Future Trends in Commercial Aviation Engines’ Combustion

Abstract

This article gives an overview of the current rich-dome combustion system design, requirements and challenges; followed by the first alternative to rich-domes that have been successfully introduced as products; the lowest levels of achievable NO_x (so called entitlement) as determined from small scale rig testing; summary of recent engine emissions data with “green” alternative fuels; description of the 2nd alternative to rich-dome products that may be of interest to the OEM’s for the N+2 and N+3 generation aviation engines; a brief discussion on the modeling and correlation accuracy expectations from future efforts in this area; the 3rd alternative to rich domes which was shown promising for autoignition times closer to 0.2 ms. The article concludes with a short section on operability and dynamics. Several large low-NO_x rich domes’ takeoff NO_x emission index is reproduced well by a simple correlation NO _{x RD −L} = 0.0303PR ^1.9722 w / R ² = 0.9906 including Talon II and Trent1000. However, the LTO NO_x is correlated well by a similarly good quality curve only for the group of combustors without Trent1000, DP / F ₀₀ = 0.6793PR ^1.2241 compared to lower value expression for Trent1000 alone given by DP / F ₀₀ = 0.1292PR ^1.6327. Consistent with the NO_x stringency pattern set by CAEP4, CAEP6 and CAEP8 and longtime goal for achieving 85 % reduction in takeoff NO_x at 30OPR, we propose the long-term LTO regulatory standard of CAEP / 18 = −37.763 + 2π effective December 31, 2033. The combustor inlet temperatures for desired overall pressure ratio at sea-level standard day static condition can be estimated by using T _3,SSS = 317.544OPR ^0.2704; 320.1955OPR ^0.272, respectively for the N+1 and N+2 generation engines. This along with generally accepted requirements for combustor operability, we have to manage significantly increased range of P₃, T₃ and fuel/air ratio, viz. T₃: 216.67–1084 K; P₃: 0.33–60 atm; ΔP: ~ (0.1–1.2) ΔP_design; and FAR: FAR_min - FAR_max. The numerical values of the dome design pressure drop (ΔP_design), minimum and maximum fuel air ratios (FAR_min and FAR_max) depend upon the combustion system design and its potential applications. We will assume typical values of these variables, respectively, 3–5 %, 0.005–008 and 0.025–0.040 for the aviation engines. The 1st generation of lean dome products met their original objectives of achieving lower NO_x within the specified design constraints including cooling technology. They were immediately followed by the 2nd generation lean dome products known popularly as TAPS in GEnx in addition to planned LEAP-X and GE9X. The takeoff NO_x of GEnx is given by NO_{x GEnx} = 1.079 × 10^-5 OPR ^3.971 W / R ² = 0.991. These products will be able to meet the proposed long-term LTO NO_x regulatory standard within the generally accepted design modification and refinement process. TALON-X and recently introduced P&W TALON-Axially Staged Combustion concept will provide credible alternatives to TAPS; an interesting technologies competition. The 2nd generation lean domes produce an order of magnitude lower exhaust smoke number than the rich domes. For all other design requirements both the lean and rich domes have comparable characteristics. The effect of FT fuel blends on combustion efficiency and NO_x is insignificant; but its benefits in regard to particulate emissions are enormous in terms of both the number density and mass emissions due primarily to significantly lower aromatic and sulfur contents. Future CFD and semi-empirical model should be developed with the proposed long term accuracy goal expressed in term of the standard deviation σ goal: 3 % of takeoff NO_x, 7.5 % and 15 % respectively of idle CO and HC emission indices.