ABSTRACT Heat transfer via phase change is a major contributor to heat removal in numerous engineering applications. Thin films of liquid result in increased heat transfer due to a reduction of conduction resistance, in addition the pressure jump at the liquid-vapor interface also affects the rate and direction of the rate of phase change. Because of these effects the morphology of the substrate surface is expected to affect the film shape, hence heat transfer, especially in thin films. In this study, the influence of surface characteristics on the rate of phase change from micron- and submicron-sized 2D droplets – i.e. films extending to infinity – forming on a substrate are modeled. Surface film profiles are generated on both flat and nonflat surfaces, triangular or wavy in nature, and a kinetic model for quasi-equilibrium phase change is applied. In the case of wavy surfaces, the surface is assumed to be a harmonic wave with an amplitude equal to the surface roughness and a wavelength corresponding to values commonly encountered in applications. Due to the presence of intermolecular forces at the contact line, which renders the solution of the augmented Young-Laplace equation stiff, an implicit scheme is employed for the numerical integration. To verify the method, the predictions of a molecular dynamics (MD) simulation of a nano-sized droplet present on a V-grooved surface are compared to the continuum model. The augmented Young-Laplace equation is solved numerically along with a phase change model originating from kinetic theory to calculate the shape of the two-phase interface forming the droplet and study the effect of various parameters on the rate of phase change. Results are obtained for droplets with liquid pressures higher and lower than that of vapor, resulting in opposite contribution to phase change due to the pressure jump at the interface. The results show that the heat-transfer rate can be substantially altered due primarily to the combined effects of surface morphology and disjoining pressure. It is also concluded that wavy surfaces with short amplitudes are preferable to ones with longer amplitudes for enhancing the rate of evaporation or condensation.

中文翻译：

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表面形态对微米和亚微米尺寸二维液滴相变速率的影响

摘要 在众多工程应用中，相变传热是除热的主要因素。由于传导阻力的降低，液体薄膜导致传热增加，此外，液-气界面处的压力跳跃也影响相变速率的速率和方向。由于这些影响，预计基材表面的形态会影响薄膜形状，从而影响热传递，尤其是在薄膜中。在这项研究中，模拟了表面特性对在基板上形成的微米和亚微米尺寸 2D 液滴（即延伸至无限大的薄膜）的相变速率的影响。表面薄膜轮廓在平坦和非平坦表面上生成，本质上是三角形或波浪形的，并应用了准平衡相变的动力学模型。在波状表面的情况下，表面被假定为谐波，其振幅等于表面粗糙度，波长对应于应用中常见的值。由于在接触线上存在分子间力，这使得增强的 Young-Laplace 方程的解变得僵硬，数值积分采用隐式方案。为了验证该方法，将 V 形槽表面上存在的纳米尺寸液滴的分子动力学 (MD) 模拟的预测与连续模型进行比较。增强的 Young-Laplace 方程与源自动力学理论的相变模型一起进行数值求解，以计算形成液滴的两相界面的形状，并研究各种参数对相变速率的影响。结果是液体压力高于和低于蒸汽的液滴，由于界面处的压力跳跃导致对相变的相反贡献。结果表明，主要由于表面形态和分离压力的综合影响，传热速率可以显着改变。还得出结论，在提高蒸发或冷凝速率方面，具有较短振幅的波浪表面优于具有较长振幅的波浪表面。结果是液体压力高于和低于蒸汽的液滴，由于界面处的压力跳跃导致对相变的相反贡献。结果表明，主要由于表面形态和分离压力的综合影响，传热速率可以显着改变。还得出结论，在提高蒸发或冷凝速率方面，具有较短振幅的波浪表面优于具有较长振幅的波浪表面。结果是液体压力高于和低于蒸汽的液滴，由于界面处的压力跳跃导致对相变的相反贡献。结果表明，主要由于表面形态和分离压力的综合影响，传热速率可以显着改变。还得出结论，在提高蒸发或冷凝速率方面，具有较短振幅的波浪表面优于具有较长振幅的波浪表面。