Numerical Simulation Study Of Heat Transfer Between Molten Salt And Superctritical CO₂ In CSP System

As the scale of non-renewable energy such as traditional fossil energies is shrinking,developing harmless and friendly renewable energy has become an urgent need at present.Solar energy is an important renewable energy type and one of the ways to utilize solar energy effectively is the solar thermal power（STP）.Molten salt is a common heat transfer and heat storage medium and has been applied both at home and abroad.Heat exchanger（HX）is one of the important equipment in the solar thermal power system.Hairpin HX that has higher heat exchange performance.It has the characteristics of single-pipe and single-shell as well as relatively more compact structure.Therefore,hairpin HX is very suitable to be used in STP system based on supercritical CO₂ Brayton cycles.This paper aims at the hairpin HX used in STP systems,and simplifies the hairpin HX to U-shaped double-pipe HX.Choosing binary chloride molten salt as the heat transfer medium and supercritical CO₂ as the working medium,the heat transfer characteristics between molten salt and supercritical CO₂ in the U-shaped double-pipe HX are analyzed by using the numerical simulation method.The influences of different inlet supercritical CO₂ parameters,inlet molten salt parameters and HX structural parameters on the heat exchange characteristics between supercritical CO₂ and molten salt.First,the heat exchange process between supercritical CO₂ and molten salt in the U-shaped double-pipe HX under the fixed parameter condition is simulated.The analysis results show that after the supercritical CO₂ enters the HX,the temperature of supercritical CO₂ gradually increases.The closer to the wall area,the higher the supercritical CO₂ temperature is.The distributions of CO₂ temperature,density and velocity are generally centrally symmetric when CO₂ does not pass through the turning portion of the HX.But in the bend of the HX,due to the influence of centrifugal force,the velocity of CO₂ near the upper side of the inner pipe of the HX goes far away from that near the lower side of the inner pipe of the HX,and the temperature and density differences occur.After entering the upper pipe through the bending part,this phenomenon gradually disappears with the increase of distance.The average CO₂ temperature increases from 740 K to 857.8 K of the outlet.The local heat transfer coefficient at the elbow has undergone a sudden change,especially at the upper and lower generatrices.That is,the outermost local convective heat transfer coefficient is the largest,with a maximum value of 6274.08 W·m^-2·K^-1,while the innermost convective heat transfer coefficient is the smallest,with a minimum value of 2349.58 W·m^-2·K^-1.Secondly,the effects of three inlet supercritical CO₂ parameters on the heat exchange characteristics between supercritical CO₂ and molten salt are analyzed.The results show that with the inlet CO₂ mass flow rate risen from 0.2 kg·s^-1 to 0.6 kg·s^-1,the local cross-section average convective heat transfer coefficient and average Nusselt number on the CO₂ side both increase significantly,the maximum local average convective heat transfer coefficient rises from 2914.43W·m^-2·K^-1 to 7119.98 W·m^-2·K^-1,and the maximum local average Nusselt number increases from737.81 to 1892.71.With the inlet CO₂ temperature increased,the local cross-section average convective heat transfer coefficient on the CO₂ side rises,but the average Nusselt number decreases.The effects of inlet CO₂ pressure on the local cross-section average convective heat transfer coefficient and average Nusselt number on the CO₂ side are not obvious.When the inlet CO₂ pressure rises from 20 MPa to 24 MPa,the maximum local average convective heat transfer coefficient on the CO₂ side rises from 5092.60 W·m^-2·K^-1 to 5155.01 W·m^-2·K^-1,but the maximum local average Nusselt number decreases from 133.09 to 1321.56.After that,the influences of two inlet molten salt parameters on the heat transfer characteristics between supercritical CO₂ and molten salt are analyzed.The results indicate that when the inlet molten salt mass flow rate rises,the average outlet CO₂ temperature increases,but the local average convective heat transfer coefficient has no obvious change,while the local average Nusselt number decreases slightly.With the inlet molten salt temperature risen,the average CO₂ temperature increases along the route,the local average convective heat transfer coefficient of CO₂ along the straight pipe before the bend slightly decreases,while that along the bend and subsequent straight pipes slightly increases,but the overall change is very small.And the local average Nusselt number has obvious decrease when the inlet molten salt temperature rises.With the inlet molten salt temperature risen from 900 K to 980 K,the maximum local average Nusselt number decreases from 1347.32 to 1315.65.Finally,the effects of inner diameter of inner pipe and elbow radius of the U-shaped double-pipe HX on the heat transfer characteristics between supercritical CO₂ and molten salt are analyzed.The results show that when the inner diameter of inner pipe and elbow radius change,the outlet CO₂ and salt temperatures both have no obvious changes,which are kept at about 857 K and 889K.In general,the variations of inner diameter of inner pipe and elbow radius can affect the local convective heat transfer coefficient,local Nusselt number and CO₂ pressure change along the route,but basically have no effects on the overall heat transfer behavior between CO₂ and molten salt.