Are Wall Shear Stress Fluctuations the Missing Hydrodynamic Parameter in Gas-Assisted Membrane Distillation?

Abstract

Gas–liquid two-phase flow is widely recognized as an effective strategy for enhancing membrane distillation (MD) performance by mitigating thermal and concentration polarization. However, most studies interpret this enhancement using time-averaged hydrodynamic quantities, such as mean wall shear stress. Here, we propose that dynamic hydrodynamic fluctuations inherent to slug flow may constitute an overlooked transport mechanism contributing to MD performance enhancement. By reanalyzing recent CFD results from the literature, we identify evidence of a possible relationship between trans-membrane permeate flux enhancement and the amplitude of wall shear stress fluctuations generated by the alternating passage of liquid slugs and elongated bubbles. Although the available evidence remains limited and does not permit definitive conclusions, the observed trend is consistent with findings previously reported for ultrafiltration and nanofiltration under gas–liquid two-phase flow. We therefore argue that wall shear stress fluctuations should be considered as a testable hydrodynamic parameter in future MD research, alongside conventional time-averaged quantities. If validated, this hypothesis would imply that transport intensification depends not only on wall shear stress magnitude but also on its temporal dynamics. This perspective could improve understanding of slug flow-enhanced transport and support the rational design and optimization of gas-assisted MD modules and operating strategies in future applications.