GB2340189A

GB2340189A - A turbomachine shroud seal having baffles

Info

Publication number: GB2340189A
Application number: GB9816842A
Authority: GB
Inventors: John D Denton; Anne Marjorie Wallis
Original assignee: Siemens PLC
Current assignee: Siemens PLC
Priority date: 1998-08-04
Filing date: 1998-08-04
Publication date: 2000-02-16
Also published as: GB9816842D0; WO2000008306A1

Description

2340189 IMPROVEMENTS TO OR RELATING TO TURBOMACI11NERY The present

invention relates to turbomachinery, and in particular to improving the efficiency of such machinery.

The presence of steam leaking around the ends of turbomachinery blading is a large source of aerodynamic loss in steam turbines.

Detailed designs already exist to reduce the quantity of the leakage flow passing around shrouded moving blade rows. These sealing arrangements typically take the form of an arrangement of knife-edges located above the turbine shroud in a cavity known as the shroud cavity as shown in Figure 1. One feature of most shrouded sealing arrangements are large axial gaps between the shroud of the blades and the casing of the machine. These gaps are necessary to allow the turbine shaft to expand in the axial direction and may be found both upstream and downstream of the blade. It has been found that, particularly with effective sealing arrangements, fluid from the main blade annulus is able to enter the cavity through the axial gaps and circulate in a circumferential direction around the cavity without passing through the sealing arrangement. This fluid then re-enters the main blading annulus introducing large losses in doing so.

An aim of the present invention is to provide means which can be easily incorporated in the design of such turbomachinery which reduces the above mentioned losses.

According to the present invention there is provided turbomachinery including at least one stage, each stage comprising a stationary blade row and a moving blade row, each moving blade row having a shroud on an end section of the blade row, said end section being located within a cavity formed by a casing of the turbomachinery and having a sealing arrangement positioned in the cavity to 2 substantially reduce the flow of a fluid through the cavity, characterised in that a row of baffles are attached to the turbomachinery in a position adjacent to the trailing edges of the moving blade row, and spaced from the shroud.

According to an aspect of the invention, baffles are formed by thin plates and aligned in an axial direction of the turbomachinery.

A plurality of baffles are provided per blade pitch, sufficient, to align the flow into the same direction of a mainstream flow.

Each baffle has an aspect ratio of 0.75-1.00.

The present invention provides the advantage of preventing the mainstream flow of fluid from penetrating the cavity, and also aligns the leakage flow into the direction of the mainstream flow before it re-enters the cavity.

An embodiment of the present invention will now be described with reference to the accompanying drawings, wherein:

FIGURE I shows a cross-section of turbomachinery known in the art, FIGURE 2 shows a computer prediction of the type of circulatory flows that occur in the large axial gaps upstream and downstream of the blade row incorporated in the prior art turbomachinery, and,

FIGURE 3 is a schematic diagram of turbomachinery including baffles in accordance with the present invention.

Referring to Figure I there is shown part of turbornachinery comprising a mo-,'ing blade row 2 having a shroud 4 on the end section of the blade row. This end section is housed within a cavity 10 formed by the turbine casing and an adjacent stator blade section comprising stationary blade rows 6. Fluid passing through the main turbine annulus is turned as it moves through the moving blade row 2 from the left hand side A to the right hand side B. A pressure gradient exists between the sides A and B, which also represent the leading and trailing edges of 3 each blade of the moving blade row 2, respectively. This pressure gradient encourages fluid to leak over the top of the moving blade row 2 across the shroud 4. It passes through a sealing arrangement 8 and re- enters the mainstream downstream of the moving blade row 2. As it has not been turned by the moving blade row, it is flowing in a different direction to the mainstream flow and losses are generated as the two flows mix out. Although the existence of leakage fluid reduces the quantity of mass flowing through the turbine blade reducing its power output and increasing mixing losses when the two flows combine, it can also perform the ftinction of sealing the shroud cavity from the mainstream fluid in the turbine annulus. Its effectiveness in performing this fimction may be reduced as the sealing effectiveness increases.

