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热学微流控系统中瞬态相变传热研究
其他题名Study of Transient Phase Change Heat Transfer in Thermal Microfluidic Systems
张伟
导师徐进良
2008-05-31
学位授予单位中国科学院研究生院
学位授予地点北京
学位名称博士
学位专业热能工程
关键词Mems 微流控 相变 沸腾 冷凝 微汽泡动力学 Marangoni效应 Microfluidic Phase Change Boiling Condensation Microbubble Dynamics Marangoni Effect
摘要Microscale phase change heat transfer exists extensively in different kinds of microfluidic systems such as microreactor, microfuelcell, microevaporator, microcondenser, micro heat pipe, microbubble actuator. It is important to study the microscale phase change phenomena in microfluidic systems in order to design and operate these systems well. In the present study, three typical microscale phase change phenomena including microscale flow boiling, microscale flow condensation and microbubble dynamics on a microheater were studied at length. All the test sections used in present study were fabricated by standard MEMS technology, which ensures the smooth surface instead of rough surface by the common mechanical fabrication process. Considering the extensive usage of microchannels with complicated structures, a microfluidic chip consisting of intercrossed microchannels was designed and fabricated to investigate the microscale flow boiling phenomenon using acetone as the working fluid. It was found that all the microchannels repeated the flow patterns in a timescale of milliseconds, with three substages: single phase liquid refilling stage, transient stratified flow stage and partial dry-out stage. In a separated microchannel zone, the liquid film behaved a non-uniform distribution along the flow direction due to the large evaporation momentum force and dry-out always took place from the upstream to the downstream. Stratified flow was observed inside the microchannels due to the low liquid Froude numbers. The temperatures on the backside surface of the chip could almost keep synchronous with those on the inner wall surface in that the Biot number was small. Although the IR camera was not fast enough to catch up with the transient flow patterns, it could still figure out the temperature fluctuations during the periodical flow pattern transitions. A single channel micro condenser with a low aspect ratio was fabricated in order to acquire the dynamic flow behaviors during water steam condensation. Two different cooling rates were used: cooling by air natural convention and cooling by chilled water with a temperature of zero centigrade. It was found that the flow patterns inside the microchannel were also cyclic with the cycle period of milliseconds. There existed a quasi-stable elongated bubble slug attached at the channel inlet that pinched off into a train of bubbles at the upstream. Enhancing the cooling rate would result in large frequencies of the bubble emission and small diameters of the detached bubbles. The periodical bubble emission from the upstream elongated bubble slug was due to the large Weber number at the vapor/liquid interface. The nearly stable bubble diameter during the bubble moved toward the outlet was because the resident time for the bubble moving inside the microchannel was only one percent of the total time when the bubble was fully condensed. A microcondenser with three parallel microchannels was designed and fabricated in order to clear up the multichannel effect on microscale flow condensation. It was found that the flow patterns in all the three micochannles were similar to those in the single channel microcondenser, i.e. there were nearly stable elongated bubble slugs at the upstream and bubble trains in the downstream. Three different modes of vapor bubble detachment were identified: single vapor thread break-up mode, dual vapor threads synchronous break-up mode and dual vapor threads asynchronous break-up mode. The three parallel microchannels had different temperature distributions due to the heat conduction of silicon substrate. In the center channel, the temperature distribution was symmetric against its centerline, while for the side channels, the high temperature zones deviated toward the center channel. The temperature difference on the whole microcondenser was small due to the good heat conduction performance of silicon, but this small temperature difference could still lead to large temperature gradient along the channel width direction. Under this condition, the dual vapor threads in the center channel was always symmetric against the centerline of the channel and broken up synchronously; the dual vapor threads in the side channels were always deviated toward the center channel and broken up firstly from the side adjacent to the center channel. The marangoni convection effect induced by the temperature gradient in the channel width direction made the bubble moving toward the zones with high temperatures after its detachment from the elongated bubble slug. During the investigation of microbubble dynamics on a microbubble actuator, a thin Platinum film microheater of was fabricated. It was found that the boiling incipience on this Platinum film microheater reached the superheat limit, indicating the homogeneous nucleation. In a wide range of the pulse heating parameters, there existed three different type of bubble patterns :( 1) explosive boiling and quick shrinking at boiling incipience, followed by the bubble regrowth and recondensation; (2)explosive boiling, bubble break-up, attraction and coalescence ;(3)periodical oscillation of bubble size followed by continuous growth and obtaining a stable diameter eventually. In the first type bubble patterns, the heat stored inside the glass substrate during the heating process played an important role in the bubble regrowth. For the second type of bubble patterns, the marangoni convection effect led to the bubble attraction and coalescence. In the third type of bubble patterns, the bubble diameter reached stable in that the vapor mass generated inside the vapor bubble was balanced by the vapor mass condensed on the vapor/liquid interface. The results of this paper will provide a scientific guidance for the design and operation of the microfluidic systems with phase change phenomenon such as microreactor, microfuelcell, micro heat exchanger, microbubble actuator.
