Study On Heat Transfer Characteristics Of Supercritical CO₂ And Inhibition Mechanism Of Heat Transfer Deterioration

With the advantages of compact structure,low cost and high efficiency,supercritical carbon dioxide power cycle is regarded as one of the most promising energy conversion systems in the emerging energy field.Since CO₂ does not have gas-liquid boundary in supercritical state,its thermophysical properties change dramatically when it crosses the pseudo-critical temperature,thus showing special heat transfer characteristics.In some trans-critical conditions,the heat transfer deterioration will occur with the heat transfer coefficient dropping sharply,which not only reduces the heat transfer efficiency,but also endangers the system safety.Therefore,the supercritical CO₂heat transfer deterioration in pseudo-critical region has always been the focus of academic researches.To study the flow and heat transfer characteristics of supercritical CO_2,and to explore the mitigation mechanism of heat transfer deterioration has become an urgent need for the development of supercritical CO₂ Brayton power cycle.Therefore,this article studies the flow and heat transfer characteristics of supercritical CO₂ and mitigation mechanism of heat transfer deterioration in vertical tube using numerical simulation and experimental measurement.In the numerical study,the SST k-ωturbulence model was used to study the effect of complex vortex structure on the supercritical CO₂ flow and heat transfer deterioration.In experimental study,the experimental platform for supercritical CO₂ heat transfer was designed and set up independently.The influence of inlet temperature,mass flow rate,wall heat flux and tube diameter on heat transfer and drag coefficient were carefully analyzed.The main contents of this paper are as follows:1.By comparing with the experimental results,it is proved that the numerical method adopted in this paper can accurately capture the characteristics of supercritical CO₂ heat transfer deterioration.On this basis,the mechanism of the transverse vortex structure mitigating supercritical heat transfer deterioration was studied by using the numerical model.Transverse vortex structure can hinder the development of thermal boundary layer and enhance turbulent heat transfer.Compared with the flow and heat transfer parameters of no gravity condition,the inhibiting mechanism of the transverse vortex structure on heat transfer deterioration is preliminarily clarified.Results show that the buoyancy effect and flow acceleration are important factors for heat transfer deterioration,and the transverse vortex structure can avoid the relaminarization by weakening the influence of buoyancy effect on flow parameters.In addition,the effects of different rib shapes,rib heights,rib pitch on the supercritical CO₂ flow structure and heat transfer deterioration were also studied.Finally,the flow mechanism of supercritical CO₂ heat transfer enhanced by transverse vortices is explained by the field synergy principle.When the heat transfer deteriorates,the synergy between velocity and temperature gradient is weakened,and the synergy angle increases significantly.The disturbance of the vortex system effectively reduces the local field synergy angle and enhances the fluid heat transfer capacity.2.Numerical study was carried out on the effect of longitudinal vortex structure mitigating heat transfer deterioration,and the effect of longitudinal vortex structure on the flow field of supercritical CO₂ was also studied.The results show that the radial secondary flow generated by the longitudinal vortex system disturbs the thermal boundary layer,optimizes the radial velocity distribution,enhances the turbulent heat transfer,and intensifies the mixing of hot and cold fluids,thus delaying and weakening the heat transfer deterioration.Although the longitudinal vortex structure can inhibit heat transfer deterioration over a long distance downstream of the rib,its effect decreases rapidly along the flow direction.In addition,the number of radial vortex pairs is closely related to the rate of vortex pair broken into wake region.When the axial vortex system can only cover a part of the inner wall,the second heat transfer deterioration may occur in the tube,which is caused by the axial density gradient and flow acceleration.When the longitudinal vortex system completely covers the inner wall,there is no large axial density gradient in the buffer layer,and the buoyance effect is effectively suppressed.In addition,the Bo^*number representing the buoyancy effect and the Ac number representing the flow acceleration can be used as the key parameters to determine the degree of heat transfer deterioration.3.The composite vortex structure is used to restrain the deterioration of supercritical CO₂ heat transfer in vertical tubes and improve the heat transfer.It is found that the horseshoe vortex structure caused by the impact effect of incoming flow greatly enhances the local convective heat transfer.The hairpin vortices and longitudinal vortices induced by the lattices increase the mixing of fluids in the boundary layer and bulk flow,reduce the radial density gradient and buoyancy effect.Meanwhile,the flow acceleration is weakened by the secondary flow caused by the composite vortex structure.When heat transfer deteriorates,the composite vortex system structure not only suppresses the local wall temperature peak,but also contributes to heat transfer enhancement.However,in normal heat transfer mode,the composite vortex structure has limited improvement on the comprehensive thermal efficiency,which may be caused by the instability and breaking of the vortex system in high velocity flow.In addition,the comprehensive heat transfer efficiency in the tube can be improved by moving the position of the first row of lattice towards the inlet,increasing the diameter of lattice,increasing the number of lattice and increasing the spacing of lattice.4.An experimental platform for supercritical CO₂ flow and heat transfer characteristics was designed and built independently,and an experimental study was conducted on the effects of different mass flow rate,inlet temperature,wall heat flux,tube diameter and tube length on the supercritical CO₂ heat transfer characteristics in a vertical smooth tube.The results show that heat transfer deterioration is more likely to occur in large diameter tube.When the inlet temperature is increased,the location of heat transfer deterioration will be closer to the pipe inlet.Under the condition of low mass flow rate and high heat flux,heat transfer deterioration may occur.Under the condition of medium or high mass flow rate,the heat transfer may be enhanced by flow acceleration.In the condition of medium mass flow rate,the heat transfer may first enhance and then deteriorate with the increase of the wall heat flux.The friction factor decreases rapidly with the increase of inlet flow,but increases slowly with the increase of wall heat flux.In this paper,numerical and experimental studies were carried out on the mitigation mechanism of different vortex structures on heat transfer deterioration,and the heat transfer characteristics of supercritical CO₂ in smooth tube.The evolution of supercritical CO₂ flow field,the influence of heat transfer and the mitigation mechanism of heat transfer deterioration can provide reference for the design and optimization of supercritical CO₂ heat exchanger.