocity in the tube were analyzed,and the effects of the lead and the blade number of the rotor on the flow fields were compared.Finally,the enhancement mechanisms of heat and mass transfer and mixing in circular tube with assembled rotors were summarized based on the flow fields in the tube.(2)Research on the heat transfer enhancement and anti-fouling performance of assembled rotorsThe experimental apparatus for heat transfer enhancement was verified accurate with the empirical equations of Gnielinski and Filonenko,then the experiments on heat transfer enhancement and friction characteristics of the new helical two-blade rotors with holes,circular grooves and cuts in turbulent region were carried out.The Nusselt number(Nu),the resistance coefficient(f),and the comprehensive evaluation factor(PEC)with the Reynolds number(Re)were acquired,and the comprehensive heat transfer enhancement of the new helical two-blade rotors was analyzed.The numerical simulation methods,especially the simulation methods of turbulence,were introduced next,and then the deposition process of CaCO3 crystallization fouling in the tube with helical two-blade rotors and the smooth tube were numerically simulated with the software ANSYS FLUENT.The accuracy of the simulation method was verified.The deposition rate,the removal rate,the net rate and the thermal resistance of the fouling with time in the tube with helical two-blade rotors were acquired,and the mass transfer enhancement and anti-fouling characteristics of helical blade rotors were revealed.(3)Research on the application of assembled rotors in microalgae cultureThe flow fields and the trajectories of microalgal cells in the tube with helical blade rotors and the smooth tube were numerically simulated with the software ANSYS FLUENT,and the mixing and the light characteristics of microalgal cells in the tube with helical blade rotors were analyzed.Then an experimental apparatus of tubular photobioreactor was built,and the cultivation experiment of Chlorella sp.was conducted.The effects of helical blade rotors on the cultivation of Chlorella sp.in tubular photobioreactor were investigated.To sum up,the basic characteristics of rotation and disturbance and the heat transfer enhancement and anti-fouling performance of assembled rotors were systematically studied in this thesis,with the aim of providing theoretical and technical reference for the industrial applications of assembled rotors.Also a preliminary study on the application of assembled rotors in microalgae culture was carried out,so as to optimize the performance of tubular photobioreactor,improve the cultivating efficiency of microalgae,and eventually provide a new method and possibility for the large-scale culture of microalgae and the utilization of microalgae biomass energy. |
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