The following gives general information regarding proper procedure and remedies for preventing damage. Examples from technical literature are used for clarification.
Tailpipe (thrust nozzle):
Figure "Weak points of fighter afterburners" (Ref. 11.2.4-17 ): In this afterburner of a widely used fighter engine, improvements to the afterburner were made after two million fleet flight hours. This included measures to prevent the liner from buckling inward (middle diagram). These deformations occurred at high flight speeds close to the ground (top diagrams). The cause was the large pressure difference between the hot gas flow and the airflow between the liner and afterburner pipe. Improved strengthening rings were used to stiffen the liner (bottom diagram). These proved to be effective during later operation.
Figure "Conclusions from pressure vibrations in after burner pipe" (Ref. 11.2.4-13): Suitably located pressure transducers (top diagram) can be used to analyze the force and direction of pressure vibrations of unstable combustion. A phase shift is marked in the diagram by the lines indicated by arrows (schematic depiction). The conclusion reached in this case was a pressure wave that moved forward towards the turbine against the direction of flow, parallel to the central axis of the afterburner. Evidently only a small part of the pressure wave was reflected by the turbine. This resulted not in a standing wave, but in an advancing wave. These waves have also been observed in other cases in mixed-flow afterburners (Fig. "After burner designs and 'buzz'").
The pressure vibrations were reduced with a simple change to the injection system. The remedy was improved distribution of the fuel/air ratio, which was accomplished by optimizing the arrangement of the injection nozzles (Fig. "Minimizing by design 'buzz' tendency in after burners")
Figure "Minimizing by design 'buzz' tendency in after burners" (Ref. 11.2.4-18): In order to achieve the desired combustion behavior, it is important to control the location of the fuel cloud and its movement in the gas flow as precisely as possible. This is especially difficult in mixed-flow afterburners (Fig. "After burner designs and 'buzz'"), in which a cold airflow from the bypass duct is mixed in. Because the fuel droplets do not vaporize quickly enough ahead of the flame, they are too large for the gas flow to redirect them properly. This prevents them from reaching the areas behind the flame holders, where the flame stabilizes thanks to the sufficiently low flow speeds (Fig. "Flame holder in afterburner"). Instead of vaporizing, the droplets wet the walls of the afterburner or the flame holder and fuel injector. For this reason, the injectors are not positioned ahead of the flame holder, but rather in the same plane as the flame holder, which allows the fuel to be injected directly into the flame. This requires the nozzles integrated into the flame holder (wake injection nozzles) to be supplied with fuel via the flame holder. The literature does not indicate why injection nozzles are arranged between the wake injectors. It can be assumed that sufficiently even distribution of the fuel would require so many wake injector rings, that the cross-section through which the gas flow passes would be unallowably constricted.
Figure "Remedies for hiogh-frequency combustion vibrations ('screetch')" (Refs. 11.2.4-13, 11.2.4-18): The best option would be the ability to predict combustion vibrations with the aid of calculations. This would allow suitable constructive measures to be included in the design of the afterburner. The continued occurrence of damage indicates that this is not yet safely possible, at least not in the engine development phase. However, it is evidently possible to determine helpful predictions as to expected frequencies and modes (Ref. 11.2.4-16). This allows statements to be made regarding the influence of the gas throughflow, the inlet temperature, the pipe length, and the fuel/air ratio. However, all of this is hardly sufficient to allow satisfactory conclusions regarding the intensity and danger of vibrations. The prerequisite for specific remedies is an understanding of the type of vibration and its causes. While the frequency of vibration (sonic frequency) does reveal something about its mode (transverse, longitudinal; see Fig. "Gas vibrations inafterburner cause stalls in the fan"), further important characteristics may be of interest with regard to remedies. Pressure transducers arranged in an axial direction from the turbine exit to the tailpipe could supply additional information. For example, this arrangement could determine whether the wave was standing or travelling (Ref. 11.2.4-14). The remedies must be specific to the type of wave. The case depicted in the bottom left diagram involved a traveling low-frequency wave (buzz). The remedy was reversed deflectors for the fuel jets (bottom center diagram). This resulted in a more even distribution of the fuel/air ratio aft of the injection system (bottom right diagrams), which acceptably reduced the vibrations.
Several measures have been shown to be effective remedies for high-frequency (screech) and low-frequency (buzz) gas vibrations during the combustion process (Fig. "Excitement of combustion vibrations") in afterburners:
If both low-frequency and high-frequency vibrations are occurring, then remedies are especially difficult. These conditions are very common in the afterburners of modern fighter engines. The reason for this is the mixing of the cold bypass flow with the hot gas flow from the engine core. The vibration-promoting conditions occur when cold airflows stabilize the fuel droplets. Screech liners are in widespread use today. They have been shown to be extremely effective in suppressing gas vibrations. A screech liner is a perforated cylinder of metal sheeting on the hot gas side. Air for the inner cooling air film passes through the openings. The hole pattern on this type of afterburner wall can be optimized for the specific frequencies to be damped. Two matched concentric perforated liners can dampen a wide range of frequencies (top diagram). An absorption of about 20 % is already enough to suppress the vibrations completely.
Figure "Fighter thrust nozzle weak points" (Ref. 11.2.4-17): In this case, separation swirls excited vibrations in the outer afterburner nozzle flaps (middle diagram). The result was increased wear. The wear zones were coated with hard materials as a remedy. Separate sealing strips (bottom diagram, left; Ref. 11.2.4-21) were replaced with wear strips that were integral with the flaps. These overlap with the neighboring flap like shingles (bottom diagram, right).
