The steam turbine stage efficiency is significantly affected by clearance losses due to leakage flow re-entering the main flowpath. The impact, in terms of delta of efficiency Δη, is usually assessed by assuming the proportionality Δη = K × (mLeak/mMain), being mLeak/mMain the fraction of the leakage flow relative to the main mass flow rate and K the constant evaluated by experimental and numerical evidence. Several studies are present in the literature concerning low aspect ratio blades, where this constant K is found to be equal to 1, or even higher for very low percentage leakage massflow.

The main purpose of the current work consists of assessing this factor in the case of steam turbine reaction blades of the HP sector, characterized by a high aspect ratio. A numerical analysis has been set up where the detailed 3D geometry of two HP stages has been modeled and simulated by a 3D-CFD approach. Then a sensitivity analysis has been carried out by varying the leakage flows coming from stator and rotor labyrinth seals, realized by changing the radial gap of the clearance. This analysis has been repeated for several working points in order to obtain the leakage impact in the whole operating range of the reaction stages. The K factor comes out to be close to 0.8 at design point, whilst it increases up to 1 for highly loaded off-design condition.

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