10.5 Augmenter Cooling Systems & Exhaust Noise Suppression

10.5 Augmenter Cooling Systems & Exhaust Noise Suppression

Quick Answer: An augmenter cooling system uses engine exhaust as a pump. FAA-H-8083-32B describes it directly: the exhaust collector tubes terminate at the exhaust ejector openings at the firewall and are tapered to deliver the exhaust gases at the proper velocity to induce airflow through the exhaust ejectors, and the exhaust ejectors consist of a throat-and-duct assembly that utilizes the pumping action of the exhaust gases to induce a flow of cooling air through all parts of the engine compartment (augmenter tube action). On a large radial, a combination exhaust manifold and augmentor assembly discharges collected exhaust into augmentor bellmouths, and the augmentors are designed to produce a venturi effect to draw an increased airflow over the engine. On the turbine side, the same exhaust stream is the dominant noise source, and noise suppressors in current use are either of the corrugated-perimeter type or the multi-tube type, both of which break up the single, main jet exhaust stream into a number of smaller jet streams.

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Part 1: Augmenter Cooling

The Cooling Problem Augmenters Solve

Section 10.3 covered pressure cooling: a sealed cowling creates a high-pressure plenum above the cylinders and a low-pressure region below, and dynamic ram air is forced down across the fins. Pressure cooling has one structural weakness: the pressure differential that drives the cooling air comes from airspeed. In the exact flight condition where the engine makes the most heat and needs the most cooling — a full-power climb at best-rate-of-climb speed — the ram pressure available is near its lowest.

Cowl flaps are the conventional answer: open the lower cowl exit to raise the mass flow. An augmenter attacks the same problem from the other end. Instead of enlarging the exit, it actively pumps the exit.

How the Ejector Works

An augmenter is a jet pump. The exhaust gas is the primary (motive) flow; the nacelle cooling air is the secondary (induced) flow.

                        AUGMENTER (EXHAUST EJECTOR) ACTION

    Cooling air drawn from the engine compartment
          \        \        \        \
           v         v         v         v
    ====================================================
    |                                                  |
    |   ==>  [ tapered collector tube nozzle ] ==>     |   AUGMENTER TUBE
    |         high-velocity exhaust gas jet            |   (throat and duct)
    |                                                  |
    ====================================================
          THROAT (low static pressure)      -->  mixed gas + air overboard

1. The exhaust collector tubes are tapered where they terminate at the firewall, which accelerates the gas to the velocity the ejector needs.
2. The high-velocity jet discharges into the throat of the augmenter tube. By Bernoulli's principle, that high velocity produces a low static pressure at the throat.
3. That low pressure is connected to the engine compartment, so ambient air is drawn through the cowling, across the cylinders and accessories, and into the augmenter.
4. The mixed exhaust and cooling air expands through the diverging duct and discharges overboard.

On the large radial installation, two exhaust assemblies and two augmentor assemblies are used on a typical 18-cylinder engine; each manifold assembly collects exhaust gases from nine cylinders and discharges the gases into the forward end of the augmentor assembly, with the gases directed into the augmentor bellmouths. The augmentors produce a venturi effect to draw an increased airflow over the engine.

Why It Matters Operationally

The pattern to remember: an augmenter's cooling authority scales with engine power, not with airspeed. That is the opposite of a pure ram system, and it is why augmenters appeared on installations that spend time at high power and low speed.

Some Augmenters Also Feed Cabin Heat

Where a cabin heater exhaust shroud is installed on the collector tube or augmenter, ram air is warmed by contact with the hot gas path and ducted to the cabin. That combination raises the stakes on inspection: any crack in the gas-carrying wall inside the shroud leaks exhaust gas directly into the cabin supply. Section 16.3 covers the carbon monoxide hazard and the pressure-test technique in full, and it applies to augmenter installations without modification.

Inspecting an Augmenter System

The ACS lists skill elements covering inspection of the exhaust augmenter cooling system and identification of augmenter-cooled engine components. The inspection is an exhaust-system inspection with cooling consequences layered on top.

• Collector tubes and slip joints. Look for cracks, especially at welds and at the transition where the tube tapers. Slip joints must be free to absorb thermal growth without leaking.
• Ejector nozzle and throat geometry. A dented, eroded, or misaligned nozzle no longer delivers the gas at the velocity the ejector needs, and cooling performance falls even though nothing looks broken. Note the position marks and clamp alignment before disturbing anything.
• Augmenter tube interior. Burn-through, buckling, and loose internal baffles change the duct geometry.
• Security and clearance. Augmenter tubes run hot and carry vibration. Check mounting, clamps, and clearance to fuel lines, hydraulic lines, electrical harnesses, and structure.
• The cooling symptom. An augmenter leak upstream of the throat dumps motive energy before it can do useful work. Rising cylinder head temperatures at unchanged power settings and cowl-flap position, with no other cooling fault, point at the augmenter system long before the leak is loud enough to notice.

