Preventing supercritical CO2 boiler overtemperature: A thermal inertia ratio (Xi) criterion for predicting heat transfer deterioration in gas-like region

Supercritical carbon dioxide (S-CO2) cycle technology provides high efficiency and flexibility, making it a promising option for advanced coal-fired power generation. However, uncertainties in convective heat transfer characteristics of S-CO2 under typical operating conditions (pressures of 9–33 MPa and temperatures of 90–650 °C, gas-like region) cause furnace wall overheating issues. This study investigated convective heat transfer behavior in the gas-like region S-CO2 within a vertical upward tube through experimental and numerical methods, focusing on heat transfer deterioration mechanisms. Results indicated that deterioration occurs under low mass flow rate, high heat flux, and inlet temperatures near the pseudo-critical point. Theoretical analysis suggested that the suppression of boundary layer turbulence mixing is the primary reason for this deterioration. Building on our previous work defining the thermal inertia ratio number Xi to indicate relative turbulence intensity in non-isothermal S-CO2, we proposed a new discriminated method for heat transfer deterioration. Additionally, a high-accuracy correlation for convective heat transfer was established using 2592 data sets, with 76.50 % and 96.80 % of calculated values falling within 10 % and 20 % error ranges. The established criterion and correlation for gas-like region S-CO2 offer theoretical support to address furnace overheating issues, improving safety and economic performance of S-CO2 boilers.