Towards a better understanding of high Reynolds number flow in U-tube heat exchangers: Insights from particle image velocimetry and computational fluid dynamics

• Turbulence intensity depends on U-tube bend radius and flow rate. • Increased turbulence boosts heat transfer but also erosion risk. • Experimental PIV validated CFD for flow in curved heat exchanger pipes. • Turbulence stabilizes downstream within a distance of 7 pipe diameters. • Regression model links flow rate and bend radius to turbulence intensity. Efficient and durable operation of heat exchangers requires understanding the flow behavior at high Reynolds numbers, especially in curved pipe segments such as U-bends. The study is motivated by quantifying turbulence-related erosion and flow maldistribution in compact heat exchangers. It is hypothesized that turbulence intensity downstream of the bend significantly depends on the pipe curvature and flow rate. To verify this, experimental velocity field measurements using Particle Image Velocimetry and numerical simulations based on the Reynolds-averaged Navier–Stokes equations with the k–ε model were performed for various U-bend radii (0.009–0.039 m) and flow rates (0.05–0.30 m 3 /h). The results show that smaller bend radii produce up to 45 % higher turbulence kinetic energy, while the flow stabilizes within 7 pipe diameters downstream. The relative increase in turbulence was expressed as a function of curvature and flow rate using a linear regression model, with R 2 > 0.99 for experimental and >0.89 for numerical results. These findings support improved thermal–hydraulic design of tubular heat exchangers by linking geometry with both performance enhancement and erosion risk mitigation.