Effect Of Rib Roughness On Convective Heat Transfer Of Supercritical Carbon Dioxide

With the rapid development of modern industry,the demand for energy has gradually increased.Facing the crisis that fossil fuels will eventually dry up,there is an urgent need to develop new energy sources and develop efficient energy systems.Compared with the traditional Rankine cycle based on water vapor,the supercritical pressure carbon dioxide（SCO₂） Brayton cycle system is more convenient,economical and simple.However,the design and optimization of the supercritical pressure carbon dioxide（SCO₂） Brayton cycle system heater is facing some challenges.What needs to be urgently understood is that the convection heat transfer coefficient near the pseudo-critical point in the fluid convective heat transfer process at supercritical pressure The local changes are more sensitive,and the local heat transfer coefficient will be extremely deteriorated.This phenomenon will lead to the instability of the supercritical fluid during the heat transfer process.It is necessary to study the mechanism of supercritical heat transfer deterioration and heat transfer enhancement.This paper uses ANASYS FLUENT to simulate the effect of rib roughness on supercritical carbon dioxide convective heat transfer.The model uses a modified shear stress transfer model and a variable turbulent Prandtl number model.The main contents are as follows:（1）Throughout this chapter,numerical simulations of the vertical rising flow of supercritical dioxide in smooth pipes with different pipe diameters are conducted.The exchange and flow characteristics of supercritical carbon dioxide in different pipe diameters are analyzed.Compared with experiments,numerical simulation can more directly observe the flow of supercritical carbon dioxide in the tube.This chapter is of great significance to the next.（1）By numerically simulating the vertical rising flow of supercritical carbon dioxide in a smooth pipeline,the heat transfer characteristics and flow characteristics of supercritical carbon dioxide under different buoyancy conditions are studied and analyzed.Compared with experiments,numerical simulation can more intuitively observe the flow state of supercritical carbon dioxide in the tube,and have a further understanding of the mechanism of supercritical heat transfer.（2）The heat transfer of supercritical pressure carbon dioxide in pipelines with discrete double diagonal ribs（DDIR）was numerically studied.The numerical simulation was carried out at 7.58 MPa,a mass flux of 200-800 kg/（m²·s） and a heat flux of 56.7k W/m².The results showed that DDIR severely suppressed the strong buoyancy effect present in the smooth tube,and no serious deterioration occurred.Sensitivity analysis of geometric parameters shows that the rib spacing is most sensitive to the deterioration of heat transfer.The effect of the tilt angle is much smaller than the height and spacing of the ribs.When Bo* is about 4.5×10-6 and PEC is 3-4,the overall thermal performance of the DDIR tube is the best.In areas where buoyancy is very strong or negligible,the value of PEC is much lower between 1-2.The main role of DDIR is to eliminate the negative effect of buoyancy on heat transfer,and they cannot effectively increase the positive effect of buoyancy on heat transfer.Finally,it is found that Bo* is suitable for performance evaluation and optimization of supercritical heat transfer enhancement technology.（3）Systematic study of the convective heat transfer characteristics of supercritical carbon dioxide in a single pipe under different rib roughnesses.The heat transfer of supercritical carbon dioxide in an internally threaded tube with continuous ribs on the inner wall is simulated.The flow heat transfer characteristics of supercritical carbon dioxide in a single pipe under the same rib roughness were compared,and the applicability of different rib roughness under different working conditions was analyzed.The results show that under mixed convection conditions,the PEC value of DDIR is 4 and the PEC value of IRT is 2.7.The longitudinal vortex induced by discrete ribs has a more obvious effect on supercritical heat transfer enhancement than the vortex induced by continuous ribs.Under natural convection and forced convection conditions,both rib reinforcement effects are not obvious.