Study of the heat transfer deterioration of supercritical CO2 in vertical pipes using a hybrid RANS/LES method

Abstract

Heat transfer deterioration (HTD) is one of the core factors affecting the efficiency of the supercritical carbon dioxide (S–CO2) Brayton cycle. When HTD occurs in a heat exchanger, the local wall temperature increases sharply, which may lead to pinch points in the heat exchanger of the circulation system and excessive thermal resistance, resulting in safety problems during the operation of Brayton cycle power generation systems. In studies concerning the HTD of S–CO2, most simulations currently use Reynolds-averaged equations (RANS) to predict and analyze the S–CO2 flowing upward in vertical tube transfer deterioration problems. In this paper, a hybrid RANS/LES method is used for simulations. The RANS model is used to describe the HTD in the near-wall region, and the LES model is used to calculate the flow details in the bulk region and accurately describe the flow regime and vortex structure of a compressible fluid with variable properties. Firstly, the growth and evolution process of HTD is analyzed from the transient-state, and then the corresponding relationship between HTD temperature and velocity field is expounded from the steady-state. Finally, combined with the buoyancy effect, acceleration effect and relaminarization caused by the change of S–CO2 physical properties, the mechanism of the occurrence and recovery of HTD flowing in vertical tubes and the second peak of wall temperature is expounded.