Experimental investigation of internal flow structures and permeate flux behaviour in direct contact membrane distillation

Understanding internal hydrodynamics of Direct Contact Membrane Distillation (DCMD) is crucial for desalination performance and mitigating temperature polarization. Despite its importance, quantitative experimental characterization of hydrodynamics governing boundary-layer development and transport enhancement within DCMD channels remains limited. This study presents the first spatially resolved Particle Image Velocimetry PIV measurement of velocity fields inside a smooth, actively operating flat-sheet DCMD channel, linking observed hydrodynamic structures to permeate flux. Two-dimensional velocity fields were captured at different streamwise positions for feed flow rates ranging from 0.7 to 4 L·min −1 , enabling analysis of flow evolution and turbulence characteristics relevant to mass transfer. The results reveal strong deviations from the bulk velocity assumption of one-dimensional transport models, with average velocity overpredicted by up to 55% in channel cross sections. Increasing flow rate intensified near-membrane shear and rotational structures: channel-averaged turbulent kinetic energy increased from: 5 × 10 −5 to 6.5 × 10 −4 m 2 s −2 , accompanied by vorticity up to −370 s −1 and Reynolds shear stress reaching 2.8 × 10 −4 m 2 s −2 near the membrane surface. These changes reflect the emergence of strong shear layers and rotational structures that enhance mixing. Permeate flux increases from 4.10 to 15.47 LMH across the tested range, demonstrating that performance enhancement evolves in parallel with the strengthening of internal shear and rotational structures rather than bulk velocity alone. These findings provide quantitative experimental evidence of DCMD hydrodynamics and show that their evolution parallels the nonlinear increase in permeate flux, providing a basis for improving transport models and module designs reflect realistic flow behaviour.