| Supercritical fluid(SCF)is widely applied in various applications including power systems,and the accurate prediction of flow and heat transfer is important for engineering design.For example,heat transfer deterioration may lead to serious overshoot of wall temperature,large pressure drop of heat exchanger will lead to the reduction of cycle efficiency,and flow instability may cause thermal fatigue failure or deteriorated heat transfer of equipment.However,because the micro-and macroscopic physical behaviors of SCFs are still unclear,the heat transfer and flow mechanism of SCF has not been well understood.Classically,it is believed that the supercritical region is homogeneous and single-phase at molecular scale.Only recently,the macroscopic transition from liquid-like to gas-like supercritical states when crossing the Widom line(WL)is successfully detected by experiment.However,the pseudo-boiling phenomenon has not attracted enough attention,and the pseudo-boiling theory has not been comprehensibly used to study the flow and heat transfer of SCFs.In this dissertation,within the framework of SCF pseudo-boiling,heat transfer and flow of SCF is studied theoretically and experimentally,focusing on heat transfer,pressure drop,and flow instabilities.The heat transfer mechanism of pseudo-boiling is described,and a set of dimensionless numbers are proposed to analyze SCF heat transfer quantitatively.Based on the traditional theory of SCF convective heat transfer,the effect of buoyancy and flow acceleration on the flow and heat transfer is investigated again,and it is found that the effect of buoyancy and flow acceleration effect is not enough to reveal the mechanism of SCF convective heat transfer.Therefore,based on the pseudo-boiling framework,the ? theorem criterion of multi-parameter system is introduced and thirteen dimensionless numbers are proposed through the analysis of flow and temperature fields.Two important dimensionless numbers are further selected,namely,the velocity number which represents the competition between the growth velocity of the near-wall gas-film and the bulk flow velocity,and gas-film temperature gradient number which represents the temperature gradient in the near-wall gas-film.A larger velocity number indicates the rapid production of morehigh-temperature fluid in the near-wall region and a larger temperature gradient number causes the gas film to cover the wall surface and to thicken.Heat transfer deterioration may be occurring when the cooler liquid-like fluid of the core region cannot rewet the hot wall efficiently,similar to subcritical film boiling.Heat transfer is analogized between supercritical pressures and subcritical pressures to create a new dimensionaless supercritical-boiling-number SBO,qualitatively representing the bubble expansion induced momentum force against the inertia force.This new dimensionless number can explain heat transfer deterioration of SCF induced by pseudo-boiling successfully and the similarity between heat transfer deterioration of SCF and subcritical departure from nuclear boiling,which provides a theoretical analysis method for the study of supercritical pseudo-boiling heat transfer.The onset of heat transfer deterioration and the distribution of local wall temperatures in forced convection of supercritical CO2(sCO2)in a vertical heating tube are investigated experimentally,with tube diameters including 8 mm,10 mm and 12 mm.The effects of tube diameter on heat transfer is fully revealed,and the experiment shows that the tube diameter has no effect on the onset of heat transfer deterioration,and sudden changes from normal heat transfer(NHT)to heat transfer deterioration(HTD)with obvious temperature peak occur when crossing a critical SBO,which is 5.126×10-4 for sCO2.The tube diameter influences the degree of heat transfer deterioration:the larger the tube diameter,the higher the wall temperature.The onset criterion of CO2 heat transfer deterioration is extended to H2O,R134a and R22,which is 2.018×10-4,1.653×10-4 and 1.358×10-4,respectively.Based on the mechanism of heat transfer deterioration induced by pseudo-boiling,a new method is proposed to predict the location and value of wall temperature peaks when heat transfer deterioration occurs,and the mechanism of multiple wall temperature peaks in forced convection heat transfer is explained in vertical heated tube.The experimental data show that Pr,which is determined by bulk temperature,has an important effect on the heat transfer of SCF.In addition,the multi-peak phenomenon of wall temperature in the forced convection heat transfer is systematically studied.It is caused by the alternating control of local evaporative momentum force or local inertia force along flow direction,which leads to the oscillation of film thickness and wall temperature.In recent years,most attention has been paid to the heat transfer in the flow channel under supercritical pressures,but the flow pressure drop is relatively less reported.The effects of physical properties are usually considered in the study of the flow pressure drop of supercritical fluid.However,it is not an effective method to simply apply correction terms in the traditional empirical correlations of isothermal fluids.In this dissertation,the flow pressure drop of sCO2 in the vertical tube under high temperature and high pressure are investigated experimentally.The influences of various parameters on the frictional pressure drop are discussed.Experimental results found that friction factor and heat transfer mode display the two regimes distribution:a normal heat transfer(NHT)regime with smaller friction factors at smaller SBO,and a heat transfer deterioration(HTD)regime accompanying sharply increased pressure drops beyond a critical SBO.We propose an orifice contraction effect due to the strong vapor expansion,explaining the HTD induced rise of pressure drops.We show that K can be the similarity criterion number to connect pressure drop and heat transfer.A new correlation of friction factors is developed for sCO2,which is suitable for both NHT and HTD.Although there is no first-order gas-liquid phase transition under supercritical pressures,SCF can still be divided into two regions of gas-like and liquid-like,and it is still possible for SCF to undergo violent unstable flow when crossing pseudo-critical temperature Tpc.In this dissertation,the flow instability phenomenon of sCO2 in the vertical heated tube is investigated experimentally.The essential characteristic,influencing factors and occurrence conditions of the flow instability are studied comprehensively.A dimensionless density ?*is proposed to characterize the nonuniformity of axial density.The flow stability boundary is discovered,and the mechanism of the flow instability observed in the experiment is explained.Based on the flow and heat transfer characteristics as well as the relative magnitude of outlet temperature Tb.out with respect to the pseudo-critical temperature Tpc,the flow and heat transfer can be categorized into five types.The experiment observed that the supercritical CO2 flow instability occurs only after heat transfer deterioration,the outlet temperature Tb,out is higher than and near the Tpc,and the phenomenon is associated with pseudo-boiling heat transfer.When the gas-film fully expands near the wall,the space at flow upstream is occupied by heavy liquid-like fluid,and when the outlet temperature is higher than and near the Tpc,the space at flow downstream is occupied by gas-like fluid,which forms a gas plug.Due to large density difference,the plug cannot stably flow at the outlet,and it is pushed out of the tube by the upstream liquid-like fluid,generating a large pressure pulse which enters the whole system.At this time,the heat transfer deterioration becomes more serious and flow instability occurs.When the heat transfer gradually recovers,the gas plug appears at the outlet again.When the phase difference between mass flux fluctuation and pressure drop fluctuation is 90°,the flow instability remains unchanged. |
PDF Full Text Request