ХИМИЧЕСКАЯ ФИЗИКА, 2GG4, том 23, № 9, с. 37-45
ГОРЕНИЕ И ВЗРЫВ
УДК 541.126
THE PROSPECTS OF A COMBINED AIRFRAME-ENGINE SIMULATION ENVIRONMENT FOR THE PREDICTION OF AERO-GASTURBINE
EXHAUST EMISSIONS
© 2004 S. A. Shakariyants, J. P. van Buijtenen, W. P. J. Visser
Delft University of Technology, Mekelweg 2, 2628 CD, Delft, The Netherlands National Aerospace Laboratory, Flight Division, A. Fokkerweg 2,1059 CM, Amsterdam, The Netherlands
Received 01.10.2003
The year 2003 marks a century of the engine-powered flight. Although being born just 100 years ago, aviation has experienced rapid growth and expansion towards an integral and vital part of modern society. At the same time, this growth has raised worries about the associated environmental impact. An essential area of concern in this respect is a lack of methods and tools for aero-engine exhaust emission prediction and analysis, which would enable to respond the problem of aviation pollution. In a commitment to contribute to the solution of the problem, a Research Project has been launched at the Thermal Power Engineering Section of the Delft University of Technology in July 2002, named "Aero-Gasturbine Emission Reduction and Simulation Technology". The Project is being conducted in collaboration with the Dutch National Aerospace Laboratory (NLR) and the Netherlands Ministry of Transport, Public Works and Water Management. The Conceptual Study of the Subject is represented in the current paper.
INTRODUCTION
Aircraft engines produce exhaust emissions as a result of the combustion of fossil fuel. The engine exhaust is composed of carbon dioxide (CO2), water vapor (H2O), unburned hydrocarbons (UHC), carbon monoxide (CO), particulate matter or smoke emission (up to 96% carbon, C), oxides of nitrogen (NOX), and excess atmospheric oxygen (O2) and nitrogen (N2).
It is widely recognized that aircraft technology has gained major advances in the past 40-50 years, steadily bringing significant gains in the reduction of exhaust emissions at the source. Thus, since the 1960s, UHC emissions are reduced by some 98 percent, the levels of CO are down around 90 percent, emissions of NOx are lowered by almost half (Ref. [1]), and smoke reduction measures have resulted in a usually "transparent" modern aero-engine exhaust plume. The typical levels of
primary-pollutant emissions (UHC, CO, NOX, C) from in-production turbofan engines, offered to power commercial jet airplanes of a 100+ passenger capacity, are provided in Table.
Except for the technological advances, a remarkable progress has been made in operation optimization, regulatory, and market-based measures, directed to reduce the level of emissions. However, the cumulative amount of aircraft emissions is expected to inevitably increase, as air transport is projected to grow. This increases the need for further progress in developing analytical methods and simulation techniques to predict the amount and composition of exhaust gases from aircraft engines. This progress will help to better assess the alternatives for reducing emissions, and better inform decision-makers, manufactures and operators.
Typical Emission Levels for In-Production Turbofan Engines (data from ICAO, Ref. [2])
Species Units Power Settings
Take-Off Climb Descent Idle
UHC (as CH4) g kg fuel 0.008-0.69 0.008-0.94 0.01-9.1 0.06-11.8
CO g kg fuel 0.02-4.48 0.02-12.18 0.4-38.4 6.48-49.75
NO, (as NO2) g kg fuel 13.25-65.84 10.41-46.31 6.37-17.34 3.4-7.78
Smoke Smoke Number 0.3-16.8 0.3-14.6 0.5-6.7 0.04-9.9
PKPs in trillion 10 r
9 8 7 6 5 4 3 2 1
History Forfecast _L 1
Prior to economic slpwdo Growth rate 5% I
- T Growth rate 4.9% After economic slowdoWn
&90
1995 2000 2005 2010 2015 2020
Years
Fig. 1. World civil air traffic growth.
CO2 emissions, Gt C/year 0.5 r
0.4 0.3 0.2 0.1
Optimistic forecast: -Traffic Growth Rate 5%
Pessimistic forecast: Traffic Growth Rate 2.2%
1990 1995 2000 2005 2010 2015 2020
Years
Fig. 2. Annual carbon dioxide emissions produced by civil aircraft.
1. ENVIRONMENT AND SUSTAINABILITY
As the world economy has been growing over the past 50 years, the civil aviation industry has experienced rapid expansion. Although, starting from 2001, a declining world economy has slowed down air travel growth, it is widely believed that the basic airline industry positive trend will remain un-changed. Certainly, a number of scenarios could be postulated, however, if air travel grows, the associated environmental impact will grow with it.
