GB2398608A

GB2398608A - Turbine blade with enhanced heat transfer

Info

Publication number: GB2398608A
Application number: GB0304023A
Authority: GB
Inventors: Donald Walker
Original assignee: Alstom Technology AG; Alstom SA
Current assignee: General Electric Technology GmbH
Priority date: 2003-02-21
Filing date: 2003-02-21
Publication date: 2004-08-25
Also published as: GB0304023D0

Abstract

The turbine blade includes an aerofoil having a convex sidewall (4) and a concave sidewall (6), each extending from a leading edge to a trailing edge of the aerofoil, and having longitudinal webs (16,17) which extend between the sidewalls (4,6) and which, together with the sidewalls, define longitudinal channels for a gaseous coolant. The surface of at least one web delimiting a channel through which the coolant flows in a longitudinal (radial) direction is provided with ribs (47) which are skewed in the same sense with respect to this direction. In the coolant gas flowing along the channel, vortices caused by Coriolis forces (49) and vortices caused by forces (48) due to the skewed ribs (47) tend to reinforce one another.

Description

TURBINE BLADE WITH ENHANCED HEAT TRANSFER

This invention relates to a turbine blade with longitudinal channels for a gaseous coolant and in particular relates to an arrangement of ribs in such channels for the purpose of enhancing heat transfer from the blade to the coolant.

Figures 1 and 2 of the accompanying drawing show a known air cooled turbine blade for a gas turbine. The blade includes an aerofoil 1 having a radial axis 2 and is joined to a rotor disk (not shown) by a conventional fir-tree root 3. The rotor disk rotates about its axis 19 in the direction indicated by the arrows 20, resulting in blade motion in the direction indicated by the arrow 21. The aerofoil 1 has a convex sidewall 4 at the leading or suction side and a concave sidewall 6 at the pressure side. The sidewalls 4, 6 extend from a leading edge 7 to a trailing edge 8 of the aerofoil 1 and extend longitudinally from a root 9 to a tip 11.

The turbine blade includes an internal passageway for channelling compressed air as a gaseous coolant to cool the aerofoil 1. The coolant is conventionally conducted from a compressor of the gas turbine through the fir-tree root 3 and radially into the aerofoil 1.

The passageway includes a leading edge channel 1 2a which extends from the root 9 towards the tip 11, where the coolant turns into a mid-chord channel 12b in which the coolant flows longitudinally towards the root 9, where the coolant turns into a trailing edge channel 12c which extends to the tip 11. The coolant is discharged from the trailing edge channel 12c through trailing edge openings 13 and tip openings 14. The channels 12a to 12c are defined by longitudinal webs 16, 17 which extend between the aerofoil sidewalls 4 and 6.

During operation of the gas turbine, the blade is heated by combustion gases and is subjected to stresses due to rotation. In order to provide acceptable service life of the aerofoil, turbulence inducing ribs 18 are formed on the internal surfaces of the aerofoil - t sidewalls 4 and 6 (and are staggered) so as to enhance heat transfer in the region of the aerofoil which is subjected to maximum temperature and stress in combination, in order to increase creep resistance in this region. However, since the ribs extend into the passageway carrying the coolant, they necessarily produce pressure losses.

Furthermore, the ribs increase the weight of the blade and thereby increase centrifugal stresses.

The present invention provides a turbine blade including an aerofoil having a convex sidewall and a concave sidewall, each extending from a leading edge to a trailing edge of the aerofoil, and having longitudinal webs which extend between the sidewalls and define longitudinal channels for a gaseous coolant, wherein the surface of at least one web delimiting a channel through which the coolant flows in a longitudinal direction is provided with ribs which are skewed in the same sense with respect to the said longitudinal direction.

In operation of the turbine, vortices caused in the coolant gas flowing in the said longitudinal direction in the said channel by Coriolis accelerations and vortices caused by the skewed ribs tend to reinforce one another.

