Outlet guide vane

The invention relates to an outlet guide vane for an axial compressor extending along a rotor axis, comprising an air foil extending in a span direction from a radially inner end at 0% height to a radially outer end at 100% height, the air foil comprising a suction side and an opposite pressure side, both sides extending in a chord direction from a leading edge to a trailing edge, wherein for each profile of the airfoil a stagger angle between the chord and the rotor axis is de fined. The invention further relates to an axial compressor having a plurality of outlet guide vanes.

A conventional gas turbine engine includes in serial flow communication an axial compressor, a discharge flow path hav ing a stage of compressor outlet guide vanes (OGVs) , disposed between annular inner and outer walls, which in turn are mounted in an outlet guide vane support structure mechanical ly tied into an engine casing. Outlet guide vanes typically have airfoil like cross-sections that include a leading edge, a relatively thick middle section, and a thin trailing edge. If the compressor is part of a gas turbine, downstream of the outlet guide vane stage is a combustor diffuser, a combustor, a turbine nozzle and a turbine. The outlet guide vanes stage is usually provided after all other compressors stages in or der to straighten the flow from the compressor and direct it appropriately to the combustor.

During engine operation, the compressor compresses inlet air flow, which is therefore heated thereby. The discharged com pressed and heated airflow is then channeled through the out let guide vanes and the diffuser to the combustor. In the combustor it is mixed with fuel and ignited to form combus tion gases. The combustion gases are channeled through the turbine nozzle to the e.g. high pressure turbine which ex tracts energy therefrom for rotating and powering the com pressor . The compressor diffuser of a gas turbine converts dynamic pressure into static pressure. The more dynamic pressure is converted, the better the efficiency of the compressor and thus of the gas turbine. The conversion from dynamic to stat ic pressure is done by decelerating the flow.

The velocity profile of the flow is of great importance for improving the deceleration in the diffuser of an axial com pressor. If the air flows through the diffuser at the same average velocity in a uniform block profile, it contains less kinetic energy than in a profile with a distinct "velocity peak". A uniform velocity profile results in a lower compres sor outlet total pressure at a certain static pressure, i. e. with less energy input, which has a positive effect on the efficiency of the gas turbine engine.

However, due to the previous compressor stages and the wall friction within the compressor, the flow at the diffuser in let generally has an unfavorable velocity profile.

US 2007/231149 A1 discloses a guide vane having a particular design, due to which design the static stress in the brazed joint formed between the vane and the outer shroud is de creased .

Therefore, the object of the present invention is to provide a more favorable air flow profile at the outlet of the com pressor .

The object of the invention is achieved by the independent claims. The dependent claims describe advantageous develop ments and modifications of the invention.

In accordance with the invention there is provided an outlet guide vane for an axial compressor extending along a rotor axis, comprising an airfoil extending in a span direction from a radially inner end at 0% height to a radially outer end at 100% height, the airfoil comprising a suction side and an opposite pressure side, both sides extending in a chord direction from a leading edge to a trailing edge, wherein for each profile of the airfoil a stagger angle between the chord and the rotor axis is defined, wherein a stagger angle dis tribution in the span direction has a curved course having a minimum located between 40% and 60% in the span direction, a first maximum at 0% and a second maximum at 100% in the span direction .

In accordance with the invention there is also provided an axial compressor having a plurality of such outlet guide vanes .

The present invention is based on the idea to use a new three-dimensional design of the outlet guide vane in order to enhance the vortices in the secondary flow which cause an ex change of momentum within the flow and thus generate a smoother velocity profile at the diffuser outlet. Due to the proposed new geometry of the outlet guide vane a radial rear rangement of the velocity profile to the side walls in the direction of the suction side is achieved and a "block shaped" velocity profile is generated.

