Measuring interfacial waves on film flowing down a vertical plate wall in the entry region using laser focus displacement meters

Waves on a film flowing down a vertical wall appear in many processes. The resulting interfacial waves show fascinating nonlinear phenomena, including solitary waves and complex disordered patterns. Measurements have often been made of these phenomena using electrical resistance or electrical capacitance methods, optical methods, and laser beam methods. This paper presents a new way of measuring the interfacial waves on a film flowing down a vertical plate wall in an entry region, using two laser focus displacement meters. The purpose of the study was to clarify the effectiveness of the new method for obtaining detailed information on the waves, and to investigate the effect of the entry length on the phenomena. With this method, accurate measurements of film thickness were achieved in real time with a sensitivity of 2 μm and 1 kHz. The error caused by refraction of the laser beam passing through a transparent wall was clarified. The present results for wave velocity and maximum film thickness agreed well with past experimental and theoretical studies. In short entry length conditions, the average measured film thickness and wave velocity agreed with those calculated using Nusselt’s Law, indicating that the flow is laminar even at a high flow rate. As a result of this study, an empirical equation expressing wave frequency in the entry region was formulated.