Reduced shock transonic airfoil

Details Reduced shock transonic airfoil Reduced shock transonic airfoil

2001-10-05

2004-01-27

Look, Edward K. (Department: 3745)

Rotary kinetic fluid motors or pumps

Means, disposition or arrangement for causing supersonic...

C415S191000, C415S208200, C416S22300B, C416S243000

Reexamination Certificate

active

06682301

ABSTRACT:

TECHNICAL FIELD
The invention concerns airfoils, such as those used in gas turbines, which operate in a transonic, or supersonic, flow regime, yet produce reduced shocks. One reason for reducing the shocks is that they produce undesirable mechanical stresses in parts of the turbine.
BACKGROUND OF THE INVENTION
A simple analogy will first be given which explains how repeated pressure fluctuations can induce vibration.
FIG. 1
shows an acoustic loudspeaker
3
which produces pressure waves
6
. Each wave
6
contains a high-pressure, high-density region
9
, and a low-pressure, low-density region
12
. When the waves
6
strike an object
15
, each high-pressure region
9
applies a small force to the object
15
, and the succeeding low-pressure region
12
relaxes the force. The sequence of
. . . -force-relaxation-force-relaxation- . . .
causes the object
15
to vibrate.
Shocks produced by rotating airfoils can produce similar vibrations, as will now be explained.
FIG. 2
illustrates a generalized shock
23
produced by a generalized airfoil
26
. The shocks as drawn in
FIG. 2
, as well as in
FIGS. 3 and 4
, are not intended to be precise depictions, but are simplifications, to illustrate the principles under discussion.
One feature of the shock
23
is that the static pressure on side
29
is higher than that on side
32
. Another feature is that the gas density on side
29
is higher than on side
32
. These differentials in pressure and density can have deleterious effects, as will be explained with reference to
FIGS. 3 and 4
.
FIG. 3
illustrates a generalized gas turbine
35
, which extracts energy from an incoming gas stream
38
. Each blade
41
produces a shock
23
A in
FIG. 4
analogous to shock
23
in FIG.
2
. The blades
41
in
FIG. 4
collectively produce the shock system, or shock structure,
47
.
Similar to the shock
23
in
FIG. 2
, each individual shock
23
A in
FIG. 4
is flanked by a differential in pressure and gas density: one side of the shock
23
A is characterized by high pressure and high density; the other side is characterized by low pressure and low density.
When the shock structure
47
rotates, as it does in normal operation, it causes a sequence of pressure pulses to be applied to any stationary structure in the vicinity. This sequence of pulses is roughly analogous to the sequence of acoustic pressure waves
6
in FIG.
1
.
For example, stationary guide vanes (not shown) are sometimes used to re-direct the gas streams exiting the blades
41
in
FIGS. 3 and 4
, in order to produce a more favorable angle-of-attack for blades on a downstream turbine (also not shown). The pulsating pressure and density pulses can generate vibration in the stationary guide vanes.
As a general principle, vibration in rotating machinery is to be avoided.
The preceding discussion is a simplification. In general, shocks
23
A in
FIG. 4
will be accompanied by expansion fans, and the overall aerodynamic structure will be quite complex. Nevertheless, the general principles explained above are still applicable.
SUMMARY OF THE INVENTION
In one form of the invention, substantially all curve on the suction surface of a transonic turbine blade is located upstream of a throat defined by the blade and an adjacent blade. Downstream of the throat, the remaining curve on the suction surface is no more than 6 degrees, and preferably no more than 2 degrees.


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