5.3.1 Aerodynamic and mechanical constraints
In early machines, the last few low pressure moving blades were of constant cross-section. The stress in this type of blade increased quadratically from the tip to the base and attained its maximum value in the transition from blade to root: this placed a restriction on the length of blade that could be used at synchronous speed.
Modern last-stage blades have a cross section which reduces exponentially with the square of the radius. The tensile stress due to centrifugal stress is then virtually constant over the greater part of the blade length and this has enabled blades up to 940 mm long to be used on 3000 r/min machines.
On modern blading, the tip diameter is typically about double the base diameter, so that at the mean, the blade pitch, i.e., the circumferential distance between adjacent blades, is about 1.5 times the pitch at the base diameter. The peripheral speed is also 1.5 times as great as at the base and the effect of this increased blade speed is to change the direction of the incident velocity of the steam relative to the moving blade. The moving blade inlet angle is therefore set to line up with the direction of the incoming steam flow and the moving blade section is also changed. This reduces the outlet angle so that a pressure drop develops across the moving blades and the steam leaves the moving blades at a higher speed to offset the higher peripheral speed, enabling the steam to leave the blades with the minimum of swirl. The stage is designed to have a fairly low degree of reaction at the base and, since the pressure drop across the fixed blades decreases in response to the increasing pressure drop across the moving blades, reaction increases with blade height. The radial tension due to centrifugal force and the aerodynamic effect of change of steam flow results in a highly-twisted moving blade, having a robust low reaction section at the root and a slim high reaction section at the tip, see Fig 1.85.