Figures 1 and 2 represent a projected increase in air traffic, based on airline statistics (Ref. [3, 4]) and industry forecasts (Ref. [5]), and the associated increase in carbon dioxide emissions, expressed in giga-tones (Gt) of carbon per year. The emission of carbon dioxide clearly illustrates environmental concerns, as it directly reflects the amount of fuel burned. Its forecast is essentially based on the data from IPCC's report "Aviation and the Global Atmosphere" (Ref. [6]), subtracting the emissions from military aviation, estimated at about 25% from the total amount of CO2 produced by aviation.
The increased amount of aircraft engine emissions will aggravate the aviation environmental impact. Eventually, environmental issues could undermine growth of air transport either directly, such as through limits and constraints, or indirectly through public opposition to the infrastructure expansion. Thus, there is a direct link between paying sufficient attention to environmental performance and sustainable growth. This link stipulates that, while satisfying our own needs, we should not compromise the abilities of the future to meet their demands.
2. FROM MEASUREMENTS TO SIMULATION
2.1. Current Certification Practice
To achieve a maximum compatibility between the safe and orderly development of air transport and the quality of the environment, a mandatory character of
civil aircraft jet engine exhaust emission certification is recommended by the International Civil Aviation Organization (ICAO), (Ref. [7]) and enforced by the National Aviation Authorities. Every engine type must be certified by means of being run on a stationary test bed throughout the power settings, corresponding to a reference landing-takeoff (LTO) cycle. The LTO cycle represents aircraft operation in the vicinity of an airport below 915 m of altitude (3000 ft), including taxi-out, take-off, climb, approach, and taxi-in.
The emissions, which are specified to control, are:
- Gaseous Pollutants: UHC, CO and NOX; and
- Smoke
Gaseous Pollutants emitted during the reference LTO cycle shall be measured and reported not to exceed the mandatory levels.
For the emissions of unburned hydrocarbons and carbon monoxide, these limits were initially specified by the ICAO in terms of mass of emissions (D , g) per unit of engine thrust (F0, kN), (Eq. (1) and Eq. (2)), (Ref. [7]) and remained not revised so far.
Unburned Hydrocarbons (UHC): Dp/F0 = 19.6 (ICAO, Ref. [7]),
Carbon Monoxide (CO): Dp/F0 = 118 (ICAO, Ref. [7]).
(1)
(2)
While the standards for oxides of nitrogen emissions, additionally taking into account the engine overall pressure ratio (n0), were amended a number of times. The NOX standards, given in a time perspective, are outlined in Fig. 3.
To carry out engine emission certification, the ICAO also prescribes stationary measurement techniques applicable to the pollutants of consideration (Ref. [7]).
Smoke emissions are prescribed to measure and report in terms of smoke number (SN). The mandatory value of the SN was specified by the ICAO as the ear-
Dp/F0 = 32 + 1.6n0
31-12-95
31-12-03
31-12-99 Dp/Fo = 40 + 2no
Legend:
Production Start
Time
Dp = f(Fo, no)
Fo no
<30 30...62.5 >62.5
26.7...89 Dp/F0 = 37.572 + + 1.6n0 - 0.2087F0 Dp/F0 = 42.71 + + 1.4286n0 - 0.4013F0 + 0.00642n0F0 + ^ o m II ^O O h, +
>89 Dp/F0 = 19 + 1.6n0 Dp/F0 = 7 + 2.0n cf
- Applicability
Fig. 3. NOx Emissions regulatory levels (data from ICAO, Ref. [7]).
liest aircraft engine emission standard, which remained as follows (Ref. [7]):
Regulatory Smoke Number:
(3)
SN = min{83.6(F0)-0'274; 50} (ICAO, Ref. [7]).
The ICAO-prescribed certification method for smoke emissions is based upon the measurement of the reduction in reflectance of a filter, when stained by a given mass flow of exhaust sample. A reflectometer-mea-sured reduction in reflectance is then expressed as smoke number.
2.2. Airframe-Engine Simulation Environment
The National Aviation Authorities of ICAO Contracting States have been implementing the recommended aero-engine emission certification practices since the late 1970s - early 1980s. However, despite the rising concern about the potential environmental and health harm associated with aero-engine emissions, the regulations were never swept over the whole variety of aircraft operational cycles. Besides, the recommended measurement techniques, as well as another standard methods, based on sampling exhaust gases and processing the samples to analyzers, are not feasible for inflight application.
As a result of extensive studies, carried out throughout the world, an option was offered to trail an aircraft engine under investigation with another aircraft equipped with gas sampling and gas analysis instrumentation. Yet, in the measurement results, the dilution of emissions in the environment has to be eliminated, which introduces a considerable uncertainty in measurements (Ref. [8]).
Eventually, as the complexity, required labor, and cost of measurement techniques increase, the ability to predict aero-engine emissions by means of analytical tools becomes more urgent. Although, simulation technology may not eliminate a need in measurements, its role in optimization process and sensitivity analysis turns crucial.
The complexity of the above outlined problem requires obtaining a combined airframe-engine simula-
tion environment for a comprehensive study of aeroengine exhaust emissions (Fig. 4).
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