Newly Developed Method for Liquid Thin Film Thickness Measurement Using Optical Fiber–Based Reflective Probe

A new ray-tracing–based calibration method for an Optical fiber–based Reflective Probe (ORP) was developed. This technique enables thickness measurement in micrometers in wavy thin liquid film flow, which is simpler and quicker than other liquid film measurements. First, the relationship between the film thickness and ORP signal was calculated through the ray-tracing simulator. The signal trend showed a steep rate of change within a few-hundred-micron thicknesses, thanks to the emission nature of the step index multimode fiber. The ray-tracing–based calibration was established using the calculated relationship. Second, the calibration method was validated under quiescent conditions. The calibrated ORP measured the thickness and then was compared to visualization. Good agreement was confirmed between the two results at a maximum difference of 20% under 1000 μm in thickness. Finally, thickness measurement for the wavy thin film flow was performed. Airflow (j_G = 40 to 75 m/s) was introduced into the rectangle test section, and a small amount of tap water (Q = 30 to 90 mL/min) was injected into the channel plate. The difference in the measured thickness between ORP and high-speed visualization was around 20%. The effectiveness of the new calibration method and ORP measurement including its uncertainty will be discussed.