An optical method for the measurement of unsteady film thickness

I. B. Özdemir*, J. H. Whitelaw

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

8 Citations (Scopus)


This paper describes a measurement technique to quantify temporal variations in the thickness of an unsteady liquid film with a resolution which is independent of the thickness. The optical transformation function has been derived for fringes of equal inclination and, for a temporally varying film, allows the unsteady component of film thickness to be measured in terms of frequency modulated signal analysis of light intensity variations. As it does not require calibration, the method is suited to in-situ measurements of complex and rapidly varying films as encountered in engineering two-phase flows. It requires the inversion of the frequency time-series of the light intensity observed by a photodetector which represents the absolute values of the time-derivative of the thickness variation. The technique has been used to measure the thickness of the film formed as a result of impingement of a pulsating two-phase jet onto a heated flat plate with surface temperatures of 150 °C and 240 °C and located 143 nozzle exit diameters downstream of the nozzle. The angle between the jet axis and the surface normal was 20 degrees and the injection frequency was 16.7 Hz corresponding to a flow rate of 7.2 mm3 per injection. The results along the line of incidence showed that the ensemble-averaged space-time structure of the film was qualitatively independent of the plate temperature with three peaks, two of which occurred at large radial distances and disappeared in less than 10 ms. The third peak was close to the impingement region and persisted for more than 50 ms due to the small velocities of the incoming two-phase jet as the nozzle needle closed and the low momentum wall jet which was unable to transport the droplets radially outwards. At the higher surface temperature, the rate of evaporation and the amplitude variation of the unsteady component of the overall film thickness increased, and the film covered a smaller area.

Original languageEnglish
Pages (from-to)321-331
Number of pages11
JournalExperiments in Fluids
Issue number5
Publication statusPublished - Sept 1992
Externally publishedYes


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