Abstract:

Evaporative phase transitions are widely present in industrial production and daily life such as thin film processes and crystal growth. The evaporation of the liquid layer and the thermocapillary convection affect each other and restrict each other, making the energy transfer mechanism of the evaporation interface very complicated. To understand the evaporation characteristics of water in its low-pressure pure vapor environment, a series of experimental studies were carried out on the temperature distributions and evaporating rate of water evaporation in the annular pool. The cylinder temperature of the annular liquid pool is controlled between 3℃ and 15℃, and the evaporation environment pressure ranges from 394 Pa to 1467 Pa, when the temperature measurement starts, the depth of water is 10 mm. The results show that the temperature of the vapor side on the liquid-vapor interface is higher than that of the liquid side and there is an obvious temperature jump across the vapor-liquid interface. With the decrease of the pressure ratio, the evaporation rate increases, and the interface temperature jump is enlarged. Meanwhile, with the increase of the distance from the cylinder, the local evaporation rate decreases, thus, the temperature jump decreases. At the same pressure ratio, as the cylinder temperature increases, the heat flux from vapor side decreases, the temperature jump decreases at all measurement points. Within the experimental controlled parameters, the maximum temperature jump obtained in the measurements is 2.56℃. Due to the coupling effect of evaporation cooling and thermocapillary convection, there is a uniform temperature layer with a thickness of about 2 mm under the evaporation interface. The thickness of the uniform temperature layer near the cylinder is always larger than that in the middle of the evaporation interface. In the uniform temperature layer, the thermocapillary convection induced by radial temperature gradient transfers heat from the cylinder to the liquid-vapor interface to compensate for the latent heat of evaporation. Below the uniform temperature layer, the temperature rises rapidly due to heat conduction and buoyancy convection.

Key words: evaporation, interface, convection, low pressure, temperature jump

摘要：

蒸发相变广泛存在于薄膜过程及晶体生长等工业生产和日常生活中，液层表面蒸发和热毛细对流相互影响、相互制约，使得蒸发界面能量传递机制变得非常复杂。为了深入了解水在低压纯蒸汽环境中的蒸发特性，对环形液池内水蒸发时的温度分布和蒸发速率进行了一系列实验研究。环形液池壁温控制在3~15℃之间，蒸发环境压力在394~1467 Pa之间变化，开始测量时液层深度为10 mm。结果表明，蒸发界面气相侧温度总是高于液相侧，气液界面存在明显的温度跳跃。随着压比减小，蒸发速率增加，界面温度跳跃随之增大；随着距壁面距离增加，局部蒸发速率降低，温度跳跃值减小；相同压比下，随着壁面温度的升高，气相侧热通量减小，蒸发界面温度跳跃值整体降低；在实验范围内测得的最大温度跳跃值为2.56℃。由于蒸发冷却效应和热毛细对流的耦合作用，蒸发界面下液相侧存在一个厚度为2 mm左右的温度均匀层，且壁面附近温度均匀层厚度大于中间区域厚度。在温度均匀层内，径向温度梯度诱导的热毛细对流将热量从壁面传输至气液界面以补偿蒸发所需汽化潜热；在温度均匀层以下，浮力对流和导热共同作用使得液相温度迅速升高。

关键词: 蒸发, 界面, 对流, 低压, 温度跳跃