• multiwavelength light extinction
• droplet sizing
• condensing flow
• optical measurement

The purpose of the present work is to assess the feasibility and the effectiveness of an optical measurement technique for the characterization of a condensing free flow. In the present case a water vapour flow undergoes a free expansion in air downstream a sonic nozzle; due to the consequent decrease in temperature, condensation occurs and a droplet cloud forms in front of the nozzle. The optical technique employed to characterize the flow is the multiwavelength light extinction, as it is able to simultaneously measure the number density and the size of the droplets. Multiwavelength light extinction requires a particular data inversion technique to be employed to retrieve the final data, thus a specific mathematical algorithm is developed to achieve this goal. A facility is built to produce the desired flow and a dedicated optical set-up is developed to implement multiwavelenght light extinction technique on the facility. Results are presented as obtained using two different light sources in the measurements. Further insight in the free expansion and condensation phenomenon is also gained investigating the dependence of the results on the nozzle inlet pressure. Quality and reliability of the results are discussed.