FLUG REVUE March 2003: Emission reduction work continues

EMISSION REDUCTION IN MODERN ENGINES

By Patrick Hoeveler

Probably only on the surface of Venus can it be more inhospitable: in the combustor of a BR715 engine, the pressure climbs to 35 bar at a peak temperature of up to 2000ºC at take-off. As if the design of such a highly loaded element were not challenge enough, the engine manufacturers are constantly developing new ideas for reducing further the emissions of noxious substances from modern jet engines.

A modern combustor has to satisfy a large number of requirements. In extreme situations, it must reignite following flame-out without problems and without any external help (windmilling). Just how important this can be is demonstrated by the example of a Boeing 747-400 which in December 1989 encountered an ash cloud from the Redoubt volcano in Alaska which caused all four engines to flame-out simultaneously. After a rapid descent, the crew managed to restart the engines.

However, the possibility of flame-out exists in cold and wet conditions as well, for example in a hailstorm. On such occasions the pilots switch over to continuous ignition of the two redundant igniters, as otherwise in normal operation these are ignited only on start-up, after which self-ignition of the air-fuel mixture is sustained. Cutting the fuel flow back quickly can be especially dangerous, as this can cause conditions in the combustor to suddenly become very lean. Because proportionately less fuel is available, the lean-extinguish threshold is exceeded, and as a result the process of self-ignition stops.

In addition to the requirement for stable combustion, the combustor must not only of course be compatible with the other components of the engine, but it must also remain mechanically stable irrespective of its size. Finally, on top of everything else, the engine must be economic with regards to fuel consumption and maintenance costs. Thus, for example, today's combustor liners and burners must have a service life of 10,000 hours.

Given the changes in global climate, pressure for environmental acceptability has increased rapidly in recent decades. Nitrogen oxide (NOx) accounts for the lion's share of the noxious emissions from jet engines. At high thrust and very high temperatures, NOx is produced through the combustion of hydrocarbons and the ensuing chemical reaction with the residual air. High temperatures must be avoided if emissions are to be reduced. But here the designers face a dilemma, since if they choose too low a temperature, then the engine will produce more of the equally undesirable carbon dioxide CO₂.

In the latest products from Rolls-Royce Deutschland in Dahlewitz, this balancing act has successfully been achieved, with combustion efficiency about 98% at idle and over 99% at full throttle. "With the existing technology we are on the safe side,” confirms the head of the combustor department, Dr. Hans-Jörg Bauer. Nevertheless they are not stopping here, but by the year 2010 they aim to have cut NOx emissions down to 50% of the CAEP/2 limits.

Already engineers have investigated the idea of a staged annular combustor which divides the two functions of the combustor into separate areas. In the pilot stage which has less combustion space, the fuel-air mixture is retained for a relatively long period. This results in stable combustion, as this stage is used in all modes of the engine. The reduction in nitrogen oxide occurs in the main stage, which cuts in from medium thrust levels upwards. Here the mixture has only a short retention time. In this way one avoids the stoichiometric area of the combustion plot, in which, thanks to an air:fuel ratio (AFR) of 15:1 combustion is most effective, and hence the hottest.

However, in the primary combustion zone the temperature can be as high as 2,200ºC, well above the melting point of the material. Moreover, this system requires significantly more air for combustion and has a greater combustor liner surface area. This makes cooling of the liner a problem. Up to now the air has flown almost tangentially into the liner through approx. 0.7mm wide holes and forms a protective film on it. In addition to this film cooling, Rolls-Royce Deutschland has developed another method, as the head of technology at the company, Dr. Helmut Richter, explained. "We are using effusion cooling to achieve a highly homogenous cooling film.” The laser drilled holes are cut obliquely and thus have a longer opening, in which heat transfer can already take place.

For the ANTLE programme, the experts from Dahlewitz are producing tiles from a superalloy based on nickel or cobalt, which also have a double wall and hence offer supplementary impingement-cooling as the air strikes the far side. The tiles have a ceramic coating with very effective insulating properties, enabling it to withstand temperatures of up to 1300ºC.

Despite all the progress, the staged combustor is so complex, for example, with its independent flow of fuel and twice the normal number of burners, that it has serious disadvantages as regards cost and weight. "We have not yet advanced far enough,” says Dr. Bauer, explaining the expenditure that will be necessary to develop the relevant research findings to the stage whether they can be applied to production. "We plan to go still further and at the same time to simplify the architecture.” Specific applications are already in the foreground.

The steps to be taken have already been determined in general terms, according to Richter, it is just the precise implementation that still requires further definition. "The gap between laboratory results and concrete use is very wide.” Finally, the "lean” concepts must work faultlessly for all flight envelopes and eventualities such as compressor surge.

One possible route is to use lean direct injection (LDI), under which the air is rotated up to a peak speed of 100m/s at the side of the combustor by radial swirlers before then meeting a film of fuel droplets.

The next step for Rolls-Royce's Vision 20 technology generation, as it is called, could be lean pre-mixed pre-vaporisation (LPP). Essentially this entails vaporising the fuel and mixing it completely with the air before it flows into the combustor. In this way, the designers are hoping to achieve a long retention period (but still in the region of milliseconds) and as homogenous a mixture as possible.

However, the retention period must not be too long as otherwise there is a danger that the fuel will spontaneously ignite before the actual burning space, causing a flashback of the flames, with catastrophic consequences. To avoid this, according to Bauer, long-term research work is necessary, especially as only half as much cooling air is still available since three-quarters of the total air flows through the burner.

Another possibility is to stage the nozzles instead of the combustor. According to Richter, if there are two separate fuel lines in one burner, producing one primary and one secondary flame zone, the number of nozzles can be reduced to fewer than 20. In present BR715 engines, the combustor possesses 20 nozzles. A staged variant would have 40 units. However, this method brings with it the problem of combustion stability.

On the other hand the other components of future engines are also affected by these developments. Because of the reduced cooling and the greater flow turbulence, the first disc in the turbine has to endure a significantly greater load, in a "dramatic change”, as Richter puts it. The phenomenon of clocking, the interaction of the thermal and aerodynamic fields of the individual components, especially the turbulence fields, requires careful investigation, as otherwise there might be a loss of turbine efficiency.

Another difficulty is thermoacoustic vibration, which in a feedback process reinforces the acoustic shock waves generated during combustion through resonance in the combustor. The engine specialists are using the cooling elements to counter this, since these also serve as acoustic resonators.

However, the push to increase overall pressure ratios is not making the sought-for improvements any easier. By 2020, Richter believes, ratios of up to 60:1 will be possible on large engines. The most powerful engine around today, the GE90-115B from GE Aircraft Engines, manages 42:1. Higher overall pressure ratios in turn mean higher inlet temperatures into the combustors, while higher bypass ratios will entail more and more air bypassing the core, resulting in a richer fuel flow through the combustor. Thus, the combustor specialists can hardly complain that there are not enough challenges. "It will take us until 2020,” says Richter.

From page 98 of FLUG REVUE 3/2003

Last updated 15 February 2003

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