Numerical analysis of axial heat conduction effect on combined forced and free laminar convection in horizontal tube

Axial heat conduction plays a significant role in microchannels of compact heat exchangers. However, the interplay between gravity-driven natural convection and axial heat conduction remains a subject of considerable debate, with the critical threshold for significant effects proving particularly elusive. This study numerically investigates axial heat conduction effects on mixed laminar convection in horizontal tube under constant heat flux. Using water as working fluid, the outer radius to tube length ratio, Re (Reynolds number), Gr (Grashof number), and heat flux range in this study are 25–50, 14–140, 0–10,000, and 1000 W/m², respectively. The results demonstrate that natural convection-induced secondary flows improve heat transfer, especially in the entrance region. Axial heat conduction also redefines the Nu (Nusselt number) distribution, shortens the mixed developing region length. K_sf (solid-to-fluid conductivity ratio) and δ (dimensionless wall thickness) show positive correlations with axial heat conduction effects, whereas Pe (Peclect number) exhibits an inverse relationship. Axial heat conduction exhibits heightened sensitivity to variations in K_sf when K_sf ≤ 16.7. Under conditions where δ > 0.3 and Pe < 200, axial conduction significantly affects mixed convection flow heat transfer. The findings provide critical insights optimizing microchannel design of compact heat exchangers.