Researches On Cooling Heat Transfer Of CO₂ At Supercritical Pressures And Microtube Heat Exchangers

As the demand for energy is increasing and more and more attention has been paid to the sustainable development of the environment,the supercritical carbon dioxide Brayton power cycle and transcritical carbon dioxide heat pump,which use carbon dioxide as the working fluid,have shown broad application prospects with the characteristics of clean,efficient,energy saving and environmental protection,and have attracted extensive attention.High-pressure and compact heat exchanger is an important part of supercritical pressure carbon dioxide cycle system.On the one hand,the variable physical properties of supercritical fluid near the critical point lead to the unique flow and heat transfer characteristics of carbon dioxide in the tube;on the other hand,the non-uniform heat capacity and variable characteristics of heat transfer and pressure drop make the optimization method of heat exchangers different from conventional heat exchangers.In addition,the high-pressure characteristic of supercritical pressure carbon dioxide has put forward higher requirements for the strength of heat exchanger.The microtube heat exchanger is a new type of high-performance compact heat exchanger,which has good pressure bearing capacity and cost performance in supercritical/transcritical carbon dioxide heat transfer.The microtube heat exchanger is the main research object in this paper and both numerical simulation and experimental research are conducted.The cooling and heat transfer mechanism of supercritical carbon dioxide in the microchannel is studied.The fluid flow and heat transfer law of the shell side of the microtube bundle is analyzed and summarized,and the accuracy of different carbon dioxide heat transfer correlations in engineering application is compared.According to drastic change characteristics of physical properties,the pinch point problem of the heat exchanger and the improvement of flow and heat transfer performance are comprehensively optimized,which can provide theoretical support for the design and application of carbon dioxide microtube heat exchangers.The main results are as follows.(1)The cooling heat transfer mechanism of supercritical carbon dioxide under constant heat flux and coupled heat transfer conditions was studied by numerical simulation.The drastic changes of physical properties such as thermal conductivity and specific heat caused by the change of temperature during the cooling process lead to the sharp thinning of thermal boundary layer thickness and heat transfer enhancement of the bottom layer near the pseudo-critical point.The change of density leads to the secondary flow under the buoyancy effect,which has a unique impact on the local heat transfer characteristics.When coupled heat transfer occurs in the microtube heat exchanger between supercritical carbon dioxide and cooling water,different flow patterns of the cooling medium outside the tube results in different internal and external temperature fields,which further affects the heat transfer and pressure drop characteristics of supercritical carbon dioxide in the tube.The secondary flow velocity of carbon dioxide under the condition of downward flow of cooling water is 5%higher than that under the condition of upward flow of cooling water.Based on the numerical results,the accuracy of the existing correlations of supercritical pressure carbon dioxide heat transfer coefficients and pressure drop is compared and analyzed,which provides a theoretical basis for the design of heat exchanger.(2)The experimental platform of supercritical pressure carbon dioxide heat exchangers was built and the experimental research of supercritical pressure carbon dioxide microtube heat exchangers was carried out.The results show that the shell side heat transfer coefficient of the microtube bundle with baffles can reach 20000?30000 W·m-2·?-1,which is 3?4 times higher than that without baffles,and the shell side heat transfer coefficient of the microtube bundle can reach more than 2 times of that predicted by the conventional shell side heat transfer correlation.The correlation was modified for the shell side heat transfer of the microtube tube bundle.The new shell side heat transfer coefficient correlation was used to analyze heat transfer between supercritical pressure carbon dioxide and cooling medium in shell side.The results show that there are some errors between the existing correlations of supercritical pressure carbon dioxide and the experimental results,and the deviation between the prediction results of Dang's correlation and the experimental results is within 10%,indicating that the accuracy of Dang's correlation is relatively high.Larger pressure drop of the supercritical pressure carbon dioxide occurs at the inlet and outlet of the heat exchanger,while the friction pressure drop and acceleration pressure drop in the tube are relatively small.(3)A simulation method of flow and heat transfer in microtube heat exchanger was developed.This method is based on the heat transfer correlation of supercritical pressure carbon dioxide and the numerical simulation of flow in shell side,which can effectively reduce the calculation cost and save the calculation consumption on the basis of ensuring the overall calculation accuracy,and is suitable for the calculation of heat exchanger with larger amount of tubes.The comparison between the simulation results and the experimental results shows that Dang's correlation can obtain a more reliable analysis of the coupled heat transfer of the microtube heat exchanger.In the microtube heat exchanger with baffles,due to the different characteristics of shell side flow near the baffles,there will be enhanced heat transfer region and weakened heat transfer region.In the enhanced heat transfer region,the wall temperature is relatively low and the carbon dioxide in the tube is closer to the critical point due to the strong shell side heat transfer,so the heat transfer in the tube is enhanced synchronously,making the overall heat transfer enhanced.While in the weakened heat transfer zone,the wall temperature is relatively high and the carbon dioxide in the tube is far away from the critical point due to the weak shell side heat transfer,so the heat transfer in the tube is reduced synchronously,making the overall heat transfer reduced.The difference of heat flux in different heat transfer regions is more than 4 times.(4)The optimization criterion of heat exchanger with drastic change of physical properties is proposed and the supercritical carbon dioxide microtube heat exchanger is optimized.The temperature curves in the heat exchanger are different from that in the conventional heat exchanger due to variable physical properties.Improper flow distribution is easy to lead to the heat transfer deteriorate at pinch point.Increasing the ratio of cold and hot fluids mass flow rate and reducing the inlet temperature of cold fluid can effectively avoid the heat transfer deterioration at pinch point.According to the theory of entransy dissipation,the design of cold fluid split flow can further reduce the irreversible heat transfer loss of the cooler and improve the effectiveness of the cooler on the basis of avoiding the heat transfer deterioration at pinch point.The total pressure drop and the pump power consumption can be reduced by improving the uniformity of pressure drop gradient with the change of heat exchange area of each segments through local internal and external heat transfer and pressure drop adjustment.Based on the guidance of optimization theory,the baffle spacing of supercritical carbon dioxide microtube cooler was rearranged.The overall simulation results show that the optimized microtube cooler can effectively reduce the total pressure drop by 10%on the shell side with the same heat transfer and heat transfer area.(5)The flow and heat transfer characteristics of supercritical carbon dioxide in spiral tubes and that of water in shell side with different baffles were numerically studied.Centrifugal force and density change lead to non-uniform distribution of supercritical pressure carbon dioxide in the cross section of the spiral tube,resulting in the heat transfer reduction in inner region and the heat transfer enhancement in outer region.The overall heat exchange of carbon dioxide in spiral tubes is 10%higher than that in the straight tube.Different types of baffles have different comprehensive performance under different water mass flow rates,which should be judged according to the working conditions.The synergy angle can evaluate the heat transfer of different structures to a certain extent,which plays a guiding role in structure design to enhance heat transfer.In this paper,the cooling heat transfer mechanism of supercritical carbon dioxide and the comprehensive performance of heat transfer and pressure drop of the microtube heat exchanger are systematically studied by numerical simulation and experimental methods in the way of coupled heat transfer unit-the whole heat exchanger-optimal design.The accuracy of different carbon dioxide heat transfer coefficient correlations is compared,and the heat transfer correlation of microtube bundle is proposed.A reliable and efficient numerical simulation method of the microtube heat exchanger is developed,and coupled heat transfer mechanism between supercritical carbon dioxide and water in the microtube heat exchanger is clarified.The optimization criteria of heat exchanger with variable physical properties is established,which provides research basis and theoretical support for the design and application of supercritical pressure carbon dioxide microtube heat exchanger.