Experimental Investigation Of Heat Transfer In Vertical Upward And Downward Supercritical Carbon Dioxide Flow In Circular Tube

Compared with traditional thermal generator set,supercritical carbon dioxide(scCO2)power generation technology has particular advantage and can be applied for the next generation of coal-fired technologies,advanced energy systems and nuclear reactors.In order to apply supercritical carbon dioxide in engineering,we need to have an in-depth study of the convective heat transfer characteristics of scCO2.The existing researches on the convective heat transfer characteristics of scCO2 mainly focus on the vertical upward flow,and only a few schloars have studied the downward flow.In this paper,two experimental sections with different pipe diameters of 8mm and 10mm are used.Within the parameter range of pressure 7.8-21.0MPa,mass flow rate 500-1000kg/m2s,and heat flux density 50-300kW/m2,experiments have been conducted on the convective heat transfer characteristics of scCO2 in the vertical upward and downward flows.A large-amplitude and large-period flow instability phenomenon has been observed in the vertical upward flow.Experimental results show that heat transfer deterioration is more likely to occur at lower mass flow rates or larger heat flux densities.Increases of pressure or mass flow are beneficial to the improvement of the heat transfer performance,while the heat transfer deterioration is much more serious.Compared with the flow in the vertical ascending pipe,the process of the vertical descending flow is less prone to heat transfer deterioration.The experimental data and existing buoyancy and flow acceleration effect criteria were compared and analyzed.The results find that the common dimensionless criteria for measuring buoyancy and flow acceleration effects Bo*and Kv are difficult to predict normal heat transfer and heat transfer deterioration.There are certain deficiencies in the analysis of buoyancy and flow acceleration.By using pseudo-boiling theory,the temperature gradient number π13,which indicates the temperature gradient of the gas-like film near the wall,explains the reason for the different heat transfer characteristics in the upward and downward flow.The results show that when the heat transfer deterioration occurs,and π13 has also reached the peak point.The increase of the temperature gradient makes the gas-like film adhere to the wall surface,and the increase in the thickness of the local gas-like film leads to the deterioration of heat transfer.Through the data obtained in the experiment,the pseudo-boiling phenomenon is considered,and a new correlation formula including the pseudo-boiling number is fitted.The new correlation formula is applicable to both rising and falling flows.After we compare the new correlation formula with the correlation in the literature,we find that the new correlation formula has higher accuracy.The investigation of the correlation between supercritical heat transfer deterioration and heat transfer provides a reference for engineering design,which can more effectively avoid component burnout caused by local over-temperature and ensure the safe operation of equipment.In engineering practice,unstable flow will cause thermal stress fatigue of the equipment,which will seriously affect the service life of the equipment.In the experimen of a 10mm vertical upward circular tube,we find an oscillation with a large period and a large amplitude.This paper studies the influence of heat flux,inlet pressure and mass flow rate on the instability The results indicate that the flow instability occurs only in a certain heat flux range,and steady flow occurs when the heat flux is higher or lower than a certain value but the heat flux has no significant effect on the oscillation period.The study also shows that increasing the pressure and mass flow rate can improve the stability of the system.