Abstract
The flow of water in the draught tube of a water turbine is quite different from that of the ordinary pipe flow. In the draught tube the water whirls round the axis tracing a helical course down to the tail race, and the more the quantity of discharge differs from that of the ideal normal the more this action is augmented. This action depends also on the radius. Hydrodynamical calculations are made with three kinds of draught tubes, cylindrical, conical and hyperboloidal, and it is proved that the pressure just in contact with the inner surface of the draught tube is unchanged by varied discharge, but the pressure inside it is very much influenced by the quantity of dischrge and also by its form. The distribution of pressure on any cross-section of the tube is parabolic, the pressure at the axis being sometimes minimum or sometimes maximum. Uniform pressure is realized only when the dischage is equal to a certain fixed amount, and when the discharge is much less than this amount a cavity is produced inside the tube forming a vacuum space which is not detected from the outside by means of pressure gauges usually attached on the tube wall. This is destructive of the effective action of the tube. In order, therefore, to avoid producing the cavity the turbine should be set so low that the pressure parabola formed on the uppermost cross-section of the tube in the case of the least available discharge lies entirely above the possible vacuum line.
