An experimental study with high-speed PIV to characterize the laminar - turbulent transition in helically coiled reactors

Helically coiled reactors (HCRs) are widely used in process engineering and biochemistry, especially in micro-reactor applications, to enhance heat and mass transfer. Their design promotes excellent radial mixing with minimal axial back-mixing. At moderate Reynolds numbers, characteristic Dean vortices form within HCRs. Increasing flow velocities lead to more complex vortex structures. The transition from laminar to turbulent flow significantly impacts reactor performance. Various experimental and numerical studies have attempted to characterize this transition, often using a critical Reynolds number based on the curvature ratio δ = d/D. Although it is generally accepted that HCRs have higher critical Reynolds numbers than straight tubes, reported values differ widely, possibly due to variations in experimental setups and inlet conditions. We propose a novel experimental setup designed to minimize the influence of inlet and outlet conditions. Reynolds numbers from 460 to 9,650 were examined. Initial Laser Doppler Velocimetry (LDV) measurements reveal that velocity fluctuations are weaker near the inner wall and stronger near the outer wall. High-speed Particle Image Velocimetry (PIV) measurements corroborate these findings. Additionally, we demonstrate how inlet conditions influence the transition point and discuss different markers for the laminar-turbulent transition in HCRs through pseudo-3D visualizations, frequency analysis, and qualitative flow analysis.