A new, optical measuring technique for the determination of the vapour film thickness during film boiling

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.