Flow field and heat transfer investigation in tubes of heat exchangers with motionless scrapers

Flow pattern and thermal-hydraulic characteristics in an innovative tube insert have been experimentally and numerically investigated. The insert device is a concept envisioned for reciprocating scraped surface heat exchangers. It consists of a concentric rod, on which is mounted an array of semi-circular plugs fitted to the inner tube wall. In motionless conditions, the insert works as a turbulence promoter, enhancing heat transfer in laminar regime. Fundamental flow features in the symmetry plane of the tube have been assessed with Particle Image Velocimetry technique. A general model of the flow mechanism has been defined, which allows the identification of three regions along a geometrical pitch: recirculation bubbles, flow acceleration and transverse vortex. Results have been complemented with experimental data on pressure drop and heat transfer. The transition onset is clearly identified, and the mechanisms that promote turbulence at low Reynolds numbers are investigated and discussed. CFD simulations for different Reynolds numbers provide a further insight into the relation of the flow structures with wall shear stress, and their role in the local heat transfer augmentation.