The modelling of drop behaviour in siphon riser tubes in paper drying cylinders

Abstract

A theoretical model has been developed for the drop behaviour in siphon riser tubes in paper drying cylinders. Analytical solutions to the equations are presented for the systems air—water at atmospheric pressure and saturated steam—water. Numerical simulations are performed for a large number of input data. Calculated drop trajectories indicate that the drops travel only a small part of the riser tube length before they are deflected to the frontside of the riser tube. For the system air—water the drop radial velocity when hitting the tube does not exceed 50% of the gas velocity, while with the system steam—water the radial velocity can be up to 90% of the gas velocity. The azimuthal drop velocities are in the range 2–10 m/s. The condensate slip relative to the cylinder results in an initial backward motion of the drops before they are accelerated to the frontside. Thus, part of the drops will hit the backside wall resulting, in the formation of a liquid film. This film will also be subjected to the coriolis force resulting in a liquid redistribution to the frontside wall. A criterion for cylinder flooding to occur, based on the fundamental differential equations, is presented. Good agreement with experimental data is presented for the system air—water in a 1.5 m model cylinder.