摘要
随着科学技术的进步,现代工业得到飞速发展,同时人们也面临着越来越严重的能源短缺和环境污染问题。如何降低工业装备的能量消耗、在现有基础上进一步提高热量质量传递设备的效率、进而减轻对环境的不良影响十分重要。另外,随着大量高粘度、具有生物活性的流体介质越来越多地出现在生物化工、制药以及生命医学等领域,也亟待找到一种在低剪切层流条件下的热质传递强化技术。提高传递设备的效率和促使设备单元的小型化与微型化,采用脉动流动技术是实现质量传递过程强化的重要途径。
由于目前尚未搜索到有关伴随反向流的脉动流动对传质强化影响的研究报导,因此本文以波壁管为研究对象,采用电化学技术完成了波壁管内不同控制参数下的质量传递速率测量,并提出了伴有反向流的脉动流动的质量传递强化机理;采用铝粉法流动可视化技术与定时刻拍照技术相结合的手段完成了对伴随反向流的脉动流动的流动结构观察,研究了不稳定流动对传质强化的影响;利用FLUENT软件及其二次开发接口,对一些实验测量不能观测的物理量进行了数值分析,为研究波壁管的传质性能提供了一种新思路。
为了观测波壁管内伴随反向流的脉动流动特性,设计加工并组建了波壁管、脉动装置和可视化及图片记录系统装置等以满足研究需要。通过测量定常流场下的流体力学性能,得到了波壁管内不同流动状态的临界雷诺数等标志量;等功耗条件下波壁管与直壁圆管的比较结果表明,当流动进入过渡流流域后,波壁管具有更好的质量传递性能。
系统研究了控制参数对伴有反向流的脉动流场下的质量传递强化的影响,并发现,在一定的脉动流振动分率范围内,质量传递强化效果随振动分率的增加而增强;最有效的质量传递强化发生在净流动进入过渡流流域之前的中等雷诺数下;与最大传递强化效果对应的最佳振动频率随净流动雷诺数的增加而减小,且几乎与流量的振动分率无关。据此提出了伴随反向流的脉动流场下的质量传递强化机理,即当入口速度和波长的比值接近脉动振动频率时会产生最有效的质量传递强化效果,并据此得到了确定最佳操作条件的依据和方法。
为建立传质强化与不稳定流动之间的联系,对伴有反向流的脉动流动进行了可视化研究,结果发现,一个脉动周期内同时存在稳定和不稳定两种流动现象,而不稳定流动状态持续的时间越长,无序混合越强烈,对应的质量传递强化效果就越好。这表明不稳定的流动结构对质量传递强化的贡献最大。
作为实验研究的验证和补充,数值模拟给出了系列分析结果。数值结果不仅与现有的实验结果吻合,同时说明了数值结果和实验结果的合理性,而且得到了实验测量很难观测的物理量,如流动分离、涡强度、浓度分布等。结果发现,脉动流动比定常流动更早发生流动分离;涡强峰值总是出现在惯性控制的中等振动频率下;浓度边界层的分布与质量传递强化过程相对应。研究结果表明,脉动流动技术是实现传质强化的有效手段之一,同时涡强度与浓度分布均可反映质量传递过程,为探究其它管内的传递性能提供了新思路和新方法。
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.
引文
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