Measurement of liquid water film thickness on opaque surface with diode laser absorption spectroscopy

Liquid film is usually formed on opaque surfaces in various industrial processes, and the investigation of liquid film is very important for understanding and optimization of the relevant processes. Here, a novel on-line measurement method based on diode laser absorption spectroscopy (DLAS) was developed to determine the film thickness on opaque surfaces. The evaporation process of static liquid films both on a silver-coated mirror (smooth opaque surface) and an aluminum alloy plate (rough opaque surface) were investigated with the proposed method. Ultrasonic pulse-echo method (UPEM) was simultaneously applied to validate the measurement accuracy of DLAS method, it revealed that the average thickness deviation between these two methods during film evaporation process on silver-coated mirror were 4.9% and on aluminum alloy plate were 7.1%, respectively. A flow liquid film on aluminum alloy surface was then studied, it was found that the average thickness deviation between DLAS and UPEM at low flow speed were 6.5% and at higher flow speed were 9.5%, respectively. And the fluctuation frequency of the liquid film at both flow speeds were also in good agreement with these two methods.