T型微通道内液滴形成过程及长度的实验研究
Experiment study on formation and length of droplets in T-junction microchannels
收稿日期: 2019-04-16
Received: 2019-04-16
作者简介 About authors
张井志(1989—),男,助理研究员,博士,从事微流控及多相流研究.orcid.org/0000-0001-5672-8349.E-mail:
利用高速摄像机研究截面为400×400 μm的正T型微通道内液-液两相流动特性,离散相(硅油)和连续相(质量分数为0.5%的十二烷基硫酸钠SDS蒸馏水)的体积流量范围分别为1~5、2~110 mL/h. 结果表明,两相流型主要为弹状流和滴状流,前者的形成机理为挤压机理,后者为剪切机理. 液滴的长度随离散相体积流量和离散相与连续相体积流量之比的增大而增大,随连续相的体积流量和毛细数的增大而降低. 液柱长度的变化规律与液滴长度相反. 液滴生成时间随离散相与连续相的体积流量的增大而逐渐降低,剪切机理生成液滴所需时间小于挤压机理. 依据实验结果,采用离散相与连续相体积流量比和连续相的毛细数,总结出无量纲液滴、液柱长度及液滴生成时间的预测关联式.
关键词:
Flow characteristics of liquid-liquid two-phase flows in T-junction microchannels with 400×400 μm cross-section were experimentally studied using a high-speed camera. Silicon oil and distilled water with 0.5% sodium dodecyl sulfate (SDS) were used as dispersed phase and continuous phase, respectively. Volume flow rates of the disperse phase ranged from 1 to 5 mL/h, while those of the continuous phases ranged from 2 to 110 mL/h. Slug flow and droplet flow patterns were observed in the experimental work. Results show that the formation of dispersed slugs is controlled by the squeezing mechanism, while the shearing mechanism domains the liquid droplet formation process. The length of liquid droplet increases with the increase of volume flow rate of dispersed phase and the volume flow rate ratio of dispersed phase to continuous phase, and decreases with the increase of volume flow rate and capillary number of continuous phase. The change of lengths of liquid column is opposite to that of liquid droplet. The droplet generation time decreases with the increase of volume flow rates of dispersed phase and continuous phase, and the formation time for shearing mechanism is less than that of squeezing mechanism. Based on the experimental results, the volume flow rate ratio of dispersed phase to continuous phase and the capillary number of continuous phase are adopted to develop the predictive correlations of dimensionless liquid droplets, column length and droplet generation time.
Keywords:
本文引用格式
张井志, 陈武铠, 周乃香, 雷丽, 梁福顺.
ZHANG Jing-zhi, CHEN Wu-kai, ZHOU Nai-xiang, LEI Li, LIANG Fu-shun.
T型微通道构造简单,广泛应用于微液滴生成装置. 依据离散相与连续相入口的夹角,T型微通道主要分为两类:正T型(夹角为180°)和侧T型(夹角为90°). Garstecki等[17-18]利用数值模拟方法,分析侧T型微通道内液滴的生成过程,指出液滴生成主要存在2种不同的机理:挤压机理和剪切机理. Fu等[13]观测侧T型微通道内部的流型,指出长气泡主要由挤压机理生成,离散气泡主要由剪切机理形成. Yao等[19]的实验结果表明对于正T型通道,当连续相无法较好地润湿通道壁面时,挤压机理向剪切机理转变的临界毛细数低于润湿性表面的工况. 液滴与液柱的长度直接影响微液滴的传热、传质特性,Wu等[14, 17, 19-22]分析微通道内液滴长度的变化规律,并利用离散相与连续相的体积流量之比和连续相毛细数,总结出无量纲液滴长度的表达式.
虽然现有文献已有涉及到微通道内液滴生成规律的研究,但对于正T型通道的研究不足,尤其是在液柱长度及液滴生成时间方面. 采用不同工作介质的微流动系统,其内的液滴、液柱长度与液滴生成时间的规律有一定的不同. 本研究主要以硅油和质量分数为0.5% 的十二烷基硫酸钠(sodium dodecyl sulfate, SDS)蒸馏水作为离散相和连续相,分析截面为400×400 μm的正T型微通道内液滴生成规律,并总结液滴、液柱长度及液滴生成时间的预测关联式.
1. 实验系统及数据处理
T型微通道液-液两相流动实验系统及测试段如图1所示. 离散相(硅油)和连续相(0.5% SDS蒸馏水)由2台注射泵驱动,流入T型微通道实验段. 在通过实验段后,两相混合液体流入集液器. 实验段与注射泵及集液器之间采用内径为3 mm的硅胶管连接. 工质的黏度由乌氏黏度计(LVDV-II,Brookfield,USA)测量,连续相与离散相的黏度分别为0.000 92、0.010 00 Pa·s. 两相之间的表面张力由表面张力仪(DCAT11EC,Dataphysics,Germany)获得,为0.021 N/m. 利用高速摄像机拍摄微通道内的两相流型图片,采用150 W的背光源提供清晰拍摄所需的光照强度. 实验段的材质为亚克力玻璃,采用精密机械加工的方法刻出实验所需的微槽道(截面尺寸为W×H=400×400 μm)及螺栓孔. 用相同尺寸的亚克力玻璃作为盖板,盖板和刻槽板之间利用螺栓压紧密封.
