Numerical Study On Heat Transfer Characteristics Of Supercritical Carbon Dioxide And Its Mixture

In order to improve the efficiency of energy utilization,supercritical CO₂and its mixed working fluids have become a research hotspot in recent years because of their excellent heat transfer performance.In this paper,the flow and heat transfer characteristics of supercritical CO₂and supercritical CO₂/propane mixture in horizontal and vertical tubes are studied by using the numerical simulation method.The calculation conditions are 90/10,92/8,95/5 and 97/3 of the mixture concentration ratio of CO₂/propane;P=7.5～9.5Mpa;G=150～200kg·m^-2·s^-1;q=20～30kw·m^-2;T_in=293～303k.The distribution of velocity field and temperature field was analyzed,and the effects of pressure,mass flow,heat flux and mixing concentration on the heat transfer characteristics of supercritical CO₂and supercritical CO₂/propane mixture were studied.A new heat transfer correlation for the flow and heat transfer of supercritical CO₂and supercritical CO₂/propane mixture in horizontal and vertical tubes was fitted through the numerical simulation results.The main work is as follows:1.Four turbulence models were verified,and the results showed that RNG k-εThe turbulence model is more consistent with the experimental results,and the final turbulence model is RNG k-εturbulence model.2.The velocity and temperature fields of supercritical CO₂single working fluid in horizontal and vertical tubes were obtained.The effects of pressure,heat flux,mass flow rate and other factors on the heat transfer characteristics of supercritical CO₂single working fluid in horizontal and vertical tubes were studied.It was found that the heat flux density had the greatest impact on the flow and heat transfer of supercritical CO₂single working fluid in horizontal tubes,and the pressure had the greatest impact on the flow and heat transfer of vertical tubes.3.The velocity and temperature fields of supercritical CO₂/propane mixture in horizontal and vertical tubes were obtained.The effects of mixing concentration,pressure,heat flux density,mass flow rate and other factors on the heat transfer characteristics of supercritical CO₂/propane mixture in horizontal and vertical tubes were studied.It is found that the heat flux density has the greatest impact,the mixing concentration and mass flow rate have a greater impact,and the pressure change has a smaller impact on the heat transfer characteristics.4.Four heat transfer correlations applicable to the heat transfer of supercritical CO₂single working medium and supercritical CO₂/propane mixed working medium in horizontal and vertical tubes were obtained by using the least square method.The error of the obtained heat transfer correlation system is small,and the accuracy of the correlation formula is high.