454
12 Thrust Gas Turbines
stream of the combustion zone, more air is added as dilution air. The liner of
Fig. 12.16 is a schematic representation of a thin-wall shell of a nickel-base alloy, covered with a thermal barrier coating (TBC) and cooled by injection of air
through arrays of a large number of small holes, either at about 30° in the wall
(called effusion cooling) or perpendicular in rings (called diffusion cooling).
Heat shielding and cooling are realised by the films formed at the inner side of
the liner, but the liner is also cooled by heat removal within the holes. Such liners are nowadays still in development phase for larger engines. More common
is a double-wall liner being a metal shell covered at the inside with ceramic tiles
cooled by a combination of convection, impingement and film cooling.
The traditional principle for keeping emissions of NOx and CO low is a sequence of rich burning, quick quenching of the flame and lean burning. The mixture
in the primary zone is very rich (about 12 % of the air) so that the combustion temperature is rather low (the combustion temperature is the highest for stoichiometric
combustion). This ensures that not much NOx is formed, but, of course, production
of CO and soot (carbon) is high. With addition of the secondary air, the mixture
passes quickly from rich to lean (about 50 % of the air in the primary and secondary zones together). This process is called quenching. The mixture passes through a
stoichiometric zone with high production of NOx but this zone is kept very short. In
the lean zone, CO and soot are burnt. With this combustion principle, emissions of
CO and NOx are higher than with the Dry Low Emission (DLE) systems of power
gas turbines. Staged combustion systems with pilots ensuring stable combustion
(diffusion flame) combined with main burners optimised for low NOx production
(lean mixture) are developed by all manufacturers nowadays. These systems are
similar to the ones used in DLE systems of power gas turbines: either rows of pilot
burners next to rows of lean burners or rows of burners with a pilot core and a lean
part around it. These systems are still not fully mature so that it is not meaningful to
discuss them in a basic book.
12.6.4 Mixer and Thrust Reverser
Figure 12.17 (left) renders a lobed surface for mixing the hot and the cold flows,
typical for low bypass rate engines. Figure 12.17 (right) demonstrates how, by pushing back the cold flow nozzle, the cold flow is deflected to generate reverse thrust
for braking immediately after landing.
12.7 Exercises
12.7.1 Single-Flow Jet Engine
With this engine, a single thrust flow is generated by a thermodynamic cycle of
compression, heating and expansion. Calculate the thrust per unit of mass flow
12 Thrust Gas Turbines
stream of the combustion zone, more air is added as dilution air. The liner of
Fig. 12.16 is a schematic representation of a thin-wall shell of a nickel-base alloy, covered with a thermal barrier coating (TBC) and cooled by injection of air
through arrays of a large number of small holes, either at about 30° in the wall
(called effusion cooling) or perpendicular in rings (called diffusion cooling).
Heat shielding and cooling are realised by the films formed at the inner side of
the liner, but the liner is also cooled by heat removal within the holes. Such liners are nowadays still in development phase for larger engines. More common
is a double-wall liner being a metal shell covered at the inside with ceramic tiles
cooled by a combination of convection, impingement and film cooling.
The traditional principle for keeping emissions of NOx and CO low is a sequence of rich burning, quick quenching of the flame and lean burning. The mixture
in the primary zone is very rich (about 12 % of the air) so that the combustion temperature is rather low (the combustion temperature is the highest for stoichiometric
combustion). This ensures that not much NOx is formed, but, of course, production
of CO and soot (carbon) is high. With addition of the secondary air, the mixture
passes quickly from rich to lean (about 50 % of the air in the primary and secondary zones together). This process is called quenching. The mixture passes through a
stoichiometric zone with high production of NOx but this zone is kept very short. In
the lean zone, CO and soot are burnt. With this combustion principle, emissions of
CO and NOx are higher than with the Dry Low Emission (DLE) systems of power
gas turbines. Staged combustion systems with pilots ensuring stable combustion
(diffusion flame) combined with main burners optimised for low NOx production
(lean mixture) are developed by all manufacturers nowadays. These systems are
similar to the ones used in DLE systems of power gas turbines: either rows of pilot
burners next to rows of lean burners or rows of burners with a pilot core and a lean
part around it. These systems are still not fully mature so that it is not meaningful to
discuss them in a basic book.
12.6.4 Mixer and Thrust Reverser
Figure 12.17 (left) renders a lobed surface for mixing the hot and the cold flows,
typical for low bypass rate engines. Figure 12.17 (right) demonstrates how, by pushing back the cold flow nozzle, the cold flow is deflected to generate reverse thrust
for braking immediately after landing.
12.7 Exercises
12.7.1 Single-Flow Jet Engine
With this engine, a single thrust flow is generated by a thermodynamic cycle of
compression, heating and expansion. Calculate the thrust per unit of mass flow