Pressure drop at low Reynolds numbers in woven-spacer-filled channels for membrane processes: CFD prediction and experimental validation

摘要

The energy consumption due to pumping power is a crucial issue in membrane processes. Spacers provide mechanical stability and promote mixing, yet increasing pressure drop. Woven spacers and their behaviour at low Reynolds numbers are less studied in the literature. Nevertheless, they are typical of some membrane technologies, as reverse electrodialysis (RED). RED is a promising technology for electric power generation by the chemical potential difference of two salt solutions within a stack equipped by selective ion-exchange membranes. The mechanical energy required for pumping the feed solutions, can dramatically reduce the net power output. In this work computational fluid dynamics (CFD) simulations of spacer-filled channels at low Reynolds numbers were carried out in parallel with an experimental campaign focused on the collection of data for model validation. Woven spacers 280-480 mu m thick were investigated at the flow rates typical of RED channels. The construction of the computational domain was based on measurements made by optical microscopy and micrometer. Fully developed flow conditions were assumed, thus, periodic boundary conditions were adopted (unit cell approach). The experiments were carried out in a flow cell with one channel. Pressure drops were measured with and without the spacer, in order to quantify the effect of inlet-outlet channel and identify the distributed pressure drops due to the woven nets. Experimental results showed that the distributed pressure drop along the spacer-filled channel for the cases investigated is around 40% of the overall pressure loss. The significant contribution of the manifolds is due to the relatively high velocity of the fluid entering and leaving the channel in radial direction in the inlet and outlet holes, as in the RED stacks commonly used. However, an improved geometry of the distribution and collection system can easily result in a significant reduction of hydraulic loss in this part of the stack. Therefore, the optimization of the spacer geometry is crucial. In this regard, a good agreement between CFD results and experimental data on hydraulic loss along the channel was found, thus confirming that a simple CFD model (as the one presented in this work) can be a powerful and cheap tool, able to efficiently evaluate the pressure drops within spacer-filled channels of any customised geometry.