Assessing selected technologies and operational strategies for improving the environmental performance of future aircraft

Abstract

The aviation industry is expected to grow at a rate of 4-5% in the next 20 years. Such a growth rate may have important impacts on local air quality, climate change and community noise. This work assesses selected technologies and operational procedures aimed at improving the environmental performance of future airplanes. Two different studies are presented: estimating turbine durability benefits from jet engine water injection and evaluating improvements in fuel burn and operating costs from using advanced technology, high bypass ratio engines and varying the design cruise speed. Water injection in commercial airplane engine combustors lowers operating temperatures and can lead to significant reductions in NO, and soot emissions, potentially improving engine hot section life. With increasing fuel prices and possible introduction of emissions trading in the future, fuel burn may become a more critical aircraft design driver. Increasing engine bypass ratio and lowering cruise speed can lead to reduced fuel consumption. The dominant mechanisms of failure in turbine blades are low cycle fatigue, creep and oxidation. The Universal Slopes method is used to evaluate possible fatigue life benefits from water injection for three representative blade materials.

(cont.) For a 67K change in turbine inlet temperature, metal temperature changes up to 47K are expected. Life improvement with a 47K change in metal temperature is possible up to a factor of 1.90 for Inconel 625, up to 1.46 for Inconel 706 and up to 2.85 for Ren 80 depending on the strains imposed. Blade life effects of creep and oxidation for varying temperatures are presented based on a literature review. The absolute value of possible benefits strongly depend on material properties, metal and gas temperatures (internal and external to the blade) and stress levels. In addition, a maintenance cost analysis is performed to evaluate and compare benefits resulting from engine de-rate and water injection using an engine cycle program (GasTurb) and airline data for a typical 1970's technology mixed flow turbofan engine. A 67K change in Tt4 from water injection corresponds to an average de-rate value of 8.4%. Material maintenance costs in 2004 dollars are reduced by 16.52% to 2.86% for a 1-hour and 12-hour flight length respectively. Results show that shorter range flights, with more takeoffs per day, experience larger benefits.

(cont.) Engine durability analysis capabilities of a numerical simulation design tool - the Environmental Design Space (EDS) are examined. EDS currently does not have the capability for durability analyses and given the inherent difficulty in finding reliable physics-based models for part life prediction that do not require proprietary data, it seems unlikely that EDS will be able to develop such a capability. The engine bypass-ratio and cruise speed trade study is conducted for a 737-sized future airplane using Boeing internal tools and data. At higher cruise speeds a clear optimum bypass ratio value for minimizing fuel burn is found; for the UEET engines this optimum BPR is about 14. As cruise speed is lowered, fuel burn continually decreases with increasing bypass ratio for the engines examined. At a fixed bypass ratio, flying slower seems most beneficial for very high fuel prices for minimizing both fuel burn and operating costs.