Numerical Study On The Turbulent And Heat Transfer Of Low Thermal Conductivity Thermal Storage Materials

Renewable energy sources such as solar energy and wind energy are subject to fluctuations in the season.In order to ensure the smooth operation of the power grid and improve the utilization rate of renewable energy,it is of great engineering value to properly carry out thermal energy storage.Recently,peppled bed sensible thermal storage systems have been widely studied and applied due to their simple operating principle and low cost.However,due to the low thermal conductivity of the thermal storage material,which affects the thermal storage efficiency of the system,the turbulent and heat transfer mechanism of the low thermal conductivity thermal storage material and its internal exergy transfer and entropy generation characteristics are studied in depth,and the operating parameters of the thermal storage system are determined.The improvement of thermal storage efficiency is of great significance.Because the structure of the thermal storage medium and the structure of the thermal storage tank are very different,the porous media model is used to study the convective heat transfer characteristics inside the thermal storage system.In order to study the flow and heat transfer mechanism inside the porous media,a two-dimensional periodic array arrangement model was established.For the circular,square,longitudinal elliptical and transverse elliptical structures,ten sets of geometric structures were selected in the range of porosity?=0.64~0.87,Re=100~20000 and two turbulence models RNG k-εand SST k-ωwere selected for comparison.The average Nusselt number of solid wall and the variation of flow velocity,turbulent kinetic energy,pressure drop,temperature and heat transfer efficiency in the flow field are analyzed under the conditions of Reynolds number and structure change.The results show that the results of the SST k-ωmodel are more reliable when Re<1000.When Re>1000,the results of the two turbulence models are similar.Among the ten sets of geometries studied,a square structure with?=0.64 has greater turbulent kinetic energy,pressure drop,and surface average Nusselt number.When the heat transfer efficiency is used as the evaluation standard,the lower Reynolds number and the higher porosity are favorable for the flow heat transfer between the systems.After that,the porous media model is introduced into the thermal storage system,and the thermal storage system is simplified into a three-dimensional staggered sphere array arrangement model.Ten sets of working conditions were selected in the range of inlet flow velocity 0~1m/s,Re=122~1220,and Pr=0.7,1.0,1.3.The average Nusselt number,drag coefficient and pressure drop of the solid wall in the flow field are analyzed with the Reynolds number and the Prandtl number,and compared with the empirical formula.The results show that with the increase of Reynolds number and Prandtl number,the average Nusselt number of the solid wall increases,the pressure drop of the flow field increases,and the resistance coefficient of the flow field first decreases and then stabilizes.The final fitting yields the correlation of the parameters of the model.In order to analyze the thermal storage performance of the system,the exergy transfer and entropy generation characteristics of the three-dimensional model of the thermal storage system are analyzed from the perspective of the second law of thermodynamics.The average exergy transfer Nusselt number Nu_e,the entropy generation number N_s and the variation of the MF function with Re number,and sphere diameter d are calculated.The results show that Nu_e decreases as the Re number increases,and its value decreases from a positive value to 0 and then to a negative value.When Nu_e=0,there is a critical Reynolds number,which increases with the increase of d,which is the upper limit of the Reynolds number of the system operation under the exergy transmission evaluation angle.The number of entropy generation N_s decreases first and then increases with the increase of Re number,and the variation range tends to be flat with the increase of d.The critical Reynolds number is the optimal operating Reynolds number.In the range of the diameter d=10~40mm,when Re<Re’,the smallest diameter sphere medium in the allowable range is selected for thermal storage system,which the efficiency is the highest.When Re>Re’,the maximum diameter sphere medium within the allowable range is selected,which is the most efficient.