Blade shrouding gives an opportunity to increase the HPT (high pressure turbine) first stage efficiency by 2–3 %. However, if high gas temperature and high circumferential velocity are at the stage, shrouding can be problematic due to load increasing at blade/disk attachment and high temperature of the shroud itself.
To make blade/disk attachment more reliable the shroud axial width has to be decreased by increasing a relative pitch of airfoil cascades t (t = t / b, where t – pitch, b – chord) at the blade tip span. According to experience for a flow with β1 = 50 – 85°, M2 = 0.8 – 1, and Re = (0.8 – 1)•106 high efficient cascades with t = 0.93 – 1.05 can be designed. Application of such a profiling for GTE (gas turbine engine) turbine is demonstrated here. In the turbine meridian flow path the blade was drastically tapered to the tip (tip width was 53 % of the mean width and 46 % of the hub width).
To lighten the blade a partial shrouding can be also applied. Model turbine tests showed that local cuts at the front shroud area and the aft shroud area at the airfoil pressure side influenced the efficiency weakly.
Required shroud temperature is provided with a cooling. The aircraft turbine with a governed cooling system and a radial clearance control is an example here.
In this case the shroud had 3 labyrinth ribs. The shrouding decreased radial clearance by 0.8 mm at main design modes that increased efficiency by ∼ 1.5 %.
To cool down the shroud the air downstream the compressor was fed into the cavity behind the front labyrinth rib. At maximal mode with full cooling the relative coolant mass flow (to the compressor mass flow) was mc = 1.3 % and gas leakages through the labyrinth were 0.2 %. It gave acceptable mixed temperature of 530°C in the cavity over the shroud. At cruise high altitude mode and a lower gas temperature and partial cooling with mc = 0.4 % and gas leakages of 0.1 % the mixed temperature also did not exceed 530°C over the shroud.
The assessment with taking into account changes of the clearance, the coolant mass flow, and gas leakages showed that the shrouding provided the engine economy improvement by 0.7 – 0.9 % for both modes.
For GTPU (gas turbine power unit) the first blade shrouding can be more complicated. However, even the slight turbine efficiency increase provides considerable profits due to GTPU huge power output and long term running.
So, when GTE and GTPU designing starts, it is reasonable to consider the turbine first blade shrouding. Here the integral evaluation criterion, which includes the assessment of a possible income from the unit full life cycle running, has to be applied.