The Role of Discrete Capillary Rings in Mass Transfer From the Surface of a Drying Capillary Porous Medium

Hafiz Tariq Mahmood,Evangelos Tsotsas,Abdolreza Kharaghani

doi:10.1007/s11242-021-01635-w

Hafiz Tariq Mahmood, Evangelos Tsotsas + Show 1 more

Open Access

https://doi.org/10.1007/s11242-021-01635-w

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Abstract

The mass exchange between the surface of a model capillary porous medium and the adjacent gas-side boundary layer is studied in the limiting condition of isothermal, slow drying. In order to quantify the role and significance of liquid films in the mass exchange process, three-dimensional pore network Monte Carlo simulations are carried out systematically in the presence and absence of discrete capillary rings. The pore network simulations performed with capillary rings show a noticeable delay in transition from the capillary-supported regime to the diffusion-controlled regime. These simulation results differ significantly from the predictions of classical pore network models without liquid films, and they appear to be more consistent with the experiments conducted with real porous systems. As compared to classical pore network models, the pore network model with rings seems to predict favorably the spatiotemporal evolution of wet and dry patches at the medium surface as well as of their relative contributions to the net mass exchange rate. This is apparent when the analytical solution of the commonly used Schlünder’s model is examined against the numerical simulations conducted using classical and ring pore network models.

Highlights

Drying is a mass transfer process that involves successive removal of a volatile liquid by evaporation from a moist porous medium
The ring pore network model (RPNM) presented here is reported in our earlier work, in which we studied the impact of capillary rings on intraparticle mass transport during drying of quasi-two-dimensional networks (Vorhauer et al 2015) and of three-dimensional particle packings (Kharaghani et al 2021)
Even though the surface pores are invaded by the gas phase and the breakthrough predicted by both classical 3D pore network model (CPNM) and RPNM occurs more or less at the same time, the drop is completely circumvented and the evaporation rate remains at the highest initial value for a significant period of drying in RPNM

Summary

Introduction

Drying is a mass transfer process that involves successive removal of a volatile liquid by evaporation from a moist porous medium. The pore-scale physical effect in connection with the present work is film flow which has been studied numerically and experimentally in systems ranging from a single capillary tube (Chauvet et al 2009, 2010), two-dimensional pore networks (Laurindo and Prat, 1998; Yiotis et al 2004), to quasi-two-dimensional micromodels (Vorhauer et al 2015; Chen et al 2017, 2018), as well as to much more complex systems such as three-dimensional (3D) particle packings (Wang et al 2012; Kharaghani et al 2021) In those model porous systems, liquid films have the form of either corner films or capillary bridges. Details of Schlünder’s model in connection to closure relationship and CPNMs have thoroughly been described in previous works (e.g., Lehmann and Or, 2013; Moghaddam et al.2018; Talbi and Prat, 2019; Lu et al 2021) While these works contributed to a much better understanding of the mass transfer at the surface, they disregarded the impact of the liquid films developing in geometrical singularities of the pore space during drying.

Ring Pore Network Model

Results and Discussion

Summary and Conclusions