Analysis of the effect of turbulence promoters in hollow fiber membrane distillation modules by computational fluid dynamic (CFD) simulations

Abstract

As an extended exploration of process enhancing strategies, nine modified hollow fiber modules with various turbulence promoters were designed and modeled using a two dimensional computational fluid dynamic (CFD) heat-transfer model to investigate their potential in improving heat transfer and module performance for a shell-side feed direct contact membrane distillation (DCMD) system.With the aids of turbulence promoters, the feed heat-transfer coefficient h f of the modified modules generally showed much slower decreasing trends along the fiber length compared to the original (unmodified) module. A 6-fold h f enhancement could be achieved by a modified module with annular baffles and floating round spacers. Consistently, the temperature polarization coefficient (TPC) and mass flux distribution curves of these modified modules presented increasing trends and gained an optimal improvement of 57% and 74%, respectively.With the local flow fields and temperature profiles visualized in CFD simulations, it was confirmed that an appropriate selection of turbulence promoters could promote intense secondary flows and radial mixing to improve the shell-side hydrodynamics and enhance heat transfer. Moreover, an increase of flow velocity was used and compared as a conventional approach to improve hydrodynamics. It was found that a well-designed module could bring more significant enhancement for a liquid-boundary layer dominant heat-transfer process.Finally, the hydraulic energy consumption (HEC) caused by the insertion of turbulence promoters or the increase of circulating velocity was compared. Configurations with attached quad spacers or floating round spacers achieved a good compromise between enhanced permeation fluxes and modest HECs. Overall, the TPC decreases with increasing MD coefficient (C) values and operating temperatures; while the thermal efficiency increases dramatically with increasing C and operating temperatures in a MD system.

Optimization of microstructured hollow fiber design for membrane distillation applications using CFD modeling

“…Also, the module specifications and the operating conditions are the same as used previously [38]. The heat-transfer model has been verified previously [38,41,42],based on an established DCMD system for a series of experimental settings, including various feed inlet temperatures, fiber lengths and flow velocities, and modified module configurations with external aids. Based on our previous verification results, the comparisons between the CFD simulation results and experimental data of mass flux (N m ) and feed pressure drop (¨P f ) showed good agreement with small relative errors of ± 5 %.…”

“…A recent CFD study, which proposed an improved heat-transfer model to couple the latent heat to the energy conservation equation and combine it with the Navier-Stokes equations, was performed by our group to address the transport relationships between the fluids (feed and permeate) and the membrane in a single-fiber MD module [38]. The model has also been used to analyze the effectiveness of different process enhancement strategies by 7 identifying the controlling local resistances in MD under various circumstances [41,42].…”

“…(5) both contribute to the permeation flux N m . Based on the prior studies, the C is an intrinsic mass-transfer coefficient of the membrane [43] and is calculated based on a combination of structural parameters (thickness, porosity, pore size and pore size distribution) [42]. Therefore, in the current study a general C value for the selected original straight and wavy designs is force of the system [3]; while the HEC (J·kg -1) and water production (kg·day -1 ) are used to assess the performance of these microstructured fibers in terms of the pumping electricity cost and water production.…”

“…Based on an analysis of the controlling heat-transfer resistance in different MD systems, external hydrodynamic aids can be employed to achieve a significant enhancement when the heat transfer through the liquid-boundary layers plays a dominant role [42]. Hence, in this study, a similar DCMD system [highly permeable membrane with membrane coefficient C = Table 1 were simulated, and while for the gear-shaped design, the improved heat transfer at the permeate side was credited 14 to the better hydrodynamic conditions on the corrugated outer membrane surface (i.e., feed side), which has much higher wall temperatures and hence a more efficient thermal diffusion across the thermal boundary layers at both sides.…”

Experimental and theoretical analyses of temperature polarization effect in vacuum membrane distillation

“…Researchers has taken advantage of the availability of powerful computers along with advanced Computational Fluid Dynamic (CFD) modeling packages and started to simulate the behavior of the fluid thin film near the membrane surface [11][12][13][14][15][16][17][18][19]. The central focus of these simulation studies has been to investigate the turbulence promoter's effect on the fluid temperature, velocity and pressure profiles near the membrane surface to see how these promoters could enhance the water vapor flux.…”