Study On The Heat Transfer Characteristics And Physical Properties Of Supercritical Propane/CO₂ Mixture Near The Pseudo-critical Point

The suercritical organic Rankine cycle（SORC）has become one of the main development directions in the field of industrial waste heat recovery due to its high thermal efficiency and high output work.The physical properties of supercritical working fluid not only affect the thermal efficiency and economy of the system,but also meet the requirements of environmental protection,safety and stability.Compared with the traditional HFCs,HCs has higher critical temperature and better matching with the heat transfer from the heat source in the supercritical heat transfer process.The high critical temperature and low critical pressure of propane are very suitable for the SORC system with low temperature residual heat,but it is inflammable.In order to ensure the safe operation of the system,an appropriate amount of CO₂ can be added to propane as a flame retardant to form a mixed working fuid for SORC system.The SORC system is a front direction in the field of low-medium-grade waste heat conversion technology.The physical properties of supercritical propane/CO₂ in the pseudo-critical point region change dramatically,which is easy to cause abnormal heat transfer characteristics.At present,the research on the heat transfer characteristics and thermophysical properties of non-azeotrope mainly focuses on the subcritical state,but there are few experimental and theoretical studies on the heat transfer characteristics and thermophysical properties near the critical point/pseudo-critical point,and it is difficult to accurately predict the thermophysical properties of supercritical state by the theoretical model established under the subcritical condition.Therefore,it is of great significance to study the heat transfer characteristics and thermal properties of propane/CO₂ mixing fluid near the critical point/pseudo-critical point to improve the performance of SORC system and the optimization design of heat exchanger.In this paper,the heat transfer characteristics of propane/CO₂ mixture near the pseudo-critical point in helical-coiled tube under the conditions of heating with composition of 35.4:64.6 and 71.6:28.4 are experimentally studied.The heat transfer characteristics of propane/CO₂ under different ratio,heat flux,mass flow and pressure are studied by numerical simulation.Molecular dynamics（MD）simulation is used to calculate the critical point properties of propane/CO₂ binary mixing system（30:70,70:30）.The radial distribution function（RDF）between propane molecule and CO₂molecule is discussed from the microscopic point of view.Based on the existing binary interaction parameter correlations,the theoretical research on the interaction parameter of supercritical alkane/CO₂ binary mixing systems in the pseudo-critical region is carried out.The following conclusions can be drawn:（1）The experimental results show that the peak value of heat transfer coefficient of working fluid with a ratio of 71.6:28.4 near the pseudo-critical temperature is about twice that of the heat transfer coefficient of working fluid with a ratio of 35.4:64.6,and there is a small fluctuation.（2）Numerical simulation results show that the heat transfer coefficient of propane/CO₂is higher before the pseudo-critical point when the composition is 30:70,and higher after the pseudo-critical point when the composition is 70:30.（3）Based on molecular dynamics（MD）simulation,density and the specific heat capacity parameters of propane/CO₂（70:30 and 30:70）at different temperatures are obtained.According to the temperature corresponding to the maximum specific heat capacity at constant pressure,the critical temperature of propane/CO₂（p_c=6.1 MPa）with a composition of 70:30 is about 355 K,and the critical temperature of propane/CO₂（p_c=6.8 MPa）at a composition of 30:70 is about 318.5 K.（4）The binary interaction cosfficient k_ij varies monotonically with the contrast temperature,the correlation formula of the predicted interaction parameter of the alkane/CO₂ in the pseudo-critical region is obtained.Compared with the experimental data,the minimum absolute deviation is 0.10%,and the maximum absolute deviation is 3.09%.