Aerodynamics and strength of a two-tier stage based on a fork-shape blade

Abstract

The specific metal consumption is a metal amount spent per an installed power unit. This criterion is important for turbine quality assessment. This criteria drops rapidly when a turbine power increase remarkably leaves behind its total metal consumption growth. This event comes to reality in design of a low pressure turbine with maximal steam flow capacity. This report shows that this solution inevitably hurts the LP turbine efficiency. The two-tier blade is an alternative solution of this problem. The lower tier airfoil carries a plate with two upper tier airfoils that have relatively small chords. This presents a complete solution of the fan factor aerodynamic problem. On the other side this 1200 mm span blade may cause operational reliability problems. This report discloses strength analysis for this blade and the structure improvements that combine fulfilment of the reliability requirements and the high stage efficiency. KEYWORDS LOW-PRESSURE TURBINE, STEAMTURBINE, TWO-TIER TURBINE NOMENCLATURE l blade length, [mm] Dmean mean diameter, [mm] Dr root diameter, [mm] u mean velocity, [m/s] u/cf kinematic parameter φ velocity coefficients at vane assembly, [-] 𝜓 velocity coefficients at blade assembly, [-] η blade efficiency, [%] H0z accessible enthalpy drop in the last stage, [J/kgK] Dmean/l relative mean diameter, [-] Δl absolute overlap of the vane and blade airfoil grids, [mm] Δl/l relative overlap, [-]