Condensate flow inside paper dryer cylinders

The rimming flow of condensate in horizontal rotating dryer cylinders has been studied computationally by solving the full Navier-Stokes equations coupled with a volume of fluid method for tracking the free surface. It was shown that significant variations in both condensate velocity and thickness exist at moderate dryer speeds, whereas at higher speeds the variations are of less significance. Regardless of dryer speed or condensate film thickness, the film can be divided into two distinct regions: a viscous sub-layer adjacent to the cylinder wall and an inviscid, oscillating layer close to the free surface. The thickness of the viscous layer decreases as the dryer speed increases, whereas, for a certain speed, it is independent of the total film thickness. The computational results are compared with measurements of both the film thickness and the pressure normal to the cylinder wall. In both cases the agreement is excellent. Some implications for heat transfer through the condensate film are briefly discussed.