Investigations On Flow And Heat Transfer Behaviors Of Supercritical H₂O/CO₂ Mixtures

Supercritical H₂O/CO₂ mixtures are the working fluids in a novel power generation system power with coal gasified in supercritical water.Their flow and heat transfer behaviors are important to design heat transfer devices and improve system efficiency.In this novel system,the supercritical H₂O/CO₂ mixtures have characteristics of high operating pressure and temperature,significantly molecular polarities between H₂O and CO₂,rather limited solubility of CO₂ in water and high CO₂ mass fractions（about 20%）.The research on the flow and heat transfer behaviors of supercritical mixtures with these characteristics is still lacking.Here,based on the experimental research and theoretical analysis,the heat transfer and hydraulic resistance of supercritical H₂O/CO₂ mixtures were investigated systematically,their unique flow and heat transfer mechanisms were analyzed.1.A high-temperature and high-pressure apparatus was established for the first time to investigate the flow and heat transfer behaviors to supercritical H₂O/CO₂ mixtures.The design pressure,design temperature and maximum CO₂ mass fraction of the apparatus was 25 MPa,650℃and 25%respectively.The unique problems of component concentration reliability,stability and safety for the high-temperature and high-pressure apparatus were solved.The oscillations of pressure,temperature and pressure drop of the apparatus in near-critical region are below±0.03 MPa,±0.3℃,±0.4 k Pa respectively,The relative uncertainties of the measured convection heat transfer coefficients and frictional pressure drops are 16.8%and1.3%.It provides hardware support for obtaining precision experimental data and revealing the flow and heat transfer mechanisms of supercritical H₂O/CO₂ mixtures.2.The phase equilibrium of H₂O/CO₂ mixtures was theoretically studied by SRK EOS combined with MHV2 mixing rule.Results show that the gas and liquid saturation lines of H₂O/CO₂ mixtures get close with the temperature increasing and converge at the critical miscible point.For subcritical temperatures,the two-phase region exists in the phase diagram.For temperatures above the critical miscible point,the mixtures can be totally miscible and become to homogeneous supercritical fluids.Also,the isobaric heat capacities and densities of H₂O/CO₂ mixtures were measured after reforming the test section of the experimental apparatus.The variations of the thermophysical properties of the mixtures were analyzed and the pseudo-critical points of the mixtures with different CO₂ mass fractions were obtained thereby.These investigations on the thermophysical properties of H₂O/CO₂ mixtures provide support for the further analysis of the flow and heat transfer mechanisms.3.The experimental investigations on heat transfer behaviors of supercritical H₂O/CO₂mixtures were carried out.In supercritical region,the heat transfer behavior of H₂O/CO₂mixtures is similar to supercritical water,which follows the homogeneous single-phase model.The forced convection heat transfer coefficients of the mixtures can be well predicted using the correlation developed for supercritical pure fluids.However,in near-critical region,heat transfer to H₂O/CO₂ mixtures shows an anomalous heat transfer enhancement,which could be characterized by a“double peak”phenomenon of heat transfer coefficients.The first peak temperatures decrease with the increasing CO₂ mass fractions and correspond to the pseudo-critical points of the mixtures,this enhancement mechanism is similar to that of supercritical pure fluids,i.e.the thermophysical property variation induced mechanism.The second peak temperature corresponds to the critical miscible point of mixtures,and thereby independent of the CO₂ mass fractions.The heat transfer enhancement mechanism corresponding to the second peak is unique for supercritical mixtures and could be summarized as the dissolution induced mechanism.4.The hydraulic resistance of H₂O/CO₂ mixtures were investigated experimentally.It is found that when fluid temperatures are higher enough than the pseudo-critical point,the friction factors of supercritical H₂O/CO₂ mixtures also conform to the homogeneous single-phase model and can be predicted by using the conventional correlations and the mixture thermophysical properties.In near-critical region,the friction factor curve for supercritical H₂O/CO₂ mixture features a pit near the pseudo-critical point,which is more significant and wider compared to supercritical pure water.Based on the analysis of the distinct pit segment,a new friction factor correlation for H₂O/CO₂ mixtures was proposed.This new correlation has a high prediction accuracy under the experimental conditions and can precisely describe the pit segment of friction factors in near-critical region.