Numerical Simulation Study Of Heat Transfer Characteristics Of Phase Change Materials Inside Cased Heat Accumulators And Their Structural Optimization

Phase change materials are widely used in many fields such as aerospace,building heating,communication and power,and refrigeration equipment due to their high thermal conductivity.Steam condensation is a typical phase change process.The phase change occurs when steam condenses,and the latent heat released is of great value.Steam condensation and its intensification technologies involving phase change play an irreplaceable role in energy conservation,environmental protection,energy chemistry,seawater desalination,refrigeration and air conditioning,heat storage for power generation,and weapons for military industry.The current research on steam condensation is mostly on the characteristics of steam condensation itself,but if the latent heat released by the phase change of steam condensation can be utilized,it will have a positive effect on improving the heat transfer efficiency.In this thesis,we investigate the heat transfer characteristics of steam,a phase change material,in a cased heat accumulator,and conduct a phase change heat accumulation study in combination with steam-enhanced heat transfer for the ultimate purpose of heat accumulator structure optimization.First,by establishing a physical model of the inner tube of the cased accumulator,using FLUENT software,loaded with self-programmed UDF,to explore the condensation characteristics of steam in the inner tube of the cased accumulator model,to obtain the flow characteristics of pure saturated steam and non-condensable gas containing steam and heat transfer characteristics of the law of change,the main factors affecting condensation to analyze the law of change of steam condensation characteristics.Secondly,on the basis of steam condensation research,the study of enhanced condensation heat transfer is carried out,respectively,from different methods such as structural changes in the tube wall,disturbance of the tube insert and increasing the pressure inside the tube to improve the condensation heat transfer coefficient and achieve the goal of enhanced condensation heat transfer.Then,the study of heat storage coupled with pure saturated steam as heat transfer fluid(HTF: Heat Transfer Fluid)and paraffin wax as phase change material(PCM: Phase Change Material)is explored to investigate the melting process of PCM and the effect of HTF flow heat transfer characteristics on the melting process during the heat storage process,and to compare the effect of steam and water heat transfer processes.Finally,based on the steam condensation study,a comprehensive steam condensation enhanced heat transfer method is used to optimize the structure of the accumulator and enhance the heat transfer efficiency from the perspective of steam condensation enhancement,and an attempt is made to propose a model of the accumulator based on the optimized conditions studied in this thesis.Through the above study it was found that:(1)Steam condensation characteristics,condensation heat transfer coefficient with the pure steam inlet flow rate increases,with the pure steam inlet temperature increases;condensation heat transfer coefficient with the mixed gas inlet flow rate increases,with the mixed gas inlet temperature increases,with the noncondensable gas mass fraction increases and decreases.(2)steam condensation to enhance heat transfer,change the wall structure,tube inserts and increase the pressure inside the tube three methods can effectively enhance the condensation heat transfer coefficient.Corrugated tube diameter to length ratio of 0.02,the best condensation enhancement effect,condensation heat transfer coefficient than smooth vertical tube enhanced 29.37%;the same size serrated than corrugated wall has a more effective and efficient condensation heat transfer effect.The tube insert can effectively increase the condensation heat transfer coefficient,and under the same conditions,the condensation heat transfer coefficient is 36.4% higher than that of the smooth tube.In terms of increasing the pressure inside the tube,when the pressure increases by 9KPa,the condensation heat transfer coefficient increases by 16.4%.(3)The melting rate of PCM is accelerated with the increase of vapor inlet flow rate and heat transfer temperature difference,the PCM physical parameters have a greater impact on its melting process,the vapor as HTF accelerates 55.84% compared to the liquid water melting time.(4)The melting time of the phase change accumulator increases with the increase of L/r,and the heat storage rate decreases with the increase of L/r;the melting time increases with the increase of R/r,and the heat storage rate decreases with the increase of R/r.(5)The optimal parameters obtained from the optimization of the heat accumulator under the calculated working conditions in this thesis are:L/r=8.3,R/r=3,Np=8.The work in this thesis will provide some reference significance for the flow and heat transfer characteristics of steam condensation containing non-condensable gases,strengthen heat transfer,and optimize the structure of the casing type phase change accumulator,so that steam,a typical representative of phase change materials,can further serve for production and life,and contribute to energy saving and consumption reduction through phase change.