Flow orientation controls gravitational convection and desalination performance in spacer-free electrodialysis

Achieving a high degree of desalination at minimal driving force remains a central challenge in electrodialysis. This objective is hindered by concentration polarization, which inherently develops at ion-exchange membranes facing the diluate channels. The formation of thin, nearly salt-depleted boundary layers significantly increases electrical resistance, thereby requiring higher applied potentials to sustain current. In practice, flow spacers are introduced to enhance mixing and mitigate polarization; however, this strategy comes at the cost of increased pumping energy, reduced residence time, and a decrease in the effective membrane area. In this work, we experimentally compare upward, downward, and horizontal feed-flow orientations and use a coupled transport model to examine the continuous change in the axial gravity component between the upward and downward limits. The superposition of pressure-driven and gravitational convection reorganizes the concentration and velocity fields in the diluate channel without substantially affecting current efficiency. In the upward configuration, gravitational convection assists the pressure-driven motion within the low-density ion-depleted layers and flattens the axial velocity profile. In the downward configuration, it opposes the pressure-driven flow near the membranes, focuses the feed toward the more concentrated channel interior, and increases polarization losses. The differences become stronger with increasing NaCl concentration. For a 0.1 M NaCl feed at an applied voltage of 50 mV, the reduced-order angular simulation predicted a continuous decrease in mean current density from approximately 35.0 A m −2 in the upward limit to 29.0 A m −2 in the downward limit; the value at the zero-axial-gravity limit was approximately 33.7 A m −2 . The fully upward orientation therefore provided the highest calculated current density under the modeled conditions, but the broad high-performance range at upward and moderately inclined orientations does not indicate a sharply defined or universally optimal angle. The robust design implication is that orientations containing an increasingly strong downward flow component become progressively less favorable.