Experimental And Numerical Study On Pulsatile Flow And Mass Transfer Enhancement In A Wavy-walled Tube

With the advancement of science and technology,modern industry is developed rapidly, then the energy source shortage and environment pollution are getting more and more serious. It is extremely important to reduce power consumption,improve heat and mass transfer rate and lessen the blight of surroundings.Furthermore,a higher heat and mass transfer enhancement technique without turbulent flow need to be found urgently because numerous high viscous and biology active liquid mediums come forth in biochemical,pharmacy and biomedical field.The high(?)r efficiency and miniaturization of transfer devices as well as pulsatile flow are main means for achieving above aims.To date,there is no experimental study on pulsatile flow with backward flow.Utilizing electrochemical technology,the mass transfer characteristics for pulsatile flow with backward flow under different controls parameters in a three dimensional wavy-walled tube are discussed,and transfer enhancement mechanism is brought out.The flow structures are observed using aluminum dust method and timing flow visualization technology,thus the relationship between unsteady flow and mass transfer enhancement is explored.Subsequently, some physical quantities,which cannot measured through present experiment system,are simulated with soft FLUENT and its second development interface,therefor a significant reference is obtained for the exploiture of higher transport devices and practical engineering applications.To investigate the characteristics of pulsatile flow with backward flow,a experimental system is designed and constructed,including a wavy-walled tube,pulsatile device, visualization equipment and so on.Then critical Reynolds numbers and flow structure characteristics are found and described.Moreover,mass transfer performance under equal pumping power condition is explored for wavy-walled and straight-walled tubes.The result indicates that the wavy-walled tube has a higher transfer performance after entering transitional flow regime.The effect of controls parameters on mass transfer enhancement under pulsatile flow with backward flow condition is investigated.It is found that transfer enhancement is increased with increasing oscillatory fraction within a limited fraction range,the optimal transfer enhancement is occurred in moderate Reynolds number.On the other hand,the optimal oscillatory frequency value,corresponding to the optimal transfer enhancement,is decreased with increasing Reynolds number,which independent on oscillatory fraction. Furthermore,mass transfer enhancement mechanism for pulsatile flow with backward flow in a wavy-walled tube is pointed out,that is the most effective transfer enhancement is obtained when the ratio of imposed inlet velocity and wavelengh is close to the forced oscillatory frequency.Hereby the basis and method of optimal operation condition for pulsatile flow are confirmed.In order to establish relationship between flow structure and mass transfer enhancement, flow structures for pulsatile flow with backward flow are observed through flow visualization technology.It is found that steady and unsteady flow structures are all exist during one pulsatile cycle,and the unsteady flow structures lasts more long time,chaotic mixing is stronger,mass transfer enhancement effect is better.Therefore,unsteady flow contributes great to mass transfer enhancement in the wavy-walled tube.As a validation and supplement of experimental study,some analysis results are obtained through numerical simulation.The numerical and experimental results are in agreement,and showing their rationality.Moreover some physical quantities are simulated,which cannot measured through present experiment system.It is found that flow separation is occurred at smaller Reynolds number with pulsatile flow than steady flow,and the peak value of vortex intensity is always appeared in moderate oscillatory frequency during deceleration phase with inertia domination.At the same time,the distribution of concentration boundary layer with different forced oscillatory frequency is agreement with the variation of mass transfer enhancement.The numerical results show that pulsatile flow is one of effectual methods for transfer enhancement,besides,both vortex intensity and concentration distribution can be used to reflect the variation of mass transfer process,which provides a new idea and method for other tubes' study.