Mid wave infrared thermography of water films in condensing and evaporating environments

Mid-wave infrared thermography was investigated to measure the surface temperature of water films flowing in condensing and evaporating environments. The measurements were performed through a sapphire window and a confined environment with the presence of water vapor. An in situ real time calibration was developed to account for the light attenuation and emitted radiation caused by the absorbance of steam and sapphire on the optical path of detection. Thermal mapping reveals spatiotemporal fluctuations measured with a sensitivity of 47 mK for evaporation and 82 mK for condensation. The thermal images were synchronized with the film thickness mapping obtained from the imaging absorption of near infrared light in the film. The images from both techniques were dewarped and cropped to obtain identical measurement fields. Superposed temperature and thickness mappings were recorded with an image size of 246 × 180 pixels, a projected pixel size of 0.63 mm and frame rates of 125 and 250 fps, respectively. Measurements were performed in the LINX facility at the Paul Scherrer Institute (PSI). Liquid films were created on a temperature controlled wall placed vertically inside a pressure vessel with controls on thermal-hydraulic conditions. Films were produced either by condensation or via a film injector. The results obtained for selected tests are presented which involve condensation and evaporation on a wavy films with steady state conditions. Measurements with the surface partially wetted by an evaporating rivulet as well as drop wise condensation are reported. Confirmation could be brought that the perturbations caused by the presence of waves on the film impact locally the surface temperature of the film. The convolution and synchronization of mid-wave and near infrared imaging represent a strong analytical tool for the study of two phase flows involving liquid films subject to heat and mass transfers.