Nihon Kikai Gakkai ronbunshu (May 2024)
An attempt to estimate liquid film thickness adhered on wall surface using ultrasonic method
Abstract
In order to estimate the thickness of adhered thin liquid film on lubricating surface using ultrasonic method, a synthetic wave of reflected waves at the solid-liquid interface and multiple reflected waves in the thin liquid film was calculated and the fundamental relationship between the amplitude (echo ratio) and frequency spectrum (resonant frequency) was investigated with respect to the film thickness. As the film thickness increases, the echo ratio begins to decrease from a thickness around 1/8 of the wavelength λ2 in the liquid, reaches a minimum around λ2/4 and then increases in the thick film region. On the other hand, the resonant frequency during this process only decreases monotonically as the film becomes thicker, unlike the echo ratio. Therefore, the estimation of the film thickness becomes easier by using the resonant frequency. However, it is impossible to estimate the film thickness below λ2/8 using these indices. For thickness estimation of such a thin adherent film, the difference wave obtained from the difference in amplitude between the reflected wave at the dry surface and that of adhered film is effective, and the difference echo ratio determined by its maximum amplitude decreases monotonically as the film becomes thicker in the region below λ2/8. These fundamental relationships were similarly observed in the experiments with ethanol as the adhered film, and they almost agree with the calculation results for each of the above-mentioned indices using the actually measured ultrasonic frequency spectrum. Thus, the difference echo ratio is effective for estimating the adhered film thickness in very thin film region, while the resonant frequency determined by the period of the resonant or difference wave and the echo ratio of the reflection or difference wave are effective for the estimation of the adhered thickness in thicker film region.
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