Flow and Heat Transfer Features of Supercritical Pressure CO₂ in Horizontal Flows under Whole-Wall Heating Conditions

Based on the first and second laws of thermodynamics, the heat transfer and flow (thermohydraulic) characteristics of horizontal supercritical pressure CO₂ (S-CO₂) in a circular pipe under heating conditions were investigated numerically. Heating flows in two different diameters (d) of 4 and 6 mm were simulated in pipes with pressures of 8 MPa, mass fluxes (G) of 300 and 400 kg/(m²·s), and heat fluxes (q) of 50, 75 and 100 kW/m². In the d = 4 mm pipe, the peak heat transfer coefficient (h_b) was about 3 times higher than in the d = 6 mm pipe, while the entropy production due to fluid friction in the 4 mm pipe was on average 1.1 times higher, and the entropy production due to heat transfer was on average about 67% lower. A 4 mm tube was employed to further evaluate the influence of the applied wall heat flux, the results demonstrated that the irreversibility due to heat transfer was on average more than 4 times higher when heat flux density was 100 kW/m² than when the heat flux density was 50 kW/m², while the peak of heat transfer coefficient increased by 1.4 times as q was decreased from 100 to 50 kW/m². The effect of thermal acceleration was ignored, while the buoyancy effect resulted in secondary flow and significantly affected the flow and heat transfer features. The jet flows were found in the vicinity of the lower wall of the pipe, which made the two fields of velocity and temperature gradient more synergistic, leading to an enhancement in heat transfer in the vicinity of the upper wall. The aggravation of heat transfer resulted in high irreversibility of heat transfer in the cross-sectional area near the wall, while the local friction irreversibility was less affected by the buoyancy effect, and the distribution was uniform. The uneven distribution of thermophysical properties also confirmed that the enhanced heat transfer occurred near the wall area at the bottom of the pipe.