The Effect of Circumferential Pitch of Steam Turbine Blades on Torque as Compared with “Biplane Effect” on the “Lift” of Aerofoils

It is known that the “lift” of biplanes and triplanes per unit of wing surface is inferior to that of a monoplane, and that the degree of departure depends upon the gap-chord ratio. Published data indicate that the lift increases with increasing gap, apparently towards some limiting value. Regarding steam turbine wheels as rings of multiplanes, it may be asked, Is there any direct evidence that the torque on each blade is influenced by the presence of the other blades?.

Many experiments have been made to determine the optimum circumferential pitch for a given blade shape, but most of these have aimed simply at arriving empirically at the best efficiency, without inquiry into the factors involved. The experiments described in the paper were carried out with low-velocity reaction blading, and were aimed at eliminating as many factors as possible, so as to make a clear issue of the effect of spacing the runner blades closer or wider apart. These experiments indicated that the effect of circumferential spacing on maximum torque was fairly critical, and that from the optimum point the torque declined less rapidly with increasing spacing than with decreasing spacing.

The decline in torque with increasing spacing is due probably to lack of proper guidance of the steam, only a portion of which gives up energy to the runner blades, the remainder doing little useful work. On the other hand, the curve of force per blade is found to be increasing with increased circumferential pitch, towards some optimum value (beyond the range of the experiments). This curve resembles closely that published for the increased “lift” on a biplane as the planes are spaced farther and farther apart.

The experiments are of interest on account of the above analysis and comparison, but no definite conclusion is possible without further investigation.