摘要
随着各种热力系统尺寸日益减小的趋势,微通道越来越广泛的应用于微换热器以及微型燃料电池等器件中。微通道内的流动凝结机理研究,对于这些高新技术产业具有重要的应用前景和学术价值。对微通道流动凝结换热的传热机理和特性认识尚处于起步阶段,有限的研究表明,微通道内流动凝结的主要流型是弥散流、环状流、喷射流和塞/泡状流。表面张力取代重力在微通道中起到主要作用,常规通道中因重力作用而导致的分层流不再出现。几何形状、表面粗糙度以及润湿性等表面特性,在微通道流动冷凝中显得尤为突出。常规通道与微通道在重力、表面张力和剪切力的相对量级上有重大差异,导致微通道内的两相流机理和流型转变与传统管道有很大不同;未考虑表面张力的常规尺度冷凝模型已不能完整描述微通道内的流动凝结过程。而将大直径圆管的结论外推到小直径非圆形管道,会造成压降和换热系数预测的极大误差。
本文以揭示微通道内流动凝结换热的机理为目标,以流型、压降和换热特性为研究重点,对微通道内蒸汽流动凝结特性进行实验研究和理论分析。本论文首先利用高速CCD可视化成像系统,对微通道内水蒸汽凝结相变过程的流型和喷射流出现频率进行可视化实验。研究结果表明:微通道内存在bubbling和jetting两种形式的凝结喷射流模式。随着水蒸汽的质量流量增大,以及微通道横截面宽高比、冷凝速率和微通道水力直径的减小,凝结喷射流的发生频率会增大;喷射流发生位置作为微通道内环状流和塞状流区域的分界点,随着质量流量增大或冷凝速率减小而向微通道的下游移动,发生点的蒸汽干度也减小。意味着微通道内的环状流区域增大,塞状流区域减小;反之则情形相反。
其次,对去离子水在不同横截面宽高比光滑梯形硅微通道中的层流压降进行实验研究和数值模拟,将实验结果与文献中的单相流压降关联式以及理论解进行对比;而后对水蒸气凝结相变压降进行了测量,发现凝结压降随着微通道水力直径的减小、质量流量和干度的增大而增大;Chisholm常数C随着微通道水力直径的减小而减小;现有的小通道和常规通道的压降关联式往往高估了本文的实验数据;基于无量纲分析方法,合理选择了Chisholm常数C的控制参数,通过大量的实验数据,修正了Lockhart-Martinelli的两相流压降模型,给出了Chisholm常数C的新实验关联式。
基于MEMS加工技术,在两根具有相同横截面宽高比的微通道内制作集成Pt热电阻直接测量微通道内壁面温度,有效减小了水蒸气凝结换热系数的实验误差。实验结果表明:凝结换热系数随着微通道的水力直径减小、质量流量和干度的增大而增大;基于湍流边界层分析建立了剪切力驱动的环状流凝结换热半理论模型,该模型与本文实验结果吻合较好;理论分析了因为常规通道压降实验关联式高估了微通道的凝结压降数据,导致了其相应的换热系数关联式必然高估微通道换热系数的原因。这个结论可以用来检验微通道的凝结摩擦压降和换热系数实验结果的一致性和正确性。
最后为了深入地研究微通道内的凝结喷射流现象,借助高速CCD对无相变微气泡在梯形截面硅微通道中的产生进行可视化实验研究。分析了梯形横截面几何形状,水和空气流量对气泡喷射频率的影响特性以及两相流型的转变曲线及区域划分。发现了bubbling及jetting两种不同的均匀气泡形成模式,针对不同的模式根据实验结果得到了各自的气泡频率和长度的拟合公式。观察气泡在下游T型通道处的分离行为,发现了断裂(breaking)及不断裂(non-breaking)两种气泡分离模式,得到了两种模式的区域划分,且与理论吻合理想。
本文的研究不仅有利于提高对微通道内蒸汽凝结换热特性及机理的理解,还有助于微冷凝器的开发以及优化设计。
With the growing trend in miniaturization of various thermal systems, the investigation of microscale fluid flow and heat transfer has been a hot research topic during the past decade. In particular, condensation heat transfer in microchannels has important applications to the design of compact and micro-heat exchangers for cooling of thermal devices. Our present understanding of mechanism and characteristics of condensation heat transfer in microchannels, however, is still in its infancy. Since surface tension dominates over gravitational forces in microchannels, condensation flow patterns in microchannels are greatly simplified as compared to those in macrochannels. For example, stratified flow induced by gravity in conventional channels does not occur in microchannels. Recently, it was found that the condensation flow patterns in microchannel include mist flow, annular flow, injection flow, and plug/slug flow. Of particular interest is the occurrence of the injection flow which does not exist in condensation in marcochannels. Furthermore, surface properties such as geometry, surface roughness and wettability become increasing important in condensation in microchannels as its diameter becomes smaller. Because of the significant differences in the relative magnitudes of gravity, shear and surface tension forces in conventional channels and microchannels, flow regime transitions in microchannels are considerably different from those conventional channels. The correlation equations for condensation in macrochannels, which did not take surface tension into consideration, is not applicable to condensation in microchannels. Thus, the extrapolation of correlations for condensation pressure drop and heat transfer obtained for large round tube to smaller diameters of non-circular channels will introduce substantial errors.
