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1. Generating buoyancy-driven convection in membrane distillation

   https://doi.org/10.1016/j.memsci.2024.123043  

   Mabry, Miles; Municchi, Federico; Liu, Yiming; Wang, Jingbo; Cath, Tzahi Y; Turchi, Craig S; Heeley, Michael B; Hoek, Eric MV; Jassby, David; Martinand, Denis; et al (September 2024, Journal of Membrane Science)

   NA (Ed.)

   Membrane distillation (MD) is a thermally-driven desalination process that can treat hypersaline brines. Considerable MD literature has focused on mitigating temperature and concentration polarization. This literature largely neglects that temperature and concentration polarization increase the feed density near the membrane. With gravity properly oriented, this increase in density could trigger buoyancy-driven convection and increase permeate production. Convection could also be strengthened by heating the feed channel wall opposite the membrane. To investigate that possibility, we perform a series of experiments using a plate-and-frame direct contact MD system with an active membrane area of 300 cm2 and a feed channel wall heated using a resistive heater. The experiments measure the average transmembrane permeate flux for two gravitational orientations, feed Reynolds numbers between 128 and 1128, and wall heat fluxes up to 12 kW/m2. The results confirm that with gravity properly oriented, wall-heating can trigger buoyancy-driven convection for a wide range of feed Reynolds numbers, and increase permeate production between roughly 20 and 130 %. We estimate, however, that at high Reynolds numbers (𝑅𝑒 > 800), more than 70 % of the wall heat is carried out of the MD system by the feed flow, without contributing to permeate production. This suggests the need for longer membranes and heat recovery steps in any future practical implementation. 

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2. Implementation of the Accurate Conservative Phase Field Method for two-phase incompressible flows in a finite volume framework, Proc. of the 9th Int. and 49th National Conf. on Fluid Mechanics and Fluid Power (FMFP), December 14-16, 2022, IIT Roorkee, Roorkee-247667, Uttarakhand, India. FMFP2022–1300.

   Jain, S.; Tafti, D. K. (December 2022, Proc. of the 9th Int. and 49th National Conf. on Fluid Mechanics and Fluid Power (FMFP))

   The phase field method provides a simple mass conserving method for solving two-phase immiscible - incompressible Navier-Stokes Equations. The relative ease in implementing this method compared to other interface reconstruction methods, coupled with its conservativeness and boundedness makes it an attractive alternative. We implement the method in a parallel structured multi-block generalized coordinate finite volume solver using a collocated grid arrangement within the framework of the fractional-step method. The discretization uses a second-order central difference method for both the Navier-Stokes and the phase field equations. A TVD-based averaging technique is used for calculating density at cell faces in the pressure correction step to handle high-density ratios. The simulation framework is verified in standard test cases: Zalesak Disk, a droplet in shear flow, Solitary Wave Runup, Rayleigh Taylor Instability, and the Dam Break Problem. A second-order rate of convergence and excellent phase volume conservation is observed. 

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3. Flow and transport in a pleated filter

   https://doi.org/10.1063/5.0102940  

   Fong, Daniel; Sanaei, Pejman (September 2022, Physics of Fluids)

   A pleated membrane filter consists of a porous membrane layer, which is surrounded by two supporting layers, and the whole structure is pleated and placed into a cylindrical cartridge. Pleated membrane filters are used in a variety of industrial applications, since they offer more surface area to volume ratio that is not found in equivalent flat filters. In this work, we introduce a novel three-dimensional model of a pleated membrane filter that consists of an empty region, a pleated region, and a hollow region. The advection diffusion equation is used to model contaminant concentration in the membrane pores along with Darcy's law to model the flow within the membrane and support layers, while the Stokes equation is used for the flow in the empty region and the hollow region. We further use the key assumptions of our model based on small aspect ratios of the filter cartridge and the pleated membrane to simplify the governing equations, which can be easily solved by numerical methods. By performing these steps, we seek to discover an optimal pleat packing density to find the optimum filter performance, while not exceeding a threshold for the particle concentration at the filter outlet. 

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4. An efficient tree-topological local mesh refinement on Cartesian grids for multiple moving objects in incompressible flow

   https://doi.org/10.1016/j.jcp.2023.111983  

   Zhang, W.; Pan, Y.; Wang, J.; Di Santo, V.; Lauder, G.; Dong, H. (April 2023, Journal of Computational Physics)

   This paper develops a tree-topological local mesh refinement (TLMR) method on Cartesian grids for the simulation of bio-inspired flow with multiple moving objects. The TLMR nests refinement mesh blocks of structured grids to the target regions and arrange the blocks in a tree topology. The method solves the time-dependent incompressible flow using a fractional-step method and discretizes the Navier-Stokes equation using a finite-difference formulation with an immersed boundary method to resolve the complex boundaries. When iteratively solving the discretized equations across the coarse and fine TLMR blocks, for better accuracy and faster convergence, the momentum equation is solved on all blocks simultaneously, while the Poisson equation is solved recursively from the coarsest block to the finest ones. When the refined blocks of the same block are connected, the parallel Schwarz method is used to iteratively solve both the momentum and Poisson equations. Convergence studies show that the algorithm is second-order accurate in space for both velocity and pressure, and the developed mesh refinement technique is benchmarked and demonstrated by several canonical flow problems. The TLMR enables a fast solution to an incompressible flow problem with complex boundaries or multiple moving objects. Various bio-inspired flows of multiple moving objects show that the solver can save over 80% computational time, proportional to the grid reduction when refinement is applied. 

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5. Membrane distillation at the water-energy nexus: limits, opportunities, and challenges

   https://doi.org/10.1039/C8EE00291F  

   Deshmukh, Akshay; Boo, Chanhee; Karanikola, Vasiliki; Lin, Shihong; Straub, Anthony P.; Tong, Tiezheng; Warsinger, David M.; Elimelech, Menachem (January 2018, Energy & Environmental Science)

   Energy-efficient desalination and water treatment technologies play a critical role in augmenting freshwater resources without placing an excessive strain on limited energy supplies. By desalinating high-salinity waters using low-grade or waste heat, membrane distillation (MD) has the potential to increase sustainable water production, a key facet of the water-energy nexus. However, despite advances in membrane technology and the development of novel process configurations, the viability of MD as an energy-efficient desalination process remains uncertain. In this review, we examine the key challenges facing MD and explore the opportunities for improving MD membranes and system design. We begin by exploring how the energy efficiency of MD is limited by the thermal separation of water and dissolved solutes. We then assess the performance of MD relative to other desalination processes, including reverse osmosis and multi-effect distillation, comparing various metrics including energy efficiency, energy quality, and susceptibility to fouling. By analyzing the impact of membrane properties on the energy efficiency of an MD desalination system, we demonstrate the importance of maximizing porosity and optimizing thickness to minimize energy consumption. We also show how ineffective heat recovery and temperature polarization can limit the energetic performance of MD and how novel process variants seek to reduce these inefficiencies. Fouling, scaling, and wetting can have a significant detrimental impact on MD performance. We outline how novel membrane designs with special surface wettability and process-based fouling control strategies may bolster membrane and process robustness. Finally, we explore applications where MD may be able to outperform established desalination technologies, increasing water production without consuming large amounts of electrical or high-grade thermal energy. We conclude by discussing the outlook for MD desalination, highlighting challenges and key areas for future research and development. 

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