Thermal analysis of supercritical CO₂ in a horizontal circular tube subjected to axially non-uniform heating

During flight, aircraft engine pre-cooler pipes typically experience uniform heating under stable external conditions. However, sudden environmental changes can introduce axially non-uniform heat flux, a scenario that remains underexplored in existing research. This numerical study investigates the flow and heat transfer characteristics of supercritical carbon dioxide (sCO₂) in a horizontal circular tube subjected to non-uniform axial heat flux conditions: monotonically decreasing (Type (a)) and monotonically increasing (Type (b)). Using the standard k-ε turbulence model with enhanced wall treatment, the study achieves reasonable wall temperature predictions compared to experimental data. Results highlight complex heat transfer behaviors under non-uniform heating compared to the uniform heat flux case (Type (c)). In Type (a) cases, heat transfer deterioration (HTD) near the inlet intensifies with increasing heat flux amplitude (A_i), with secondary flow effects playing a crucial role. Specifically, a vortex near the top wall impedes heat transfer, exacerbating HTD when T_b = T_pc. As A_i increases, the vortex center shifts closer to the top wall, intensifying the phenomenon. In contrast, HTD is absent in Types (b) and (c) at T_b = T_pc. Additionally, regions with high secondary flow kinetic energy (K > 5) are localized along y-axis symmetric walls, with peak K values increasing as heat flux rises at T_b = T_pc. These findings provide new insights into non-uniform heating effects in sCO₂ cooling systems, which are critical for aircraft thermal management.