Commercial Aircraft Propulsion and Energy Systems Research Reducing Global Carbon Emissions (2016) / Chapter Skim
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3 Aircraft Gas Turbine Engines
3 Aircraft Gas Turbine Engines
Pages 35-50
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From page 35... ...
Thus, any discussion of reducing carbon emissions from commercial aircraft will need to consider the potential for improvement of gas turbine engines. To that end, this chapter will delineate the current state of the art of aircraft engines, discuss the potential for and constraints on gas turbine improvement over the next three decades, and suggest research directions to achieve such improvement.
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From page 36... ...
Motor thermodynamic efficiency of commercial aircraft engines has improved from about 30 percent to over 50 percent over the past 50 years, as shown in Figure 3.3. Most commercial airline engines are designed to maximize efficiency at cruise, since that is where most fuel is burned.
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From page 37... ...
Gas Turbine Characteristics Gas turbine engines have several characteristics that distinguish them in important ways from other power plants such as internal combustion engines or electric drives. All engines produce waste heat that must be rejected.
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From page 38... ...
90% 80% Propulsive Efficiency 70% 60% ~0.3 percent per year 50% 1960 1970 1980 1990 2000 2010 2020 Entry Into Service, Year FIGURE 3.4 Trend with time of commercial aircraft turbofan propulsive efficiency at cruise.
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From page 39... ...
This means that compared to requirements at cruise, extra propulsion weight need not be carried nor drag incurred for an airliner to be able to take off. Also, since most gas turbine engines are already optimized for minimum fuel burn during cruise, there is little to be gained by better matching of the engine to the airplane characteristics.
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From page 40... ...
Perhaps most importantly, since as airplane and engine efficiency improves, less power is needed for flight, the engine size and power required at constant airplane capability will decrease in the FIGURE 3.6 Variation of motor thermodynamic efficiency at cruise with engine size (in terms of sea level power) for existing aircraft turbine engines.
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From page 41... ...
Practical intercooled or recuperated cycles could increase efficiency by another 4.4 Improved fans and propellers could also increase propulsive efficiency by 10 percent.5 Of course, the practical limits to propulsive efficiency cannot be addressed at the engine level alone without reference to airplane configuration and propulsion integration, as discussed in Chapter 2. To summarize, aircraft gas turbine engines have considerable room for improvement, with a potential to improve overall efficiencies by 30 percent or more over the best engines in service today, with the potential for improvement of propulsive efficiency being about twice that of thermodynamic efficiency.
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From page 42... ...
Work in this area is important no matter the choice of motor to power the propulsor. The second part is improving the motor thermodynamic efficiency of an aircraft gas turbine engine.
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From page 43... ...
, propulsor-duct solutions must be found that are acoustically and aeromechanically acceptable and in which the losses due to distortion are small compared to the gains from wake cancellation. Improving Thermodynamic Efficiency There is a vast literature on aircraft gas turbine engines and the improvements needed to reduce fuel burn.
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From page 44... ...
Also, as discussed above, gas turbine engines for smaller aircraft are less efficient than engines for larger aircraft. Materials and Manufacturing The history of the aircraft gas turbine engines is the history of advanced material development specifically aimed at improving gas turbines; some highly successful examples include forged titanium alloys (now widely used in aircraft structure as well)
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From page 45... ...
Also, as the overall pressure ratios (OPRs) of engines have been increased to improve thermodynamic efficiency, the flow areas and thus the dimensions of airfoils in the core, especially at the rear of the compressor and in the highpressure turbine, have shrunk dramatically.
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From page 46... ...
As engine overall pressure ratios are increased to improve thermodynamic efficiency and reduce CO2, combustor design will be further challenged to meet both emissions and mechanical integrity goals. Areas that may be helpful include new design concepts and improved modeling tools, especially physics-based approaches capable of accurate prediction of regulated emissions.
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From page 47... ...
As aircraft subsystems become more electrical and as fan pressure ratios drop to improve propulsive efficiency, this challenge will be exacerbated. The inefficiency of current fuel pumps consumes much of the heat capacity of the fuel flow that would otherwise be available for the cooling needed by other aircraft heat sources.
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From page 48... ...
CHALLENGES Aircraft gas turbine challenges were discussed above to elucidate some of the many opportunities available to improve engine performance. These opportunities are often presented in a traditional, disciplinary sense: • Materials and manufacturing, • Turbomachinery -- aerodynamics and structural concepts, • Heat exchangers, • Low-emissions combustion systems operating at very high pressure ratios, • Controls and accessories,
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From page 49... ...
The areas whose promise for reducing CO2 emissions over the next three decades justifies the most investment are summarized in the following challenges: Technical Challenges Propulsive Efficiency Low fan pressure ratios are needed to reduce exhaust velocities and thereby improve propulsive efficiency, regardless of whether the fan is driven by a gas turbine or an electrical motor. For a constant level of thrust, this requires that the effective fan area increase so as to avoid commensurate increases in weight, drag, and integration losses.15 Thermodynamic Efficiency Enabling higher operating temperatures is a prerequisite to achieving significant improvement in gas turbine engine thermodynamic efficiency, and a major impediment to achieving higher operating temperatures is the difficulty of developing advanced materials and coatings that can withstand higher engine operating temperatures.
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From page 50... ...
Engine Materials and Coatings Develop materials and coatings that will enable higher engine operating temperatures. Key research topics for this project are advanced materials that could lead to the reduction or elimination of turbine film cooling as well as to compatible coatings for environmental protection, erosion prevention, ice rejection, and thermal barriers.
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