May 16, 2007 - 1. A Comparison of Two Methods for Predicting Emissions from. Aircraft Gas Turbine. Combustors. Doug Allaire, Ian Waitz, and Karen Willcox.
A Comparison of Two Methods for Predicting Emissions from Aircraft Gas Turbine Combustors Doug Allaire, Ian Waitz, and Karen Willcox May 16, 2007
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Outline • • • • • • •
Background and Motivation Objectives Modeling Methodologies Assessing Predictive Capabilities Comparison of Emissions Estimates Comparison of Emissions Predictions Conclusions
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Background •
Environmental concerns expected to be one of the most important constraints on air transportation system growth
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There is a need take into account important interdependencies when considering policy options – Interdependencies among emissions (CO, NOx, and PM) – Interdependencies among various environmental impacts (community noise, local air quality, and climate change)
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U.S. Federal Aviation Administration is developing a suite of tools to assess these interdependencies – Requires the development of parametric representations of future vehicles – Developing a robust method for assessing the influence of various aircraft and engine parameters on pollutant emissions is one of the greatest challenges
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Emissions Modeling Emissions prediction techniques fall into four general categories – – – –
Empirical models Semiempirical models Simple physics-based models High-fidelity simulations
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Model Requirements For an emissions modeling method to be useful in a policymaking setting, the method should: 1. Represent the physical relationships among operating conditions, simplified combustor design parameters, and pollutant emissions in a consistent way 2. Use high-level design parameters that would be convenient for an expert to use in projecting future technology 3. Be general in the sense that it can be applied to estimate trends in combustor designs across engine manufacturers 4. Have well-understood uncertainties and limitations
Among the four general methods for estimating emissions, the empirical and physics-based approaches appear to have the most potential for meeting these requirements
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Objective
Develop and compare an empirical emissions modeling approach to a physics-based modeling approach for aircraft gas turbine combustors for application to understanding tradeoffs and interdependencies in a regulatory policy-making setting.
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Empirical Methodology • Desirable to use one type of correlation across all combustors rather than different forms of correlations for every type of combustor Common industry practice [Lecht et al.]:
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Empirical Methodology (cont.)
NOx Model
CO Model
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Physics-Based Methodology • Perfectly Stirred Reactor (PSR) [Turns] – Models combustion occurring with a Damkohler number of zero – Useful approximation for regions of high turbulence intensity and recirculation
• PLUG Flow Reactor (PLUG) [Turns] – Models combustion processes occurring in a uniform flowing fluid – Useful approximation for one-dimensional reacting flow
PSR
PLUG 9
Physics-Based Methodology (cont.)
Inputs: • T3 • P3 • mass flow rate • fuel flow rate • flow splits • zone volumes • primary zone unmixedness
Outputs: • EINOx • EICO
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Physics-Based Methodology (cont.) Primary Zone Unmixedness (s)
[Sturgess et al.]
s=
! "
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Physics-Based Methodology (cont.) Determining the number of PSRs required to model the primary zone
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Physics-Based Methodology (cont.)
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Responses to T3 variation
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Estimating Emissions
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Predicting Emissions
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Conclusions • Both models successfully estimate NOx emissions of current and potential future gas turbine combustors at high power conditions • Neither model estimates CO emissions of current and potential future gas turbine combustors well, but both models predict the correct trends • A case can be made for selecting either model for use in a policy-making setting – Physics-based: greater number of physical inputs – Empirical: simplicity
• More work must be done in assessing the predictive capabilities of each model • Work must be extended to different combustor designs and different pollutant emissions (unburned hydrocarbons and particulate matter) 17
Acknowledgements • The authors would like to thank Joe Palladino, Sean Bradshaw, and Stephen Lukachko for assisting with this work • The work was funded by the U.S. Federal Aviation Administration Office of Environment and Energy, Grant No. 03-C-NE-MIT, Amendment Nos. 011, 015, 018, 022, and 025. • FAA Project Manager Joe DiPardo
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Questions?
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References [1] Lecht, M., Doepelheuer, A., Madden, P., Norman, P., Oblaza, S., Park, K., Penanhoat, O., and Plohr, M. “In-Flight Engine Emissions Determination.” NEPAIR/WP2/WPR/01, August 2003. [2] Turns, S., 2000. An Introduction to Combustion: Concepts and Applications. McGraw-Hill, USA. [3] Sturgess, G., Zelina, J., Shouse, D., and Roquemore, W., 2005. “Emissions Reduction Technologies for Military Gas Turbine Engines.” Journal of Propulsion and Power, 21(2), March-April, pp. 193-217.
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Response to Primary Zone Equivalence Ratio Variation
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The Primary Zone • Designed to anchor flame and achieve close to complete combustion of the fuel – Requires sufficient residence time, high temperatures, and high turbulence – Typically have large recirculation regions of flow with high turbulence intensity
• Model the primary zone with perfectly stirred reactors
[From Ghoniem]
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