Numerical study of flow-network interconnections and header geometry in a multi-channel tube-in-tube heat exchanger

Compact refrigerant–water heat exchangers in residential appliances often operate under severe packaging constraints, where header-induced flow maldistribution and pressure-drop penalties limit performance. However, the effects of flow-network topology in mixed serial–parallel configurations remain relatively unexplored. This study examines the effects of flow-network topology and header geometry on the flow distribution and thermo-hydraulic performance of a compact multi-channel tube-in-tube heat exchanger formed by covering a serpentine stainless steel tube with an outer casing to create water-side annular passages. A key feature is an inter-pass connected (IPC) configuration that links selected bends, converting a serial layout into a mixed serial–parallel network. Water-side CFD was conducted using an isothermal tube outer wall (45 °C) assuming near-isothermal condensation of R290 inside the tube. In Stage 1, three configurations (IPC1, IPC5, and IPC15) were evaluated over 𝑅 ⁢ 𝑒 = 3126–15626 using the goodness factor 𝐺 ⁡ 𝐹 * and Uniformity Index 𝑈 ⁢ 𝐼 . In Stage 2, IPC5 was optimized by varying three compact header concepts (IAH, IOAH, and IIOAH) and header angle. Flow distribution was validated via fluorescent dye visualization, showing good agreement with CFD. IPC connectivity reduces hydraulic resistance, and IPC5 provides the best thermo-hydraulic balance. Angled headers improve uniformity; however, at higher Reynolds numbers, local header losses reduce the benefit of improved distribution. Finally, correlations for the Colburn 𝑗 and Fanning 𝑓 factors are proposed as functions of Reynolds number and normalized header angle 𝜃 * for each header concept, providing a practical basis for preliminary design screening of similar compact heat exchangers.