Inter-laboratory optical measurements of film thickness during annular flow condensation in a small diameter channel

In the present study, liquid film thickness measurements are performed during vertical downflow condensation inside a 3.38 mm inner diameter channel at 40 °C saturation temperature. Condensation tests are carried out with HFE-7000 in two different test apparatuses, the first being composed of two counter-flow tube-in-tube heat exchangers separated by an adiabatic borosilicate glass tube for flow visualizations and liquid film thickness measurement, while the second consisting of a diabatic sapphire tube coupled with an infrared camera for simultaneous local heat flux and film thickness measurements at low mass flux (5–30 kg m−2 s−1). Chromatic confocal imaging and shadowgraphy are applied to the adiabatic borosilicate glass tube for evaluating the liquid film thickness with high temporal and spatial resolution, while punctual optical sensors (chromatic confocal and interferometer) are used in the diabatic sapphire test section. The liquid film thickness results obtained in the two independent laboratories are compared at the same operating conditions, showing an excellent agreement. The effect of mass flux (from 50 to 150 kg m−2 s−1) and vapor quality on the liquid film thickness and interfacial waviness of HFE-7000 is discussed by means of both statistical and spectral analyses. The liquid film thickness data of HFE-7000 are compared with those of R245fa and R134a taken during previous experimental campaigns. HFE-7000 generally displays lower film thickness values compared to R245fa and R134a due to the smaller vapor density. At low mass flux (50 kg m−2 s−1), such comparison shows the effect of the small liquid viscosity on the thinning of the liquid film in the case of R134a. The accuracy of well-known models for the prediction of the mean film thickness is assessed with comparison to the experimental data of HFE-7000.