The life span of parts with surfaces exposed to the hot gas flow was increased through the use of thermal barrier coatings and improvements to the cooling air distribution.
The literature indicates that these problems are typical for afterburner nozzles and also occur in other engine types (Ill. 11.2.4-16; Ref. 11.2.4-19).
11.2.4-1 S.W. Kandebo, “USAF Targets Engine Mishaps”, periodical “Aviation Week & Space Technology”, March 29, 1999, pages 84 and 85.
11.2.4-2 “GE Finds Angled Fuel Tubes Solve Afterburner No-Light Problem”, periodical “Aerospace
Propulsion”, February 4, 1993, page 5.
11.2.4-3 “Goodbye F100 stall stagnation”, probably from periodical “Flight International” late 1979 or early1980.
11.2.4-4 “F100 Problems Spur Durability Stress”, periodical “Aviation Week & Space Technology”, December 10, 1979, pages 24 and 25.
11.2.4-5 “Low-life F100 cause F-15/F-16 engine shortage”, periodical “Flight International”, December 8, 1979, Seite 1896.
11.2.4-6 J.Ott, “Lauda Crash Probes Focus On Midair Thrust Reversal”, periodical “Aviation Week & Space Technology”, June 10, 1991, pages 28 and 29.
11.2.4-7 M.A. Dornheim, “767 Thrust Reversers Equipped to Counter in-flight Deployments”, periodical “Aviation Week & Space Technology”, June 10, 1991, pages 29 and 30.
11.2.4-8 K.Hünecke, “Flugtriebwerke - ihre Technik und Funktion”, Motorbuch Verlag Stuttgart, pages 239 and 240.
11.2.4-9 “The Jet Engine”, Rolls-Royce plc 1986, Volume 1996, ISBN 0 902 121 2 35, pages 172 and 200.
11.2.4-10 B.Gunston, “The Development of Jet and Turbine Aero Engines”, Patrick Stephens Ltd. pages 49-62.
11.2.4-11 C. Thoyer-Rozat, “Thrust Reverser Reliability”, Paper of the “ Aero-Engine Overhaul & Maintenance Conference and Industry Promotion”, Paris, September 22-23, 1993.
11.2.4-12 I.E. Traeger, “Aircraft Gas Turbine Engine Technology”, Second Edition, publisher: Glencoe, ISBN 0-07-065158-2, pages 164-167.
11.2.4-13 J.M. Bonnell, R.L. Marshall, “Combustion Instability in Turbojet and Turbofan Augmentors”. AIAA Paper No. 71-698 of the “Propulsion Joint Specialist Conference”,Salt Lake City, Utah, June 14-18, 1971, pages 1-8.
11.2.4-14 P.J.Langhorne, “Reheat buzz: an acoustically coupled combustion instability. Part2, Theory”. periodical “Journal of Fluid Mechanics”, Cambridge University Press, Volume 193, August 1988, pages 417-443.
11.2.4-15 P.J.Langhorne, “Reheat buzz: an acoustically coupled combustion instability. Part1, Experiment”. periodical “Journal of Fluid Mechanics”, Cambridge University Press, Volume 193, August 1988, pages 445-473.
11.2.4-16 A.P.Dowling, “Reheat Buzz - an Acoustically Coupled Combustion Instability”. Proceedings AGARD-CP-450, of the conference on “Combustion Instabilities in Liquid Fuelled Propulsion Systems”, pages 10-1 to 10-15.
11.2.4-17 S.F.Powell IV, “On the Leading Edge: Combining Maturity and Advanced Technology on the 404 Turbofan Engine”. Proceedings Paper ASME 90-GT-149, of the “35th International Gas Turbine and Aero Engine Congress and Exhibition”, Brussels, Belgium, June 11-14, 1990, pages 1-5.
11.2.4-18 A.Sotheran, “High Performance Turbofan Afterburner Systems”, Proceedings Paper AGARD-CP-422 of the conference on “Combustion and Fuels in Gas Turbine Engines”, pages 12-1 to 12-9.
11.2.4-19 “F100-PW220, The Power of Technology” , publication of United Technologies, Pratt & Whitney“, page 14.
11.2.4-20 J.Markham, P.Jalbert, W. Atkinson, E. Suarez, “Turbine Engine Augmentor Screech and Rumble Sensor”, Paper AIAA 2001-3766 of the “37th AIAA/ASME/SAE/ASEE Joint Propulsion Conference” Salt Lake City, Utah, July 8-11, 2001, pages 1-8.
11.2.4-21 “Coating helps keep engines flying”, periodical “Machine Design”, October 24, 1994, page 46.
11.2.4-22 “FAA orders thrust reverser changes on DC-10s”, periodical “Flight International”, 9-15 May, 2000, page 9.
11.2.4-23 N. Ionides, “Australia's ATSB warns of RB.211 nozzle failure”, Sept. 1998, from internet address: http://www.rati.com/news.
11.2.4-24 NTSB Identification ATL88IA037, microfiche number 35744A, incident on Novermber 15, 1987.
11.2.4-25 D.J. Stromecki, “An Assesment of Gas Turbine Engine Augmentor Technology and Needs for the 89's”, Proceeding AIAA-80-1200 of the AIAA/SAE/ASME 16th Joint Proulsion Conference, June 30-July 2, 1980/Hartford, Connecticut, pages 1-7.