Viscous stabilization of drying front: three-dimensional pore network simulations
Advancing the chemical engineering fundamentals
Transport Phenomena in Porous/Granular Media - III (T2-7c)
Keywords: capillary porous media, invasion percolation, moisture profiles, drying rate curve
In this study, a recently developed pore network drying model, which accounts for liquid viscosity, is applied to three dimensions for the first time. Isothermal convective drying is simulated for a cubic network (25x25x50) with pore throats of mono-modal radius distribution. The role of liquid viscosity is assessed by comparison with non-viscous drying of the same network. Simulation results are presented as moisture profiles and drying rate curves.
If viscous effects are negligible, liquid is always pumped out of the largest meniscus throats by capillary forces and drying can be modelled as an invasion percolation process. Due to random distribution of throat size, the liquid phase gradually splits up into many (temporarily trapped) clusters. However, as long as the liquid is connected over the whole network, capillary pumping experiences no constraint. During this period, no drying front occurs, but saturation level drops more or less uniformly throughout the network. In the presence of a diffusive gas-side boundary layer, lateral vapour transfer ensures that liquid is evaporated at a constant rate. Gas penetrates into the depth of the network until a saturation is reached at which the liquid phase consists of small disconnected clusters. Then, if film flow is not considered, these clusters will evaporate one by one starting from the product surface, and a receding evaporation front is observed. Only in this period, drying rate will drop significantly due to complete drying out of the surface and additional internal mass transfer resistance.
In the case of important viscous effects, differences in capillary pressure are not enough to pump liquid from the largest meniscus throats to other evaporating meniscus throats. As a result, within each liquid cluster, many menisci can be moving. Concerning viscous effects on drying behaviour, several periods of the drying process must be distinguished. At the very beginning, when only some of the near surface throats are emptied, distances for capillary flow are short so that capillary forces still dominate over viscosity. Later on, as gas penetrates deeper into the drying body, distances for capillary flow get longer; additionally, surface saturation drops so that local evaporation rates at the remaining surface menisci increase (due to increased lateral vapour diffusion in the boundary layer). Both effects contribute to viscous stabilization of the drying front. As drying proceeds, this front recedes into the porous medium and gradually widens up because of reduced overall evaporation rates. In this limit, the role of liquid viscosity becomes negligible again.
In conclusion, viscous effects are decisive for phase distributions and drying rates in intermediate stages of the drying process, whereas initial and final behaviour are similar to the non-viscous case. Especially the duration of the first drying period is determined by the relative size of viscous effects, which are conveniently quantified by a Capillary number.
See the full pdf manuscript of the abstract.
Presented Wednesday 19, 11:20 to 11:40, in session Transport Phenomena in Porous/Granular Media - III (T2-7c).
