| The pollution caused by fossil energy burning makes the global environmental problems increasingly prominent.The development and utilization of clean and efficient renewable energy is an effective way to reduce environmental pollution and sustainable development.Among them,solar thermal utilization technology is a hot research topic at home and abroad.Solar heat absorber is the core component of photothermal conversion in tower solar thermal power generation system,and its performance directly affects the thermoelectric conversion efficiency of the whole system.At present,the research of heat absorber mainly focuses on how to improve the structure of heat absorber to improve the flow and heat transfer characteristics of working medium,so as to obtain high temperature medium and improve the thermal performance of heat absorber.In this paper,the particle flow and heat transfer characteristics in the internal circulating fluidized bed solid particle absorber are studied.The flow and heat transfer characteristics of particle phase in the recirculated fluidized bed particle absorber with built-in drainage tube are studied by numerical simulation.A model of gas-solid two-phase flow in a solid particle solar absorber in an internal circulating fluidized bed was developed based on computational particle hydrodynamics.The drag model and Euler-Lagrange method are used to describe the gas and particle flows,and the interaction of the radiation field in the radiation source and heat absorber is described by the radiation model and heat transfer coefficient.The unsteady gas-solid two-phase flow was calculated by numerical simulation,and the heat transfer characteristics of gas-solid two-phase flow in the heat absorber were analyzed.In addition,on the one hand,a two-dimensional model was established to describe the flow and heat transfer process of particles in the internal CFB heat absorber in detail,and the specific flow patterns and heat transfer process(particle volume fraction,particle temperature,gas temperature,radiation amount)of particles and gases in the heat absorber were analyzed.On the other hand,a three-dimensional model was established to study the performance of the solid particle heat absorber in the internal circulating fluidized bed,and the variation rules of gas temperature,thermal efficiency,absorption efficiency and total solar energy conversion efficiency at the outlet of the heat absorber were analyzed under different factors.The results show that the forced recirculation system at the bottom of the heat absorber strengthens the circulation effect of gas-solid two-phase flow,prolonging the residence time of gas in the heat absorber,and promoting the convective and radiative heat transfer between particles and gas.The plane(z=0.27m)near the bundle port of the built-in bundle port drainage tube in the heat absorber is greatly affected by the drainage tube bundle port.Some particles will hit the wall and bounce down when they rise to the drainage tube bundle port,resulting in the aggregation of particles in the drainage tube.It is found that with the increase of inlet gas mass flow rate,the outlet gas temperature decreases and the total solar energy conversion efficiency decreases,while the thermal efficiency and absorption efficiency increase.The larger the aperture at the top of the heat absorber leads to the increase of reradiation loss,which further affects the working performance of the heat absorber.Therefore,the aperture size is 200 mm.With the increase of particle volume fraction and recirculation rate,the air temperature and thermal efficiency at the outlet of the heat absorber firstly increase and then decrease.When the particle volume fraction exceeds 0.006% or the recirculation rate exceeds 0.4kg/s,the air temperature and thermal efficiency at the outlet of the heat absorber decrease.Therefore,there exists an optimal particle concentration and recycling rate corresponding to the maximum thermal efficiency. |
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