The determination of the liquid film thickness during annular flow condensation in a small diameter channel is complex and very rare in the literature, although important to understand the heat transfer mechanisms involved during the phase change process. In the present paper, the liquid film thickness and heat transfer coefficients have been measured during vertical downflow condensation inside a 3.38 mm inner diameter channel. Condensation tests have been run with R245fa at 40 °C saturation temperature and mass velocity ranging from 30 kg m⁻² s⁻¹ to 150 kg m⁻² s⁻¹. The test section is composed of two heat transfer sectors connected with a glass tube. The glass tube has been realized with a special external shape that produces a magnification of the liquid film thickness and allows the use of a chromatic confocal sensor. The liquid film thickness is determined by coupling a shadowgraph technique with the measurements performed by the chromatic confocal sensor. An ad-hoc algorithm has been developed for the statistical characterization of the wave structures in terms of amplitude, frequency and velocity, with the aim to better understand how the interfacial waviness affects the heat transfer.

尽管对于了解相变过程中涉及的传热机理很重要，但在小直径通道中进行环形流动冷凝过程中液膜厚度的确定是复杂且很少见的。在本文中，在3.38毫米内径通道内的垂直向下冷凝过程中测量了液膜厚度和传热系数。使用R245fa在40°C饱和温度下进行的冷凝测试和质量速度范围为30 kg m ^-2 s ^-1至150 kg m ^-2 s ^-1。测试部分由与玻璃管连接的两个传热扇区组成。玻璃管已经实现为具有特殊的外部形状，该外部形状会放大液膜厚度，并允许使用彩色共焦传感器。通过将阴影图技术与彩色共焦传感器执行的测量结合起来来确定液膜厚度。为了更好地了解界面波纹度如何影响传热，已经开发了一种自组织算法来对波结构的振幅，频率和速度进行统计表征。