The unique characteristics of supercritical CO2 (SCO2) make it have promising potential in chemical engineering, energy conversion and other fields, but the drastic variations in thermophysical properties also bring great challenges to the prediction of heat transportation processes. The thermal-hydraulic performance of supercritical CO2 (SCO2) flows in horizontal pipes during heating processes is investigated numerically from the viewpoints of the first and second laws of thermodynamics. Heated flows through pipes with a diameter of 4 mm and mass flux of 400 kg/(m2·s), at a pressure of 8.0 MPa, with three heat fluxes (50 kW/m2, 75 kW/m2 and 100 kW/m2) are simulated. The results showed that the heat transfer irreversibility is more than 4 times higher on average when the highest heat flux of 100 kW/m2 is applied relative to the lowest heat flux of 50 kW/m2, while the peak heat transfer coefficient
increases by ~1.4 times when the heat flux decreases from 100 kW/m2 to 50 kW/m2. The thermal acceleration effect is negligible, while the buoyancy effect leads to secondary flows and affects the heat transfer and flow characteristics significantly. A jet flow in the near-wall region at the bottom of the pipe improves the synergy between the temperature gradient and velocity fields, leading to a higher (more than 2 times) heat transfer coefficient in this region than in the near-wall region at the top of the pipe. The present work provides insights into the mechanisms and characteristics of SCO2 flow heat transfer as well as practical guidance on the design and optimisation of relevant components.