The aim of this thesis is to study mechanisms of condensation in microchannels, with particular emphasis on flow regimes, pressure drop and heat transfer. A visualization study, using a high-speed CCD and a microscope, is conducted to observe steam condensation flow patterns and occurrence frequency of injection flow in microchannels. It was found that there are two types of injection flow patterns: bubbling and jetting. The occurrence frequency of injection flow is found to be increasing with mass flux of steam and decreasing with aspect ratio of microchannel section, condensation rate and hydraulic diameter of microchannels. The location of breakup point, which is the dividing point of annular flow and slug flow regimes in microchannels, moves downstream with increase of mass flux or decrease of condensation rate, and quality at this location decreases. This means that annular regime extends and slug flow regime shrinks.
Experimental and numerical research on frictional pressure drop of deionized water in silicon microchannels with different aspect ratios are first conducted, which is followed by measurements of pressure drop in steam condensing in microchannels. It is found that pressure drop in the condensing flow decreases with hydraulic diameter of microchannels and increases with mass flux and quality. The existing correlations of pressure drop in mini- and macro-channels overestimate experimental data in microchannels. The data of condensation pressure drop in microchannels is correlated in the form of Lockhart-Martinelli correlation. The Chisholm constant C in the correlation is found to be increasing with the hydraulic diameter of microchannels. The controlling parameters of the Chisholm constant C are properly chosen based on a dimensionless analysis. A new correlation of Chisholm constant C is developed in terms of appropriate parameters by fitting the experimental data.
For condensation heat transfer study, integrated Pt thermoresistors are fabricated by MEMS technologies on the wall of the microchannels. These Pt thermoresistors are used to measure the internal-wall temperature of microchannel directly in our experiments. This would reduce the estimated temperature on the wall in previous condensation experiments in microchannels where temperatures were measured by thermocouples embedded on the bottom of the microchannels. The condensation heat transfer coefficient is found to be decreasing with hydraulic diameter of microchannels and increasing with mass flux and quality. Semi-analytical method, based on thermal boundary layer theory of turbulent flow, is used to derive the condensation heat transfer coefficient of annular condensation flow, which is shown in good agreement with experimental data obtained in this thesis. The overestimation of correlations for pressure drop in conventional channels on that in microchannels results in certain overestimating of its corresponding correlations for heat transfer coefficient on heat transfer coefficient in microchannels are analyzed theoretically. This conclusion can be used to verify the consistency or validity of experimental results on frictional pressure drop and heat transfer coefficient in microchannels.
Finally, in order to gain a deeper understanding of the occurrence of injection flow in condensing flow in microchannels, the formation of micro air bubble in the co-flowing of air and water in microchannels is investigated with the aid of a high-speed CCD and microscopy. The effects of geometrical parameters and volume flow rate of air/water on bubble formation frequency are investigated experimentally, and the transition curve and flow map of the two phase flow without phase change are illustrated. It was found there are two different bubble formation patterns: bubbling and jetting. Regression analyses were performed on data obtained for bubble formation frequency of these two formation patterns. Two kinds of bubble separation mode, breaking and non-breaking, are observed at T junction downstream. The regions of the two separation modes are distinguished, which are in good agreement with theory.
The result of the present study not only contributes to the improved understanding of condensation in microchannels, it also provides data base for optimal design of micro-condensers for cooling of micro-devices in emerging technology.
引文
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