Flow pattern assessment in tubes of reciprocating scraped surface heat exchangers

Flow pattern in the tubes of an innovative scraped surface heat exchanger with reciprocating scrapers has been experimentally investigated. The scraper consists of a concentric rod inserted in each tube of the heat exchanger, mounting an array of semicircular plugs that fit the inner tube wall. A hydraulic piston provides the scraper with constant-velocity reciprocating motion. Phase-averaged velocity fields have been obtained with PIV technique for both scraping semi-cycles, with special emphasis on the effect of the scraping velocity (velocity ratio) and Reynolds number. Visualization results have been contrasted with experimental data on Fanning friction factor, obtaining a clear relation between flow patterns, pressure drop augmentation and turbulence promotion. CFD simulations for quasi-steady laminar flow provide a further insight into the relation of the flow structures with wall shear stress, and the contribution of pressure forces to global head losses, for each scraping semi-cycle.