Analogical Study On Heat Transfer Characteristics Of Supercritical And Subcritical CO₂ In Horizontal Tubes

As a fluid with excellent properties,supercritical carbon dioxide is widely used in many fields such as drug production,food processing and pollution treatment.In the field of power generation,the supercritical CO2 Brayton cycle has its unique advantages compared with the traditional Rankine cycle and it has a good application prospect in the future power generation technology.Therefore,it is particularly important to study the heat transfer characteristics of supercritical CO2 in tubes as a working fluid.At present,experimental research on heat transfer characteristics of supercritical CO2 mainly focus on vertical tubes,while horizontal tubes are few.Meanwhile,the similarity between supercritical fluids and subcritical fluids has been known,but the comparative studies between them are relatively few,especially in the large parameter range of heat transfer characteristics.In this paper,an experimental section of 8 mm is used to conduct experimental studies under supercritical and subcritical pressures of horizontal pipe heating,and two different pipe diameters of 10 mm and 12 mm are supplemented.The experimental parameters of all pipe diameters are as follows:Pressure range 5.51-21.23 MPa,mass flux range 480.0-1346.1 kg/m2s.heat flux range 97.34-404.1 kW/m2.The heat transfer characteristics of CO2 horizontal tubes under supercritical and subcritical pressures were analyzed experimentally,and the similarity of heat transfer between supercritical and subcritical tubes was compared under the newly proposed supercritical three-regime-model,and the different characteristics of heat transfer in different regions were highlighted.Finally,the collected CO2 correlation formula in tubes was compared and analyzed.The experimental results of supercritical and subcritical heat transfer comparison show that there is a difference in heat transfer in the direction of horizontal pipe due to the density difference between light and heavy fluids.The subcritical pressure heat transfer far below the critical pressure presents a typical subcritical flow boiling heat transfer with obvious peak wall temperature and the corresponding heat transfer coefficient decreasing.The subcritical pressure heat transfer at near critical pressure is not consistent with the typical subcritical flow boiling heat transfer but it appears to be close to supercritical heat transfer.The wall temperature along the supercritical pressure did not appear obvious peak value,and the wall temperature gradually increased along the supercritical pressure.The supercritical three-regime-model establishes a connection between the heat transfer between supercritical and subcritical.By comparing the experimental Nu with the hypothetical model of the single-phase model,it is found that the deviation between supercritical and subcritical in the two-phase/two-phase-like region is large for the single-phase model,while the deviation in the single-phase region is small.On the one hand,it reflects the similarity of heat transfer under two kinds of pressure,and on the other hand,it also reflects that heat transfer under supercritical pressure also has obvious differences in different regime.In order to illustrate the necessity of supercritical partitioning and compare the existing correlations,experiments under supercritical pressure of 10 mm and 12 mm pipe diameters are added.The experimental results show that there are differences in heat transfer between different pipe diameters,and the larger pipe diameters are generally worse in heat transfer.In addition,increasing mass flux and decreasing heat flux can appropriately weaken the heat transfer difference between different diameters.After supplementing the data,the Nu ratio diagram under supercritical pressure was drawn.The results were consistent with the trend of the supercritical subcritical comparison diagram,which further verified the obvious differences in heat transfer in different regions under supercritical pressure,especially between the two-phase-like region and other regions,and also showed the rationality of such partition.Finally,the existing correlations are compared with the obtained experimental data,and the results show that there are great differences in the prediction of correlations in different regimes,which is due to the different heat transfer mechanisms in different regimes.The heat transfer correlation with good prediction in each region is selected to predict some working conditions,and a good prediction is obtained,which further demonstrates the necessity of heat transfer partition.This study illustrates the similarity of heat transfer in tubes between supercritical and subcritical,and verifies the obvious differences in heat transfer in different supercritical regimes by using pseudo-vapor mass quality as partitioning basis,which makes preparation for the proposal of a new correlation and provides a basis for the design of corresponding equipment.