The Flow And Heat Transfer Characteristics Of Supercritical CO₂ In Microchannels

In the background of carbon peaking and carbon neutrality goals and industry trends of miniaturization of thermal equipment,supercritical CO₂ has been widely used in all kinds of power cycle systems with high efficiency and compact structure because of its unique physical properties.Microchannel heat exchangers using supercritical CO₂ as heat transfer medium is an important part of future high-efficiency energy system.However,the flow and heat transfer characteristics of supercritical CO₂ in tubes become more complicated due to the drastically changing physical properties of CO₂ near the pseudo-critical point and the complex structure of the new microchannel heat exchangers.Based on the first law and the second law of thermodynamics,the flow and heat transfer characteristics of supercritical CO₂ in microchannels with different structures under heating conditions were studied,in order to reveal the influencing laws and internal heat transfer mechanism.According to the thermal physical properties of supercritical CO₂ under different pressures,the influence of channel scale on the selection of numerical solution method was analyzed combined with Knudsen number.The mathematical model of supercritical CO₂ flowing in microchannel was established.The reliability of SST k-ωturbulence model was verified by comparing the simulation results with experimental data and the calculation results of empirical correlation.The effects of operating parameters such as the inclination angle,heat flux,mass flow rate,inlet temperature and pressure on the flow and heat transfer characteristics of supercritical CO₂ flowing in circle microchannel were discussed by comparing three average heat transfer coefficients,frictional pressure drop and entropy generation number along the flow direction.The internal mechanism of buoyancy effect on heat transfer was discussed.What’s more,the phenomenon of local heat transfer deterioration was also discriminated and analyzed.The results show that when the flow direction is not vertical,the buoyancy effect causes uneven heat transfer in the direction of gravity,and the secondary flow intensity is an important factor while the effect of flow acceleration on heat transfer can be ignored.Reducing the angle between the flow direction and the gravity direction,increasing the mass flow rate,reducing the heat flux or reducing the operating pressure can help to improve the heat transfer performance and reduce the irreversible loss.The frictional pressure drop has the largest contribution to the total pressure drop,which is about 82%～90%,and the flow acceleration pressure drop cannot be ignored under the condition of high mass flow rate and low operating pressure.The effective thermal conductivity distribution in the boundary layer near the wall has a great influence on the heat transfer performance,and the fluid with low effective thermal conductivity accumulates near the upper wall may lead to the local heat transfer deterioration.Based on the simulation results in circular microchannel,the effects of heat flux,mass flow rate and tube placement on flow and heat transfer characteristics of supercritical CO₂ flowing in horizontal semicircular microchannel of the printed circuit heat exchanger（PCHE）were studied.Because of the effect of buoyancy,when compared with the lower semicircular wall,the effective thermal conductivity and heat transfer coefficient of the fluid near the upper wall are lower while the heat transfer entropy generation rate is higher.When the heat flux decreases or the mass flow rate increases,the angular effect of the two corners is weakened,and the heat transfer effect is improved while the irreversible loss is reduced,but the friction pressure drop at the same bulk temperature will increase.When the geometry of the upper curved-wall and the lower flat-wall is adopted,the temperature distribution on the circumferential wall is more uniform,the temperature of two corners is reduced,the heat transfer performance is improved,and the irreversible loss is reduced.The overall performance is better than the other two placement methods,which can be considered in the design of heat exchangers.In order to further explore a new path to enhance heat transfer for PCHE,a physical model of supercritical CO₂ flowing in a sinusoidal microchannel was established,and the effects of operating parameters and structural parameters on flow and heat transfer characteristics were analyzed.The results show that compared with the straight channel with equal heat exchange area,the heat transfer performance in the sinusoidal microchannel is better,but the resistance loss is larger.In a certain range,reducing the wavelength or increasing the amplitude is helpful to improve the comprehensive performance.Considering heat transfer performance,pressure drop and irreversible loss,the sinusoidal microchannel with amplitude of 7 mm and wavelength of 50 mm has the best comprehensive performance.The temperature distribution,flow field distribution and turbulent kinetic energy distribution of the typical cross-sections of the fluid domain can well explain the phenomenon that heat transfer can be enhanced by changing the pipe structure.The RANS numerical simulation method was used to explore the flow and heat transfer characteristics of supercritical CO₂ in microchannel,and its unique flow and heat transfer mechanism was analyzed,which could provide theoretical reference for the design and optimization of microchannel heat exchangers.