Analysis & Control of Profile Losses

1. Analysis & Control of Profile Losses P M V Subbarao Professor Mechanical Engineering Department More Reasons to modify blade profile……

2. Losses Contributed by Profile

3. Front Loaded Vs Aft Loaded Blades

4. Aft Loaded Blades with High Diffusion

5. Generation of Separation Bubble • Higher diffusion rates are responsible for creation of separation bubble. • Low Reynolds number/ laminar flow tend to create long separation bubble. • This action promote aft-loading to a significantly higher degree. • In order to avoid substantial trailing edge loss, however, careful studies have to be carried out to ensure that turbulent reattachment is completed before reaching the trailing edge.

6. Control of Reattachment Point • A lower diffusion rate can readily be achieved by shifting the profile loading to the front part of the profile (front-loaded profile, FLP). • This has very interesting features of a low diffusion rate on the suction side of such profiles. • However, the low level of adverse pressure gradient causes the transition point to be unstable (higher transition length). • This length is more sensitive to inlet turbulence variations. • Theoretical predictions based on 2D blade-to-blade boundary layer calculations and on wind tunnel measurements have clearly confirmed that a lower profile loss can be reached with FLPs at the design point.

7. Problems with Blade with Large Adverse Pressure Gradient Separation Bubbles & Reattachment

8. Flow deceleration on the suction side • Improvement of transitional behaviour and growth of boundary layer on the front-loaded profiles is important. • Arrange the curvature distribution on the suction side to create a pronounced local adverse pressure gradient but a very mild diffusion. • This configuration enforces a well defined transition location and the ensuing turbulent boundary layer is relaxed, through very mild diffusion. • These measures improve the profile loss level and transitional behaviour but do not normally improve the structural weaknesses of these profiles.

9. Local Tuning of Blade Shape The flow potential is perturbed and hence the local flow is influenced by the airfoil shape/blade shape.

10. Control of Flow Deceleration on Suction Side

11. Control of Flow Deceleration on Suction Side

12. Control of Flow Deceleration on Pressure Side • For a given profile loading, a certain amount of initial deceleration on the pressure side is helpful • This allows reduction in the maximum velocity level on the suction side, thereby reducing the overall diffusion on the latter. • Local changes in design feature in terms of both profile contour change and the corresponding Mach number distribution is helpful in this regard.

13. Control of Flow Deceleration on Pressure Side Note that, owing to the lower dynamic head on the pressure side, substantial geometry change is required to produce a measurable effect on the suction side.

14. Rules to Contain Profile Losses : Tuning of Suction side Profile

15. Rules to Contain Profile Losses : Tuning of Pressure side Profile

16. Key design parameters of a turbine Blade profile incascade • From BL flow point of view, a profile must be considered as another entity. • In many situations it is useful to work with some more dimensionless parametersthat characterize a profile corresponding to BL flow features.

17. The Most Important Dimensionless Parameter

18. Frictional Losses • Roughness is caused by contaminates in the steam which deposit on the surface of the partitions. • Roughness is also caused when foreign particles collide against partition surfaces, leaving behind small indentations in these surfaces. • Stage efficiency losses due to an increase in the measurable roughness of a blade surface will be a function of; • the ratio of the height of the projections to the thickness of the boundary layer. • Whether the boundary layer is laminar or turbulent (Reynolds Number). • The thinner the boundary layer (higher Reynolds Number), the more significant the friction loss becomes, even for small projections.

19. Loss in stage efficiency as a function of surface Roughness

20. Profile loss ratio Vs Reynolds number effect The loss effect of Re on blade cascade performance is pronounced. Thus any correlation of cascade data which neglects the Reynolds number of test is of little value. Typically in the range of Re between 2  l04 and 2  l05 the loss will be halved. A general prediction method for use in steam turbine analysis requires that the effect of Re should be predictable up to values of Re, equal to about 4  l06 which are now obtained at the inlet of modern high pressure (h.p.) cylinders.