Deterioration of boiling heat transfer on biphilic surfaces under very low pressures
Deterioration of boiling heat transfer on biphilic surfaces under very low pressures
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DOI：
10.1016/j.expthermflusci.2019.110026
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发表时间：
2020-05
影响因子：
3.2
通讯作者：
B. Shen;Tomosuke Mine;Naoki Iwata;S. Hidaka;Koji Takahashi;Y. Takata
中科院分区：
文献类型：
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作者：
B. Shen;Tomosuke Mine;Naoki Iwata;S. Hidaka;Koji Takahashi;Y. Takata
近年来，表面润湿工程作为增强沸腾传热的有效工具引起了越来越多的关注。特别是在润湿性图案（所谓的双亲性）表面上，低于大气压沸腾几乎不会导致传热速率严重下降，而这种情况在平面表面上往往会普遍存在。在减压条件下令人惊讶的一致性能可归因于疏水性和亲水性表面之间边界处的三相接触线（TPCL）的相当强的钉扎，这基本上消除了气泡循环之间的等待期。只有当压力降低到足够低时，双亲表面上才会最终发生向不期望的间歇沸腾模式的转变。本研究的目的是研究极低压力下混合润湿表面传热恶化的物理机制。为此，我们在不同的表面过热度和系统压力下，对涂有单个疏水性聚四氟乙烯（PTFE）点的光滑铜表面上的气泡成核和生长过程进行了高速可视化实验。结果显示 TPCL 行为与气泡生长动力学之间存在有趣的相关性。具体来说，在某个特定的压力阈值下，在特别快速的气泡膨胀下，TPCL 越来越有可能从其在双亲表面上的固定位置移出。结果，可能会发生疏水表面的完全淹没，这被认为是疏水点作为可行成核位点暂时失活的原因。此外，基于跨异质润湿性的 TPCL 传播的扩散界面模拟，对不同气泡膨胀速率的情况进行比较提供了定性证据，支持这种加速气泡生长速率在驱动 TPCL 克服润湿性鸿沟处产生的能量势垒方面的关键作用。
Surface wettability engineering has attracted growing attention in recent years as an effective tool to enhance boiling heat transfer. On wettability-patterned (so-called biphilic) surfaces in particular, subatmospheric boiling has been shown to be nearly free of the severe degradation of heat transfer rate that tends otherwise to prevail on plain surfaces. The surprisingly consistent performance under reduced-pressure conditions can be attributed to the rather strong pinning of the three-phase contact line (TPCL) at the border between the hydrophobic and hydrophilic surfaces, which essentially eliminates the waiting period between bubble cycles. Only when the pressure is decreased sufficiently low does the transition to the undesired mode of intermittent boiling eventually occur on the biphilic surface. The purpose of the present study is to investigate the physical mechanism for the heat transfer deterioration on a mixed-wettability surface at very low pressures. To that end, we performed high-speed visualization experiments of the process of bubble nucleation and growth on a smooth copper surface coated with a single hydrophobic polytetrafluoroethylene (PTFE) spot, under different surface superheats and system pressures. The results show an interesting correlation between the TPCL behavior and the bubble growth dynamics. Specifically, under some certain threshold of pressure, it would become increasingly likely for the TPCL to be dislodged from its pinned position on the biphilic surface under a particularly rapid bubble expansion. As a result, full flooding of the hydrophobic surface might ensue, which is deemed responsible for temporary deactivation of the hydrophobic spot as a viable nucleation site. Furthermore, based on the diffuse-interface simulations of TPCL propagation across heterogeneous wettabilities, a comparison of cases with different bubble expansion rates offered qualitative evidence supporting the critical role of such accelerated bubble growth rate in driving the TPCL to overcome the energy barrier raised at the wettability divide.