  Optimization and design of full-scale membrane distillation (MD) systems
usually require Sherwood and Nusselt correlations that are developed from
lab-scale systems. However, entrance effects in lab-scale systems can
significantly impact heat, mass and momentum transfer in the reactor, therefore
affect the accuracy of the developed experimental Sherwood and Nusselt
correlations. Here, Computational Fluid Dynamics (CFD) simulations using
OpenFOAM are performed to understand the effects of right-angled bends and
inlet design on flow dynamics, temperature and concentration polarization in MD
systems. Simulation results show that the presence of right-angled bends and
inlets with sudden expansions lead to the formation of Dean vortices. Dean
vortices enhance perpendicular mixing in MD systems and reduce both temperature
and concentration polarization. Temperature and concentration polarization
coefficients in MD systems with right-angled bends and inlets with sudden
expansions vary significantly for the same volumetric flow rate. Our studies
show that lab-scale systems with the same volumetric flow rate but different
designs lead to significantly different Nusselt and Sherwood correlations. This
study demonstrates the importance of CFD-informed design of lab-scale systems
to minimize entrance effects and suppress Dean vortices for consistent model
development and calibration across multiple scales.
