Numerical Study On Heat Transfer Characteristics Of Microencapsulated Phase Change Material Slurries In Helical Corrugated Tube And Tube With Interts

Microencapsulated phase change material slurries(MPCMs)is a kind of liquid slurry consisting of encapsulated phase change material particles and carrier fluid in a certain proportion.In the early stage,due to its strong latent heat capacity and considerable phase change heat,it was used as a regenerative temperature control regulation to effectively improve energy efficiency.In recent years,MPCMs are becoming more and more widely studied in the field of heat transfer.The phase change materials are abundant and diverse,mainly give priority to the heat transfer of organic polymer MPCMs.However,the coefficient of thermal conductivity of organic polymers is small,with the increase of slurry concentration,the adverse heat transfer factors caused by the small thermal conductivity of seriously affect the Convection heat transfer between fluid and wall.Therefore,solving the unfavorable heat transfer problems caused by the low coefficient of thermal conductivity of MPCM is of great significance for the wide application and promotion of MPCMS.In this paper,MPCMs was used as the working fluid to study the convective heat transfer of the slurry in helically corrugated tube and the tube with twisted tape inserts.The helically corrugated tube adopts the internal concave helically corrugated tube.The existence of grooves greatly enhances the convective heat transfer of the fluid near the wall surface,which can transfer the heat to the core area of the fluid flow faster.The width of the twisted tape is equal to the inner diameter of the tube,which not only promotes the disturbance of the fluid,but also acts as fins in the tube,thereby enhancing the convective of the fluid and wall heat transfer in the tube.In the flow analysis of MPCMs in the helical corrugated tube,the fluid inlet temperature is 299 K,and it can enter the phase change state soon after it is heated in tube.The study of convective heat transfer in helical tube is mainly carried out from the perspectives of Re number and slurry concentration c,and the fluid flow in smooth tubes under the same conditions is taken as the comparison.The results represent that the helical tube has a promoting effect on the convection heat transfer of MPCMs.Compared with the flow of the slurry in the smooth tube,the existence of the groove on the wall surface of the helical tube can increase the radial temperature gradient of the tube and improve the convection of fluid in the central area and enhance the heat transfer of fluid near the wall,so that the heat near the wall can be transferred to the central area of the fluid more quickly.In the smooth tube,the Nu* of 20% concentration slurry is 0.54 times higher on average than that of 10%concentration slurry,and in the helical tube flow,the average increase of Nu* is about 1.40 times.The Nu* of 20% concentration slurry in the helical tube is 4.57 times higher than that of 20% concentration slurry in smooth tube,indicating that the helical tube had a significant effect on the enhanced heat transfer of MPCMs.In the flow analysis of MPCMs in the smooth tube with twisted tape inserts,the flow heat transfer of MPCMs was studied under different Reynolds number,different slurry concentration,and different twist rate parameters of twisted tape.The results showed that when γ=4 in the insert tube,the 20% concentration of the slurry Nu* increased on average by 0.87 times than the 10% concentration slurry;The Nu* of 20% concentration slurry in tube with twisted tape inserts is 1.99 times higher than that of 20% concentration slurry flow in the smooth tube,and the heat transfer enhancement effect is better.It is found that the two types of tubes have an enhanced effect on the heat transfer of slurry flow in the tube,especially the higher the concentration of slurry,the better the heat transfer enhancement effect,because compared with the slurry flow in the smooth tube,the two types of tubes make the latent heat capacity of the slurry phase change as much as possible,and improve the heat transfer effect.Different from the phase distribution of the slurries in smooth tubes and helical corrugated tube,when the slurries in the inserted torsional zone enters the liquid state region from the phase transition zone,the fluid near the central area of the tube first enters the liquid state region,and the liquid state region is entered at the latest near the wall.The heat transfer enhancement of the twisted tape to the tube is mainly realized by the "finned" effect of the twisted tape.At low Reynolds number,the heat transfer enhancement effect of spiral corrugated tube and tube with twisted tape inserts is similar to that of integrated heat transfer enhancement,but with the increase of Reynolds number,helically corrugated tube has better heat transfer enhancement effect and comprehensive heat transfer enhancement performance.