Numerical Study On Cooling Heat Transfer Characteristics Of Supercritical CO₂ In Spirally Grooved Tube

At present,with the rapid development of the global economy,the consumption of energy has increased dramatically,and environmental pollution problems such as greenhouse effect and smog have become increasingly prominent.Therefore,energy conservation and emission reduction draw global attention.In order to promote China’s green development to a new level,the CPC Central Committee has put forward the strategic goal of"peak carbon dioxide emissions and carbon neutralization",which is the inherent requirement of the great rejuvenation of the Chinese nation and the objective need of the sustainable development of mankind.In the field of refrigeration,despite favorable thermodynamic properties,traditional chlorofluorocarbon（CFCs）and hydrochlorofluorocarbon（HCFCs）refrigerants may also result in severe greenhouse effect upon discharge into the environment.Therefore,new types of environmentally friendly refrigerants are required to replace the traditional refrigerants.CO₂is an environment-friendly natural refrigerant with stable chemical properties.If CO₂,an industrial by-product that is originally discharged into the air,is recycled and used as working medium for heat transfer or refrigeration,it will achieve the most direct"emission reduction",and also bring the effect of"energy saving"due to favorable thermophysical properties.Due to its excellent heat transfer property,supercritical CO₂has been widely used in heat exchanger（mainly gas cooler）of heat pump water heaters.In order to enhance the heat transfer effect,spirally grooved tube is being gradually developed and applied.Therefore,it is necessary to conduct detailed study on the heat transfer characteristics for supercritical CO₂flowing in spirally grooved tubes.At present,this topic has been studied on the assumption of supercritical CO₂flowing outside the spirally grooved tube in both simulation and experimental studies,and the characteristics of supercritical CO₂flowing inside the tube are not involved.Therefore,in this paper,CO₂is injected inside and outside the spirally grooved tube to flow,the heat exchange characteristics of the two flow modes are simulated under different working conditions by CFD and compared carefully.The main works and conclusions are as follows:（1）Firstly,the model is established,and then the reliability of the simulation calculation is verified based on the experimental data published in the literatures.Secondly,supercritical CO₂is injected into the inner tube and outer tube of spiral heat exchanger to flow respectively.It is found that the average heat exchange coefficient for flowing in the inner tube will be slightly smaller than that for flowing in the outer tube by comparison of the simulation results.From the perspective of the pressure on the outer wall,the pressure on the outer wall can be effectively reduced when supercritical CO₂is injected into the inner tube to flow.At this time,the materials of the outer tube are less demanded and more selective,even if they remain unchanged,the thickness of the tube wall can also be greatly reduced.At the same time,as hyperthermia CO₂is injected into the inner tube to flow,the unnecessary heat dissipation of the whole double-tube heat exchanger will be reduced,and the energy utilization rate of hot water produced will be effectively improved.Therefore,there are many benefits for supercritical CO₂flowing inside the inner tube,such as the enhanced safety,costing saving,reduced heat dissipation and excellent economic feasibility.（2）The supercritical CO₂ is injected into the inner tube for flowing,subsequently the heat transfer performance of the supercritical CO₂in cooling condition is compared under various inlet pressure and flow rate.The study shows that the overall heat exchange coefficient will increase with increasing pressure at constant inlet flow rate;meanwhile at constant pressure,the overall heat exchange coefficient will also increase with the increased flow rate.The maximum heat exchange coefficient appears near the pseudocritical state point.（3）Multiple types of spirally grooved tubes including triple,quadruple and sextuple are used for studying the cooling heat exchange characteristics.In addition,grooves in two structures are designed,including vee groove and trapezoidal-groove.The heat exchange performance is studied by simulation for the supercritical CO₂flowing respectively in the inner and outer tubes in the same cooling boundary condition.It is found that the primary influence of the rotating flow in spirally grooved tube on the heat exchange characteristics of supercritical CO₂is ascribed to the synthetic action of strong Coriolis force along groove walls,the buoyancy lift of fluid and the gravity.The more number of the spiral trough is,the stronger rotation will be,the influence of buoyancy lift will reduce;（4）In overall consideration of the influence of heat transfer coefficient,pressure drop,heat transfer area and mass flow rate,a new comprehensive evaluation factor is put forth in this paper.It is clear by comprehensive evaluation with the evaluation factor above that the supercritical CO₂shows higher comprehensive property when flowing in the inner tube than that when flowing in the outer tube and the optimal comprehensive property when flowing in the inner tube of the triple spirally grooved tube.（5）In view that when changing the flowing space,different cross sectional areas of the inner and outer tubes will lead to varied mass flow rates of working medium before and after changing the flowing space,and with an aim of accurately reflecting the influence of changes on density of heat flow on the heat exchange characteristics of the supercritical CO₂in two different flowing modes,change the diameter of outer sleeve first to get the same cross sectional area of the inner and outer tubes,thereby ensuring the same mass flow rate under the same flow rate before and after the working medium changes the flowing space.The cooling heat exchange characteristics of the supercritical CO₂flowing inside and outside the spirally grooved tube under three different densities of cooled heat flow-20,000W/m²,-40,000W/m²and-60,000W/m²are compared carefully.The study reveals that with the same cross sectional area of the inner and outer tubes,the local heat exchange coefficient tends to increase with the increased density of heat flow.Nevertheless,in indicates in overall consideration of the heat exchange coefficient and on-way resistance that the supercritical CO₂shows the optimal comprehensive property when flowing in the inner tube with the density of heat flow of-40,000W/m².