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.

摘要

通过传热相变是许多工程应用中除热的主要因素。液体薄膜由于降低了传导电阻而导致增加的热传递，此外，在液-气界面处的压力跳跃还影响了相变速率和方向。由于这些作用，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，模拟了表面特性对微米级和亚微米级2D液滴（即延伸至无限远的膜）在基板上形成的相变速率的影响。表面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，并应用了准平衡相变的动力学模型。对于波浪形表面，假定该表面为谐波，其振幅等于表面粗糙度，并且波长对应于应用程序中常见的值。由于在接触线处存在分子间力，这使得扩充的Young-Laplace方程的解变僵硬，因此采用了一种隐式方案进行数值积分。为了验证该方法，将V型槽表面上存在的纳米级液滴的分子动力学（MD）模拟预测与连续模型进行了比较。扩展的Young-Laplace方程与源自动力学理论的相变模型一起通过数值求解，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。对于具有高于和低于蒸气压力的液体压力的液滴，获得了结果，由于界面处的压力跳跃，导致了相变的相反作用。结果表明，主要由于表面形态和解体压力的综合作用，传热速率可以显着改变。还得出结论，为了增强蒸发或凝结的速度，具有短振幅的波浪形表面比具有较长振幅的波浪形表面更可取。