Numerical Study On Heat Transfer Of Supercritical Fluid In Printed Circuit Heat Exchanger

The rapid development of society has raised demands for efficient energy utilization and reducing energy waste has become a current theme.The performance of heat exchangers has a crucial impact on the energy consumption of industrial production today.Printed Circuit Heat Exchanger(PCHE),a new type of compact heat exchanger,has been widely used in fields such as sodium-cooled fast reactors,refrigeration and air conditioning,and thermal energy storage due to its small size and high heat transfer efficiency.The complex convective heat transfer process inside microchannel heat exchangers is key to studying their performance.The property change characteristics of supercritical carbon dioxide have brought new challenges to the study of PCHE.In this paper,a combination of theory and numerical simulation is employed to explore the influence of the geometric structure of heat exchangers and the property changes of the working fluid on the flow and heat transfer performance of PCHE using supercritical CO2,and to analyze its enhanced heat transfer mechanism.This study has great significance for the development of PCHE.This article focuses on the investigation of factors influencing the flow and heat transfer performance of a zigzag channel under different inlet velocities,using numerical simulations.The research mainly focuses on the effects of channel depth and crosssectional shape on the thermal and hydraulic performance of the channel.By comparing different indicators such as fluid temperature,local velocity,convective heat transfer coefficient,and frictional resistance for channels with different structures,the performance of the printed circuit heat exchanger(PCHE)is evaluated.The results show that increasing the channel depth is beneficial for improving the overall heat transfer performance of the zigzag channel,while reducing the pressure drop.For two channels with depths of 0.55 mm and 0.95 mm,respectively,the average Nu number of the latter increased by 35.9%,and the friction factor decreased by about 8.9%.Under the same hydraulic diameter,channels with a semi-circular or square cross-section have better heat transfer performance,while the square cross-section has better overall performance.The elliptical cross-section has poorer performance.Secondly,the study investigates the distributed PCHE channel with airfoil fins and establishes a hot and cold fluid coupling model of the airfoil channel to explore the enhanced effect of near-critical region supercritical CO2 property changes on its local convective heat transfer coefficient.Through the local analysis of the airfoil channel,it was found that the property changes caused by temperature and pressure fields lead to uneven heat transfer performance along the flow direction.At higher Reynolds numbers,the heat transfer coefficient at the tail end increased by about 80.9%.However,the Reynolds number affects this local unevenness,and reducing the Reynolds number will slow down this growth trend.In addition,it was found that increasing the working pressure of supercritical CO2 or reducing the working temperature can effectively improve the overall heat transfer performance of the channel.Supercritical CO2 operating at 9 MPa can increase the convective heat transfer coefficient by about 28.4%and the Nu number by 9.75%compared to supercritical CO2 operating at 7.5 MPa.However,raising the inlet temperature to 373.15 K will increase the overall heat transfer rate but result in a decrease in Nu of about 9.63%.Finally,an gradient airfoil fins channel structure was developed based on the density variation characteristics of supercritical CO2.Numerical simulation results showed that compared with a uniform channel,the gradient channel could improve the comprehensive performance by 5.19%at higher Reynolds numbers.The enhancement effect was analyzed using field synergy theory and boundary layer theory,which showed that the structure could effectively increase the field synergy number of the channel and have a smaller local field synergy angle.In addition,the gradient channel thinned the boundary layer near the airfoil,which could effectively improve the overall heat transfer performance.Therefore,when exploring the heat transfer performance of supercritical fluids near the critical point,using a gradient channel structure can provide a new optimization idea.