| Membrane distillation technology has a wide range of applications,which has great potential for further development,especially in the field of seawater desalination.Among many strengthening methods,gas-liquid two-phase flow has obvious advantages in membrane distillation process.That method enhances membrane distillation process by increasing instability of fluid flow.It is difficult to analyze the transport mechanism involving complex interaction between gas and liquid phase only by experimental measurements when we study the enhancement process of two-phase flow in membrane distillation.Hence the computational fluid dynamics method can be used to abstract,model and solve the actual two-phase flow enhancement process of membrane distillation,and to assist in the analysis of its transfer mechanism.Afore mentioned study provides a theoretical basis for the further promotion and application of gas-liquid two-phase flow method in the field of membrane distillation enhancement.Therefore,based on the experimental study of two-phase flow enhanced membrane distillation,CFD simulation was carried out to explore the transfer mechanism of strengthening process.Contents of this study are as follows.First of all,two-phase flow enhancement air gap membrane distillation experiment was conducted,where the hydrophobic tubular coal-based carbon membrane made in laboratory,sodium chloride aqueous solution(35g·L-1),N2 gas and low pressure water vapor were employed as separation membrane,separating medium and intensifying medium respectively.Results are as follows.The maximum permeation flux with N2 gas and low pressure water vapor injection can reach up to 4.70 and 5.46 times of that without gas injection respectively.Under the same experimental condition,permeation flux with low-pressure water vapor injection was greater than that of N2 injection.Compared with the N2 gas-feed liquid flow pattern,vapor-feed liquid flow pattern showed obvious hysteresis.In conclusion,interaction between N2 gas and feed liquid is quite different from that between water vapor and feed liquid.In terms of increasing permeation flux,enhancement effect of low-pressure water vapor is better than that of N2 gas.Secondly,physical model of two-phase flow enhanced air gap membrane distillation with strong universality was proposed.Then FLUENT software and VOF model were used to simulate the two-phase flow enhanced membrane distillation process.Permeation flux,flow pattern,local membrane wall shear stress,turbulence intensity,Nusselt number,temperature polarization coefficient?TPC?,mass transfer coefficient and concentration polarization coefficient?CPC?were calculated and studied respectively to analyze transport mechanism.Results are as follows.With gas injection,local shear stress shows a fluctuating state,and turbulence intensity increases significantly,where the maximum turbulence intensity with N2gas and low pressure water vapor injection can reach up to 150%and 50%respectively.Temperature polarization coefficient increases significantly,where the maximum TPC with N2gas and low pressure water vapor injection can reach up to 1.64 and 2.37 times of that without gas injection respectively.And concentration polarization decreases,where the maximum CPC with N2 gas and low pressure water vapor injection can down to 0.62 and 0.22 times of that without gas injection respectively.In conclusion,two-phase flow technology can enhance the transport process,and the enhancement effect of N2 gas and water vapor on momentum,heat and mass transfer process is quite different.Finally,strengthening effects of different gas media are compared.The results show that when the low pressure water vapor is used as the strengthening medium,utilization efficiency of total temperature difference driving force is higher and the degree of concentration polarization is lower.When gas holdup is less than 0.33 or greater than 0.56,low-pressure water vapor is more economical as a strengthening medium for membrane distillation.When gas holdup is between 0.33 and 0.56,economy of two strengthening media is similar. |
PDF Full Text Request