Fluid dynamics of spacer filled rectangular and curvilinear channels

Salinity Gradient Power by Reverse Electrodialysis (SGP-RE) technology allows the production of electricity from the different chemical potentials of two differently concentrated salty solutions flowing in alternate channels suitably separated by selective ion exchange membranes. In SGP-RE, as well as in conventional ElectroDialysis (ED) technology, the process performance dramatically depends on the stack geometry and the internal fluid dynamics conditions: optimizing the system geometry in order to guarantee lower pressure drops (DP) and uniform flow rates distribution within the channels is a topic of primary importance. Although literature studies on Computational Fluid Dynamics (CFD) analysis and optimization of spacer-filled channels have been recently increasing in number and range of applications, only a few efforts have been focused on the analysis of the overall performance of the process. In particular, the proper attention should be devoted to verify whether the spacer geometry optimization really represents the main factor affecting the overall process performance. In the present work, realized within the EU-FP7 funded REAPower project, CFD simulations were carried out in order to assess the effects of different parameters on the global process efficiency, such as the choice of spacer material and morphology, and the optimization of feed and blowdown distribution systems. Spacer material and morphology can affect the fluid dynamics inside each channel. In particular, the appropriate choice of net spacer material can influence the slip/no-slip condition of the flow on the spacer wires, thus significantly affecting the channel fluid dynamics in terms of pressure drops. A Unit Cell approach was adopted to investigate the effect of the different choices on the fluid flow along the channel. Also, the possibility of choosing a porous medium to substitute the net spacer was theoretically addressed. Such investigation focused on the porosity and the fiber radius required to respect the process constrains of pressure drops and mechanical stability. On the other hand, the overall pressure drops of a SGP-RE or ED stack can be considered as resulting from different contributions: the pressure drop relevant to the feed distributor, the pressure drop inside the channel, and the pressure drop in the discharging collector. The choice of the optimal stack geometry is, therefore, strongly related to the need of both minimizing each of the above terms and obtaining the most uniform feed streams distribution among the stack channels. In order to investigate such aspects, simulations were performed on a simplified ideal planar stack with either 50 spacer-less or 50 spacer-filled channels. The effect of the distribution/collector channel *Corresponding author. (2012) 390-403 Octoberthickness and geometry on single-channel flow rates and overall pressure drops in the system was analyzed and a significant influence of distributor layout and size on the overall process performance was found.

Section: Introductionsupporting

confidence: 68%

“…According to Ranade and Kumar [8], a spacerfilled channel can be considered as a set of connected periodic cells. They proposed to model via CFD one of these representative periodic cells (Unit Cell approach) in order to allow fine computational grid to be employed and small-scale flow structures to be consequently predicted.…”

Section: Introductionmentioning

confidence: 99%

CFD analysis of the fluid flow behavior in a reverse electrodialysis stack

Gurreri¹,

Tamburini²,

Cipollina³

et al. 2012

“…However, while in the literature many works considered the elementary cell as a single set of crossing filaments [10,12], in this case the cell consisted of a small portion of a spacer filled channel, containing a number (between 5 and 6) of longitudinal and transversal filaments, thus allowing the simulation of the thermo fluid dynamics also among the rows of filaments.…”

Section: Models and Methodsmentioning

confidence: 99%

“…More recently, Ranade and Kumar [10] performed RANS simulations (using the standard k-ε turbulence model) in order to analyse the effects of channel curvature due to the spiral wound configuration of several types of membrane modules. Their results showed that no significant difference with flat channels is found for common curvature radius.…”

Section: Cfd Modelling Of Spacer-filled Channelsmentioning

confidence: 99%

Membrane distillation heat transfer enhancement by CFD analysis of internal module geometry

Cipollina¹,

Micale²,

Rizzuti³

2011

Module geometry optimisation can be a crucial matter in all separation processes using selective or hydrophobic membranes, e.g. Reverse Osmosis (RO), Membrane Distillation (MD). In fact the choice of suitable channel shape and size can dramatically affect the performance of the process. With reference to the membrane distillation process, temperature polarization phenomena and pressure drops along the channels largely affect the process efficiency (i.e. the efficient use of temperature driving force for the passage of vapour through the membrane) as well as pressure distribution, module mechanical resistance and pumping costs.Several works have been presented so far in literature on the fluid flow characterization of spacer-filled channels for the case of Reverse Osmosis modules, but only few works deal with the problem of MD modules optimization.The present work aims at the CFD simulation of the fluid flow and temperature fields within spacer-filled MD module channels for a variety of spacer geometries. Both commercial and custom made geometries have been simulated in order to identify the most important parameters affecting process efficiency. The commercial CFD code ANSYS CFX11.0 has been adopted to perform simulations.Results provide valuable information to identify the main features which an optimised spacer should possess in order to minimise T-polarization and pressure drops along the channel.

“…This is the case of the study of thermo-fluid dynamics in channels adopted for membrane separation processes, in which the smallness of characteristic lengths does not allow for the introduction of probes and for the experimental analysis of the system. Several studies have been performed for the analysis of RO and NF channels in which only the fluid dynamics was investigated, highlighting the differences in spacers geometry [4,5]. In most of these works the focus was on the analysis of pressure drops along the channel and the turbulence level generated by the spacers.…”

Section: Fig1 Transfer Phenomena In Direct Contact Membrane Distillamentioning

confidence: 99%

CFD simulation of a membrane distillation module channel

Cipollina

Miceli

Koschikowski

et al. 2009

The interest towards the use of Membrane Distillation (MD) processes for seawater desalination has been rising recently due to the ease of coupling MD with waste and/or solar thermal energy. Notwithstanding the flexibility of the process and its potential for further developments in membrane performances, one of the main drawbacks is the thermal efficiency reduction caused by temperature polarization. Because of such phenomenon, only a small amount of the driving force potentially available for the separation process, i.e. the temperature difference between evaporating and condensing fluids, is actually used for the separation. In order to reduce temperature polarization a study on the effects of spacer and channel geometry has been performed using Computational Fluid Dynamics (CFD) techniques. A simple reference geometry has been built to simulate the flow and temperature fields of a portion of a Spiral Wound MD module channel. Results show how spacers can significantly affect temperature gradients within the channel, therefore modifying the effective driving force between the faces of the membrane. The main features, which an optimal spacer should possess, have been thus indicated.