Some studies of laser doppler anemometry in wet steam

Abstract

This study concerns the use of counter based laser Doppler anemometry in a wet steam flow of variable wetness fraction. Velocity measurements across the flow were made under different steam conditions. Comparison was made with a theoretical profile based upon a simple flow analysis. A small radial turbocharger was used as a means of extracting enthalpy homogeneously from a dry superheated flow of steam using the compressor as a brake. The wetness fraction of the exhaust was estimated using measured values of the thermodynamic properties. A laser extinction method was used to determine the number concentration and mean radius of the water droplets acting as natural scatterers in the wet steam. A laser anemometer was designed which made use of the properties of a propagating gaussian beam to produce a small probe volume. This was required to reduce the number of water droplets likely to be present simultaneously in the measuring volume. Good Doppler signals were obtained and these have been presented for a range of wet steam conditions. A computer model was developed to predict the scattering of laser light through wet steam. Results have shown that this can be accurately modelled using a particle size distribution function. The program written to perform the simulation takes into account both single and multiple scattering events. The parameters used for the distribution function required a knowledge of the wetness fraction and so provided a useful means of checking the estimate based upon the thermodynamic measurements. It has been demonstrated that the ability to obtain Doppler signals from the wet steam can be predicted by computation of the signal-to-noise ratio for the medium. Good results were obtained for the wet steam conditions under investigation.