On the influence of pore shape, contact angle and film flows on drying of capillary porous media

As pointed out in several previous works, thick liquid film flow can represent a major transport mechanism in drying. The effect of films is to greatly reduce the drying time compared to situations where they cannot develop. Using pore network simulations, we explore the influence of pore shape and contact angle on drying rates during the isothermal drying of porous materials in relation with the effect of liquid films when viscous effects are important in the films but not in the liquid saturated pores. It is shown that the overall drying time is greatly affected by the pore shape and contact angle when film flows are important and that incorporating the film effect in the pore network model leads to a much better agreement with experimental results. Film flows can significantly contribute to the occurrence and/or the duration of the constant rate period (CRP), which is a classical feature of convective drying. When film flows are important, the quantitative prediction of drying rate becomes very difficult for it depends on tiny details of the pore space geometry and is affected by possible changes in the local wettability conditions. This contributes to explain why the accurate prediction of drying rate still remains essentially an open question, at least when the effect of films cannot be neglected. (c) 2006 Elsevier Ltd. All rights reserved.