Seminar on "Lean Direct Injection for Aircraft Gas Turbines"

Abstract:

The desire to reduce the environmental impact of aviation is unrelenting. Lean burn technologies provide a likely route to achieving future emissions targets; primarily by reducing flame temperature whilst limiting (theoretically) the quantity of fuel introduced. Unfortunately, there are a number of significant practical obstacles to lean burn technologies which must be overcome in order to make such a system tenable.

The major difference in lean burn systems (compared to traditional rich burn architectures) is in the design of the fuel injector. Lean burn combustion, by definition, requires increased air admission to the reaction zone. Up to approximately 70% of the compressor delivery air may be used for fuel preparation and oxidation, more than twice that of a rich burn system. In order to maintain acceptable levels of system pressure drop (and hence maintain acceptable levels of fuel burn) both the injector and the flametube into which it issues must increase in effective area. In addition, since aircraft typically deal with significant variation in fuel flow rates, lean injectors typically employ some form of staging, to ensure reliable ignition. Resultantly, lean burn injectors tend to be diametrically large and consist of concentrically mounted “pilot” and “main” modules.

Although the injectors within lean burn systems are much larger than their rich burn counterparts, the delivery of compressor air is achieved in substantially the same manner. As a result, the downstream flowfield (within the combustor) tends to become very non-uniform. Pollutant emissions are heavily dependent on the ability of the injector to generate a homogenous reaction zone. Thus understanding the effects of the injector-feed interaction becomes important if lean burn technologies are to realise the postulated improvements in emissions.

Velocity data have been acquired via the use of hot-wire anemometry to examine the magnitude of the non-uniformity induced by the upstream conditions within an isothermal non-reacting rig at near atmospheric conditions. In addition, several design modifications are proposed to mitigate the feed induced non-uniformities. These have been designed numerically and subsequently investigated within an experimental facility.

Typically gas turbine injectors consist of several concentric swirling jets, which aid in fuel preparation and downstream mixing. The flowfield issuing from a lean injector is incredibly complex and highly time dependent; containing secondary flow features generated from the injector geometry in addition to the feed induced non uniformities. The concentric swirling streams interact with one another and are host to a number of unsteady processes. The importance of the time dependent flowfield cannot be understated. Not only because pollutant formation depends on the instantaneous flowfield, but because combustion instability is one of the largest impediments to lean burn technology. It is fair to say that injector generated, multi-annular swirling flowfields are not yet fully understood, though they provide a very interesting fluid dynamic problem. The hot-wire technique employed to measure (time mean) non-uniformity also provides information pertaining to the time dependent nature of the flowfield and gives a tantalising glimpse into the unsteady behaviour of lean burn injector modules which is also discussed.