Study On Phase Change Characteristics And Particle Size Distribution Of Direct Contact Thermal Storage Material With Ejection

With people’s reflection on the current issues on energy and environment of the world,the development and utilization of renewable energy has become the consensus of countries around the world.Heat storage technology has been used as an important method to solve the problem of intermittence and fluctuation in the process of renewable energy application.Direct contact thermal storage technology has the advantages of fast thermal storage speed and high heat exchange efficiency,which is one of the research hotspots in the field of heat storage.To solve the issue that the flow of heat transfer fluid(HTF)is blocked in the charging process of traditional direct contact thermal storage,a new method that ejecting the molten phase change material(PCM)into the HTF is proposed in this paper.The experimental study on the melting and solidification characteristics of PCM has been carried out.The research works in this dissertation deepen the understanding of the ejection PCM and influence factors and rules of particle size distribution,provide evidence for optimization and promotion of direct contact thermal storage.In the experiment,sodium acetate trihydrate(SAT)was selected as the PCM and the conduction oil as the HTF.A high-speed camera system was used to photograph the broken process of SAT jet into the conduction oil,measuring and counting the particle size distribution by Image Pro Plus.Under certain conditions,changing different ejection parameters can change the particle size distribution,the experimental results show that the particle size increases with the increase of nozzle diameter and ejection temperature,the maximum variation of average particle size is 4.0 mm and 0.7 mm,respectively.However,the relationship between above particle size and ejection velocity is on the contrary,the maximum variation average particle size is 2.0 mm.Under the experimental conditions of this paper,the nozzle diameter has the greatest effect on the particle size,followed by the ejection velocity and the ejection temperature.In this paper,Normal,Log-Normal and Rosin-Rammler distribution function were used to fit the particle size distribution measured in the experiment.The fitting results show that the measured particle size distribution is reflected by these three functions and the absolute errors are not more than 10%.The Normal distribution function values and Log-Normal distribution function values are closer to the measured values,the calculated results are in good agreement with the actual particle size distribution.Relatively speaking,the error of Rosin-Rammler distribution function values is a little larger.In the charging experiment,the suspension phenomenon and melting process of broken particle in the HTF is recorded and observed by camera.The relationship between operation parameters,including inlet flow of HTF and inlet temperature of HTF are quantificationally analyzed.The results show that the suspension height of broken particle is mainly affected by inlet flow and will increase with the increase of inlet flow,and the maximum increase is 320 mm.The particle melting efficiency was positively correlated with inlet flow rate and inlet temperature,and the influence of inlet temperature was greater than that of inlet flow rate.The maximum reduction of melting time was 15 min.