The invention will be described further, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a schematic respective view, partly in section, of a known turbine blade; Figure 2 is a section taken on line II - II in Figure 1; Figure 3a is a diagrammatic cross-section through part of the aerofoil of a turbine blade without turbulence inducing ribs; Figure 3b is a diagrammatic section taken on line B-B in Figure 3a and showing the velocity distribution of a flowing gaseous coolant; Figure 4a is a diagrammatic section through part of the aerofoil, in a plane normal to the axis of rotation of the rotor disk; Figure 4b is a diagrammatic section taken on line B-B in Figure 4a and showing the Coriolis acceleration distribution of the flowing gaseous coolant in the rotating aerofoil; Figure 4c is a view similar to Figure 4b but showing secondary-flow vortices caused by Coriolis acceleration; Figure 5c is a view similar to Figure 4a but with turbulence inducing ribs on the aerofoil sidewalls; (not in accordance with the invention) Figure 5b is a section on line B-B in Figure 5a; Figure 5c is a section of line C-C in Figure 5a, showing secondary-flow vortices caused by the ribs when the aerofoil is not rotating; Figure 6a is a view similar of to Figure Sa but with turbulence inducing ribs on i the webs (in accordance with the invention), being a section on line A-A in Figure 6b, the ribs sweeping toward the convex sidewall of the aerofoil; Figure 6b is a section on line B-B in Figure 6a; Figure 6c is a section on line C-C in Figure 6a, showing secondary-flow vortices caused by the ribs when the aerofoil is not rotating; Figures 7a-c are similar to Figures 6a-c, respectively, but with the ribs sweeping toward the concave sidewall of the aerofoil; Figure 8a is a diagram similar to Figure 5a, showing the longitudinal flow of gaseous coolant in a rotating blade with ribs on the aerofoil sidewalls (not in accordance with the invention); Figure 8b is a diagrammatic section on line B-B in Figure 8a; Figure 8c is a diagrammatic section on line C-C in Figure 8a; showing the Coriolis and rib-induced forces acting on the coolant flow; Figure ad is a view similar to Figure 8c but showing the secondary flow field; Figure 8e is an enlargement of the detail E in Figure 8c, showing interaction between Coriolis-driven and rib-driven streams; Figure 9a-d are similar to Figures 8a-d, respectively, but with the ribs being on the webs (in accordance with the invention) and sweeping toward the concave sidewall of the aerofoil (as in Figures 7a-c); and Figures 1 Oa-d are similar to Figures 9a-d, respectively, but with the ribs sweeping toward the convex sidewall (as in Figures 6a-c).

Flow Mechanisms In an internally ribbed aerofoil I of a rotating turbine blade, in a longitudinal channel of basically rectangular cross-section, typified by the channel 12b, there occurs the interaction of three flow mechanisms in the cooling stream: the primary flow 5 and two secondary flows. The primary flow 5 is shown in Figure 3b, the secondary flows caused by Coriolis acceleration in Figure 4c, and those caused by ribbed walls in Figures 5a, 6c, and 7c. The effects of superposition of these mechanisms not in accordance with the invention are shown in Figure Ed, and those in accordance the invention in Figures 9d and I Od. The details of these flows will now be described.

Primary flow The pressure-driven primary flow 5 is the dominant flow in the cooling channel 12b and the distribution of its radial velocity component, v, is shown at 22 in Figure 3b.

The effects of turbulence and viscosity can be seen to result in lowvelocity boundary- layers 23, with v = zero at the walls. (The length of each arrow in the distribution 22 is proportional to the local velocity.) Secondary flows 1) Coriolis-induced vortices The variation of the Coriolis Inertia effect across the cooling channel 1 2b creates secondary flows within that channel. For a particle of cooling gas the Coriolis acceleration component, C, relative to a frame of reference rotating with the rotor, is given by: C = 2 Q v, where Q = the angular velocity of rotor rotation, and v = the local radial component of the cooling flow velocity (according to the distribution 22 in Figure 3b).

The direction of the inertia force due to this acceleration is upwards with the flow and rotor rotation directions shown in Figure 4a.

Because v is zero in the boundary layer and a maximum within the mainstream typically as shown in Figure 3b, C varies similarly across the stream (distribution 24 in Figure 4b), resulting in a pump effect forcing the centre of the flow upwards towards the convex sidewall 4. This creates a pair of counter-rotating vortices 25 with their axes 26 lying along a line parallel with the rotation axis 19, as shown in Figure 4c.

2) Rib-induced vortices When the pressure-driven primary flow 5 passes skewed ribs 27 attached to a wall, in absence of blade rotation, the flows 10 near to that wall very quickly adopt the skewness 30 of the ribs (i.e. the angle relative to the longitudinal direction). With the ribs 27 attached to the aerofoil side walls 4,6 as shown in Figures 5a-c, two strong, elongated, counter-rotating vortices 28 are formed with their axes 29 lying vertically one above the other (Figure 5c).

When the ribs are attached to the webs 16,17 as shown in Figures 6a-c and 7a-c the axes of the vortices 35,36 lie side by side along a line parallel with the aerofoil faces, but depending on the skew direction (31 in Figure 6a and 33 in Figure 7a), the wall flows and 36 are towards the convex or concave aerofoil surfaces 4, 6 respectively. The ribs 32 (or 34) are applied to both opposing walls 16, 17 in directly opposing positions and with the same skewness so that when viewed from the side they appear superimposed.

