Numerical Simulation Of Flow And Heat Transfer Instability Of Supercritical Carbon Dioxide In A Heating Tube

A supercritical fluid is a kind of fluid whose pressure and temperature are above the critical value.The fluid of this state is neither liquid nor gaseous and has the characteristics of almost liquid high density and nearly gaseous low viscosity.These characteristics significantly improve system efficiency as the working medium of the thermal power generation cycle.Therefore,the energy conversion technology,using supercritical carbon dioxide（SCO₂）as a working medium,which has the advantages of high thermal power conversion efficiency,compact equipment,and low operation cost,has become one of the research hotspots in the field of energy.Although there is no phase transition process in the energy conversion system of SCO₂,the thermophysical properties of SCO₂ at near-critical pressure change dramatically near the quasi-critical temperature.The drastic change of physical properties will cause the instability of flow and heat transfer in the pipe,make the heat exchange device form periodic mechanical stress and thermal stress,cause adverse consequences such as fatigue damage of mechanical components,and affect the operation safety of the system seriously.However,understanding the influence law of SCO₂flow and heat transfer instability is incomplete,and there is a lack of corresponding instability boundary discrimination formula.Therefore,this work further carries out numerical simulation research on SCO₂ flow and heat transfer instability from three angles and obtains the boundary map of SCO₂ flow and heat transfer instability to provide theoretical support for the application of SCO₂..Firstly,the flow heat transfer instability of SCO₂ in vertical pipe was studied,and the influence of inlet temperature,inlet pressure,inlet and outlet pressure drop and pipe diameter on the flow heat transfer instability of SCO₂ was explored.It is found that the system stability has a critical threshold with the change of operating parameters,and the system instability occurs when the critical threshold is exceeded.Finally,by integrating the data,the discriminant correlation of SCO₂ flow heat transfer instability with pseudo-critical number(N_TPC)as the instability boundary measurement parameter was obtained by Mcquardt method.Secondly,when studying the flow and heat transfer instability of SCO₂ in non-uniform heating pipe,the effects of heating mode and inclination angle are mainly discussed.The study found that half-ring heating is more stable than full-ring heating.In the inclined pipe,the system stability in the descending pipe decreases with the increase ofɑ,while that in the rising pipe improves with the increase ofɑ,while that in the horizontal pipe has the lowest instability.By integrating N_TPC comparison,the discriminant correlation for the occurrence of SCO₂ flow heat transfer instability in non-uniform heating pipes was obtained.Finally,the influence of inlet temperature,operating pressure and pipe diameter on the instability of flow and heat transfer of SCO₂ in large pipe diameter was studied.The results show that,under the parameters set in this paper,there is a maximum operating parameter for the instability of the system,and no increase of density wave oscillation is observed in the system when the maximum operating parameter is greater than the maximum operating parameter.By comparing the occurrence condition of supercritical water（SCW）and SCO₂instability with the actual application condition,it is found that the actual operation condition of SCO₂ deviates more from the unstable condition than the critical water.Based on the data fitting,the correlation equation for determining the SCO₂ flow heat transfer instability boundary under large pipe diameter was obtained.