An Experimental Heat And Mass Transfer Research On Enhancement Of Membrane Distillation By 3D-Printed Spacers

Membrane Distillation(MD)is open source incremental fresh water preparation process,which deeply couples the mechanism of thermal and membrane water treatment processes.It is not limited by the concentration of saline solution,and can be operated under normal temperature and pressure.It can be driven by low-grade heat source for wastewater treatment.The flux and efficiency of the existing process are lower,so it needs to be optimized and strengthened extensively to promote its commercial application in the water treatment industry.In this thesis,the mechanism of heat and mass transfer in the process of MD was researched experimentally.Through basic experiments,the relationship between the two configurations of water flux and thermal efficiency and the influencing factors was verified.The water flux of DCMD is greatly affected by the temperature of hot feed liquid,and the thermal efficiency is positively and negatively correlated with the temperature of feed liquid and the inlet flow rate,respectively.There is a critical vacuum value in the vacuum condensation side of the VMD process,and within a certain range above it,the water flux is far better than that of the DCMD type the critical vacuum value,and the transmembrane heat conduction loss is also small.A novel 3D-printed spacers suitable for membrane module channel was developed by 3D printing technology in this thesis.Six kinds of basic spacers with different structural dimensions were designed.As the factor parameters are changed,the results show that the variation trend of water flux and thermal efficiency is similar to that of the basic experiment when the 3D-printed spacers is arranged.The water flux of DCMD has been greatly improved,and the higher the temperature is.the greater the enhancement amount is.Compared with the basic experiment,the thermal efficiency has a certain decline,but it was controlled within the range of about 5～6%.The best effect of mass transfer enhancement is that the flow attack angle α is 45° and the angle β between the upper and lower layers’ gratings is 90°.According to the results of strengthening exploration experiment,the structural design with the best strengthening effect is optimized.The optimized line contact linear and curved 3D-printed spacers are applied to the optimization and strengthening experimental study.The strengthening effect of the curved spacers is better than that of the linear and other basic spacers.The enhanced mass transfer effect is affected by temperature and flow rate,and the enhanced effect is better with the increase of temperature.The process thermal efficiency of the optimized 3D-printed spacers is higher than that of the unoptimized spacers and the unorganized channel.Under the same flow conditions,the arrangement of the 3D-printed spacers enhancing the MD process is not by increasing the average cross-section velocity,but by strengthening the turbulent behavior near the membrane surface.The key factor is 3D-printed spacers weakening the temperature and concentration boundary layer by enhancing the turbulent behavior in the near-membrane surface region.Thus,the concentration polarization and temperature polarization effects are weakened,and the mass transfer driving force and heat utilization efficiency are improved.By comparison,it is found that the basic 3D-printed spacers has a poor strengthening effect,but the pressure of feed liquid on the film surface is small,which can balance the change of material and liquid.It’s suitable for long-term operation.When the optimized type 3D-printed spacers contact with the film surface,the feed’s pressure is stronger,and the mechanical strength of the local film surface will be affected when the incoming flow fluctuates greatly or the feed liquid pressure and temperature change greatly.Based on this optimized 3D-printed spacers design layout,it can effectively strengthen the water flux and thermal efficiency of MD process.Furthermore,this has a certain development potential in the later design and application of multi-effect MD process,so as to accelerate the improvement of the process performance and perfect the application of Membrane Distillation.