A novel porous media CFD model of direct contact membrane distillation in hollow fiber modules

A computational model was developed to predict the performance of hollow fiber Direct Contact Membrane Distillation (DCMD) modules. Feed and permeate are modelled as fluids flowing through two interpenetrating porous media; each fluid may indifferently fill either the lumen or the shell side compartment. In regard to hydrodynamics, on the shell side the Darcy permeabilities are derived from previous CFD work by the present authors, based on a unit-cell approach, for both axial and cross flow, while on the lumen side they are derived from the assumption of Poiseuille flow. In regard to heat transfer, on the shell side Nusselt numbers are computed from classic literature correlations for flow past tube banks, while on the lumen side a constant value representative of parallel flow in pipes is adopted. Bulk and wall temperatures and salt concentrations on each side are alternatively computed in an iterative way and are coupled by volumetric source/sink terms describing heat and mass transfer to the fluid flowing on the opposite side. The model predicts 3-D flow, temperature and salt concentration fields, along with overall performance parameters such as freshwater yield and recovery ratio, as functions of geometry and inlet flow rates and temperatures, requiring only the membrane permeance and thermal conductivity as empirical input. It was validated against two independent sets of experimental data for tap and brackish water presented in the literature for two laboratory-scale hollow fiber DCMD modules; predicted yields and their dependence on feed inlet temperature or flow rate agreed satisfactorily with experimental measurements.