| For the solar thermal power generation and industrial heat process,compared with the traditional solar heat collection system using heat transfer oil as the intermediate medium,parabolic trough collector system with direct steam generation(DSG)is of high efficiency,low cost and no pollution,and thus is a type of solar heat collection technology with good development potential.Due to the intermittent and random nature of solar irradiation,working conditions during the operation frequently exhibit large changes in the DSG parabolic trough collector system,the nonlinear characteristics of the system are very remarkable,and as a result the system control problems are very complicated.Establishing an effective mathematical model and mastering the dynamic characteristics of the liquid phase fluid during the preheating and phase-changing process in the system are important prerequisites and foundations for the design and control of the DSG parabolic trough solar collector system.Domestic and international scholars have carried out fruitful research on DSG parabolic trough collector system and its operation characteristics.For the modeling and dynamic characteristic simulation of the DSG process in parabolic trough solar collector system,studies are generally based on the spatial control volume modeling that belongs to Eulerian method,and therefore the fluid parameter control equation and corresponding numerical model in absorber tube are established.However,in DSG collector system,as the conditions(especially solar irradiation)frequently change sharply during the operation,the position of the boundary between liquid region and two-phase region may move frequently and conspicuously as well as the one between two-phase region and superheated region.Consequently,making use of the above modeling schemes based on control volume,it is necessary to build corresponding moving boundary models for single-phase region and two-phase fluid region respectively,which not only makes the model and the related algorithm lack necessary generality,but also makes the model’s numerical stability be difficult to be guaranteed if the system operating conditions change dramatically.On the basis of the above issues,in this thesis,in order to establish a general model for DSG parabolic trough collector system,the process mathematical description,numerical analysis and simulation method,and the practical application are studied systematically and comprehensively.The main research work and achievements of this thesis are listed as follows:(1)In view of the problems existing in modeling the DSG collector system,based on the idea of Lagrangian fluid parcel-tracking(FPT),a general one-dimensional distributed parameter model of the fluid heated and phase transition process in the DSG collector system is established.A unified mathematical description of the change law of fluid thermodynamic parameters of the single-phase and two-phase fluid in the collector is constructed,which is a unified and easy-to-calculate model to describe the transient distributions of fluid temperature for single-phase region and of steam quality for two-phase region.Therefore,a general one-dimensional distributed parameter model is established for each phase(such as liquid,two-phase fluid and superheated steam)of the working medium preheating and phase transformation process in the DSG collector system.In addition,because the Lagrangian FPT idea is adopted in discribing the transient distribution of thermodynamic parameters in each region,the temporal and spatial coordinates for the fluid show a one-to-one correspondence.Therefore,the integrated description of the thermodynamic parameter distribution of the working fluid is successfully realized in both the dynamic and the static process of the DSG collector system.(2)Based on the FPT idea,an FPT calculation scheme corresponding to the above-mentioned general distributed parameter model is developed,and thus a general dynamic calculation method for fluid preheated process and phase transition process in DSG collector tubes is proposed.The transient distribution of fluid thermodynamic parameters and the transient position of fluid phase transition interface are determined through the FPT calculation.With the help of the existing static and dynamic experimental results,the reliability of the above model and FPT calculation method are verified.Numerical simulation results demonstrate that the proposed general distributed parameter model and the developed FPT calculation scheme not only have the good versatility,but also can reflect the disappearance and regeneration process of the superheated steam region and the two-phase region in the DSG collector loop under conditions with large direct normal irradiance disturbance.Therefore,to some extent,the model and FPT scheme provided in this thesis should be of practical value to analysis both the dynamic and static characteristics of the DSG collector system,as well as the design and evaluation of the corresponding control system.(3)Using the modular modeling method,the dynamic models of the relevant components(such as separator,valve,water spray desuperheater,etc.)of the DSG collection system are established,and the overall model of the DSG collector system is constructed.Taking the DISS test facility as the object,under typical disturbance conditions,the dynamic characteristics of the DSG collector system are studied both in once through mode and in recirculation mode,and the response characteristics of the outlet parameters in the DSG collector system under the two typical operating modes are also briefly analyzed.(4)Based on the aforementioned general distributed parameter model,the hydrodynamic characteristic model of the DSG collector system is established,and the hydrodynamic characteristics are studied.The effect of operating parameters such as direct solar irradiation,inlet water temperature and working pressure on the hydrodynamic characteristics of the system are studied as well as that of structural parameters including the loop length and the absorber inner diameter.The thermal and hydraulic deviation characteristics between parallel loops of DSG collector system are studied,in which the flow mass deviation and thermal deviation characteristics of parallel loops are analyzed when the direct solar normal irradiation is unevenly distributed between the loops.The general distributed parameter model and FPT algorithm proposed in this thesis not only realize the unified calculation of the single-phase and two-phase fluid thermodynamic parameters in the DSG collector system but also realize the integrated dynamic and static calculation of the DSG collector system,which not only enriches the modeling method of the DSG collector system,but also has a pretical value for the initial design of DSG collector system,the real-time analysis of dynamic characteristics,and the design and assessment of DSG system controller.In addition,the established model and FPT calculation method can also be extended to study the dynamic and static characteristics of other thermal systems or thermal equipment. |
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