Research On Heat Transfer Enhancement Of A Horizontal Casing Phase Change Unit

The current research on horizontal casing phase change units at home and abroad mainly focuses on heat storage conditions.The effect and strengthening of the natural convection of phase change materials during melting conditions were studied especially.Scholars at home and abroad have proposed and confirmed that the eccentric structure is a very effective heat transfer enhancement measure,but there is no in-depth study on the strengthening mechanism of phase change heat transfer.At the same time,there are relatively few comprehensive studies on heat storage and heat release conditions.The research on the comprehensive heat transfer enhancement of the structure is very meaningful.This paper theoretically analyzes the phase change heat transfer mechanism of a horizontal casing phase change unit,and further deepens the simulation study of heat transfer enhancement of the eccentric structure.The main contents are as follows:（1）Selection of phase change heat storage materials:this paper introduces the chromatographic analysis method into the priority sorting of phase change materials,and takes the phase change enthalpy,thermal conductivity,specific heat,supercooling degree and price as evaluation indexes.For medium temperature phase change heat storage materials with a phase change temperature of 100℃-250℃,maleic acid,Mg Cl₂·6H₂O,49%Na NO₃+51%Li NO₃ and 87%Li NO₃+13%Na Cl were selscted as alternative phase change materials.By calculating the single-layer ranking and total ranking results under the two weights of performance-oriented system and performance-price-oriented system,the comprehensive performance score of each material can be obtained.When selecting phase change materials,the performance-price-oriented system is the main judgment system.Maleic acid and Mg Cl₂·6H₂O have the highest comprehensive score.Considering that maleic acid is more acidic and more corrosive,Mg Cl₂·6H₂O is finally selected as phase change heat storage material in this paper.（2）The mechanism of phase change heat transfer analysis:in the process of phase transition,the effect of natural convection velocity field and temperature field exist at the same time,making the phase change heat transfer process of solving more complex.In view of the phase change heat transfer process,the energy equation of the solid phase and liquid phase trace were established,the boundary layer theory was proposed,and the integrand function with universal significance was proposed by dimensionless parameters.Finally,it is found that improving the Angle between velocity gradient and temperature gradient and guiding the direction of natural convection velocity are very effective ways to improve the phase change heat transfer of phase change materials.The Solidification/Melting model of numerical simulation is elaborated in detail.（3）Enhanced heat transfer analysis of eccentric structure:the melting and solidification processes of horizontal casing phase change units with eccentricity of 0,0.2,0.4,0.6,0.8 and 0.933 were simulated with Ansys fluent software.The simulation results showed that:The eccentric structure enhanced the heat transfer of Mg Cl₂·6H₂O in the horizontal phase change unit.For the melting condition,when the heating temperature T_wis 150℃and the eccentricityεis 0,0.2,0.4,0.6,0.8 and 0.933,the melting time decreases by 36.1%,54.4%,73.9%,82.3%and 83.2%respectively.Considering only the melting process,when T_wwas 130℃,140℃,and 150℃,the optimal eccentricity was 0.8,0.8,0.933 respectively.For the solidification condition,eccentricity has an inhibitory effect on the heat transfer of Mg Cl₂·6H₂O in the horizontal phase change unit.When the condensation temperature T_w is 30℃and the eccentricityεis 0,0.2,0.4,0.6,0.8 and 0.933,the solidification time increases by 28.2%,86.4%,134%,185.7%and 222.5%respectively.Considering the melting and solidification conditions,the optimal eccentricity was 0.2.（4）Heat transfer enhancement analysis of the eccentric structure with fins:the melting and solidification processes of the horizontal casing phase change unit with eccentricity of 0.2 for three different fin combinations were simulated with Ansys fluent software,and the simulation results showed that:The fin structure enhanced the heat transfer of Mg Cl₂·6H₂O in the horizontal phase change unit.For the melting condition,when n=5,7,9,melting time decreases by 65.1%,63%and 70.7%respectively,and the effects of strengthening heat transfer can achieve higher heat transfer enhancement of eccentricity of the eccentric structure.For the solidification condition,when n=5,7,9,coagulation time decreases by23.8%,45.7%and 52.2%respectively,and the effects of strengthening heat transfer even far beyond non-eccentric structure of the heat transfer effect.Considering the melting and solidification conditions,the optimal fin combination is n=9.