Study On Heat Transfer And Flow Characteristics Of Supercritical CO₂ For Micro Tube

Supercritical CO₂ power generation technology has the potential to guide the development of the industry,and has attracted the attention of scholars from various countries.The performance of supercritical CO2 power cycle is better under high temperature and high pressure.As an important part of the supercritical CO₂ power cycle system,the heat exchange equipment will seriously affect the efficiency and performance of the whole power system.Therefore,it is of great significance to study the heat transfer capability of supercritical CO₂under high parameters.The development of micro tubes participating in heat exchanger can improve the performance of supercritical CO₂ power system.Based on the supercritical CO₂power cycle as the research background,the flow and heat transfer characteristics of the supercritical CO₂ flow in horizontal micro tube are studied.The model of the horizontal tube heat transfer is built by the CFD flow simulation software.The temperature range is300-500°C,the pressure range is 22-30MPa,the range of the Re is 10⁴-6×10⁴ micro flow passage.The change rule of heat transfer and friction resistance of supercritical CO2 in horizontal micro tube is numerically simulated.The main contents are as follows:?1?Firstly,the thermal physical properties of supercritical CO₂ are analyzed.The effects of various factors such as operating temperature,pressure,mass flow and flow direction on the flow and heat transfer of supercritical CO2 are discussed.The changes of the CO2 density,specific heat,viscosity and other thermal physical parameters of CO₂ under high temperature and high pressure are plotted by querying the NIST REFPROP physical property database,and the changes of CO₂ density,specific heat and viscosity under the near critical and high parameter conditions are analyzed.According to the year,an empirical correlation between local heat transfer Nu and frictional resistance coefficient of supercritical CO₂ is carding out.?2?In this paper,the CFD fluid simulation software is used to establish the flow and heat transfer model of the horizontal tube supercritical CO₂,and compare the velocity field distribution and the previous experimental results under the same conditions.The results show that the calculated values of the model are basically the same as those in the literature,and the coincidence degree is high,which proves that the calculation model has high accuracy and reliability,and can accurately simulate the change law of flow and heat transfer in the supercritical CO2 of the horizontal micro tube.?3?A theoretical analysis of the flow characteristics and heat transfer characteristics of the supercritical CO2 in a horizontal tube is carried out by heating the constant heat flow.The change tendency and law of the gravity g,the inlet temperature t_in,the inlet pressure p_in,the mass flow m.,the diameter d to the pressure drop?p,the friction drag coefficient C_f,the convective heat transfer coefficient h,Re and local Nu along the length direction are analyzed.The results show that the gravity effect is weakened under high temperature,high pressure and high Re.The results show that under the inlet pressure p_in=22-30MPa and the high parameter boundary condition of the inlet temperature t_in=300-500°C,the effect of gravity is not obvious,the influence of the inlet temperature and pressure on the supercritical CO₂convection heat transfer in the horizontal micro tube is not significant.The increase of the flow rate or the decrease of the pipe diameter can effectively increase convection heat transfer in the tube,and help to enhance the heat exchange strength in the tube.?4?By using the heating mode of constant heat flow,aiming at the high Re of high temperature and high pressure,the empirical correlation of local convection heat transfer Nu and friction resistance coefficient C_f for the supercritical CO₂ in the horizontal micro tube is deduced.The difference between software simulation value and other empirical formulas under the same working condition is compared with the empirical formula put forward by others.The results show that the empirical correlation of the local convection heat transfer Nu and the friction coefficient C_f of the supercritical CO₂ heat transfer in the horizontal micro tube is applicable to high temperature,high pressure and high Re,and most of the simulated value error range is less than 20%.