其他摘要微尺度相变传热广泛存在于微反应器、微型燃料电池、微蒸发器、微冷凝器、微热管、微汽泡执行器等微流控器件中,研究微流控系统中的相变问题对于微流控器件的设计和运行具有重要的科学意义。本文针对三类典型的微尺度相变问题,即微尺度流动沸腾、微尺度流动凝结以及微加热器上的汽泡动力学进行了深入细致的研究,实验研究中所采用的实验件均为标准MEMS微加工工艺制作,克服了常规机械加工所造成的表面粗糙度的影响。 考虑到微流控系统中大量应用交叉型、弯曲型等复杂结构的微通道,在微尺度流动沸腾研究中,设计了一种具有交错微通道结构的微流控芯片,并以丙酮为工质,对该芯片内的流动沸腾进行了研究。发现了周期为毫秒量级微时间尺度的流型结构,整个周期包括单相液体充液、两相分层流以及部分蒸干的液膜流三个阶段;在单个微通道区域,由于蒸发动量力的作用,液膜沿流动方向呈非均匀分布,蒸干首先发生在上游;由于液相弗劳德数较小,导致微通道中依然存在分层流流型。由于毕渥数较小,芯片背面温度几乎与芯片内壁面温度保持同步变化。虽然红外热像仪的响应频率较低,但仍然可以鉴别出由于流型周期性转换导致的壁面温度脉动。 在微尺度流动凝结换热研究中,为便于获取凝结过程的动态流动特性,设计了一种低高宽比的单微通道,并以水为工质,对该微通道中的流动凝结换热进行了研究。实验中采取了空气自然对流冷却和 水强制对流冷却两种冷却强度。研究发现,该微通道中的凝结换热呈周期性,其周期在毫秒量级。在通道上游入口处,存在一个呈准静止状态的长汽弹,汽弹前端周期性脱离汽泡。增加冷却强度会使汽泡的脱离频率增大,脱离直径减小;长汽弹前端周期性脱离汽泡是由于汽液界面具有较大的韦伯数。汽泡在该微通道内的运动过程中直径基本不变是由于汽泡在通道内的滞留时间远小于汽泡完全冷凝所需的总时间。 为澄清并联通道的多通道效应对微尺度凝结换热的影响,作者设计了由三个矩形通道组成的并联微通道冷凝器。研究发现,通道中的流型结构与单通道凝结过程类似,均为上游呈准静止状态的长汽弹和下游周期性的汽泡脱离。在中间通道和侧通道中,总共发现了三种不同的汽泡脱离模式,即单汽丝断裂模式、双汽丝同步断裂模式以及双汽丝非同步断裂模式。多通道效应主要表现在由于硅基固体导热的影响,三个通道中具有不同的温度分布,中间通道的温度关于其中心线成对称分布,而两侧通道中的高温区域均靠向中间通道。虽然硅具有良好的导热性,整个硅基上的温差很小,但在微尺度下,小温差依然可以导致较大的温度梯度,造成中间通道的双汽丝关于其中心线成对称分布,并且总是发生同步断裂;侧通道中的双汽丝偏向中间通道,并且在靠近中间通道的一侧汽丝总是首先发生断裂。由于温度梯度引起的Maragnoni对流效应,侧通道中的汽泡脱离后便靠向高温侧。 在微汽泡动力学研究中,设计了一种尺寸为 的Pt薄膜微加热器,研究了脉冲控制参数对微加热器上汽泡动力学特性的影响。研究发现在该微加热器上发生汽泡核化时,核化温度均达到液体的过热极限,因此为均质核化过程。在不同的脉冲控制参数下,存在三类不同的汽泡动力学特性,即(1)汽泡爆炸性生长和冷凝以及汽泡二次生长;(2)汽泡爆炸性生长继而分裂、吸引并聚合;(3)汽泡振荡生长而后持续生长并最终达到稳定状态。在第(1)类中,汽泡二次生长是由于脉冲加热过程中在玻璃基片上储存了热量;在第(2)类中,汽泡冷凝过程中的Marangoni效应导致分裂后的汽泡互相吸引并最终聚合。在第(3)类中,汽泡尺寸最终达到稳定是由于汽泡内蒸汽的发生量与汽液界面上蒸汽的凝结量相等。 本文的研究将为微反应器、微型燃料电池、微换热器、微汽泡执行器等相变微流控系统的设计和运行提供科学指导。
学科领域热能工程
页数共145页
文献类型学位论文
条目标识符http://ir.giec.ac.cn/handle/344007/1585
专题中国科学院广州能源研究所
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张伟. 热学微流控系统中瞬态相变传热研究[D]. 北京. 中国科学院研究生院,2008.
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