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A new, optical measuring technique for the determination of the vapour film thickness during film boiling
Author(s) -
Weichert Reiner,
Jeschar Rudolf,
Klemm Walter,
Heidt Viktor,
Thibor Patrick,
Mühlenweg Heike
Publication year - 1995
Publication title -
steel research
Language(s) - English
Resource type - Journals
eISSN - 1869-344X
pISSN - 0177-4832
DOI - 10.1002/srin.199501119
Subject(s) - boiling , leidenfrost effect , materials science , optics , water vapor , boiling point , optical microscope , composite material , intensity (physics) , analytical chemistry (journal) , chemistry , scanning electron microscope , chromatography , thermodynamics , heat transfer , nucleate boiling , heat transfer coefficient , physics , organic chemistry
A non‐contact measuring technique was developed and tested to determine the thickness of a vapour film during film boiling. The technique is based on the different optical properties of vapour and water admixed with a fluorescent substance. Pictures were taken of the surroundings of the vapour film using a long‐distance microscope of high resolving power and a TV‐camera; they were recorded with a video recorder. The pictures were evaluated using methods of digital image processing and by application of a model for the intensity distribution of fluorescence‐radiation. The technique was tested on a steel sphere heated to 690°C and subsequently immersed into warm water of 55°C. The thickness of the vapour film directly after immersion was determined at 0.20 mm. The thickness decreased continuously during the cooling process and reached a thickness of 0.11 mm after 20 s, immediately before the collapse of the vapour film. The surface temperature of the steel sphere immediately before the collapse was about 420°C. The thicknesses of the vapour film calculated with a simple theoretical model were found to be about 50 % smaller than those measured.