Direct numerical simulations of heat and mass transport in membrane distillation systems

Membrane distillation (MD) is an emerging process for desalinating hypersaline brines. The process flows warm brine and cool distillate water on opposite sides of a hydrophobic membrane. The temperature difference causes water to evaporate from the brine, travel as vapor through the membrane, and condense in the distillate. Two major challenges for MD are temperature and concentration polarization. Temperature polarization represents a reduction in the transmembrane temperature difference due to heat transfer through the membrane. Concentration polarization describes the accumulation of solutes near the membrane, which leads to undesirable salt precipitation. We develop an in-house CFD method to simulate 2-D and 3- D unsteady heat and mass transport within plate-and-frame MD systems. The coupled momentum, energy, and mass transport equations are solved with an efficient projection method. For the 2-D simulations, we perform a parametric study of temperature and concentration polarization under a variety of operating conditions. We show that the temperature and concentration boundary layers show self-similar behavior that satisfies power laws for the downstream growth. We also present 3-D results demonstrating polarization occurring both in the downstream and lateral directions.