Exergy Analysis And Comprehensive Heat Transfer Performance Of Supercritical CO₂ Cooled In Helically Coiled Tube

As a natural refrigerant,carbon dioxide?CO₂?has been widely applied in CO₂heat pump and air conditioning system due to its advantages of environmentally friendly and attractive thermal physical properties.In addition,helically coiled tube plays a positive role in improving the system performance of CO₂ heat pump and air conditioning system because of its compact structure and excellent heat transfer capabilities.It is necessary to investigate the irreversible loss in heat transfer process for the reasonable utilization of available energy and the improvement of system and components performance.Considering heat transfer and friction loss comprehensively,it is of great significance to find the best working condition and the optimal geometry of helically coiled tube for the optimization design of helically coiled tube heat exchanger.Therefore,in this paper,the irreversible loss and comprehensive heat transfer performance of heat transfer process in cooled helically coiled tubes with supercritical CO₂ are analyzed by experimental investigation and numerical calculation.Its working conditions are optimized based on minimizing irreversible loss and the appropriate geometry parameters of helically coiled tube are analyzed based on the performance evaluation criteria number.Experimental investigation of the cooling heat transfer of supercritical CO₂ in helically coiled tubes with constant wall heat flux is analyzed based on exergy analysis.The experiment covers a tube diameter range of 2–5 mm,a coil diameter range of 36–120mm,an inlet temperature range of 295–330 K,a mass flux range of 159.1–954.9kg/?m²s?and a Re range of 10122–87456.The influence of diameter of helically coiled tube,heat flux,pressure and mass flux on the dimensionless exergy destruction are investigated.Experimental results show that the dimensionless exergy destruction caused by the irreversibility of heat transfer is much larger than that by flow friction for supercritical CO₂.The optimal Re is more sensitive with mass flux and tube diameter rather than heat flux.For the tube diameters of 3,4,5 mm considered,there exists an appropriate range of Re that have the smaller N_I and the better exergy utilization with certain diameter.A correlation for the optimal Reynolds number as function of main dimensionless parameters related to wall heat flux,mass flux,fluid properties and geometric dimensions is proposed.These results provide helpful information for the optimal design of helically coiled heat exchanger with supercritical CO₂ based on the thermodynamic analysis approach.In order to investigate the improvement capacities of comprehensive performance of helically coiled tubes compared with straight tubes,a calculation model of PEC for evaluating the comprehensive heat transfer performance is established by taking heat transfer and friction loss into consideration simultaneously,according to the empirical correlation of heat transfer and flow fluid of cooling in helically coiled and straight tubes with supercritical CO₂.The influence of different working conditions and geometry parameters of helically coiled tubes on PEC are investigated.The results show that PEC increase as curvature increasing,and PEC reaches maximum while the curvature is 0.3 within the selected curvature range.The critical values of curvature are found under different conditions.As a heat transfer enhancement device,helically coiled tube is effective when its curvature is larger than the critical value of curvature.The tube diameter corresponding to the maximum PEC appear at the minimum tube diameter which meet the constraint conditions.According to the calculate results,the highest comprehensive heat transfer performance is realized under the condition of d=1mm,D=3.3mm,q=5kW/m²,G=800kg/?m²·s?,T_b=320K,the maximum value of PEC is 3.1268.It is expected that these investigations could provide theoretical basis for the engineering application and optimization design of helically coiled tube heat exchanger with supercritical CO₂.