Liquid film thickness and heat transfer measurements during downflow condensation inside a small diameter tube

•Condensation of R245fa in a 3.38 mm tube during vertical downflow is investigated.•Simultaneous film thickness and heat transfer measurements are performed.•Shadowgraphy and chromatic confocal imaging are used to measure film thickness.•The liquid film thickness and interfacial waves affect the heat transfer.•Statistical analysis on wave height, frequency and velocity is performed. 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.