Paper 25: Size Sampling of Steam-Borne Water Droplets

Abstract

The results are briefly reported of a detailed study undertaken to assess the most promising method of sampling steam-borne water droplets for size. The ‘impressions’ method based upon a standard microscope glass slide smoked with the aerosol magnesium oxide was chosen for experimental development. A supporting probe was designed which exposed the slide in novel manner that avoided the ‘cut-off’ of smaller droplet sizes associated with conventional impaction techniques, without the necessity of accelerating the steam. The assembly was tested in a steam tunnel in which the wet steam mixture is prepared by introducing water droplets from a spinning disc atomizer into dry steam. The droplet size is varied by adjusting the disc speed and the tests were conducted at steam conditions up to 6 per cent wet with velocities up to 30 ft/s at pressures from 17 inHg vacuum to atmospheric.

The paper explores the corrections which must be applied to the observed crater size frequency distribution to allow for the collection efficiency of the probe, penetration of the stagnant layer inside it, as well as sedimentation and transient phase changes. Results suggest that the sampling method can be employed successfully at relatively low velocities of flow, but the corrections and processing of data are tedious. The advantages and limitations of the method are discussed at some length and a numerical example is appended to illustrate the application of the corrections.