In the past, the outlet guide vane has been designed so that the flow into the diffuser is free of swirls. Vortices in the secondary flow were either neglected or considered undesira ble. In the present invention, the outlet guide vane is spe cifically designed so that strong vortices occur. These vor tices are oriented approximately in the direction of the ro tor axis. Important for the function of these vortices is their significant expansion in the span direction, i.e. the vortices have to be as large as possible in order to

transport the flow in the direction of the walls.

In a preferred embodiment, the difference in the stagger an gle between the minimum and the first maximum is between 8° and 23°. Such design of the outlet guide vane benefits the occurrence and spread of the block-shaped velocity profile. In another preferred embodiment, the longest chord length is at the outer end.

In yet another preferred embodiment, the stagger angle in the minimum is between 1° and 7°.

Preferably, the stagger angle at the first maximum is between 14° and 26° .

Still preferably, the stagger angle at the second maximum is between 8° and 28°.

Embodiments of the invention are now described, by way of ex ample only, with reference to the accompanying drawings, of which :

FIG 1 shows in a perspective view a pressure side an outlet guide vane according to the present in vention,

FIG 2 shows in different perspective view the pres sure side the outlet guide vane according to FIG 1,

FIG 3 shows a profile of an outlet guide vane, and

FIG 4 shows the stagger angle distribution in the span direction for the outlet guide vane shown in FIG 1.

It is noted that in different figures, similar or identical elements are provided with the same reference signs.

FIG 1 and FIG 2 show an outlet guide vane 2 for an axial com pressor which is not shown in detail. The axial compressor is e.g. an industrial gas compressor or is part of a gas turbine engine and is operated under subsonic conditions. The axial compressor comprises at its rear end a ring having a plurali ty of such outlet guide vanes 2. The axial compressor extends in the direction of rotor axis, which in FIG 1 is parallel to the x-axis .

The outlet guide vane 2 comprises an airfoil 4 having an up stream-sided leading edge 6 and a downstream-sided trailing edge 8 between which a suction side (not shown) and a pres sure side 10 extend in chord direction. The radial height of the airfoil 4 is determined from its radially inner end 12 with 0% height to its radially outer end 14 with 100% height. The span direction of the airfoil 4, which is also equivalent to the radial direction of the compressor, is in FIG 1 paral lel to the z-axis.

For each height position of the airfoil 4, following the flu id streamlines, a profile can be determined. One such exem plary profile 16 is shown in FIG 3. The profile 16 represents the outer airfoil shape for a specific height of the airfoil 4 defined by a cross section, in particular parallel to the x-y plane through said airfoil 4 at said height rotor axis. For each profile a stagger angle y is determinable between a chord line C of the profile and the rotor axis x. Hereby the chord line C is an imaginary straight line joining the lead ing edge 6 and trailing edge 8 of the airfoil 4.

As can be seen in FIG 1 and FIG 2, the longest chord length for the airfoil 4 is at the radially outer end 14.

FIG 4 shows the distribution of the stagger angle g in the span direction z from the radially inner end 12 at 0% height to the radially outer end 14 at 100% height. The distribution line D has a curved, u-shaped course having its minimum A lo cated between 40% and 60% in the span direction z. A first maximum Mi of the u-shaped line D is at the radially inner end 12, i.e. at 0% height, and a second maximum M ₂ is at the radially outer end 14, i.e. at 100% height. In FIG 4 the stagger angle g in the minimum A is approximate ly 3°. In general, the stagger angle y at this point is be tween 1 and 7. The stagger angle y at the first maximum Mi (at the radially inner end 12, 0% in span direction) is ap- proximately 24° and the stagger angle g at the second maximum M2 (at the radially outer end 14, 100% in span direction) is approximately 16°. Hence, the difference in the stagger angle g between the minimum A and the maximum at the radially inner end is 21° and the difference in the stagger angle g between the minimum A and the maximum at the radially outer end is

13°. In the embodiment shown in FIG 4 also the stagger angle g in the second maximum M2 is smaller than the stagger angle Y in the first maximum Mi.