Flow boiling of the perfluorinated dielectric fluid FC-77 in a silicon microchannel heat sink is investigated. The heat sink contains 60 parallel microchannels each of 100 lm width and 389 lm depth. Twenty-five evenly distributed temperature sensors in the substrate yield local heat transfer coefficients.The pressure drop across the channels is also measured. Experiments are conducted at five flow rates through the heat sink in the range of 20–80 ml/min with the inlet subcooling held at 26 K in all the tests. At each flow rate, the uniform heat input to the substrate is increased in steps so that the fluid experiences flow regimes from single-phase liquid flow to the occurrence of critical heat flux (CHF). In the upstream region of the channels, the flow develops from single-phase liquid flow at low heat fluxes topulsating two-phase flow at high heat fluxes during flow instability that commences at a threshold heat flux in the range of 30.5–62.3 W/cm2 depending on the flow rate. In the downstream region, progressiveflow patterns from bubbly flow, slug flow, elongated bubbles or annular flow, alternating wispy-annular and churn flow, and wall dryout at highest heat fluxes are observed. As a result, the heat transfer coefficients in the downstream region experience substantial variations over the entire heat flux range, based on which five distinct boiling regimes are identified. In contrast, the heat transfer coefficient midway along the channels remains relatively constant over the heat flux range tested. Due to changes in flowpatterns during flow instability, the heat transfer is enhanced both in the downstream region (prior to extended wall dryout) and in the upstream region. A previous study by the authors found no effect of instabilities during flow boiling in a heat sink with larger microchannels (each 300 lm wide and 389 lm deep); it appears therefore that the effect of instabilities on heat transfer is amplified in smaller-sized channels. While CHF increases with increasing flow rate, the pressure drop across the channels has only a minimal dependence on flow rate once boiling is initiated in the microchannels, and varies almost linearly with increasing heat flux.
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机译：研究了全氟化介电流体FC-77在硅微通道散热器中的沸腾现象。散热器包含60个平行的微通道，每个通道的宽度为100 lm，深度为389 lm。基板中的25个均匀分布的温度传感器产生局部传热系数，还测量通道中的压降。在所有测试中，通过散热器的五种流速在20-80 ml / min的范围内进行实验，入口过冷度保持在26K。在每种流速下，输入到基材的均匀热量会逐步增加，因此流体会经历从单相液体流到临界热通量（CHF）的流动状态。在通道的上游区域，在流动不稳定期间，流量从低热通量的单相液体流发展为高热通量的脉动两相流，并在30.5–62.3 W / cm2的阈值热通量开始取决于流速。在下游区域，观察到来自气泡流，团状流，细长气泡或环形流，细细的环形和搅动流交替变化以及在最高热通量下壁变干的渐进流型。结果，在下游区域的传热系数在整个热通量范围内经历了显着的变化，据此确定了五个不同的沸腾状态。相反，在测试的热通量范围内，沿通道中间的传热系数保持相对恒定。由于在流动不稳定期间流型的变化，在下游区域（延长的壁变干之前）和上游区域的传热都得到了增强。作者先前的研究发现，在具有较大微通道（每个300 lm宽和389 lm深）的散热器中，在流沸腾过程中，不稳定性不会受到影响。因此，似乎不稳定性对热传递的影响在较小尺寸的通道中被放大。虽然CHF随着流速的增加而增加，但是一旦在微通道中开始沸腾，通道中的压降对流速的依赖性就很小，并且随着热通量的增加几乎呈线性变化。
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