Numerical study on the heat transfer deterioration of supercritical CO₂ in straight tubes under different inclination angle

The heat transfer deterioration of supercritical CO₂ in straight tube is numerically studied by SST k−ω model under different inclination angles. Through quantitative analysis, the effects of buoyancy and thermal acceleration are evaluated. The heat transfer deterioration is divided into 4 stages: the initial stage, the early stage of stability, the late stage of stability and the recovery stage. By researching the distribution of turbulent kinetic energy and velocity distribution about y+ under different tube inclination angles, it is found that the turbulent kinetic energy and velocity distribution of vertical tube in the four stages have their own characteristics. The turbulent kinetic energy of non-vertical tube is not restrained in the area of 5 < y+ < 30, the velocity distribution on the cross-section approximately follows the 1/7 velocity law curve, and the difference of velocity values on each cross-section at each angle is small, so no heat transfer deterioration arises. In addition, it is found that when the heat transfer deterioration occurs, the maximum changes of density, dynamic viscosity, and fluid temperature as well as critical temperature all appear in the regime of 5 < y+ < 30. The dimensionless number K values are introduced to analyze its relationship with wall temperature variation and heat transfer deterioration.