Figure 2 illustrates the type of circulatory motion that develops when the energy in the leakage fluid is insufficient to prevent fluid from the mainstream from spiralling around the inlet and exit gaps. The more effective the seal is, the further the mainstream fluid can penetrate into the shroud cavity generating increased losses.

Figure 2 is taken from a computational fluid dynamic simulation of the flow in the shroud cavity above a turbine rotor blade. The figure shows a view of the computational domain looking obliquely inwards onto the shroud surface, with the pressure surface of the blade to the front of the figure. The leading edge of the blade is at the right hand side of the figure so that the blade passage flow is moving from right to left. A selection of streamlines at the principal regions of interest is shown in the view. These streamlines show the movement of the leakage fluid relative to the moving blade, with the double headed arrows indicating the direction of motion.

4 The mainstream fluid approaches the rotor blade row and is deflected upwards into the shroud cavity near the leading edge of the blade. Some of the fluid moves along the cavity in the downstream direction as "conventional" leakage flow 12, but a proportion of it remains in the gap, developing into a vortical motion within the gap 14.

At the exit of the shroud cavity, the mainstream fluid again spirals around the axial gap. Some mainstream fluid enters the gap at the pressure side surface of the trailing edge and rolls up 'into a vortex which traverses along the gap 18. Close to the suction surface, the fluid re-enters the main passage 20 in a different direction to the mainstream flow causing a mixing loss.

The turbomachinery in accordance with the present invention will now be described now with reference to Figure 3 which overcomes the above mentioned leakage problem by the provision of baffles.

In Figure 3, identical parts to those shown in Figure I bear the same reference numeral. The moving blade row 2 is provided with a shroud 4 and is accommodated within the turbine casing. A row of baffles 22 is provided attached to the casing of the turbomachine at the point adjacent to the trailing edges of the blades in the moving blade row 2. The spacing of the baffles is such that the leakage flow is aligned into the same direction as the mainstream flow, with each baffle having an aspect ratio of 0.75-1.0. The gap between the inner radius of the baffle and the shroud is designed to inhibit the ingress of the mainstream fluid from downstream of the moving blade without increasing losses generated by windage.

The invention is errunently suitable for application with any turbomachinery, and in particular may be used in relation to commercial steam turbines.

1 1 5 Whilst the baffles have been described connected to the casing adjacent to the trailing edges of the moving blade row, they could be connected to the stator blade tips adjacent to the rotor surface.

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Claims

1. Turbomachinery including at least one stage, each stage comprising of at least a stationary blade row (6) and a moving blade row (2), each moving blade row having a shroud (4) on an end section of the blade row (2), said end section being located within a cavity (10) formed by a casing of the turbomachinery, and having a sealing arrangement (8) positioned in the cavity to substantially reduce the flow of a fluid through the cavity, characterised in that a row of baffles (22) are attached to the turbomachinery casing in a position adjacent to the trailing edges of the moving blade row (2), and spaced from the shroud (4).

2. Turbomachinery as claimed in claim 1, wherein the baffles are formed by thin plates, and aligned in an axial direction of the turbomachinery.

3. Turbomachinery as claimed in claim 2, where a plurality of baffles are provided per blade pitch, sufficient to align the flow into the same direction as a mainstream flow.

4. Turbomachinery as claimed in any preceding claim wherein each baffle (22) has an aspect ratio of 0.75-1.00.

5. Turbornachinery as claimed in any preceding claim, wherein the space between the baffles (22) and the shroud (4) is such that the ingress of mainstream fluid is inhibited without increasing losses through windage.

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6. Turbomachinery substantially as hereinbefore described with reference to Figure 3 of the accompanying drawings.

7. Turbomachinery as claimed in any of the preceding claims I to 5 wherein the row of baffles (22) is attached to tips of the stator blade adjacent to the rotor surface.