Study On The Thermal-hydraulic Performance Of Printed Circuit Heat Exchanger With Supercritical Carbon Dioxide

In recent years,the supercritical CO₂ Brayton power cycle attracts lots of attention because it has the advantages of high efficiency,compactness and superior economy.As the key components in the power cycle,the recuperator and pre-cooler have crucial influences on the performance of the cycle.The printed circuit heat exchanger?PCHE?can be used as the recuperator and pre-cooler because of its high thermal efficiency,compactness,high temperature and pressure resistance.Compared with regular fluid,the supercritical CO₂ properties change dramatically?especially near the pseudo-critical point?,and the flow and heat transfer characteristics of supercritical CO₂ are complex.In order to study the flow and heat transfer characteristics of supercritical CO₂ in the PCHE and promote the engineering application of the MW-scale supercritical CO₂Brayton power generation system.The paper mainly focuses on the study of the PCHE with supercritical CO₂ and supercritical CO₂,supercritical CO₂ and water as the working fluids using three methods:theoretical analysis,experimental test and numerical simulation.The main work and conclusions in the current study are listed as following:?1?.The segmental heat duty design method and segmental heat transfer area design method are employed to investigate the local and overall thermal-hydraulic performance of PCHE from the perspective of the theoretical analysis.When the heat duty is given,the local logarithmic mean temperature difference,local heat conductance and local heat transfer entropy generation number have extremums,which appear in the vicinity of the intersection point between the heat capacity flow rates of hot and cold working fluids.When the total heat transfer area is fixed,the local heat capacity rate ratio has crucial influences on the local effectiveness and the heat transfer entropy generation number.The pressure of supercritical CO₂ has little effect on the total entropy generation number at a high mass flow rate of hot fluid.The local and total entropy generation mainly depend on the local and total heat transfer entropy generation under the fixed heat duty and the heat transfer area,and flow imbalance increases entropy generation,especially when the local heat capacity flow rate of the cold side is higher.?2?.The first test platform with two supercritical CO₂ loops was built in China to study the performance of the recuperator and pre-cooler.The maximum operating temperature and pressure are 600?and 32 MPa in the CO₂ loops.100-kW recuperator module and 100-kW pre-cooler module have been tested successfully and run well on the test platform.Therefore,the design of 1-MW recuperator and 1-MW pre-cooler is reliable based on the parallel module of the 100-kW recuperator and 100-kW pre-cooler.For the recuperator,the total heat duty under all the experimental test conditions varies from 107.3 kW to 120.8 kW,the hot fluid pressure drop varies from 39.7 kPa to 47.2kPa,and the cold fluid pressure drop varies from 29.3 kPa to 40.1 kPa.The hot fluid pressure drop of the recuperator is more sensitive to the supercritical CO₂ properties and the channel structure compared with the cold fluid pressure drop.For the pre-cooler,the total heat duty under all the experimental test conditions varies from 64.5 kW to81.6 kW,the supercritical CO₂ pressure drop varies from 32.8 kPa to 87.9 kPa,and the water pressure drop varies from 22.9 kPa to 58.2 kPa.When the outlet temperature of supercritical CO₂ in the pre-cooler is close to the pseudo-critical point,the influence of the supercritical CO₂ properties is more important than that of the mass flow rates of hot and cold fluids,and the influence of the supercritical CO₂ properties on the heat transfer is greater than that on the pressure drop.?3?.The numerical simulation is used to investigate the local heat transfer and flow characteristics in the pre-cooler,and the standard k-?model is in good agreement with the experimental results.The numerical results are summarized as follows:the supercritical CO₂ temperature distribution is nonlinear and the pinch point occurs in the pre-cooler;the mass flow rates of both sides have crucial influences on the location of the peak of the supercritical CO₂ Nusselt number,and only the mass flow rate of supercritical CO₂ affects the change of the peak value;the effect of the radius of curvature on the secondary flow at the zigzag bends should not be ignored;the smaller the radius of curvature,the better the synergy of velocity and temperature gradient,and the greater the total heat transfer rate and pressure drop of the pre-cooler,the 1 mm radius of curvature is the most suitable considering the comprehensive performance of the pre-cooler;to generalize the experimental and numerical results,the empirical correlation of supercritical CO₂ is proposed and verified,and the empirical correlation is more suitable under the conditions of the drastic variations of the Prandtl number.