【作者】 付鑫；

【导师】 张鹏；

【作者基本信息】 上海交通大学 ， 制冷及低温工程， 2011， 博士

【摘要】 微细通道内的相变换热由于结构紧凑,换热效率较高,在电子、航天、生物医学等现代高新技术领域有广泛的应用,对其规律的研究也成为国际传热界的热点。本文针对微型医疗器械,超导磁体冷却等应用中涉及的微细通道内液氮的流动沸腾,采用高速摄像从微观层面上对其热物理特性与机理进行了深入的研究,并对相变过程中气泡动力学特征以及对应换热特性进行了数值模拟。主要结论如下:开展了实验难度较大的低温流动与传热的可视化研究工作,解决了低温条件可视化实验中布光和放大倍率的难题,获得了高质量的流型图片。微细通道内液氮流动沸腾的主要流型为:泡状流,弹状流,搅拌流和环状流;并且在1.042 mm和0.531 mm管内发现了受限气泡流。绘制了流型图,结果表明表面张力是影响流型转变的重要物性参数。相对于空气-水的流型图,弹状流区域很小,对应的弹状流/搅拌流,搅拌流/环状流流型转变线向较低的气体表观速度方向移动;而泡状流/弹状流的转变线向较高的气体表观速度方向移动。为了深入研究微细通道内液氮流动沸腾的机理,在微细玻璃管外表面电镀一层透明的加热膜（氧化锡铟）用来研究气泡核化,脱离等气泡动力学特征和流型转变过程。微细玻璃管的内径为1.3-1.5 mm。测量了气泡脱离直径以及气泡周期,满足关系式（ D_d ）^1.46·（1/τ）=constant,说明微细通道内的气泡脱离特征更类似于常规通道。微细通道内气泡脱离后流型的转变则受到明显的微尺度效应的影响,气泡生长受限,流型转变加快,换热系数增大。研究了微细通道内不同流型的换热系数,包括泡状流,弹状流,环状流以及倒流和干涸。结果表明微细通道内流动沸腾的主导机理是液膜蒸发。发生干涸时,换热恶化,而倒流能在一定程度上增强上游的换热系数。研究了干涸之后的流型发展过程,观测到了反泡状流,反弹状流以及反环状流等流型。发现并详细描述了微细通道内的液滴夹带现象,不同于常规通道,这种液滴夹带较多的发生于弹状流中。一般的可视化实验得到的结果只能反映二维平面上的信息,而带来三维空间上诸多重要信息的缺失。而常规尺度的三维可视化方法由于工作距离上的限制,很难应用于微尺度的三维可视化实验中。本文创造性地提出一种简洁有效的适应于微尺度成像的三维可视化光路,成功实现了微细通道内两相流动的三维可视化。该方法在实验段周围特定的位置设置一片等腰直角棱镜和一面平面镜,由此实现了一个相机同时获取两相流的正面像和侧面像。在此基础上实现了三维重建。同时对由于折射以及棱镜色散而带来的图像变形进行了定量分析,并提出了矫正方法。尽管该方法的验证实验针对可视化难度大的低温流体进行,但是同样适用于微细通道内常温流体的可视化研究。在实验研究基础上,本文对微细通道内气泡动力学特征以及对应的换热特征进行了数值研究。采用Volume-of-Fluid（VOF）模拟,将计算区域划分为主流区域和微液膜区域,采取不同的质量和能量源项来模拟相变过程。采用了一种简单的微液膜模型,实现了微液膜层内的传热传质过程。成功模拟了微细通道内气泡的核化生长过程。同时系统研究了流量等参数?物性?几何特征等因素对气泡生长以及对应的换热特性的影响。发现在较高流速下,气泡生长表现出线性规律;而流速较低时,生长曲线表现抛物形的特点。分析了热流密度对气泡生长的影响,在微细通道内气泡生长的主导机理为热控制机理。热物性如表面张力?接触角以及液气密度比对气泡生长以及流型转变有显著影响。对于小表面张力和接触角的流体,核态沸腾时气泡较容易脱离加热表面。当液气密度比增大时,气泡生长速率较快。气泡生长受到壁面限制时,换热系数增强。模拟分析了受限气泡的产生发展过程,结果表明受限气泡的换热影响区域约为受限气泡大小的两倍;受限气泡能够显著增强影响区域内的换热系数。系统研究了以不锈钢为基材的微通道热沉内液氮流动沸腾的流型特征和换热特性。通过高速摄像,获取的主要流型为泡状流,弹状流和环状流,发现不稳定倒流现象严重。在流量为50.1-880.5 kg/m²s范围内,最大换热能力达到21.35 W/cm²,增加热沉通道深度能够显著增加换热能力。测量了各个微细通道间的流量分配。发现单相条件下,各通道间的流量分配基本一致,两相流条件下,各通道间的差别较大,而且随着流速的增加,不均匀性增强。在本文的实验范围内,各通道间最大的流量差别约为18%。研究了各个通道在不同流型条件下的压降特性。发现在单相流动条件下,各通道的压降曲线基本重合在一起,进入两相状态后,在波动相位和幅度上,各通道逐渐出现偏移,甚至反相。更多还原

