A comparative study on abnormal heat transfer of supercritical CO2 heated in vertical tubes

The heat transfer characteristics of supercritical CO₂ heated in vertical smooth tube are experimentally and numerically investigated. The results show that the M-shaped velocity corresponds to heat transfer enhancement (HTE) at G = 100 kg/m²s, q = 20 kW/m², and the heat transfer coefficient (HTC) is 15% higher than that of normal heat transfer (NHT). While, the M-shaped velocity causes significant heat transfer deterioration (HTD) before the T_pc at G = 278 kg/m²s, q = 35 kW/m², and the HTC of HTD is 28%–42% of that in NHT. According to the numerical analysis on the M-shaped flow structure, it reveals that the zero-velocity gradient point of abNHT (HTE and HTD) always falls into the buffer layer (5 < y⁺ < 30). While, the zero-velocity gradient point of the heat transfer recovery (HTR) is in the log-law region (30 < y⁺ < 60). The u/u₀ of the zero-velocity gradient point of HTE are larger than 1.35. The cross-section turbulent kinetic energy structure shows that smaller TKE in the near-wall region is the dominant factor for HTD and larger TKE in the core region is the dominant factor for HTE.