Investigation of Two-Phase Rimming Flow and Heat Transfer inside Rotational Paper Cylinder Dryers using Three Multiphase Computational Models

The paper industry uses rotating cylinder dryers that employ steam to heat the paper web moving over the cylinder outer walls. As steam condenses, the condensate is accumulated inside the dryers and evacuated using siphons. The form of condensate motion occurring inside a rotating dryer consisting of three modes: puddling, cascading or rimming. To help improve the drying performance, it is important to understand the fundamental thermal-fluid physics in the rotational dryer. Thus, the objectives of this study are (a) to investigate the dynamic two-phase flow and heat transfer behavior inside the rotational dryer at different rotational speeds; (b) to employ three different multiphase computational models, the Volume of Fluid (VOF) model, the Mixture model, and the Eulerian-Eulerian (E-E) model, and compare their results. The results show that the E-E model better captures the physics of condensate behavior inside the dryer. It also predicts very well the rimming speed in comparison with the empirical correlation although it takes longer computational time than the VOF model. The mixture model doesn't adequately capture the cascade and rimming physics due to excessive liquid dispersion. Based on the results, the categorization of the thermal-flow behavior of the liquid layer is expanded from the traditional three phases to five phases: puddling, transitional cascading, cascading, transitional rimming, and steady rimming. Generally, the heat transfer increases during the initial puddling period, followed by oscillatory attenuation during the cascade period, and finally reaches the steady state after rimming is achieved.