6.5 The Impact of Simple Tangential Lean on the Flow Through the Turbine Circumferential Cascade

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Figure 6.7 Researched blade profile TC-1A
Figure 6.7 Researched blade profile TC-1A

It is known that the lean along the flow leads to increasing secondary flow losses on the periphery and to reducing them at the root. Lean against the stream leads, correspondingly, to the opposite result.

The lean to the opposite flow direction allows to alter the distribution of flow parameters along height, so that the leakages in the axial gap is reduced on the periphery, that positively affects stage efficiency.

As an object of research the circumferential guide blade cascade was chosen with the profiles TC-1A (Fig. 6.7) with the following parameters:

Figure 6.71

and centering on the input edge.

Boundary conditions:

  • inlet:P20 = 102240 Pa; T*0 = 373.15K;
  • the degree of turbulence 1%;
  • Outlet: P2= 81861Pa ;
  • the blade and the ends of the channel: impermeable wall with condition of sticking the flow;
  • working medium: air.

In the investigated guide blade cascade boundary conditions correspond to subsonic flow with Reynolds number at outlet equals 2∙106

Fig. 6.8–6.10 shows the results of calculations with lean angles γ = –30; –20; –10; 0; 10; 20; 30° (negative lean means the lean against the direction of the flow and positive – the lean in the direction of the stream).

6.10

Obviously, that the total losses are increasing when there is negative lean and decreasing with the positive lean. Exit angle of the flow and the mass flow rate is increasing as with positive and with negative lean.

The mode of the changing of the actual outlet flow angle is shown at Fig. 6.11. Positive lean leads to an increase in the actual outlet flow angle near the root and decrease on the periphery; negative lean brings to the opposite effect. Given the nature of allocation of the losses along height of the blade as a lean result (Fig. 6.12), possible to say that an increase of the outlet flow angle leads to the secondary losses reducing.

6.11
Figure 6.11 Distribution of actual outlet flow angle along the blade height.
6.12
Figure 6.12 Distribution of losses coefficient along the height behind
cascade with different blades leans.

An increase of the mass flow rate, as with a positive and with a negative lean occurs due to an increase in the integral actual outlet flow angle. In addition, with positive lean into increase of the mass flow rate also contributes reducing the integral losses and, accordingly, with a negative lean increasing of the losses slightly brings down increasing the mass flow rate, which takes place as a result of increase of the outlet flow angle.

The reasons for this significant changing of flow parameters with lean of the blades may be explained by the distribution of static pressure with various leans in the control plane (Fig. 6.13). Isolines of static pressure in all cases are located almost vertically.

Figure 6.13 The contours of the static pressure at control plane at different leans: а – 0°; b – – 30°; c – 30°.

In the cascade without lean (Fig. 6.13) the pressure gradient along the height of the blade is virtually absent, unlike the blades with lean (Fig. 6.13b and Fig. 6.13c). Negative lean (see Fig. 6.13b) leads to pressure gradient appearance along the height of the blades, directed from the periphery to the root and on the suction side of the blade it is more substantial than on its pressure side. Positive lean brings to appearance of the opposite to the direction of pressure gradient at the surface of the blade (Fig. 6.13c).

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