Experimental Study On Isobaric Specific Heat Capacity And Density Of Carbon Dioxide Under Supercritical And Subcritical Pressure

Considering that supercritical fluids have some special physical and chemical properties,with the development of understanding and research,the application scope of supercritical fluids is rapid expanding such as in the technical fields of extraction,rapid expansion technology and solvent resistance technology,refrigeration and power generation cycle and regenerative cooling technology etc.As the country has enhanced the requirements for new energy forms and energy development and transformation technology in recent years,supercritical carbon dioxide?SCO₂?has gradually arised the attention of the engineering and academic community because of its highly efficient and non-polluting.The corresponding SCO₂ Brayton cycle has become a research highspot in recent years due to its compact design,small footprint and high thermal efficiency.At present,most of the research on the SCO2 Brayton cycle focus on the cycle mode and cycle optimization,but ignore the physical properties of CO₂ under corresponding working conditions.But it is important to understand the overall cycle heat transfer process and improve cycle efficiency on the basis of the thermal physical properties of CO₂.The isobaric specific heat capacity and density data are essential for calculating other physical properties and understanding the flow state and flow properties of the fluid.CO₂'s flowing state during the circulation process accounts for that the traditional offline static measurement method cannot precisely simulate the complex and changeable state of CO₂.Therefore,it is necessary to realize the online measurement of the isobaric specific heat capacity and density under the flowing state.Based on the flow-type calorimetry and vibration method,experiment platforms for measuring isobaric specific heat capacity and density comprising the temperature range of25-80°C and the pressure range of 0.1-12 MPa are constructed respectively.The extended relative uncertainties were estimated to?1.66%-2.46?%and?1.52-2.02?%respectivelt?confidence factor k=2?.To verify the practicability and reliability of the methods,pure water,cyclohexane,pentane,and methylbenzene?from subcritical to supercritical?were tested continuely.Subsequently,the isobaric specific heat capacity and density of CO₂ in the working range of temperature 25-60?and pressure 6.7-12 MPa were measured,and the curve of the change of constant pressure specific heat and density under different working conditions was drawn and analyzed.The data provides a basis for the research and design of the SCO₂brayton cycle.At the same time,the platforms also create conditions for further study of the isobaric specific heat capacity and density of other supercritical fluids.