Experimental Research On The Heat Transfer Characteristics Of Air-sand Heat Exchanger

The continuous reduction of traditional fossil energy reserves and the deteriorating environment has caused the rapid development of various new energy technologies and the continuous increase in the proportion of global energy used.Solar thermal power generation is relatively mature in various new energy sources,but there is still the problem of instability of supply.Therefore,the development of energy storage and heat storage technology that improves its utilization is receiving more and more attention.Based on the research of the heat storage system in solar thermal power generation,sand was used as the heat storage material,and the hot air that had absorbed solar energy were used to simulate the heat source,and the heat exchange process of the gas-solid two-phase is mainly studied in this paper.This research not only has an in-depth grasp of the heat storage capacity of sand and the air-sand heat transfer performance,but also it obtains the gas-solid heat transfer coefficient correlation formula that is suitable for this research,which lays a foundation for strengthening heat transfer and giving full play to the heat storage performance of sand.First,this paper analyzes the air-sand two-phase heat transfer method and their contact form in the moving bed heat exchanger and based on heat and mass transfer and porous media theory,appropriate assumptions and simplifications are made on the actual heat transfer process,the mathematical model of the heat transfer temperature field was derived and established.Then,according to the research content of the subject,an air-sand moving bed heat exchange experiment platform was designed and built.Finally,We used the platform to conduct multiple experiments and study the influence of different factors on heat transfer process,and analyze the experimental data.The analyses of the experimental results were drawn as follows:（1）The main factors affecting the two-phase heat transfer in heat exchanger bed are airflow,sand flow,and sand particle size.The influence of airflow and sand flow on heat transfer is significant,and the change of sand particle size has little effect on heat transfer.Under certain working conditions,the solid-phase efficiency of the sand after heat exchanged can reach more than 0.9,indicating that the structure of this air-sand moving bed heat exchanger can exert the heat storage performance of sand,and it is feasible that the bed is applied to tower solar heat storage system.（2）With the increase of airflow,the gas-solid heat transfer coefficient also increases;with the increasing of sand flow,the gas-solid heat transfer coefficient also increases;with the increasing of sand particle size,the gas-solid heat transfer coefficient decreases instead.Besides,the heat transfer intensity is different at different positions in the heat exchanger,the heat transfer coefficient varies from 6to 16 W/（m²·K）,and the heat transfer coefficient gradually decreases with the direction of airflow.（3）Based on the calculation and analysis of the experimental data,the experimental correlation formula of gas-solid heat transfer coefficient that is suitable for the experimental conditions is deduced by using linear regression fitting,the experimental correlation formula is Nu=0.75ε^0.8Re^0.65Pr^1/3,the air Re of the correlation in this experiment varies in the range of 7.81～18.07,and the variation of Pr is in the range of 0.674～0.683.The average error between the experimental value and the calculated value of the fitted correlation is 2.12%.