Large eddy simulations and stereoscopic particle image velocimetry measurements in a scraped heat exchanger crystallizer geometry

The transport phenomena in scraped heat exchanger (HE) crystallizers are critical for the process performance. Fluid flow and turbulence close to the HE surface as generated by stirring elements and scraper blades are crucial in this respect as they aim at avoiding an insulating scale layer on the HE surface. For this reason we performed large-eddy simulations of the turbulent flow (at a Reynolds number of 5×10⁴) in a typical cooling crystallizer geometry with a focus on the bottom region where the heat exchanging surface was located. The flow simulations were validated with stereoscopic PIV experiments performed higher up in the crystallizer. Water at a constant temperature was the working fluid in the experiments as well as in the simulations. For reasons of optical accessibility being hindered by the scrapers, the experiments could not be done near the heat exchanging surface. The flow structures as revealed by the large-eddy simulations could explain the local occurrence of scaling on an evenly cooled HE surface, and its irreproducibility caused by instantaneous cold spots.