Study On The Design Of Precooler For Supercritical Carbon Dioxide Brayton Cycle Power System

The supercritical carbon dioxide Brayton cycle has the advantages of high efficiency,compact,high flexibility,easy optimization and control,etc.In the future,it can replace part of the steam Rankine cycle to achieve more efficient thermal conversion.The precooler part has a large energy flow,up to about 70%of the power generation.Usually the printed circuit plate heat exchanger（PCHE）is used to cool supercritical CO₂ in the form of water cooling,but there is little research on air cooling.In this paper,the flow and heat transfer characteristics of supercritical CO₂in cooling section are discussed based on previous studies.A sectional calculation model for supercritical CO₂ heat transfer is designed.The calculation program for supercritical CO₂ heat transfer is established by using MATLAB software and REFPROP software.Taking 1000MW unit model as an example,the conceptual design of PCHE and finned heat exchanger is carried out.The heat transfer performance of PCHE and finned heat exchanger is analyzed.The results are as follows:The average heat flux of water-cooled PCHE is 742W/m²,the heat exchange area is 1278000 m²,and the water consumption is 11349kg/s.The average heat flux of air-cooled PCHE is 315 W/m²,the heat exchange area is 3015,500 m²,and the air intake is 34893kg/s.The average heat flux in the finned heat exchanger tube is 4261 W/m²,the heat exchange area is 273750 m²,and the air intake is 34893kg/s.The heat transfer process of supercritical CO₂,water and air in PCHE is analyzed,and the influence of various parameters on the heat transfer performance is explored.The heat transfer in low temperature area is more than 40%;The change of thermal conductivity,Prandtl number,density and specific heat of supercritical CO₂with temperature leads to the decrease of heat transfer capacity in the inlet and outlet section.The minimum value of heat flux and the minimum value of temperature difference appeared in the middle section.Small temperature difference and small heat transfer coefficient led to serious heat transfer deterioration in the middle section.The effects of finned tube length,number of tube rows and supercritical CO₂ inlet velocity on heat transfer performance were studied.It is found that it is difficult to cool supercritical CO₂ from 90℃to 32℃when the tube length is insufficient.The more the number of pipe rows,the higher the outlet air temperature,which is conducive to saving power consumption;the larger the inlet velocity of supercritical CO₂ is,the smaller the area occupied is,and the higher the outlet wind temperature and pressure loss are.Finally,the heat transfer of supercritical CO₂ in finned heat exchanger is simulated by numerical simulation.The comparison shows that the calculated results are in good agreement with the simulation results within the allowable error range.According to the simulation results,it is found that the secondary flow and eddy current near the elbow will lead to the decrease of the heat transfer capacity in the pipe and the occurrence of large gradient pressure drop.