Analysis And Optimization Of The Hydraulic Characteristics Of The Forward Osmosis Membrane Surface Based On FBG Sensing

It’s possible to slow the CP and control the membrane fouling by changing the shear-force in the FO process.In order to obtain more information on the hydrodynamic characteristics of membrane surface and improve the permeability,a measuring method of the shear-force distribution on the membrane surface based on FBG sensing technology was proposed to investigate the hydraulic properties of membrane micro-interface combined with computational fluid dynamics in this study.The effects of FS flow rate,DS flow rate,cross-flow direction on the membrane flux and membrane shear-force distribution of forward osmosis process were investigated.Results revealed that FBG sensing technology could be used to monitor membrane boundary hydraulic properties which provide more effectively information on filtration process.A number of key conclusions could be provided as following:(1)A nonuniform flow field distribution exists inside the FO membrane module.The CFD simulation results show that there is a regionally different flat FO film surface inside the module.The maximum velocity in the flow field distribution of the membrane surface is not at the entrance,and the backflow zone is formed.(2)A nonuniform spatial variation of the shear-force distribution existed along the FO membrane surface,and the design of operating conditions requires the coordination of ECP and ICP on both sides of the membrane.FBG results from experimental work demonstrated that osmotic pressure did not affect the wavelength change of FBG.And a non-uniform spatial variation of the shear-force distribution exists along the membrane,and higher shear force is distributed in the middle position which resulted in higher diffusion load on the particular location of the membrane rind.Increasing the inlet flow rate can effectively improve the scouring effect at various positions on the membrane surface,but enhancing the shear-force simply to a certain value didn’t^result in a higher shear force and lower the effect of concentration polarization(CP).This is mainly due to the fact that high FS and DS shear forces significantly increase the solute diffusion into the support layer,increasing the eff-ect of ICP on the FO infiltration process.(3)It is better to choose counter-current mode because it showed the better hydraulic characteristics by increasing the crossflow.Compared to co-current mode,counter-current mode showed higher shear-force,faster scouring frequency and consistent shear-force distribution.Meanwhile,in the countercurrent mode,the membrane flux can be effectively improved by increasing the shearing force on both sides of the membrane.This is because the higher mass transfer rate makes the osmotic pressure on both sides of the membrane relatively larger and more uniform,which can improve the membrane surface utilization,achieving higher permeate flux.(4)The increase in unilateral shear forces has a different effect on the concentration polarization on both sides of the membrane.Increasing the flow rate on the FS or DS side alone will change the stress distribution on the membra,ne surface,resulting in a change in the membrane concentration distribution and changing the water flux.The shear force and hydraulic pressure can be improved by increasing the FS flow rate,and the solution concentration near the membrane surface is reduced to slow down the ECP effect.However,increased DS flow rate causes the osmotic pressure to decrease while increasing ICP.It’s adverse f’or the FO infiltration process.