Numerical And Experimental Research On Characteristics Of Printed Circuit Board Heat Exchanger In Supercritical Carbon Dioxide Brayton Cycle

As a new type of compact heat exchanger,printed circuit heat exchanger(PCHE)has the advantages of large specific surface area,high heat transfer efficiency,small equipment size,high temperature and high pressure resistance,which is very suitable for the heat exchange equipment such as regenerator in supercritical carbon dioxide Brayton cycle.It is of great significance to study and master the steady-state and dynamic characteristics of PCHE for the construction,operation and control of the cycle system.In this paper,a research was carried out from several aspects by means of combing simulation and experimental test,including numerical channel design,prototype test verification,design program model,dynamic simulation and test,and system coupling optimization.The numerical simulation study was carried out on a kind of new trapezoidal channel structure PCHE,and the coupled heat transfer and pressure drop characteristics were analyzed comparing with the straight,zigzag and S-shape channels.The results showed that the trapezoidal channel had the strongest heat transfer performance among the four structures with a more than 5000W/(m2·K)convective heat transfer coefficient.But at the same time,its pressure drop loss was 5times,2.50 times and 1.20 times higher than that of the straight,zigzag and S-shape,respectively.The geometric parameters of trapezoid,including the total length L,the upper straight length Ls and the bottom angle θ,were optimized to obtain a channel structure with the balance between heat transfer and pressure drop.Furthermore,aiming at the pressure loss problem,an optimized sandwich heat transfer unit was proposed,which can improve the heat recovery performance of the hot side by 5% and reduce the pressure drop to 1/4 of the original structure,so that the trapezoidal structure had a better thermal-hydraulic characteristic.The experimental test was carried out on a trapezoidal channel structure PCHE prototype,and the heat transfer correlations of both cold and hot side were obtained,respectively.The deviations between correlations prediction and the experimental results were within ±15%.The numerical simulation results were also compared and verified with experiments.The maximum deviation of outlet temperature on the cold and hot side was-2.54% and 7.39%,respectively,and pressure drop maximum deviation is-2.20% and-18.45%,respectively.The heat transfer correlations were further extended to the low Reynolds number range,and the deviations from the test results were within ±16%.The test work was carried out on a zigzag prototype as well,and their performances were compared through the comprehensive performance criteria PEC.The results indicated the trapezoidal structure had more advantages with a 18.53% higher comprehensive performance than zigzag on the cold side.The design methodology of PCHE was studied and a program model was constructed.Compared with the test prototype,the design result showed the error of outlet temperature was1.22%,the error of size and volume was 5% and the error of heat transfer coefficient was less than10%.An optimization design scheme achieved a minimum volume reduction of 35% over the existing prototype,and a minimum pressure drop scheme showed that the pressure drop was reduced by 40.39% and 46% on the cold and hot side,respectively.PCHE equipment with heat load levels of 100 k W、500 k W、1 MW、5 MW and 10 MW were designed respectively,and the design differences between trapezoidal and zigzag structure were compared.The trapezoidal device volume was reduced by 8.46% ～ 10.83%,and its compactness increased by more than 10%with a pressure drop increased less than 5.58% on the cold side.The dynamic characteristic of PCHE was studied and a dynamic response model was constructed.The steady-state results were verified with a 6% maximum deviation from the test temperature and a 6% ～ 8% deviation from the test pressure drop,indicating that the model can reflect the real working process of PCHE to a certain degree.Furthermore,the dynamic processes under varying conditions,such as variable inlet temperature,variable working pressure and variable mass flow rate,were simulated and experimentally studied.It was found that the response time of variable inlet temperature was more than 1200 s,the response time of variable working pressure was about 90 s,and the response time of variable flow is shortest,but it would affect the operation of the whole system.The experiments verified the predictions of the model.The maximum deviation of temperature was generally less than 4 ℃,and the deviation of pressure drop was less than 0.50 k Pa,which could predict the dynamic response process of PCHE under varying conditions in Brayton cycle.The coupling design and optimization of PCHE regenerator and Brayton cycle system were studied.Three typical cycle configurations,namely simple regeneration cycle,recompression cycle and pre-compression cycle,were selected to optimize the design of PCHE regenerator and system for the 100 k W class demonstration and 100 MW class commercial system,respectively.For the 100 k W class demonstration,simple regeneration cycle had the best efficiency of 31.52%,and the PCHE had the smallest pressure drop with no more than 35 k Pa and the smallest volume.For the 100 MW class commercial system,the recompression cycle showed the efficiency advantage,which could reach 51.63%,but its PCHE equipment volume were 2 ～ 3 times larger than other two cycles.The pressure drop in high temperature regenerator was also highest,which might limit the selection of the cycle parameters.Therefore,it was necessary to consider both the efficiency and the device cost when choosing the system configuration.This study is expected to provide reference for the design and selection of heat exchange equipment and system construction of supercritical carbon dioxide Brayton cycle.