7.5. Combustion Technology

7.5.1. Introduction

Section 7.4 explained how major improvements in the fuel efficiency of engines have been achieved through parallel advances in turbomachinery aerodynamics, combustion, cooling, and materials. Significant progress in each of these areas is the direct result of industrial and governmental investment in balanced, interdependent research and technology programs that respond to commercial and environmental pressures. This section focuses on combustion technology, with particular emphasis on achievements to reduce emissions and the prospects of further improvements.

We first describe the modern gas turbine combustion system and the particular requirements placed upon this important engine component. Next we address emissions from combustors. In view of the wide availability of excellent reference material on this subject, however, only a brief assessment of the principal emissions is given here. A new emphasis has been placed on current research aimed at improving the understanding of soot formation and aerosols, a matter of increasing importance (see Chapter 3). We then outline progress made in reducing emissions in recent years. We go on to describe the specific part the combustor plays in controlling NOx emissions from engines and discuss prospects and plans for further improvements.

7.5.2. The Gas Turbine Combustion System

In the cross-sectional drawing of a modern civil aircraft engine (Figure 7-13), the combustor is shown in its central position between the compressor and turbine. High-pressure air enters the combustor at a relatively high velocity. The air is first carefully decelerated to minimize pressure losses, then forced into the combustion chamber, where fuel is added. The combustion chamber is designed to allow time and space for the fuel and air to mix thoroughly and burn efficiently before entering the turbine stages. Detailed design features of the combustor control the complex burning processes, thus the completeness of the chemical reactions involved and the nature and scale of individual emissions from the engine. Therefore, the combustor has a key role in determining the impact of aircraft on climate.

7.5.2.1. Combustor Features and Requirements

Aircraft engine combustors must meet the special requirements of operations over a very wide range of pressures and temperatures. The combustor must be able to ignite and accelerate the engine over a wide operational envelope. For instance, it must be able to ignite at high altitude (up to 9 km) after an unscheduled shutdown when the air is very cold (e.g., 220 K) and pressure is low (e.g., 0.03 MPa). It must also be able to maintain stable burning over a very wide range of air velocities and fuel/air ratios to prevent "flameout" during engine deceleration. At the other end of the power range, when pressures and temperatures are very high, the combustor must be able to burn fuel so that turbine components are presented with a smooth temperature profile, to minimize damage to the blades and vanes and thereby maximize service life. Together, these requirements present a major engineering challenge because the simplest solutions to meet requirements at one end of the operational envelope often conflict with those required at the other. Relatively recent emissions requirements have added considerably to the time and cost of developing combustors that fully satisfy the operational and environmental requirements placed on today's aircraft.

Future combustors are likely to face even more challenging requirements as manufacturers respond to the continuing need to increase fuel efficiency. As explained in Section 7.4.1, higher cycle pressure ratios lead to improved engine fuel efficiency. However, the compressor delivery/combustor inlet air temperatures rise, reducing the cooling capacity of the air. A greater proportion of total airflow is then needed to cool the hottest parts (liners, blades, etc.). In turn, this requirement reduces airflow available for primary combustion and dilution, making it more difficult to control turbine inlet temperature profiles and emission levels. This cycle of primary and secondary problems, which stem from increases in engine pressure ratios, can be broken only by continuously improving the effectiveness of cooling techniques and devices and/or the use of new and improved materials.

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