Flows cooling passage not in accordance with the invention When the above three mechanisms are superimposed on the aerofoil rib geometry the resulting flows are as shown in Figure 8c-e. The upwards Coriolis forces 39 are at a maximum in the centre of the channel, and are perpendicular to the leftwards directed rib-driven forces (38 in Figure 8c). The enlarged view of Figure Be shows the flow asymmetry caused by the interaction of these forces: the smoothly merging streams 42 on the left (rib stream reinforcing the Coriolis stream) contrast with the counter- flowing streams 43 on the right (counter-Coriolis rib stream), the latter resulting in a turbulent mixing region 44. The consequence of this asymmetry can be seen in Figure ad in the asymmetry of the vortices 41 and the creation of "dead-water" regions 40.

Flows in cooling passages in accordance with the invention a) Placing the ribs on the webs 16,17 instead of the aerofoil walls as shown in Figures 9a-d and making the ribs 47 sweep towards the concave aerofoil surface (angle 45) results in co-Coriolis (49) forces 48 and a smooth merging of both rib streams with Coriolis streams to form a symmetrical pair of elongated vortices without the losses and "dead-water" regions noted above. The lower losses result in stronger vortices and enhanced heat transfer to the aerofoil walls, as well as lower coolant pumping losses.

b) Although making the ribs 51 sweep towards the convex aerofoil surface (skew angle 46 in Figure lOa) creates forces 52 which could be expected to counter the Coriolis forces 49, the configuration switches from two to four vortices, as shown in Figure 1 Od, and results in even greater heat transfer than in (a) above, for the same cooling air pumping losses.

Various modifications may be made within the scope of the invention. For example, in Figures 6a and 7a the skew angles 31 and 33 of the ribs 32 and 34 is approximately 40 and in Figures 9a and 1 Oa the skew angles 45 and 46 of the ribs 47 and 51 are approximately 52 , but the skew angles could by greater or less, the preferred range being 30 to 60 , more preferably 35 to 55 . Preferably the ribs are mutually parallel but there may be circumstances in which the skew angle can vary between successive ribs, perhaps decreasing (or increasing) in the direction of the primary flow 5 of coolant.

Although straight ribs have been shown, there may be circumstances in which non- linear ribs (e.g. arcuate or sinuous ribs) may advantageously be used. In a cooling channel delimited by two webs, it is preferable for both webs to be provided with ribs, as described above; however, ribs on only one of the webs would provide some benefit.

At the leading end and trailing end of the aerofoil, a cooling channel is delimited by a single web, which is preferably provided with skewed ribs as described above. The ribs are preferably equally spaced along the cooling channels.

A small additional improvement in performance may possibly be achieved by staggering the ribs on one web wall in the longitudinal (radial) direction relative to those on the opposite wall of the adjacent web.

Claims

CLAIMS: 1. A turbine blade including an aerofoil having a convex sidewall

and a concave sidewall, each extending from a leading edge to a trailing edge of the aerofoil, and having longitudinal webs which extend between the sidewalls and which, together with the sidewalls, define longitudinal channels for a gaseous coolant, wherein the surface of at least one web delimiting a channel through which the coolant flows in a longitudinal direction is provided with ribs which are skewed in the same sense with respect to the said longitudinal direction.
2. A turbine blade as claimed in claim 1, wherein the skewed ribs are at 30 60 to the said longitudinal direction.
3. A turbine blade as claimed in claim 2, wherein the skewed ribs are all at the same angle to the said longitudinal direction.
4. A turbine blade as claimed in any preceding claim, wherein the said channel is delimited by two webs, both provided with ribs which are skewed in the same sense with respect to the said longitudinal direction.
5. A turbine blade as claimed in claim 4, wherein each rib on one web is directly opposite a rib on the other web.
6. A turbine blade as claimed in claim 4, wherein the ribs on one web are staggered in the said longitudinal direction relative to the ribs on the other.
7. A turbine blade as claimed in any of claims 1 to 6, wherein the ribs, viewed along the said longitudinal direction, are inclined toward the concave sidewall of the aerofoil.
8. A turbine blade as claimed in any of claims I to 6, wherein the ribs, viewed along the said longitudinal direction, are inclined toward the convex sidewall of the aerofoil.
9. A turbine blade substantially as described with reference to Figures 9a-d or Figures I Oa-d of the accompanying drawings.