Research On Refined Model Of Heat Transfer And Fluid Flow In Solar Updraft Power System And Its Performance Enhancement Technology

Due to shortage of fossil energy and environmental problems,it is very significant to use solar updraft power generating technology to develop clean energy sources on vast desert regions and to promote energy transformation.Mechanism of humidity affecting the power output characteristics of solar updraft power system(SUPS),enhancing performance of the sloped-collector SUPS,synergistic mechanism of divergent chimney on the performance of SUPS and its internal flow characteristics are systematically studied and analyzed.The details of research work in this dissertation are presented as follows:(1)New governing equations of the compressible moist air flow in a SUPS are derived.The expression of the pressure potential of a large-scale SUPS containing no integral is also established while the condensation of water vapor exists in the chimney.The refined model regarding the multi-phase flow and heat transfer is then developed to realize the 24-hour transient-state simulation of the operation of the power system.MATLAB programs are written to solve the developed confined model based on the typical humid climate in Wuhan city and the reference dimensions of 100 MW SUPS,the effect of humidity on power output of the system is investigated comprehensively.In a moist climate,the condensation height denoting the condensation level in the chimney is highly related to the relative humidity of the ambient air and the heat absorbed by the air current,and the condensation will improve the power output of the system effectively.With the increase of ambient humidity,the power generation will decrease firstly and then increase.The power generation can only be improved while the condensation occurs.In addition,the existence of humidity will make the power output of the system more smoothing.The humidity is an important factor while considering the construction site of SUPS in the regions with poor solar radiation.(2)The expressions containing no integral for predicting the pressure potential of the sloped-collector SUPS under various assumption of air are established based on the compressibility of air and moisture.The theoretical model of the buoyancy and the steady flow model of the compressible moist air are developed.The enhancing performance of the sloped-collector SUPS is evaluated based on the 5 MW power system.Besides,novel concept of enhancing the performance of sloped solar collector by using natural anabatic winds is put forward.While assuming that the air in the system comes from the ambient,the power output will decrease with the increase of air humidity without the consideration of condensation.The pressure potential and power output are found to gradually increase with the increase in the ambient pressure,the collector height,or the tower height,or the decrease of the ambient temperature and humidity.As for the novel power system using natural anabatic winds,the total efficiency of the plant is improved considerably from 0.63%without heat input from the lower bare mountain slope to 1.79%with the heat input from the lower bare mountain slope five times as large as the collector area.The novel system is effective in enhancing the power generation performance.(3)Based on the Boussinesq assumption and considering the internal flow as fully developed turbulent flow,three-dimensional steady CFD simulations of flow behaviors in divergent chimney of various shapes are carried out.The recovery coefficient of the pressure potential regarding the divergent chimney are proposed.The influence of the shape parameters on the output performance and flow structures are studied.For the chimney with specified aspect ratio,there exists the optimal chimney outlet-inlet area ratio to obtain the maximum output.When the outlet-inlet area ratio is too large,the boundary layer will separate from the chimney wall which leads to occurrence of stall and backflow.The pressure loss caused by the block of flow passage by backflow and the heat loss caused by the cold ambient air are the two main reasons that are responsible for the decrease of pressure potential.The stall in the divergent chimney is the fully developed stall under the steady and fully developed turbulent flow.Large fixed vortex forms near the divergent chimney wall due to the backflow caused by the stall,the dimension of the vortex is determined by the backflow intensity,and the ratio of block area to chimney section area is an important factor measuring the backflow intensity.The Reynolds number of the internal flow in the chimney is large,and the backflow characteristics mainly rely on the shape parameters of the divergent chimney.