图 1
图 1 微液滴实验系统及测试段示意图
Fig.1 Schematic diagram of experimental test rig and test section for micro-droplets
如图2所示为实验图片的处理过程. 图中,Lc为液柱长度,即2个连续的液滴之间的长度;Ld为液滴长度;Luc为微液滴单元长度,即一组液滴与液柱长度之和. 本研究采用MATLAB软件中的Canny算法,处理微通道内流型图片,获得液滴的轮廓线,利用像素的相对大小,分析液滴、液柱的长度.
图 2
2. 实验结果与讨论
2.1. 微液滴形成过程分析
在实验过程中,离散相和连续相的体积流量分别为
图 3
图 3 不同工况下的T型微通道内液滴形成过程
Fig.3 Forming processes of droplets in T-junction microchannels under different working conditions
图 4
图 4 T型微通道内液-液两相流流动型图
Fig.4 Flow pattern regimes of liquid-liquid two-phase flows in T-junction microchannels
2.2. 液滴长度的变化规律
无量纲的液滴长度
图 5
图 5 主要因素对无量纲液滴长度的影响
Fig.5 Effects of main factors on dimensionless length of droplets
式中:μc为连续相的动力黏度;σ为两相表面张力;Cac为黏性力与表面张力的相对大小.
由图5可以看出,Ld/W随
2.3. 液柱长度的变化规律
决定液柱长度的主要因素与液滴长度的决定因素一致,有通道结构、两相工质物性、体积流量等. 如图6所示为液柱长度Lc和无量纲的液柱长度Lc/W随
图 6
图 6 主要因素对无量纲液柱长度的影响
Fig.6 Effects of main factors on dimensionless length of liquid slugs
2.4. 液滴、液柱长度的预测关联式
Yao等[19-21]根据实验结果拟合出微通道内液滴长度的预测关联式,具体关联式的形式如表1所示. 表中,
表 1 液滴、液柱长度预测公式
Tab.1
文献 | 通道结构 | 连续相 | 离散相 | 关联式 | MAD/% | MADmax/% | MRD/% |
Xu等[20] | 侧T型W=0.2 mm | 水+SDS | 正辛烷 | | 22.5 | 55.5 | −18.7 |
Yao等[19] | 正T型W=0.6 mm | — | — | | — | — | — |
甘油+SDS | 辛烷 | Case I | 16.0 | 42.8 | 14.6 | ||
辛烷+ SPAN80 | 甘油 | Case II | 13.0 | 41.2 | 7.2 | ||
魏丽娟等[22] | 侧T型W=0.4 mm | 水+甘油+SDS | 环己烷 | | 13.8 | 29.2 | −13.8 |
本研究 | 正T型W=0.4 mm | 水+SDS | 硅油 | | 6.7 | 17.7 | 5.2 |
水+SDS | 硅油 | | 14.5 | 38.2 | 2.1 |
图 7
图 7 实验值与新关联式预测值的对比
Fig.7 Comparison of experimental results and predictions of new correlations
2.5. 液滴形成时间影响规律
液滴或液柱的长度本质上是由液滴生成时间(频率)决定的,液滴生成时间越小(频率越高),在相同的
图 8
图 8 连续相与离散相体积流量对液滴生成时间的影响规律
Fig.8 Effects of volume flow rates of continuous and dispersed phases on formation time of droplets
无量纲的液滴生成时间T可以由液滴生成时间t推导得到:
式中:Vd、Vc分别为离散相和连续相的流速,A为截面积,W为通道宽度,
图 9
图 9 实验获得的无量纲时间与关联式预测值的对比
Fig.9 Comparison of experimental dimensionless time and predictive value of correlations
3. 结 论
(1)弹状流与滴状流的形成过程具有明显的不同,前者离散相基本完全填充T型通道交汇区域,而后者无法完全填充. 前者对应于挤压机理,依靠两相之间的压力差形成液滴;后者对应于剪切机理,两相之间的剪切力是形成液滴的主要作用力.
(2)Ld/W随
(3)依据本研究实验结果,利用q、Cac总结3组无量纲数Ld/W、Lc/W、T的经验公式,预测值与实验结果较吻合.
(4)本研究主要考虑液滴生成规律,并未深入分析液滴长度对传热、传质特性的影响,在后续的研究中会进一步讨论微液滴对微通道内两相流动的传热、传质特性的影响机理.
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