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.
The general problem of erosion of turbine blading under wet steam operating conditions is reviewed and remedial methods available to the design engineer are considered. Of these, direct attack on the source of erosion, steam-borne water drops, is most attractive and is examined in some detail.Five stages may be distinguished in the process culminating in erosion. They are: formation of droplets, their deposition on diaphragm blading, subsequent detachment, acceleration towards the moving blades and final impact with them.The possibilities of interrupting this process at any stage are discussed and the paper primarily deals with the experimental investigation of these possibilities for the three intermediate stages using a full sized cascade of hollow last stage diaphragm blades. This enabled various configurations of extraction slots to be investigated as well as the effects of energizing the blade wakes and heating the bladcs. The construction and operation of the cascade and associated instrumentation are discussed and results are presented in terms of blading efficiencies for the various processes.
This paper describes what is believed to be the first attempt to make a wet-steam ‘wind’ tunnel. Saturated water is sprayed from a small high-speed rotating disc so that the globules are entrained by a stream of saturated steam at the same temperature in which the disc is immersed. The tunnel is designed for a maximum steam speed of 30 ft/sec in a circular working section of 16-in diameter with steam pressures ranging from a full vacuum to 10 lb/in2 gauge. The entrained globules range in size from 5 to 100 micron, and the minimum nominal dryness fraction is 94 per cent. A sampling technique was developed based on the impressions made by the globules on a prepared slide, and an attempt is made to relate the globule size to the disc size and speed, the injection water flow and the physical characteristics of the saturated liquid and vapour. Globule trajectories were calculated by computer allowing for variation of drag coefficient and acceleration of the entraining steam. In the paper the design of the equipment and the problems encountered both in generating the wet steam and in determining the size of the globules are discussed.
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