High-speed measurement of thickness of a water film formed by a jet obliquely impinging onto a plate using an LED-induced fluorescence method

A transient thickness distribution measured with a high temporal resolution is elemental for exploring the flow characteristics and mechanism of a liquid film formed by an impinging jet. Therefore, this paper develops a high-speed Light-Emitting Diode-Induced Fluorescence (LEDIF) system based on the brightness measured directly above the liquid film. An Ultraviolet (UV) LED lamp is used to provide sufficient and continuous excitation light. Then, a system performance analysis proves that the system can continuously measure the global film thickness at a high acquisition frequency of 5000 Hz when the dye concentration is 200 mg/L. The influence of the irregularity of the excitation intensity, including the spatial non-uniformity, temporal instability, and long-term instability, on the measurement uncertainty is analyzed in detail. The analysis indicates that the system has an acceptable uncertainty of 10%. Compared with theoretical results, experimental results verify that the LEDIF system can accurately measure the global thickness of a liquid film formed by a water jet obliquely impinging onto a plate. An experimental investigation of the radial section of the raised zone demonstrates that the radial section changes from a sewing needle to an oval when the azimuth angle increases from 10° to 90°. Meanwhile, the dynamic contact angle exponentially decreases from 41.4° to 30.1°. A dynamic analysis of surface waves shows that the measured wave velocity decreases from 12 m/s to 1 m/s and the dominant frequency decreases from 1000 Hz to 10 Hz along the flow direction.