Heat Transfer Characteristics And Flow Maldistribution Of S-CO₂ In Microchannel Heat Exchanger

As the supercritical carbon dioxide（S-CO₂）Brayton cycle has the characteristics of high cycle thermal efficiency,compact system,and small footprint,it can reduce power generation costs.S-CO₂ as a working medium’s power cycle is gradually increasing globally.Become a research hotspot.S-CO₂ Brayton cycle power generation is a new utilization method of the flue gas waste heat of marine diesel engines.There is great theoretical and practical significance to master the convective heat transfer characteristics of S-CO₂ and the flow distribution rules in printed circuit board heat exchangers（PCHEs）for the improvement of system thermal efficiency,avoiding heat transfer deterioration,and ensuring the safe operation of the system have.This article focuses on the heat transfer deterioration characteristics of S-CO₂ flowing upward in a vertical tube（mechanistic research）,the thermal and hydraulic characteristics of the precooler in the Brayton cycle（experimental and theoretical research）,and the flow distribution rule in PCHE（application research）.The conclusions are as follows:（1）The phenomenon of heat transfer deterioration（HTD）when S-CO₂ flows upward in a vertical pipe is discussed.The hybrid LES / RANS model is used for transient simulation.The HTD position and temperature peaks obtained by the simulation are in good agreement with the test results,indicating that the numerical model has high credibility and can accurately capture the flow regime and vortex structure of S-CO₂ flowing in the mainstream region.Firstly,the evolution process of the occurrence,development and recovery of HTD is analyzed from the transient perspective,then the corresponding relationship between the HTD temperature field and the velocity field is explained from the steady state perspective.And finally,combining the effects of buoyancy,acceleration,and relaminarization caused by changes in the physical properties of S-CO₂,the mechanism of the occurrence,development,and recovery of HTD in the vertical flow of S-CO₂ are clarified.（2）Experiments and numerical simulations of the thermal and hydraulic characteristics of the precooler in the Brayton cycle are performed.First,S-CO₂ and water heat transfer tests are performed in the PCHE test piece,and data of different working conditions are collected for analysis.Subsequently,a CFD numerical model was established and compared with the test conditions to ensure the accuracy and reliability of the CFD numerical simulation.Finally,different S-CO₂ pressure,S-CO₂ Reynolds number,and water Reynolds number were simulated.The influence of each factor on the local and overall characteristics of S-CO₂ and water heat transfer in PCHE was analyzed.The mechanism of heat exchange between S-CO₂ and water was revealed,and the heat transfer correlation of S-CO₂ was obtained by using multiple linear regression method;（3）A numerical simulation study was performed on the flow distribution characteristics of the PCHE test piece.The flow mechanism of the uneven distribution of the flow in each layer and each flow channel in the PCHE test piece was analyzed.The inlet header is optimized to be tapered and the optimal tapered angle range is determined.Subsequently,the influence of different taper angles on the thermal and hydraulic characteristics of the PCHE test piece was analyzed.The results show that when the taper angle changes between 6 ° and 9 °,the standard deviation of the flow maldistribution in the various layers in the PCHE test piece less than 0.05,the flow distribution of each layer is relatively even,and the comprehensive heat transfer performance of the PCHE test piece can be improved by 17.3% ～ 19.7%.