Experimental Investigation On The Pressure Drop And Heat Transfer Characteristics Of Supercritical Carbon Dioxide In Horizontal Tubes

For achieving high-efficiency power generation and refrigeration,CO2 cycles are receiving great attention and have been widely studied.In these cycles,the intube pressure drop of CO2 determines the required pumping power to maintain tube wall temperature and consequently affects the system efficiency,stability,and safety,and thus it is necessary to conduct in-depth research on the prediction of CO2 pressure drop characteristics and correlation.The change of pipe diameter will affect the CO2 heat transfer and pressure drop in the tube,which is of great significance for the design of CO2 cycle.In this paper,horizontal heated circular tubes with inner diameters of 8,10 and 12 mm were used as experimental sections.The pressure drop characteristics of CO2 in horizontal heating tubes with an inner diameter of 8mm under subcritical and supercritical pressures and the effects of different pipe diameters on supercritical CO2 flow and heat transfer were studied.The pressure range is 5.53～20.52 MPa,the mass flux range is 517.2～1285.0 kg/m2s,and the heat flux range is 100.2～402.9 kW/m2.According to the experimental results,the effects of mass flux,heat flux and experimental pressure on pressure drop characteristics are discussed.The results show that the pressure drop increases with the increase of mass flux and heat flux,but it shows complex behavior with the change of pressure.Therefore,a supercritical three-regime-model is introduced,and the change of pressure drop with pressure is attributed to the two-phase effect under subcritical and supercritical pressure.In addition,the experimental pressure drop is compared with the existing correlations to predict the pressure drop,and a new friction pressure drop correlation is developed.For the subcritical pressure,the modified Blasius correlation is used to obtain the modified Chisholm correlation,which can accurately predict the experimental friction pressure drop.For the supercritical pressure,considering the supercritical two-phase effect and introducing the pseudo vapor mass quality x,and the friction coefficient will decrease with the increase of x.Based on this,a new correlation of friction coefficient is proposed,and its prediction accuracy is greatly improved.This paper proves the similarity of CO2 flow characteristics under subcritical and supercritical conditions.In addition,the use of supercritical three-regime-model,which is similar to subcritical two-phase flow,provides a new way to understand supercritical fluid flow.In practical applications of supercritical CO2 power generation,pressure drop and heat transfer characteristics of different tube diameters will vary.In the study of supercritical pressure drop and heat transfer for tube diameters of 8,10,and 12 mm,it was found that larger tube diameters resulted in lower pressure drops,higher wall temperatures,and worse heat transfer,but the flow and heat transfer differences between different tube diameters decreased at high pressures.The friction coefficient correlation previously proposed can well predict both 10 and 12 mm pipe diameters.At lower supercritical pressure and x<1,because of the strong effect of supercritical two-phase effect,the influence of pipe diameter is not obvious,making Nu decrease first and then increase along the pipe axis.In the study,it is also found that the heat transfer at the top of the horizontal tube is better than that at the bottom,and the difference of heat transfer at the top and bottom is also different under different x,and the larger the pipe diameter,the greater the difference of heat transfer between the top and bottom.This is mainly related to the action of inertia force and gravity,so the Froude number Fr is introduced to explore the mechanism.In this paper,the flow and heat transfer characteristics of supercritical CO2 in horizontal pipes are studied by using supercritical three-regime-model,and very good results are obtained.The modified subcritical pressure drop correlation and the proposed supercritical friction coefficient correlation provide a good basis for engineering construction.