Supersonic condensing flow characterization by non-intrusive measurement techniques

The scope of this work is the development of non intrusive an on-line and realtime optical technique to characterize nanoparticles and nanodroplets. In particular water nanodroplets, generated by homogeneous nucleation in vapor flow condensing downstream a sonic nozzle, is the subject of interest. The optical technique used to characterize the flow is the “Multi Wavelength Light Extinction - MWLE”, it allows to measure the granulometry of the flow and the concentration of the droplets simultaneously. This non-intrusive technique is one of the most widely used for measurements in wet steam. The MWLE is the best way in flowing steam to quantify the size and even the concentration of droplets which have sizes in the same range than light wavelengths. Using the difference of extinction depending on the ratio between the radius of a droplet “r” and a wavelength “λ”, determined by the Lorenz-Mie theory5 it is possible to infer the droplet distribution on the light path. In the laboratory of the Environmental and Applied Fluid Dynamics Department of Von Karman Institute for Fluid Dynamic (Belgium), a facility has been built to produce the desired flow and a dedicated optical set-up has been developed to implement MWLE technique. Further insight in the free expansion and condensation phenomena has been also gained investigating the dependence of the results on the nozzle inlet pressure.