Numerical Simulation Investigation On Convective Heat Transfer Of Supercritical Carbon Dioxide In Uniform And Non-Uniform Heating Channels

In recent years,China has pursued a new development philosophy,promoted high-quality development and followed the global trend of green and low-carbon development.Supercritical carbon dioxide（S-CO₂）cycle power generation technology has attracted wide international attention due to its unparalleled advantages in system high efficiency and flexibility,which has become a hot topic in line with the background of the times.The S-CO₂ thermal properties show drastic nonlinear changes near the proposed critical point,which makes the boundary layer structure of its flow in the channel more complex,and the heat transfer law more difficult to predict,which makes heat transfer deterioration（HTD）unpredictable.However,HTD will cause the wall temperature to soar,and even lead to over-temperature tube explosion,which seriously endangers the engineering safety.However,current investigations on the convective heat transfer characteristics of S-CO₂ are mainly focused on the condition of axial uniform heating and low mass flux,and there is also a lack of enhanced channels that are convenient for processing.There are few heat transfer correlations applicable to high mass flux conditions.In the investigation,the characteristics of S-CO₂ convective heat transfer in uniform and non-uniform heating channels were analyzed in detail by numerical simulation,and a new method to optimize the channel structure was proposed.Firstly,the ability of S-CO₂ convective heat transfer predicted by seven low Reynolds number turbulence models was evaluated and selected by comparing a large number of simulation results with the convective heat transfer experimental database.The SST k-ωturbulence model was identified as the optimal model,which laid a foundation for the accuracy of the numerical simulation.Secondly,the effects of different operating parameters on S-CO₂ convective heat transfer in vertical heating tubes were discussed under the conditions of cons tant heat flux and high mass flux.The results showed that the buoyancy force and flow acceleration effect could not explain the heat transfer deterioration phenomenon under the condition of high mass flux.Finally,based on the pseudo-boiling theory,the S-CO₂ boiling heat transfer model in the vertical heating tube was established to demonstrate the actual distribution of S-CO₂ in the vertical heating tube,and the thermal properties and flow field distribution of the fluid in the cross-section were combined to successfully reveal the S-CO₂ heat transfer mechanism at high mass flux and elaborate on the causes of heat transfer deterioration.By introducing the supercritical boiling number SBO,it was created about the heat transfer correlation applicable to the condition of high mass flux.In view of the actual distribution of thermal boundary conditions in the cooling wall of the S-CO₂ coal-fired boiler,the convective heat transfer characteristics of S-CO₂ under non-uniform heating conditions were explored,and the effects of operating pressure,mass flux,heat flux,and heating method on the convective heat transfer characteristics of S-CO₂ were investigated.The results showed that the non-uniform heat flux distribution would have a significant effect on the heat transfer of S-CO₂,which could inhibit heat transfer deterioration effectively.However,the effect of operating parameters on S-CO₂ convective heat transfer was consistent regardless of the uniform or non-uniform heating conditions.By correcting the equations of buoyancy force and flow acceleration effect,it was found that still could not explain the heat transfer deterioration well.Finally,based on the boiling-like theory,the heat transfer characteristics of S-CO₂ were elaborated under non-uniform heating conditions,and the heat transfer mechanism was revealed.By introducing the supercritical K-number,it was created about the heat transfer correlation applicable to the non-uniform heating condition.In order to restrain or eliminate the heat transfer deterioration that might occur in engineering applications,some new types of channels were designed based on the previous research on heat transfer enhancement methods.This kind of channel could effectively restrain heat transfer deterioration.One of the design methods was applied to the channel design of the printed circuit board heat exchanger（PCHE）.The results showed that the gradual expanded channel could effectively enhance the heat transfer compared with the uniform cross-section channel,and the total heat transfer coefficient could be increased by 101.7%under the same working condition.The implementation of this investigation could not only provide a new perspective for the research of supercritical heat transfer,but also bring important academic significance to enrich and develop the theory of supercritical heat transfer has to.From the perspective of application requirements,the results could not only provide theoretical guidance for the design of S-CO₂ boiler but also provide new ideas and directions for the optimal design of the S-CO₂ cycle power generation system.