Investigation On Effect Of Boundary Layer On Heat Transfer Deterioration Of Supercritical Fluids

The investigation on heat transfer of supercritical fluids is very important for its application in industrial field and the key issue is to avoid the heat transfer deterioration and further improve the heat transfer performance in supercritical mixed turbulent flows.With the development of supercritical heat transfer theory,the influence of boundary layer on heat transfer analysis is more critical,the analysis within boundary layer is of great significance to further deepen the supercritical fluid heat transfer theory,and help to improve the practical efficiency in industrial application.In this dissertation,a systematic study has been conducted experimentally and numerically to reveal the influence of boundary layer and buoyancy effect on heat transfer of supercritical fluids,main conclusions has been drawn as follows:Firstly,the visualized multi-purpose supercritical CO2 loop has been established in order to reveal the mechanism of supercritical heat transfer deterioration.An obvious "blurred shadow" is captured through the window when wall temperature rises.This phenomenon of local flow acceleration is one of the most intuitive evidences of supercritical heat transfer deterioration(HTD)and it is caused by buoyancy effect due to the decrease of fluid density.The local flow acceleration has proved the typical "M-shaped" velocity distribution in supercritical heat transfer deterioration and revealed the significant influence of boundary layer on heat transfer of supercritical fluids.Secondly,the supercritical CO2 convective heat transfer experiments have been carried out in a round tube with hydraulic diameter equivalent to that of the visualized experiment section to study the heat transfer performance of supercritical CO2 under a wide range of buoyancy.It has been concluded that the critical heat flux qonset,min at which HTD occurs is directly proportional to the square of mass flux G.A new universal criterion for HTD is proposed and this criterion has good universality and extrapolation.Comparing the results of tube experiments with visualization experiments,it can be found that the visualization experimental section with non-circlular flow channel shows the better heat transfer performance.Furthermore,the distribution of the normalized Nu(Nu-ex/Nu-Watts)versus buoyancy parameter Bo is analyzed.Thirdly,the performance of heat and mass transfer in the buffer layer(y+≈5～50)near to the hea(?)d wall is of great importance to the overall heat transfer of supercritical fluids:in heat transfer enhancement,the essence is the radial cumulative effect of specific heat at constant pressure cp,and when the fluid in buffer layer experiences the pseudo-critical temperature,the corresponding axial local heat transfer coefficient reaches the maximum value;in heat transfer deterioration,strong buoyancy effects may lead to the flow acceleration in boundary layer and the decrease of local turbulent shear stress,as a result,the turbulence is significantly impaired and the HTD occurs.Based on the mechanism of boundary layer effect and drastic variations of fluid properties near pseudo-critical temperature,a modification of variable Prt on traditional SST k-ω model has been proposed with further considering the effects of tube diameter and system pressure on the onset of HTD.The modified model has been verified by 26 individual supercritical CO2 experimental conditions,the obtained results not only can reproduce the wall temperature rise in HTD,but also can reflect the onset of HTD with increase of inlet temperature Tin and heat flux q.The modified model is of good applicability and its prediction accuracy is remarkably better than the existing traditional turbulence model,which means a certain popularization value for the design and industrial application of supercritical CO2 heat exchanger.Finally,the typical phenomenon in visualization experiments is reproduced by using the variable Prt model.Comparing the contours from numerical results with the visualized flow state directly observed in experiments,the comprehensive analysis on variations of fluid properties within boundary layer and the buoyancy effect on heat transfer deterioration has been carried out with detailed instructions.The applicability of modified model has been further improved by verifying the numerical heat transfer data to that of visualization experimental section(non-circular flow channel).