Hydrodynamic Characteristics And Process Enhancement Of Vortex Wave Flow

It has been received much attention that the mass,momentum and heat transfer can be produced by unsteady flows in a channel during the past few decades because of their usages in chemical,biochemical and medical engineering.As the study of fluid mechanics has evolved from steady state investigations towards the reality of unsteady phenomena,it has become increasingly clear that instantaneous whole field non-intrusive flow measurement techniques are required.Vortex Wave Membrane Bioreactor(VWMBR) combines vortex wave flow with membrane aerated bioreactor for wastewater treatment,by which mass transfer is enhanced by the action of forced convective.Compared to traditional methods of enhancing mass transfer by using turbulent flow,the shearing stress and the power dissipation in VWMBR are rather lower.VWMBR increases survival rate of microorganism cells and can also solve some problems such as clogging and concentration polarization in the traditional MBR.The understanding of spatio-temporal characteristics of the fluid flow and the performance of the VWMBR are of great practical importance.To characterize the flow instability,digital particle image velocimetry(DPIV) (FlowMap500 from Dantec Inc.) is used to measure the near-instantaneous flow fields in 5 different channels with different structure respectively.To improve our understanding on the mixing processes,fluid flow in 2D asymmetric channels with different structure or different operating parameters are investigated via Computational Fluid Dynamics simulations.In this paper,performance of the VWMBR,such as volumetric oxygen transfer coefficient and the removal rates of COD,is investigated for artificial wastewater treatment.The main contributions of the study are included as follows:(1) System study shows that the flow regime is different from Karman Vortex Street. And quantitative analysis reveals the influence of geometric parameters(such as the channel height,width,transition angle,expand ratio,deflector distance) and operating parameters(St, Re,inlet velocity) on the size of vortex wave,the vorticity and the stress.The radical reason of mass transfer enhancement lies in the adequately convective mixture of the fluid particles in the central region and near the channel wall during vortices motions.(2) Quantitative analysis on mechanism of the vortices formation and motion based on boundary-layer theory indicates that the geneses of the vortices are different.The first and the last vortices are formed owing to the adverse pressure gradient caused by the channel enlargement and contraction in section,respectively.And the others were induced by the adverse pressure gradient attributed to inlet flow deceleration.(3) A set of experimental equipment of VWMBR in batch mode is established.The mass transfer experiments of water and wastewater are carded out for different channel structures. Results of synthesis wastewater treatment are evaluated by Chemical Oxygen Demand(COD) of influent and effluent.The comparison study of the k_La and COD removal rates shows that vortex wave can enhance convective mixing.The result of water experiment is consistent with that of wastewater treatment.Through orthogonal experiments,the factors affecting k_La and COD removal rates,such as Re,Tran-Membrane Pressure(TMP) and ventilation volumeQ_G were studied.The results showed that Re had strong effect on k_La and COD removal rates. The effects of four operation factors(Re,TMP,Q_G and deflector distance) on kLa and COD removal rates are investigated.The results show that the system is good for repetitiveness.(4) VWMBR can be treated as a batch continuous stirred tank reactor with the vortex wave flow regime.Based on the analysis of the laws of conservation of mass,the degradation is shown to follow a pseudo-first order reaction:C=C₀exp(-K₁t)。The water quality of the effluent could close to the national first discharge standard 100mg/L during experiment of 3h, and meet the standard after 4.5h.Using linear regressive analysis,approximate formulas are obtained which could be used to determine C₀ and K₁.This work presents quantitatively studies for the hydrodynamics and mass transfer characteristic of the vortex wave with low Reynolds number.It provides experimental and design foundation for further study and industrialization of the VWMBR.