【Abstract】 Flow boiling heat transfer in mini/micro-channels has attracted a great deal of attention in the past a few years due to the various applications in electronics, astronautics, medical treatment, etc. With the aid of high-speed photography, the present study aims to uncover the physical mechanism of convective boiling of liquid nitrogen in mini/micro-channels. Moreover, numerical simulation of the bubble dynamics is developed to further understand the phase change phenomenon in mini/micro-channels. The main conclusions are shown below:Successfully solving the two difficulties in cryogenic visualization, i.e., illumination and magnification, the present study set up the visualization system for micro-scale two-phase flow in cryogenic temperature. It was found from the experimental results that the flow patterns were mainly bubbly flow, slug flow, churn flow and annular flow. And the confined bubbles and mist flow were also observed in micro-tubes of 1.042 mm and 0.531 mm in inner diameters in the experiments. Compared with flow regime maps for gas-water flow in tubes with similar hydraulic diameters, the region of slug flow in the present study reduces significantly. Correspondingly, the transition boundary from the bubbly flow to slug flow shifts to higher superficial gas velocity, and the transition line of churn to annular flow moves to lower superficial gas velocity.To explore the mechanism of flow boiling of liquid nitrogen in mini/micro-channels, it employed a segment of upward vertical quartz glass tube with the inner diameter range of 1.3-1.5 mm, which was coated with a layer of transparent ITO film （Indium Tin Oxide） as the heater on the outer surface. The bubble growth, departure and the following flow pattern evolution in the micro-tube were visualized and quantitatively investigated, along with the simultaneous measurement of local heat transfer coefficient around a specified nucleation site. The bubble departure diameter and bubble period were investigated and satisfy （ D_d ）^1.46·（1/τ）=constant, which showed that the tube size of the micro-tube had no notable effect on the bubble departure and the trend of the bubble departure was similar to that in macro-tubes. Whereas the following flow pattern evolution was apparently confined due to the size effect, which presented the acceleration of flow pattern transition and desirable heat transfer performance in micro-tubes. The heat transfer coefficients for different flow patterns along the micro-tube were obtained in terms of bubbly, slug, annular flow and the flow regimes of flow reversal and post-dryout. It was found that the dominant heat transfer mechanism was the liquid film evaporation which offered desirable heat transfer capability. The heat transfer performance would be deteriorated in the post-dryout regime, while flow reversal could somewhat enhance the heat transfer upstream of the nucleation site. Flow pattern evolution beyond the boiling crisis was also investigated. Post-dryout regimes such as inverted bubbly, inverted slug and inverted annular flow were observed in the micro-tube. Flow reversal and liquid entrainment, which were relevant to flow instability in the flow pattern evolution, was demonstrated clearly. Other than macro-channels, the liquid entrainment usually occours in slug flow in mini/micro-channels.Flow pattern visualization is essential for understanding the mechanism of two-phase flow in the micro-scale passages like micro-tubes and micro-channels, etc. However, the front view of the two-phase flow is the only information obtained in the most flow pattern visualization researches in micro-scale passages, so far. The two-dimensional images can only provide partial information and sometimes important information such as the bubble nucleation sites, bubble shapes and spatial distribution of the bubbles, which could provide an in-depth understanding of two-phase flow, are missed or not accurately obtained. Due to the limitation of the working distance of the conventional visualization system, it is very difficult for applying the conventional three-dimentional photography method for the visualization of the two-phase flow in mini/micro-channels. The present study proposed a simple but effective method to visualize the two-phase flow in mini/micro-channels three-dimensionally, which was validated by the difficult experiment in the cryogenic temperature and could be extended to the flow condition in room temperature. An isosceles right-angle prism combined with a mirror located 45°bevel to the prism was employed to obtain synchronously the front and side views of the flow patterns with a single camera, where the locations of the prism and the micro-tube for clearly imaging should satisfy a fixed relationship which was specified in the present study. The image deformation due to refraction and chromatic aberration due to the prism were clearly specified and corrected.Numerical simulation was conducted to clarify the flow boiling process in micro-tubes by using Volume-of-Fluid （VOF） method which was modified to include the effect of phase change. A specially treated micro-layer model was used, in which the evaporative heat flux through the micro-layer was approximated in the simulation. The effect of heat flux, mass flux, surface tension force, contact angle and channel size on the bubble growth and heat transfer were analyzed. It was found that the bubble growth rate displayed linear trend in relatively high flow rate, while the bubble growth curve shaped parabolic under low flow rate. The effect of heat flux on the bubble dynamics was specified and it showed that the controlled mechanism of bubble growth during flow boiling in micro-tubes was thermally controlled mechanism. The thermal properties such as surface tension, contact angle and density ratio played significant role in the bubble growth and the following pattern evolution. For the working fluid with small surface contact angle and surface tension, the vapor bubble departed from the heating wall easily in the nucleate boiling. Bubble growed fast as the liquid-vapor density ratio increased. The confined bubble occurred and the corresponding heat transfer performance was enhanced as the channels size reduces. The influential region of the confined bubble was specified, which covered more than two-fold area of the confined bubble and could heavily influence the region upstream of the confined bubble.The experimental study was performed on the convective boiling of liquid nitrogen through stainless steel heat sink. Heat transfer characteristics and associated flow patterns were quantified. The appearing typical flow patterns in the micro-channel heat sink were bubbly, slug and annular flow, as well as flow reversal. The greatest heat capability was up to 21.35 W/cm² in the flow rate range of 50.1-880.5 kg/m²s. The maldistribution of mass flux in each channels of the heat sink was investigated and it was found the maximum difference was around 18% in the present experimental range. Moreover, the maldistribution of mass flux in each channel became apparent as the flow rate increases. The pressure drop characteristics of different channels for different flow patterns were experimentally investigated. It was found that the pressure drop curves almost overlapped for single liquid phase. Whereas the curves diversified as the flow enters into two phase, even out of phase with each other in some cases.更多还原