BRAZING & SOLDERING TODAY Selecting Materials for Brazing a Honeycomb in Turbine Engines Labyrinth Seals To reduce leakage flows in turbo machines, tight operational clearances between stator and rotor parts are desirable. With reduced clearances, the risk of rub contact increases. In such situations, the stator is fitted with an abradable structure or coating that minimizes damage to the rotor when such rub contact occurs. A popular abradable structure is metallic honeycomb, which derives its abradability from a low structural density, typically in the range of 8 to 20% of the bulk alloy density. Honeycomb seal lands are always used as part of a 44 FEBRUARY 2014 labyrinth seal, as schematically shown in Fig. 1. Seal fins on the rotor together with the stator wall form cavities. A gas flow is accelerated as it enters into the labyrinth from the high-pressure side through an annular gap (A). Subsequently, the flow area cross section increases dramatically as the flow enters the cavity (B) causing it to dissipate flow energy through a reduction of speed and pressure in a turbulent flow. A series of such events, creating a tortuous path for the gas flow, allows it to maintain a sizable pressure difference across the labyrinth seal, although a certain amount of leakage flow is inevitable. The amount of leakage flow, however, DIETER SPORER (dieter.sporer@sulzer.com) is with Sulzer Metco Management AG, Winterthur, Switzerland. DIENTJE FORTUNA (deni.fortuna@sulzer.com) is with Sulzer Metco (U.S.), Inc., Troy, Mich. Based on a paper presented at IBSC 2012, 5th International Brazing and Soldering Conference, April 22–25, Las Vegas, Nev. depends on the size of the clearance C. Small operational clearances can only be designed if the stator wall is lined with a structure that can easily be cut by the rotating seal teeth without damaging or overheating them — Fig. 2. Engine Seal Locations In a gas turbine engine, there is a large number of locations that require sealing against leakage flows. Honeycombs are used in both compressor sealing as well as turbine sealing applications. However, air seals in low-pressure turbine (LPT) modules remain a domain of honeycomb as abradable seals. Typical five-stage, low-pressure aero engine turbine modules have ten gas path seal locations over shrouded blades and vanes, so Increasing aircraft turbine engine efficiency involves higher operating temperatures that introduce a number of engine manufacturing concerns BY DIETER SPORER AND DIENTJE FORTUNA Fig. 1 — Laybrinth seal schematic. Fig. 2 — Stepped layyrinth seal with honeycomb seal lands.
Welding Journal | February 2014
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