A Non-Invasive Liquid Film Thickness Measurement

Abstract

Wall spray interactions play an important role in the combustion efficiency prediction of turbojet or ramjet. They generate complex physical phenomena such as rebound onto wall or rebound onto wetted surface, splashing, deposition, film formation, film streaming and film atomization. ONERA/DMAE has been working on these subjects for few years, and some wall-drop interaction models have been developed and integrated into CFD-industrial-codes. In order to improve this work, a basic experimental study has been performed to analyze wall liquid film inside a combustion chamber. This is a cold flow experiment, where a liquid film is flowing on a hot tilted plate put on the bottom wall of the tunnel. Ethanol enriched with fluoresceine is used as fuel. The liquid emerges from a pipe with a diameter of 1 mm. Afterwards the film flow is canalized in a groove. It is streaming on the hot plate which temperature should be fixed from 300 K to 700 K by an element heating controlled electronically. The film thickness is measured with a non-intrusive technique based on the laser trace displacement at the liquid film interface. Indeed, when the film thickness varies, the trace of the laser plan is moving. Thus, it is enough to know the optical magnification used and the angle of the CCD camera to obtain the film thickness. This technique gives only the thickness of the film, so its velocity has to be estimated using flow rate conservation. The goal of the present experiment is to create an experimental data-base on wall liquid film behavior in terms of thickness, velocity and surface instabilities evolution (with an FFT analysis) for numerical comparison.