Transition between thin film boiling and evaporation on nanoporous membranes near the kinetic limit
Transition between thin film boiling and evaporation on nanoporous membranes near the kinetic limit
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DOI：
10.1016/j.ijheatmasstransfer.2020.119673
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发表时间：
2020-01
影响因子：
5.2
通讯作者：
Qingyang Wang;Yang Shi;Renkun Chen
中科院分区：
文献类型：
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作者：
Qingyang Wang;Yang Shi;Renkun Chen
纳米孔结构包括单纳米孔和纳米孔膜，已被用作研究基本的液-汽相变换热过程的平台，也是一种很有前途的高通量散热候选材料。以前，我们实现了纳米多孔膜来支撑一种薄的液体薄膜进行沸腾，这被称为薄膜沸腾，并实现了高传热性能。除了薄膜沸腾，通过纳米孔结构的薄膜蒸发也被证明可以获得高热流密度，但这两种机制通常被认为是在截然不同的条件下运行的两种相互排斥的机制，决定PCHT过程如何接近动力学极限的因素是难以捉摸的。在这项工作中，我们利用薄膜沸腾和通过纳米孔膜蒸发之间的独特转变来阐明相对于动力学极限条件决定热通量和换热系数(HTC)的因素。我们毫不含糊地展示了从沸腾到蒸发的可控转变，当液体退回到纳米孔中，并提供了维持在纳米孔中的毛细泵送的额外驱动力。我们证明了这种转变是普遍存在的，并且可以从我们研究的所有四种类型的流体的一个简单的流体传输模型中理解，这些流体涵盖了很大范围的表面张力(水、乙醇、异丙醇、FC-72)。更重要的是，沸腾和蒸发之间的转折点上的PCHT条件接近所有这些流体的动力学极限。然而，在过渡点以外进一步增加热流密度会导致HTC的降低和对动力学极限的偏离，这可以归因于蒸气空间和纳米孔内部的蒸汽阻力的增加。在不同蒸汽压的IPA上的实验也证实了这种蒸气阻力的增加。我们的工作可以揭示纳米孔道结构中PCHT的动力学极限，并可能推动高热流密度散热器件的发展，特别是使用介电流体。
Nanoporous structures including single nanopores and nanoporous membranes have been utilized as a platform to study fundamental liquid-vapor phase change heat transfer (PCHT) processes as well as a promising candidate for high flux heat dissipation. Previously, we implemented nanoporous membranes to support a thin liquid film for boiling, which was termed “thin film boiling”, and realized high heat transfer performance. Besides thin film boiling, thin film evaporation through nanoporous structures have also been demonstrated to achieve high heat flux, but these two mechanisms are usually considered two mutually exclusive regimes operated under vastly different conditions, and the factors dictating how close the PCHT process is to the kinetic limit are elusive. In this work, we utilized a unique transition between thin film boiling and evaporation through nanoporous membranes to clarify the factors determining the heat flux and heat transfer coefficient (HTC) with respect to the kinetic limit conditions. We unambiguously showed the controllable transition from boiling to evaporation, when the liquid receded into the nanopores and provided additional driving force from capillary pumping sustained in the nanoscale pores. We showed that this transition is universal and can be understood from a simple fluid transport model for all the four types of fluids we studied, which cover a wide span of surface tension (water, ethanol, IPA, FC-72). More importantly, PCHT conditions at the transition points between boiling and evaporation were close to those of the kinetic limit of all these fluids. However, further increase of the heat flux beyond the transition points led to decreasing HTC and deviation from the kinetic limit, which can be attributed to the increasing vapor resistance in the vapor space and inside the nanopores. This increasing vapor resistance was also confirmed by experiments on IPA with different vapor pressures. Our work could shed light on PCHT on nanoporous structures with respect to the kinetic limit, and could advance the development of high heat-flux heat dissipation devices, especially using dielectric fluids.