Investigation Of Heat Transfer Characteristics And Mechanism Of Supercritical CO₂/Propane Mixtures

The national goal of"carbon peak in 2030 and carbon neutral in 2060"puts forward higher requirements in terms of energy saving and efficiency improvement,clean energy production and the development of renewable energy.In coal-fired power plants,chemical and metallurgical fields,a large amount of heat is directly exhausted into the environment in the form of low-grade waste heat.Waste heat resources account for about 17%to 67%of its total fuel consumption.The low-grade waste heat recovery and utilization technology of transcritical CO₂ Rankine cycle has become a frontier research hotspot in various countries.The core difficulty of the transcritical CO₂Rankine cycle system is the design of the compact gas heater,because the drastic change of thermo-physical properties near the pseudo-critical region leads to the complex boundary layer structure in the near-wall,which leads to the abnormal behavior even the deterioration of heat transfer.Therefore,it is of great significance to develop a supercritical CO₂ heat transfer deterioration model,to put forward a buoyancy force criterion,and to establish the relationship between the microscopic mechanism and the macroscopic heat transfer characteristics for promoting the practical application of CO₂ and its mixed working medium in the transcritical Rankine system,and to improve the circulation efficiency of the transcritical Rankine system.In this paper,the heat transfer characterisitcs of supercritical CO₂ and its binary mixtures heated in the vertical tube with upward/downward flowing,horizontal tube and helically coiled tube are investigated by experiments,numerical simulations and theoretical analysis.The experimental conditions cover pressures 7.5?9 MPa,mass flow rates 80?1120 kg/(m²·s),heat fluxes 10 k W/m²?200 k W/m².Based on experimental data,by considering the comprehensive influence of thermo-physical properties gradient,thermal acceleration,buoyancy effect and geometric structure factors on heat transfer,the semi-empirical correlations of supercritical CO₂ in heating vertical tube,horizontal tube and vertical helically coiled tube are established respectively.The predicted values of the semi-empirical correlations are respectively compared with the experimental data,and the results show that the predicted values are in good agreement with the experimental data that enables more extensive working conditions of applicability and higher accuracy.In the vertical-downward flow,the heat transfer deterioration is only reported in forced convection in previous studies.It is found for the first time in this paper that the heat transfer deterioration occurs in vertical pipe downward mixing and forced convection.The differences of heat transfer characteristics between upward and downward flowing in heated-vertical tube are compared and analyzed by experiments.The deficiency of the classical shear stress distribution model for the heat transfer deterioration is revealed.In this paper,a new model of heat transfer deterioration is built is proposed to consider the thermal acceleration in the boundary layer based on the change in velocity profile,involving the convection term in the differential equation of motion in the boundary layer,.The new model can predict the occurrence of heat transfer deterioration more accurace than the classical model.In the vertical-donwward flow,the heat transfer deterioration is induced by the restratification of turbulence,resulting from the thermal acceleration in the boundary layer.Based on the heat transfer characteristics and mechanism of straight pipe,the influence of buoyancy force and curvature effect on the flow and heat transfer characteristics of helically coiled tube are investigated.A buoyancy criterion for helically coiled tube is established based on the influence of geometrical structures on flow characteristics by the law of energy conservation.A new flow regime map is drawn based on the new buoyancy force criterion and flow characteristics in the helically coiled tube.On these bases,the heat transfer and flow characteristics of supercritical CO₂heated in vertical pipe,horizontal pipe and vertical helically coiled tube are compared by experiments and numerical simulations.It is proposed that the coupling relationship between buoyancy and flow characteristics should be considered in the formulation and application of buoyancy criterion.The results show that the heat transfer deterioration in vertical tube is more serious than that of horizontal tube and helically coiled tube.In general,helically coiled tube has obvious advantages in terms of local and average heat transfer performance,especially under the condition of strong buoyancy force,and the scope of heat transfer enhancement by helically coiled tube is determined(?>2 and?<0.5).Finally,the heat transfer characteristics of supercritical CO₂/propane binary non-azeotropic mixtures in helically coiled tube are experimentally studied.The heat transfer performance of pure working fluid and mixed working fluid under the same working condition is compared.The experimental results show that under the same pressure ratio(p/p_cr),when the mass fraction of propane is in the range of22.24%-100%,the peak value of heat transfer coefficient increases with the increase of propane.The correlation proposed above based on the experimental data of supercritical CO₂ is used to predict the experimental data of propane/CO₂ mixture.The results show that 80%of the predicted values are within±20%and 95%of the predicted values are within±30%.