Flameholder for a turbojet engine afterburner

A flameholder structure for a turbojet engine afterburner is disclosed in which a generally "V" shaped flameholder ring has a plurality of substantially radially extending fins attached to and extending radially outwardly from its outer surface. A portion of each of the fins is disposed at an angle to the gas flow so as to impart a cylindrical rotation to that portion of the flow passing over the fins. The cylindrical rotation of the flow increases the shear effect to thereby improve the stability of the flame front and increase the efficiency of the afterburner.

BACKGROUND OF THE INVENTION 
It is well known to provide afterburner systems on turbojet engines and 
such systems typically comprises means to inject fuel into the gas stream 
mixture, downstream of the turbojet engine turbine and means to ignite the 
mixture. In turbofan engines, the afterburner system is usually located 
downstream of the point at which the hot flow of gases and the radially 
outer colder flow converge. By igniting the additional fuel and the 
exhaust gas stream, additional thrust is provided to the engine. 
Although the afterburner theory is relatively simple, it has proven most 
difficult to achieve a complete burning of the fuel and to provide a 
stable flame front for the ignition of the gases. Various attempts have 
been made to increase the efficieny of the afterburner systems. The patent 
to Vdoviak (U.S. Pat. No. 3,931,707) illustrates one such attempt. The 
structure set forth in this patent utilizes an annular passage upstream of 
a generally "V" shaped flameholder ring to induce a circumferential swirl 
by a plurality of flow vanes disposed in this annular passage. However, 
this structure increases the complexity of the afterburner system thereby 
not only increasing the overall weight of the turbojet engine, but also 
increasing its maintenance and decreasing the overall reliability. 
SUMMARY OF THE INVENTION 
The present invention relates to a flameholder having a simplified 
construction to improve the overall efficiency of afterburner performance, 
without increasing the complexity of the engine structure. The flameholder 
according to the invention comprises an annular flameholder ring having a 
substantially "V" shaped cross-section attached in the afterburner duct of 
a turbojet engine. A plurality of substantially radially extending fins 
are attached to and extend radially outwardly from only a radially outer 
leg portion of the flameholder ring. The fins impart a cylindrical 
swirling flow to the portion of gas flowing over their surfaces. 
The rotation produced by this flow segment generates a radial pressure 
gradient which serves to increase the recirculating zone downstream of the 
flameholder. Increasing the recirculation volume directly improves the 
stability range of the afterburner system. 
The recirculation phenomenon also induces shearing turbulence near the 
boundary between the rotary cylindrical flow and the main flow outside of 
the flameholder which is not subject to this rotation. Accordingly, the 
flame propagation rate is increased to thereby achieve higher operational 
efficiency. 
The present invention also enables the efficiency to be increased through a 
shorter longitudinal distance downstream from the flameholder. Thus, not 
only is the performance of the afterburner improved, the length of the 
turbojet engine duct may be made shorter, resulting in a sifnificant 
reduction in both the mass and bulk of the turbojet engine. 
According to the invention, each of the radial fins has an upstream portion 
which extends in an upstream direction from the base of the annular 
flameholder ring in a direction generally parallel to the direction of the 
gas flow. A downstream portion of each of the fins extends at an angle to 
the direction of the gas flow so as to impart the circumferential motion 
to this portion of the gas flow. 
The structure according to the invention avoids the teachings of the known 
prior art which utilize fins extending from a radially inwardly portion of 
the flameholder ring. The prior art generates a radially inner rotation in 
the gas flow which hinders the recirculation downstream of the flameholder 
due to the centrifugal speed component imparted by the fins. By utilizing 
only fins extending in a radially outwardly direction, this drawback of 
the prior art is avoided. 
The invention also encompasses a substantially cylindrical sleeve which 
interconnects the radially outermost edges of each of the fins. A flange 
extends from the cylindrical sleeve in an upstream direction beyond the 
distal edge of each of the flanges.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
A partial, sectional view of a turbojet engine with an afterburner is 
schematically shown in FIG. 1. The hot gas flow is defined internally by 
inner cowl 1 and externally by wall 3. The gases pass in the direction of 
arrow 2 from an upstream portion toward a downstream portion (to the right 
as seen in FIG. 1). Wall 3 separates this hot flow of gases from an outer, 
secondary flow, generally designated the cold flow, which circulates in 
the annular secondary duct between wall 3 and an outer wall 4. Although 
only an upper portion of the cross-section is shown, it is to be 
understood that all of these structures are symmetrical about the 
longitudinal axis of the engine and afterburner duct shown in FIG. 1. 
The afterburner structure 5 is also shown in FIG. 1 interposed between the 
inner cowl 1 nd the outer duct 4, and comprises a radially outer burner 
ring 6 consisting of flameholder 7 and a fuel injection manifold 8. The 
radially outer burner ring 6 is located near the outer wall 4 in the cold 
flow. Burner ring 9 also consists of a flameholder 10 and fuel injection 
manifold 11 and is located near the convergence of the hot and cold flows. 
Another burner ring 12, also consisting of a flameholder ring 13 and a 
fuel injection manifold 14 is located in the stream of the hot gases, as 
is radially innermost flameholder ring 15. 
The various rings are attached to the fixed wall 3 separating the hot and 
cold flows by means of link rods 16, 17, 18 and 19, provided at their ends 
with hinge attachment means 20, 21, 22, 23, 24 and 25. The system also 
includes a fuel intake device 26 located upstream of the afterburner 
structure. Although one or more of the burner rings may include a 
flameholder according to the present invention, the most advantageous 
application of the invention is to incorporate the radial fins in the 
flameholder 10 at the juncture of the hot and cold flows. 
The specific design according to the invention is shown in detail in FIGS. 
2 and 3. A plurality of substantially radially extending fins 27 are 
mounted on, and extending radially outwardly from, the radially outer leg 
portion 28 of flameholder 10. As can be seen from FIG. 2, the flameholder 
10 has a generally "V" shaped cross-section and is oriented such that the 
"V" opens in a downstream direction. The "V" shaped cross-section is 
defined by a radially outer leg portion 28, and a radially inner leg 
portion interconnected at their upstream edges by base portion 29, which 
may be curved as shown. 
Each of the fins 27 comprise a downstream portion 27a which extends at an 
angle to the direction of the gas flow so as to impart a circumferential 
component to the motion of such flow. Each fin 27 also has a portion 27b 
extending from the base portion 29 of the flameholder 10 in an upstream 
direction generally parallel to the direction of gas flow. The fins 27 are 
regularly distributed about the periphery of flameholder 10 to ensure that 
the cylindrical portion of the gas flow passing over the fins is set into 
rotation. 
The radially outermost edges of fins 27 may be connected by a continuous, 
generally cylindrical sleeve 30. Flange 31 may be formed on the upstream 
edge of outer sleeve 30 such that a distal edge of this flange extends, in 
an upstream direction, beyond the upstream edges of fins 27. This 
arrangement enhances the separation of the flow designated by arrows 32 
and 33 in FIG. 2, such that the flow passing in a direction of arrow 32 is 
set into rotation due to its passage over the fins 27, while the flow 
denoted by arrow 33 is not given such a motion. This increases the 
shearing effect of the mixture. 
The foregoing description is provided for illustrative purposes only and 
should not be construed as in any way limiting this invention, the scope 
of which is defined solely by the appended claims.