[!CAUTION] Any exhaust gas leak in an augmenter installation is simultaneously a cooling loss, a carbon monoxide hazard where a heat exchanger shares the gas path, and a fire hazard where the escaping gas impinges on fuel or hydraulic lines. Treat a suspected leak as grounding until it is located and repaired per the manufacturer's data; exhaust system repairs are not owner-preventive maintenance.

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Part 2: Turbine Exhaust Noise Suppression

Where Turbine Noise Comes From

FAA-H-8083-32B names three sources of noise involved in the operation of a gas turbine engine: the engine air intake, vibration from the engine housing, and the engine exhaust. It then states that the first two do not compare in magnitude with the exhaust.

Exhaust noise is "caused by the high degree of turbulence of a high-velocity jet stream moving through a relatively quiet atmosphere." The handbook describes the structure of that noise field:

• Close behind the nozzle (a few nozzle diameters), jet velocity is high and mixing with the atmosphere is limited. The turbulence there is fine grain and produces relatively high-frequency noise.
• Farther downstream, the jet slows and mixes, turbulence becomes coarser, and the noise frequency drops.
• As the jet's energy finally dissipates in large turbulent swirls, a greater portion of that energy is converted into noise, at a frequency near the low end of the audible range.

That last point drives everything: "the lower the frequency of the noise, the greater the distance the noise travels." High-frequency noise is weakened more rapidly by distance, by buildings and terrain, and by atmospheric absorption. The handbook's analogy is a deep-voiced foghorn carrying much farther than a shrill whistle of the same source volume.

The Two Levers: Velocity and Frequency

Lever 1 — reduce exhaust velocity. "Noise levels vary with engine thrust and are proportional to the amount of work done by the engine on the air that passes through it." An engine with low airflow but high thrust (high exhaust temperature, high pressure, or afterburning) produces a high-velocity gas stream and therefore high noise levels. A larger engine handling more air is quieter at the same thrust, and a turbofan is quieter during takeoff than the turbojet version of the same engine because the exhaust velocities at the tailpipe are slower. The fan engine's larger turbine, often with an added stage, reduces the velocity of the gas and therefore reduces the noise, because exhaust gas noise is proportional to exhaust gas velocity. The fan discharge itself is at relatively low velocity and does not create a noise problem.

Lever 2 — raise the frequency. This is what a suppressor does. "The noise suppressors in current use are either of the corrugated-perimeter type, or the multi-tube type. Both types of suppressors break up the single, main jet exhaust stream into a number of smaller jet streams. This increases the total perimeter of the nozzle area and reduces the size of the air stream eddies created as the gases are discharged into the open air."

The consequence is precise and counterintuitive: "Although the total noise-energy remains unchanged, the frequency is raised considerably." Eddy size scales down linearly with the size of the exhaust stream, which has two effects the handbook names:

1. Some of the noise may be pushed above the audibility range of the human ear.
2. High frequencies that remain in the audible range, while perhaps more annoying, are more highly attenuated by atmospheric absorption than are low frequencies, so intensity falls off faster and the noise level is lower at any given distance from the aircraft.

A suppressor does not destroy acoustic energy. It redistributes it into frequencies that do not travel.

Acoustic Nacelle Linings

Separately from the nozzle, "in the engine nacelle, the area between the engine and the cowl has acoustic linings surrounding the engine. This noise-absorbing lining material converts acoustic energy into heat." The linings "normally consist of a porous skin supported by a honeycomb backing" with a separation between the face sheet and the engine duct. Unlike a suppressor nozzle, a lining genuinely removes energy from the acoustic field by dissipating it as heat.

The maintenance consequence: an acoustic liner is a tuned structure, not trim. Crushed honeycomb, a punctured or painted-over porous face sheet, or an unapproved repair changes its acoustic properties, and repairs are governed by the structural repair manual rather than by general sheet-metal practice.

Related Devices and What They Are Not

That last row is the distinction the ACS tests. The Powerplant standards list noise suppression theory, components, and operation with mufflers, hush kits, and augmenter tubes named as examples, and they separately list the augmenter cooling system under reciprocating induction and cooling. The same hardware family appears under both headings, so be ready to say which function is being asked about: an augmenter is installed to move cooling air, while a suppressor is installed to change the noise spectrum.

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Independent Prep Note

Independent FAA AMT Powerplant prep by OpenExamPrep. Not sponsored by or affiliated with the Federal Aviation Administration (FAA). Technical data compiled from FAA-H-8083-32B, FAA AC 43.13-1B, and 14 CFR Parts 43 and 65.