A New Ray-Tracing-Assisted Calibration Method of a Fiber-Optic Thickness Probe for Measuring Liquid Film Flows

A ray-tracing-assisted (RTA) method for calibrating a fiber-optic thickness probe (FOTP) that enables the measurement of thickness in micrometers in wavy liquid-film flow is described herein. This technique calibrates the device more quickly than other liquid-film measurements and more efficiently than conventional fiber-optic methods. First, the emission natures of the multimode optical fiber were investigated through ray tracing. The step-index multimode fiber was observed to be superior to the graded-index multimode fiber regarding the thickness resolution and the uncertainty avoidance of the liquid-film shape. Second, the RTA calibration method for pseudo-thickness measurement under stagnant conditions was validated. The measured thickness of the calibrated FOTP was compared to that of visualization. Agreement between the two resulted in a maximum difference of 10% for over <inline-formula> <tex-math notation="LaTeX">$200~\mu \text{m}$ </tex-math></inline-formula> and 20% for under <inline-formula> <tex-math notation="LaTeX">$200~\mu \text{m}$ </tex-math></inline-formula>. Finally, the numerical verification of this method in virtual liquid-film flow was demonstrated. The applicable thickness range and measurement uncertainty of the FOTP, which are unattainable by experiments alone, are discussed.