Study On Heat Transfer Characteristics Of Strongly Variable-Properties Fluids And Optimization Method Of High Efficient Heat Exchanger

With the shortage of energy and the intensification of environmental pollution around the world,the development and utilization of renewable clean energy,and the improvement of energy efficiency have become the trend of world energy development.As an important device in the industrial production process,the performance of heat exchanger has a decisive influence on the system.Printed circuit heat exchanger?PCHE?has the advantages of compact structure,high effectiveness,high temperature and high pressure resistance,etc.,so it has broad application prospects in many fields such as nuclear energy,solar energy,aerospace,oil and gas mining,chemical industry and so on.Supercritical carbon dioxide?S-CO₂?undergoes drastic changes in thermo-physical properties near the pseudo-critical point,triggering complex heat transfer and flow behavior,thus making traditional heat transfer calculation and design methods no longer applicable.Studying the convective heat transfer characteristics of strongly variable-properties fluids in microchannels,and developing new types of efficient and compact PCHE structure are of great significance for the efficient use of energy and sustainable development.In this paper,firstly,the convective heat transfer of the strongly variable-properties fluid in a single enhanced channel was studied by numerical simulation,and the heat transfer enhancement mechanism was revealed through the field synergy theory and the secondary flow analysis.Secondly,the coupled convective heat transfer characteristics of strongly variable-properties fluid in hot and cold channels was investigated.The theoretical analysis methods of vector analysis and statistical analysis,combined with numerical simulation,were innovatively adopted to analyze the effect of non-uniform distribution of local heat transfer coefficients on overall performance,and related optimization design methods were proposed.Then the modified heat transfer enhancement components for PCHE,two new types of airfoil fins were proposed.Through the simulation calculation,the flow and heat transfer characteristics of S-CO₂fluid in the new airfoil fin PCHE were analyzed.Furthermore,the performance of new airfoil fin PCHE was experimentally tested on the S-CO₂ test platform,and the local heat transfer and flow friction empirical correlations were developed based on the experimental data and simulation results.In addition,the effect of heat exchanger performance on the multi-stage heat exchange system using variable-properties fluids,as well as the optimal design rules,was investigated.Analysis on field synergy and secondary flow in wavy channel shows that the corners of channel can enhance the disturbance of the internal flow,and enhance the heat transfer effectively;on the other hand,compared with the straight channel,the fluid in wavy channel is disturbed,making the velocity field and temperature gradient field change,resulting in a better synergy between these two fields and the better heat transfer.Near the pseudo-critical point,the local heat transfer of S-CO₂ is significantly enhanced due to the drastical changes in its thermo-physical properties.Considering that comprehensive influence of fluid properties and channel structure on internal convection heat transfer in wavy channel,an index ratio is proposed?De/Gr^*?to characterize the relative magnitude of centrifugal force and buoyancy force in wavy channels.The S-CO₂ fluid was adopted to investigate the coupled heat transfer characteristics of variable-properties fluids in hot and cold channels.Vector analysis and statistical analysis were used to analyze the distribution of local heat transfer coefficients along the channel,and two new parameters,a synergy angle and the overall non-uniformity of distributions,were derived.Associating with numerical simulation results,it is found that improving the synergy of local heat transfer coefficient distribution on both sides?i.e.,increasing the distribution similarity of the heat transfer coefficients on both sides?,and reducing the overall non-uniformity,can effectively improve heat transfer performance.Two new airfoil fins for PCHE were developed and numerically analyzed.The results show that the heat and flow performance of one of the new airfoil fins are improved compared with NACA 0020 airfoil fin,and thereby the overall performance is improved;the flow resistance of another new airfoil fin is significantly reduced,making a better comprehensive performance under the condition of high Reynolds number.The staggered arrangement and the proper shape of fins can reduce the influence of the boundary layer,change the velocity field,temperature gradient field and the synergy of the two fields,thereby improving the heat transfer performance;in addition,the streamlined airfoil structure can reduce the flow resistance effectively.The heat transfer characteristics between supercritical pressure CO₂ and water in the new airfoil fin PCHE were tested.Experimental results show that the inlet temperature has a smaller effect on the overall performance than the CO₂ flow rate;When the inlet Reynolds number is in the range of 10000³4000,the pressure drop of CO₂ is generally kept at a low level?less than 0.25%of the operating pressure on the CO₂ side?;increasing the pressure on the CO₂ side can increase the heat load and reduce the pressure drop.When the pressure on the CO₂ side increases from 7.62 MPa to 8.65MPa,the heat load increased by about 30.51%,and the pressure drop decreased by about 23.44%.The comparison with the zigzag PCHE test data shows that the pressure drop of the new airfoil fin PCHE is only 1/6¹/5 of that of the zigzag channel PCHE when the heat load is basically the same.The analysis of local heat transfer flow characteristics by numerical simulation shows that:the supercritical pressure CO₂ heat transfer is effectively enhanced near the pseudo-critical point;the Nu number and friction factor before and after the pseudo-critical point have opposite trends with temperature,which should be separately considered in the design.According to the simulation results of the test conditions,the correlations of local Nu number and the friction factor for CO₂ cooling conditions in the new airfoil fin PCHE were proposed.Two-stage waste heat recovery system of flue gas was investigated,and a comprehensive analysis was carried out from the perspective of the first and second laws of thermodynamics.The study of the two-stage heat exchange systems with condensation finds that in the design process,placing fluid conditions with different property changes in different heat exchange equipment,that is,placing the inflection point of the thermal property changes at the junction of the upstream and downstream heat exchangers,is conducive to improve the system performance by improving the heat exchanger performance according to the specific fluid properties.In this paper,research on convective heat transfer performance of strongly variable-properties fluid is gradually carried out in the order of"single wavy channel,coupled channels on both sides,new airfoil fin channel in PCHE,heat transfer system".This research reveals the flow law and the heat transfer enhancement mechanism of the variable-properties fluid in the wavy channel,and proposes a new indicator to measure the effects of channel structure and fluid property changes on heat transfer;Investigation explains the enhancement mechanism of coupled heat transfer on both sides of strongly variable-properties fluid from the perspective of distributed synergy innovately;two new efficient compact PCHE structures are developed,one of which is experimentally tested and the relevant empirical correlations are proposed;from the perspective of heat transfer and thermodynamics,the optimal design of the multi-stage heat transfer system with complex variable-properties fluid is studied.The present work can provide theoretical guidance and technical support for the research on the heat transfer flow mechanism of variable-properties fluids,heat transfer enhancement and channel optimization design,and development of efficient and compact microchannel heat exchangers.