Study On The Heat Transfer Characteristics Of Supercritical Carbon Dioxide In Straight And Helical Tubes

With the increasingly serious problem of energy shortage, it is particularly important to improve the efficiency of HVAC equipment. Supercritical CO2 is widely used in air conditioning and heat pump systems because of its high efficient heat transfer. Due to the dramatic changes of the thermal physics parameters of the supercritical fluid within the pseudocritical region,the heat transfer process in the straight and helical tubes become very complicated. In this paper, the numerical simulation method is used to study the convective heat transfer process of supercritical CO2 in straight and helical tubes. The conclusions of this study can provide some reference for the design and optimization of supercritical pressure heat transfer equipment.Firstly, the related background and theory of convective heat transfer to supercritical CO2 in the tube are summarized. The concept of supercritical fluid, the related theory of convective heat transfer and the related theory of numerical simulation of flow and heat transfer are briefly described.Then,the convective heat transfer process of supercritical CO2 in the straight tube under heating condition of constant wall temperature is numerically simulated. The distribution of temperature, velocity, density and secondary flow of supercritical CO2 in different sections of straight tube are obtained in the simulation. The degree of buoyancy is evaluated by the criterion of buoyancy. The variation laws of the bulk mean temperature, bulk mean velocity,local convective heat transfer coefficient, Nusselt number and skin friction coefficient of supercritical CO2 in the straight tube are analyzed combining field synergy theory. The impacts of changing the inlet Reynolds number, wall temperature, pressure and gravity on the convective heat transfer to supercritical CO2 are discussed. The results show that the influence of buoyancy on the heat transfer can not be neglected within the pseudocritical region;increasing the inlet Reynolds number and the component of gravity which is vertical to the flow direction can strengthen the convective heat transfer in the straight tube; increasing the wall temperature makes the position where the convective heat transfer coefficient reach thepeak move slightly forward; increasing the pressure in the tube will reduce the peak value ofconvective heat transfer coefficient.Finally, the convective heat transfer process of supercritical CO2 in the helical tube under heating condition of constant wall temperature is numerically simulated. The applicability of the RNG ?-? turbulence model in the simulation of convective heat transfer to the supercritical CO2 in the helical tube is verified. The distribution of temperature, velocity and density in the convective heat transfer to the supercritical CO2 in the helical tube are obtained. The variation laws of the bulk mean temperature, bulk mean velocity and local convective heat transfer coefficient of the supercritical CO2 along the axial angle in the helical tube are summarized.The impacts of centrifugal force, buoyancy and variable physical properties on convective heat transfer to the supercritical CO2 are analyzed and evaluated. The effects of inlet Reynolds number and wall temperature on the local convective heat transfer coefficient of helical tube are discussed. The results show that the influence of centrifugal force is existing in the whole convective heat transfer process, and the influence of buoyancy can not be neglected before the fluid leaves the pseudocritical region; when the axial angle ??1035°, the convective heat transfer coefficient at the bottom generatrix is the largest, the convective heat transfer coefficient at the inner generatrix is the smallest and increases to the peak value first; the peak value of the local convective heat transfer coefficient increases with the increase of the inlet Reynolds number and decreases with the increase of the wall temperature.