Investigation on liquid film of urea–water solutions with diode laser absorption spectroscopy

Measurement of multiparameter of liquid films (e.g., film thickness and concentration) is very important to understand the film formation processes in industrial applications. Here, a novel diode laser absorption spectroscopy (DLAS) sensor was developed to simultaneously measure the liquid film thickness and concentration of urea–water solutions by forming the transmittance ratio at two wavenumber positions. The performance of the sensor employed two diode lasers (6613.25 and 7187.50 cm−1) was first validated using a calibration tool providing liquid film of urea–water solutions with known film thickness (100–1000 μm) and mass fractions (5–50 wt%), and then, the sensor was applied to study a free-falling film of urea–water solutions on a vertical transparent quartz plate. Shadowgraph images were simultaneously taken as a means to obtain falling film thickness, and it was observed that measured film thickness was in good agreement with DLAS method and shadowgraph technique, and the deviation between these two techniques was 4.1 % when the falling film was stable.